Sensors Based on Nanostructured Materials

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Sensors Based on Nanostructured Materials

Francisco J. Arregui Editor

Sensors Based on Nanostructured Materials

13

Editor Francisco J. Arregui Universidad Pu´blica de Navarra Depto. Ingenieria Electrica y, Electronica Campus Arrosadia, s/n 31006, Pamplona, Spain

ISBN: 978-0-387-77752-8 e-ISBN: 978-0-387-77753-5 DOI: 10.1007/978-0-387-77753-5 Library of Congress Control Number: 2008928442 # Springer ScienceþBusiness Media, LLC 2009 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer ScienceþBusiness Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper springer.com

Acknowledgments

As Editor, I want to express my gratitude to all the members of Springer US for their assistance and help, to Angela DePina and Jennifer Mirski for their infinite patience. Steven M. Elliot, who truly believed in this project since the very first moment and who made it possible to join all the pieces of this challenging puzzle, deserves a special mention. My colleagues and friends Rick Claus and Ignacio Matias kept their support for this book from the start and I am very grateful to them. The availability of the rest of contributors of this book has been amazingly encouraging. These scientists are some of the most active authors in the world in the nano & sensor field. It has been really great to work with Professors Craig A. Grimes, Yoke Khin Yap, Manuel Vazquez, Michael J. McShane, John T.W. Yeow, Joan R. Morante, Richard O. Claus and Ignacio R. Matias. They were also the ones who convinced other colleagues to participate in this book. Of course, I would not like to forget any of the authors, with a special and thankful remark to Prof. Zheng Wei Pan who attended the request made by Prof. Yoke Khin Yap. For the rest, I want to acknowledge the work made by the people in Pennsylvania; Boston; Waterloo (Canada); Barcelona, Madrid and Pamplona (Spain); Texas and last, but not least, Virginia: N. Sinha, Kristen E. La Flamme, Abhishek Prasad, Samuel Mensah, Teresa Andreu, Jordi Arbiol, Andreu Cabot, Albert Cirera, Joan Daniel Prades, Francisco Hernandez-Ramirez, Albert Romano-Rodriguez, Ignacio Del Villar, A. Asenjo, M.P. Morales, K.R. Pirota, G. Badini-Confalonieri, M. HernandezVelez, Jennifer Lalli, Bradley Davis and Christelle Jullian. Special thanks also to Javier Goicoechea for his world-record response time. I also want to acknowledge all the students, colleagues and friends of the Universidad Publica de Navarra who have shared their time, dedication and enthusiasm with us. They have been inspiring. I am also grateful to Dr. Paul Urquhart for his always interesting suggestions and to Ms. Manjula Jude for the final editing of the book. Besides the authors, editorial staff, reviewers and others who participated in this book, we would also like to express our gratitude to all their families, friends and colleagues for their understanding. v

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Acknowledgments

If I express gratitude to the participants’ families, I cannot forget mine, just saying thanks to them is a very small thing (a nano thing!) compared to all their support over these years. Thanks to my Encarna, Modesto, Mikel, Joseba, Miriam-Larri and Isabel. Everyday they remind me that there is ‘‘plenty of room at the bottom’’.

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Francisco J. Arregui

1

2

Carbon Nanotube and Fullerene Sensors . . . . . . . . . . . . . . . . . . . . . . John T. W. Yeow and Niraj Sinha

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3

Non-carbon Nanotubes: Hydrogen Sensors Based on TiO2 . . . . . . . . Kristen E. LaFlamme and Craig A. Grimes

29

4

Alternative Nanostructured Sensors: Nanowires, Nanobelts, and Novel Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abhishek Prasad, Samuel Mensah, Zheng Wei Pan, and Yoke Khin Yap

5

Nanosensors: Controlling Transduction Mechanisms at the Nanoscale Using Metal Oxides and Semiconductors. . . . . . . . . . . . . . . . . . . . . . Teresa Andreu, Jordi Arbiol, Andreu Cabot, Albert Cirera, Joan Daniel Prades, Francisco Hernandez-Ramı´ rez, Albert Romano-Rodrı´ guez, and Joan R. Morante

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79

6

Quantum Dots for Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Javier Goicoechea, Francisco J. Arregui, and Ignacio R. Matias

131

7

Nanostructured Magnetic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . Manuel Va´zquez, Agustina Asenjo, Maria del Puerto Morales, Kleber Roberto Pirota, Giovanni Badini-Confalonieri, and Manuel Hernańdez-Ve´lez

183

8

Encapsulated Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Michael J. McShane

253

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Contents

9

Optical Fiber Sensors Based on Nanostructured Coatings . . . . . . . . . Francisco J. Arregui, Ignacio R. Matias, Javier Goicoechea, and Ignacio Del Villar

275

10

Nanostructured Flexible Materials: Metal RubberTM Strain Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Christelle Jullian, Jennifer Lalli, Bradley Davis, and Richard Claus

303

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

319

Contributors

Francisco J. Arregui Electric and Electronic Engineering Department Universidad Pu´blica de Navarra, Edificio de los Tejos Campus Arrosadı´ a, 31006 Pamplona, Navarra, Spain Teresa Andreu Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Jordi Arbiol Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Agustina Asenjo Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain Richard Claus NanoSonic Inc. 1485 South Main Street Blacksburg, VA 24060 Andreu Cabot Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Albert Cirera Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Giovanni Badini-Confalonieri Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain Bradley Davis NanoSonic Inc. 1485 South Main Street Blacksburg, VA 24060 Ignacio Del Villar Public University of Navarre, 31006 Pamplona Spain Craig A. Grimes The Pennsylvania State University Electrical Engineering University Park, PA 16802 Javier Goicoechea Electric and Electronic Engineering Department Universidad Publica de Navarra, Edificio de los Tejos, Campus Arrosadia, 31006 Pamplona, Navarra, Spain Francisco Hernandez-Ramı´ rez Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Manuel Hernańdez-Ve´lez Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain

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Contributors

Christelle Jullian Department of Materials Science and Engineering Virginia Tech Blacksburg, VA 24061 Jennifer Lalli NanoSonic Inc. 1485 South Main Street Blacksburg, VA 24060 Kristen E. LaFlamme Boston University Boston, MA Samuel Mensah Department of Physics Michigan Technological University Houghton, MI 49931 Joan R. Morante Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Ignacio R. Matias Electric and Electronic Engineering Department Universidad Pu´blica de Navarra, Edificio de los Tejos Campus Arrosadı´ a, 31006, Pamplona, Navarra Spain Maria del Puerto Morales Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain Michael J. McShane Biomedical Engineering Department Texas A&M University College Station, TX 77843 Abhishek Prasad Department of Physics Michigan Technological University Houghton, MI 49931 Zheng Wei Pan Faculty of Engineering & Department of Physics and Astronomy University of Georgia Athens, GA 30602 Joan Daniel Prades Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Kleber Roberto Pirota Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain Albert Romano-Rodrı´ guez Department of Electronics Faculty of Physics University of Barcelona Barcelona E-08028 Spain Niraj Sinha Department of Systems Design Engineering, University of Waterloo, 200 University Avenue (W), Waterloo, ON N2L 3G1, Canada. Manuel Va´zquez Instituto de Ciencia de Materiales CSIC 28049 Madrid Spain John T.W. Yeow Department of Systems Design Engineering, University of Waterloo, 200 University Avenue (W), Waterloo, ON N2L 3G1, Canada. Yoke Khin Yap Department of Physics Michigan Technological University Houghton, MI 49931

Chapter 1

Introduction Francisco J. Arregui

1.1 Some Data About Nanotechnology In this digital era in which we live, human beings are creating new devices, machines and systems that apparently make our lives easier, more pleasant and more comfortable. These new artifacts have to be incorporated in cheaper, faster, smaller and more complex ‘‘technological organisms’’ that organize themselves in cheaper, faster, smaller and more complex ‘‘technological organizations’’. Thanks to the effort that humanity is making now, perhaps the future of the world will be better but what is for sure is that we can achieve things today that were not possible yesterday. For instance, we can chat, talk and see our beloved ones when they are a world away from us, just by using a small computer in our living room. We can have access to the biggest encyclopedia that the man was able to create: Internet. We can have medicines, diagnosis tools and implants that can deal with health problems in a way that was unknown until today. This continuous progress is possible because all of us, in the Newtonian way, are able to see farther than others because we stand ‘‘on the shoulders of giants’’. As was stated above, it is obvious that the partial democratization of the technology, that is, the possibility to access computers that can connect each other in this world, has opened the door to the generation of a new kind of knowledge as a result of the cooperative effort of many. This revolution would not have been possible without the continuous innovation of the semiconductors. The Moore’s Law which very optimistically predicted that ‘‘the number of transistors that can be inexpensively placed on an integrated circuit will be increasing exponentially, doubling approximately every two years’’ has been proved not so optimistic and has been ruling the microlectronics market for more than 40 years now. That word ‘‘microelectronics’’ has been the key of all we are talking about now. The key to make possible that we (the authors of this book) could control and monitor new experiments, or just press the keyboard, to obtain strange symbols and graphs in the screen of a computer F.J. Arregui Electric and Electronic Engineering Department, Universidad Pu´blica de Navarra, Edificio de los Tejos, Campus Arrosadı´ a, 31006 Pamplona, Navarra, Spain

F.J. Arregui (ed.), Sensors Based on Nanostructured Materials, DOI: 10.1007/978-0-387-77753-5_1, Ó Springer ScienceþBusiness Media, LLC 2009

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F.J. Arregui

and some months later, maybe years, some readers (yourself!) have the chance to see what we saw after working very hard to obtain our modest results. That part of the word ‘‘micro’’ has been with us in the last decades; ‘‘micro’’ has been part of the common travel that humanity initiated. But now when we are just beginning to see that the ‘‘micro’’ world is quite big, we are just beginning to observe even smaller things. Things that go beyond the micron, things that belong to the nanometer scale, to the nano reign. Half a century has already passed since Nobel laureate Richard P. Feynman stated in 1959 that there was ‘‘plenty of room at the bottom’’, what is considered by many as the formal initiation of nanotechnology [1]. The visionary challenge and proposal of Feynman pointed out that ‘‘At the atomic level, we have new kinds of forces and new kinds of possibilities, new kinds of effects. The problems of manufacture and reproduction of materials will be quite different’’. This aspect of nanotechnology, where a number of properties of the matter change when the nanoscopic level is reached, is what is very attractive to explore for the design of materials with new properties and applications not known yet. Today, we can already find nanotechnologic applications on diverse fields such as drug delivery, hard coatings or antimicrobial textiles. Just the appearance of the word ‘‘nano’’ in referred scientific papers has exploded as is shown in Fig. 1.1. In fact, in 2008 there are more scientific journal articles that include the word nano published in just 1 day, around 115, than in the whole year 1970. According to the databases of ISI Web of KnowledgeSM and ScopusTM, the sum of all the scientific journal articles that include this simple word, nano, has surpassed a quarter million articles (around 350,000 papers including conference papers).

45,000

Number of Journal Articles

40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 07 20 06 20 05 20 04 20 03 20 02 20 01 20 00 20 99 19 98 19 97 19 96 19 95 19 94 19 93 19 92 19 91 19 90 19 89 19 88 19 87 19 86 19 85 19 84 19 83 19 82 19 81 19 80 19

Year of Publication

Fig. 1.1 Number of scientific journal articles with the word ‘‘nano’’ in the abstract, keywords or title since 1980 (data extracted from ScopusTM database)

1 Introduction

3

5%

5.18%

4.82%

3.94%

3.77%

2.99%

2.25%

1.88%

1.87%

2.08%

1.48%

1.92%

1.73%

2.45%

2.03%

0.10%

1.42%

0.12%

0.00%

0.17%

1%

0.37%

2%

2.87%

3%

4.63%

4%

0.00%

Ratio of Journal Articles (%)

6%

0% 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year of Publication

Fig. 1.2 Ratio of journal articles that include the word ‘‘sensor’’ from the ones which already include the word ‘‘nano’’ (data extracted from ISI Web of KnowledgeSM and ScopusTM databases)

Besides, not only the number of scientific publications with the word ‘‘nano’’ has increased exponentially, but in this specific group of articles around the nano-world the ratio of scientific publications related to sensors has also increased as is plotted in Fig. 1.2. After studying this data, it is easy to find an explanation to this trend, but if we have to name some of the reasons we could say that the advantages that nanotechnology introduces in the sensor field are many: obviously a decrease in the sensors’ dimensions (or at least the sensing films), an increase of surface area, a shorter diffusion time (therefore a shorter response time) and, the most interesting thing, the possibility of designing materials with tailored and new properties not presented in the bulky or macroscopic versions of the same compounds. The top 50 most cited journal articles that include the words ‘‘nano’’ and ‘‘sensor’’ when the introduction of this book is being written, May 2008, are indicated in the bibliography [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]. It is clear also that great economical efforts have been made by the governments, agencies and private foundations which have been able to share this vision. Today the tendency to dedicate funds to nanoscience and nanotechnology is increasing and numerous ‘‘nanotechnology centers’’ are appearing around the globe, usually linked to universities or research institutions. As an example of this institutional support, in 2000, the United States National Nanotechnology Initiative (NNI) was founded to coordinate federal nanotechnology research and development; the budget of this agency is plotted in Fig. 1.3. Until now, all things that have been said in this introduction with respect to nanotechnology are only advantages with respect to the consolidated technologies and with all the promising benefits of nanotechnology it is easy to fall in self-complacence. It is evident that when a new world is in front of our eyes, everything looks an adventure, a challenge and a new opportunity, and it is easy to set off the epidemic of a gold fever, the nano-fever because this is a new world

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F.J. Arregui

1,527 *

2009*

1,491 *

2008*

1,425

2007

1,351

Year

2006 1,200

2005 989

2004 863

2003 697

2002 464

2001 0

200

400

600

800

1,000

1,200

1,400

1,600

NNI budget (US dollars in millions)

Fig. 1.3 The United States National Nanotechnology Initiative budget (source the NNI web site, www.nano.gov *2008 and 2009 show the estimated and proposed budgets, respectively)

with new rules, a new world to explore and discover and a new world that needs pioneers. Inevitably, a time for quest is also a time of confusion and it is easy to talk and promise gold but all that glitters is not gold and we have to be rigorous, hard workers and have the patience to pursue real objectives. Today, it is easy to see that many applications that are presented as ‘‘nanotechnology’’ are in fact a recasting of straightforward materials science, and some researchers already talk that there may be a danger that a ‘‘nano bubble’’ will form from the overuse of the term by scientists and entrepreneurs to collect funding, spoiling the transformative possibilities of nanotechnology. In this quest, we will have to find the equilibrium with this new nano-world, learn its new language and be respectful with its laws. On the other hand, sensors are our devices for perception of sight, hearing, taste, smell, touch and many more things beyond our five senses. Sensors can be used for detection of bacteria, viruses, explosives, biological markers, magnetic fields, infrared radiation, traces of pollutants, medical diagnostic tools and many other applications. Nanotechnology and sensors are two disciplines that have in common its high degree of interdisciplinarity and can be combined very easily. Since we are able now to synthesize new materials arranged at the nanometer scale we are also able to fabricate materials with properties different to the original bulk materials. This has opened the door to new and real applications for sensing. These nanostructured materials take advantage of the arrangement of matter on the atomic and molecular scales, generally 100 nm or smaller, and the fabrication of devices that lie within that size range, an intermediate size between molecular and microscopic (micrometer-sized) structures. There are different approaches to achieve these nanostructures; an easy classification is to

1 Introduction

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make two big groups: top-down techniques and bottom-up techniques. The topdown techniques aim to fabricate smaller devices by using larger dimension objects; many of these technologies descended from the conventional semiconductor industry which is now capable of creating devices or arranging materials in dimensions smaller than 100 nm. The bottom-up techniques arrange structures from smaller components such as atom by atom or molecule by molecule. In current applications both approaches, top-down and bottom-up, tend to merge in order to take advantages of both techniques as will be seen in the different chapters of the book whose structure is described below.

1.2 About This Book This book reviews the works already presented in the literature with respect to the fabrication of sensors based on nanostructured materials. The book has been organized by topics of high interest. In order to offer a fast read of the state of art of each topic, every chapter in this book is independent and selfcontained. On the other hand, since nanotechnology is interdisciplinary by definition, some chapters overlap others and are in some way related between them. The nine chapters which follow the introduction of this book try to keep the same structure: first an introduction to the specific topic under study; second, the different fabrication methods of nanostructured materials for sensing in that particular field and, third, the sensing applications already reported in the literature of these devices. In Chapter 2 a review about different carbon nanotube (CNT) and fullerene sensors is presented. Chapter 3 deals with non-carbon nanotubes, and more specifically TiO2 nanotube arrays and its applications as hydrogen sensors, since this a less covered topic in the literature the chapter gives very detailed information. Nanowires, nanocombs, nanobelts, nanorods, nanoswords and nanosquids of mainly ZnO and SnO are presented in Chapter 4; these structures show some of the most beautiful arrangements in nature. Chapter 5 is focused on the utilization of metal oxides and semiconductors for controlling the transduction mechanisms of sensors at the nano scale, paying special attention to nanotemplates and nanowires. A review about the utilization of quantum dots for sensing is presented in Chapter 6 specially covering many of its applications in biosensing. Chapter 7 is a detailed review about the different types of nanostructured magnetic sensors: magnetic nanoparticles as well as magnetic nanowires and films. Encapsulated probes, term that refers to a class of multimolecular cocktails which have been physically encased within a protective package, are studied in Chapter 8. Optical fiber sensors based on nanostructured coatings are described in Chapter 9. The last chapter of the book, Chapter 10, deals with nanostructured flexible materials, more specifically presents some of the characteristics of Metal RubberTM, a commercially available product. Some examples of

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A

B

Fig. 1.4 Some examples of the nanostructures that are studied in this book. A TiO2 nanotubes for hydrogen sensing presented by the contributors in Chapter 2, B badmintonlike SiOx nanowires presented by the contributors in Chapter 4

these nanostructures can be seen in Fig. 1.4. The sensing applications that are covered along these nine chapters include temperature, pressure, strain, radiation, flow, magnetic fields, gas, volatile organic compounds, ions, chemicals, immunoassays, DNA detection, biological recognition, glucose, enzymatic detection, cell detection, magnetic resonance imaging (MRI) and others. Each chapter has been written by different contributors who give their personal perspective of these topics, a strategy that enriches the contents of the book because the chapters sometimes overlap and at least a few themes are dealt with from different angles. With respect to the contributors of this book, we have the great satisfaction of having convinced the most active authors in this field in the world to participate: according to ScopusTM database, 5 of the first 11 authors with more scientific journal publications with the words ‘‘nano’’ and ‘‘sensor’’ are authors of some chapters in this book. The first author of the most cited article with the words ‘‘nano’’ and ‘‘sensors’’ [2] (more than 2000 times according to Scopus) is also a contributor of this book. Most of the contributors are members in the editorial boards of journals related to the field and three of them are editors-in-chiefs of different journals. The purpose of this book is to make a humble contribution but a real one to some discipline that is emerging, thanks to the new applications of the nanotechnology: sensors based on nanostructured materials. This book does not deal ‘‘what it could be’’ but ‘‘what it is’’, already published results, a fact, tested devices and this is what, perhaps, makes this book valuable because there is not in the literature a collection dedicated to sensors based on nanostructured materials. We hope that readers enjoy it and that can be a valuable tool for those who want to have a summary of sensors fabricated with many diverse techniques, always dealing with nanostructured materials.

1 Introduction

7

Paradoxically, these nanostructures have been possible because we stood yet on the shoulders of the microdevices, and we do not have to forget that, thanks to those giants we are able to see farther; farther and deeper, in that place where there is plenty of room at the bottom.

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41. M. Trau, N. Yao, E. Kim, Y. Xia, G.M. Whitesides, and I.A. Aksay (1997). ‘‘Microscopic patterning of orientated mesoscopic silica through guided growth’’, Nature 390, 674–676. 42. A.D. McFarland and R.P. Van Duyne (2003). ‘‘Single silver nanoparticles as real-time optical sensors with zeptomole sensitivity’’, Nano Lett 3, 1057–1062. 43. B. Mayers, B. Gates, Y. Yin, and Y. Xia (2001). ‘‘Functionalized carbon nanotubes for molecular hydrogen sensors’’, Adv Mater 13, 1384–1386. 44. J. Hahm and C.M. Lieber (2004). ‘‘Direct ultrasensitive electrical detection of DNA and DNA sequence variations using nanowire nanosensors’’, Nano Lett 4, 51–54. 45. J.R. Grider, A.E. Foxx-Orenstein, and J. Jin (1998). ‘‘5-Hydroxytryptamine4 receptor agonists initiate the peristaltic reflex in human, rat, and guinea pig intestine’’, Gastroenterology 115, 370–380. 46. Z.L. Wang (2004). ‘‘Zinc oxide nanostructures: Growth, properties and applications’’, J Phys Condens Matter 16. 47. Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, and C.L. Lin (2004). ‘‘Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors’’, Appl Phys Lett 84, 3654–3656. 48. S.F. Bent (2002). ‘‘Organic functionalization of group IV semiconductor surfaces: Principles, examples, applications, and prospects’’, Surf Sci 500, 879–903. 49. S.Y. Chou (1997). ‘‘Patterned magnetic nanostructures and quantized magnetic disks’’, Proc IEEE 85, 652–671. 50. D. Routkevitch, A.A. Tager, J. Haruyama, D. Almawlawi, M. Moskovits, and J.M. Xu (1996). ‘‘Nonlithographic nano-wire arrays: Fabrication, physics, and device applications’’, IEEE Trans Electron Devices 43, 1646–1658. 51. P. Gomez-Romero (2001). ‘‘Hybrid organic-inorganic materials – in search of synergic activity’’, Adv Mater 13, 163–174.

Chapter 2

Carbon Nanotube and Fullerene Sensors John T.W. Yeow and Niraj Sinha

2.1 Introduction The first fullerene was discovered in 1985 by Sir Harold W. Kroto from the University of Sussex and Richard E. Smalley and Robert F. Curl Jr. from Rice University [1] inadvertently when they were studying the nucleation of carbon in the atmosphere of a cool carbon-rich red giant star. Fullerenes refer to a family of carbon allotropes. Each carbon molecule is composed of at least 60 carbon atoms such as C60. When the atoms are arranged in the form of hollow sphere, it is referred to as buckyballs. Fullerenes that take the form of a cylinder are referred to as carbon nanotubes (CNTs). By 1990, it was relatively easy to synthesize macroscopic quantities of C60. Donald Huffman of University of Arizona and Wolfgang Kratschmer of Max Planck Institute developed a technique by which C60 was produced by evaporating graphite electrodes via resistive heating in an atmosphere of 100 Torr helium [2]. Since the discovery of CNTs in 1991 by Iijima [3], the interest in practical applications of fullerenes has skyrocketed. Carbon nanotubes (CNTs), which can essentially be thought of as a layer of graphite rolled up into a cylinder, have shown great promise in the field of nano electromechanical systems (NEMS). The advantage of CNTs over other materials is due to their small size, high strength, high electrical and thermal conductivity, and high specific area. There are two types of CNTs: singlewalled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). These two types of CNTs differ in the arrangement of their graphene cylinders. While SWNTs have only one layer of graphene cylinder, MWNTs have many layers. Although SWNTs are structurally similar to a single layer of graphite (that is, a semiconductor with zero band gap), they can be either metallic or semiconducting depending on the tube diameter and the chirality (the sheet direction in which the graphite sheet is rolled to form a nanotube cylinder) [4, 5].

J.T.W. Yeow Department of Systems Design Engineering, University of Waterloo, 200 University Avenue (W), Waterloo, ON N2L 3G1, Canada


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J.T.W. Yeow, N. Sinha

The diameter (d) and the chiral angle () can be obtained by an integer pair (n, m) using Eqs. 2.1 and 2.2 [6]: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a m2 þ mn þ n2 p " pffiffiffiffiffi # 3n ¼ arctan 2m þ n

d¼

(2:1)

(2:2)

where a is the lattice constant in the graphite sheet. Depending on the relation between n and m, three categories of CNTs are defined: (i) armchair (n = m and chiral angle equal to 308); (ii) zigzag (n = 0 or m = 0 and chiral angle = 08); and (iii) chiral (other values of n and m and chiral angles between 0 and 308) [7]. All armchair nanotubes are metals, as well as those with n m = 3j (j being a nonzero integer). All others are semiconductors, which have the band gap that is inversely related to the diameters of the nanotubes [8].

2.2 Carbon Nanotube Synthesis Techniques For synthesis of defect-free CNTs of macroscopic lengths in desired quantities, development of reliable synthesis techniques is essential. Controlling the chirality of CNTs for a specific application is very challenging. The state-of-the-art synthesis techniques produce statistical distributions of chiralities, and hence electrical properties [9]. In general, following three techniques are used for synthesizing CNTs: (i) carbon arc-discharge technique; (ii) laser-ablation technique; and (iii) chemical vapor deposition (CVD) technique. The three techniques are discussed in detail below.

2.2.1 Carbon Arc-Discharge Technique In the carbon arc-discharge technique, two carbon electrodes are kept in a vacuum chamber. The electrodes are used to generate an arc by DC current. An inert gas is supplied to the chamber to increase the speed of carbon deposition. After the stabilization of pressure, the power supply is turned on (about 20 V) and the positive electrode is gradually brought closer to the negative electrode to strike the electric arc. The electrodes become red hot and a plasma forms. The rods are kept about a millimeter apart upon stabilization of the arc. During this period, the CNT deposits on the negative electrode. The power supply is cut off and the machine is left for cooling once a specific length is reached. The two most important parameters to be taken care of in this method

2 Carbon Nanotube and Fullerene Sensors

13

are (i) the control of arcing current and (ii) the optimal selection of inert gas pressure in the chamber [10]. Using arc-discharge technique, MWNTs were first discovered by Iijima [3] in 1991 when he saw very thin and long tubes of pure carbon under electron microscope. The growth of SWNTs for the first time was demonstrated by Iijima and Ichihashi [11] and Bethune et al. [12] in 1993 using arc-discharge technique. Arc-discharge technique produces high-quality CNTs. While MWNTs do not need a catalyst for growth, SWNTs can only be grown in the presence of a catalyst. MWNTs can be obtained by controlling the pressure of inert gas in the discharge chamber and the arcing current. The by-products are polyhedron-shaped multi-layered graphitic particles in case of MWNTs. The growth of SWNTs by arc-discharge technique was optimized by Journet et al. [13] using graphite cathode (16 mm diameter, 40 mm long), graphite anode (6 mm diameter, 100 mm long), mixture of catalysts (Ni–Co, Co–Y, or Ni–Y), helium pressure of 660 mbar, arcing current of 100 A, and voltage drop of 30 V between the electrodes. The scanning electron microscopy (SEM) revealed that the deposited material consisted of high amount of entangled carbon ropes of diameters 5–20 nm. Li et al. [14] synthesized SWNTs by modifying the arcdischarge method and by using FeS as a promoter. As evaluated by scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy, the synthesized SWNT fibers were 80% pure by volume. Later, Paladugu et al. [15] demonstrated that CNTs can be synthesized by arc-discharge in open air. As their method does not require a controlled atmosphere, the cost of production may be reduced.

2.2.2 Laser-Ablation Technique Intense laser pulses are utilized in the laser-ablation technique to ablate a carbon target, which in the presence of an inert gas and catalyst forms CNTs. An analysis by X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that the SWNTs produced by Thess et al. [16] using laser ablation were ropes (or bundles) of 5–20 nm diameter and tens to hundreds of micrometers of length. It was found by Arepalli et al. [17] that individual nanotubes of lengths tens of microns are formed in the vicinity of the target at the beginning, which subsequently coalesce into bundles. Based on spectral emission and laser-induced fluorescence measurements, Scott et al. [18] suggested that the carbon for the formation of CNTs comes from direct ablation as well as from carbon particles suspended in the reaction zone. In addition, the confinement of CNTs in the reaction zone within the laser beam leads to the purification and annealing of CNTs by laser heating. Braidy et al. [19] used pulsed KrF laser ablation of a graphite pellet to synthesize SWNTs and other nanotubular structures. It was observed by them that relatively high UV laser intensity had an adverse effect on the growth of SWNTs. By using high-vacuum laser ablation, multi-layered MWNTs were grown selectively

14


by Takahashi et al. [20] by dispersing graphite powder on a Si (100) substrate. During the growth of MWNTs, high substrate temperature was maintained by them. In general, the amount and type of catalysts, laser power and wavelength, temperature, pressure, type of inert gas present, and the fluid dynamics near the carbon target are some of the parameters that determine the amount of CNTs produced [21]. The by-products of SWNTs in case of arc-discharge and laserablation techniques are fullerenes, graphitic polyhedrons with enclosed metal particles, and amorphous carbon [21].

2.2.3 Chemical Vapor Deposition Technique In CVD, energy is imparted to hydrocarbons (the commonly used sources are methane, ethylene, and acetylene) to break them into reactive radical species in the temperature range of 550–7508C. These reactive species diffuse down to a heated and catalyst-coated surface where they remain bonded. As a result, CNTs are formed. The commonly used energy sources are electron beam and resistive heating. By catalytic decomposition of acetylene over iron particles at 7008C, microtubules of up to 50 mm length of CNTs were synthesized by Yacaman et al. [22]. Vardan and Xie [23] developed a CVD technique, which used microwave energy for synthesizing MWNTs. The use of acetylene as the hydrocarbon and cobalt as the catalyst at a temperature of 7008C resulted in MWNTs with 26 layers and average diameter of 20–30 nm. A sequential combination of radio frequency plasma-enhanced CVD (RF PECVD) and thermal CVD was used by Park et al. [24] to synthesize CNTs from acetylene and hydrogen gas mixture on stainless steel plates. Wei et al. [25] used CVD with gas-phase catalyst delivery to direct the assembly of CNTs in a variety of predetermined orientations, building them into one-, two-, and three-dimensional arrangements. The key parameters that affect the synthesis of CNTs by CVD include the nature of hydrocarbons, catalysts, and the growth temperature.

2.2.4 Purification In all the three synthesis techniques mentioned above, the CNTs come with a number of impurities. The type and amount of impurities depend on the synthesis technique that was used to produce CNTs. The purity of CNTs can be analyzed by spectroscopic techniques. For example, Raman spectroscopy is commonly used for qualitative evaluation of purity of CNTs, while near infrared spectroscopy can be used for the quantitative assessment. Carbonaceous materials are the most common impurities observed. The other types of impurities include metals. As carbonaceous impurities have high oxidation rates, the


15

impurities in the carbon arc-discharge technique can be purified by oxidation. Generally, two approaches are followed for purification by oxidation: (i) gasphase oxidation and (ii) liquid phase oxidation. Ebbesen et al. [26] used gas-phase oxidation for purification of CNTs. As low yield of purification was observed by them, liquid phase oxidation for better homogeneity was tried by Hiura et al. [27]. Bandow et al. [28] used a cationic surfactant and trapped SWNTs on a membrane filter to purify SWNTs synthesized by pulsed laser-ablation technique. Although high purity by weight (90%) was observed after purification, this technique was not found very useful for purifying large batches. To overcome this limitation, a macroscale purification technique was proposed by Rinzler et al. [29]. Xu et al. [30] developed a process for purification of SWNTs grown by CVD of carbon monoxide that included sonication, oxidation, and acid washing steps. For MWNTs grown by CVD, Biro et al. [31] used wet and dry oxidation to remove impurities and traces of catalysts. Several other techniques have been proposed to purify CNTs. However, they may change the electrical and mechanical properties of CNTs since the structural surfaces of CNTs are modified after purification. Therefore, current research focus is on producing high-purity CNTs directly.

2.3 Carbon Nanotube and Fullerene Sensors The sensor market is expanding at a very fast rate. Some of the potential applications of sensors are in biomedical industry, environmental monitoring, agricultural industry, fishing industry, food industry, automotive industry, electronics industry, defence and homeland security. All these applications require improved sensitivity, selectivity and stability beyond the commercially available sensors currently available. In order to meet these requirements, many different approaches have been considered by CNT research community, and several manuscripts related to CNT and fullerene sensors have been published [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 6162, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120]. This section presents a survey of sensors, which utilize the remarkable properties of CNTs. As is common with any emerging field, there are many proof-of-concept prototypes available for CNT and fullerene sensors. However, commercially available CNT sensors are not very common.

2.3.1 Force Sensors – Pressure and Strain For the first time, the potential of SWNTs as molecular and macroscopic pressure sensors was demonstrated by Wood and Wagner [32]. They applied high hydrostatic pressures to SWNTs by using a diamond anvil and recorded

16


the micro-Raman spectroscopy. The Raman spectrum of the CNTs was monitored under different pressures. The shifts in various Raman bands were observed simultaneously. The observed shifts in Raman peaks were found to be highly reversible, which showed potential of CNTs in pressure sensing based on mechanism of Raman band shifts. Liu and Dai [33] grew SWNTs on suspended square polysilicon membranes and demonstrated that pressure sensors can be realized by using their piezoresistive properties. A change in resistance in the SWNTs was observed when uniform air pressure was applied on the membranes. Moreover, the membrane was restored to its original condition when the gas was pumped out, which indicated that the process was reversible. In their study, Wu et al [34] demonstrated using first-principle quantum transport calculations, molecular-dynamics simulation, and continuum mechanics analysis that hydrostatic pressure can induce radial deformation. The radial deformation results in electrical transition of SWNTs. Transition from metal-to-semiconductor property in armchair SWNTs was observed when pressure was applied. This observation provides a basis for designing nanoscale tunable pressure sensors. Later, Fung et al. [35] demonstrated that piezoresistive pressure sensors can be built using MWNTs. They positioned a network of MWNTs across a PMMA membrane using dielectrophoretic (DEP) manipulation. When pressure was applied, the membrane deflected causing a bending in MWNTs. The advantage of this device is that it can be made through conventional micromachining processes. It was argued by Dharap et al. [38] that the conventional sensors have disadvantage that they are discrete point, fixed directional, and are not embedded at the material level. To overcome these limitations, a CNT film sensor for strain sensing on macroscale was presented by them. It was based on the principle that the electronic properties of CNTs change when subjected to strains. Since randomly oriented bundles of SWNTs were used, the film was isotropic in nature. The isotropic nature of CNT films helps in measuring strains in multiple locations and in different directions. A nearly linear relationship between the measured change in voltage and the strains in CNT films was observed experimentally when the films were subjected to tensile and compressive stresses. In another study, Li and Chou [39] developed SWNT-based sensors to measure strain and pressure at nanoscale. The sensors were based on the shift in resonant frequency of carbon nanotube resonator when subjected to a strain resulting from an external loading. Simulation studies by atomistic modeling revealed that the resonant frequency shifts are linearly dependent on the applied axial strain and the applied pressure. In addition, it was found that the reduction in tube length and diameter enhances the sensitivities of sensors. Berdinsky et al. have reported the use of fullerene-based sensor for temperature and pressure detection [40]. Pure fullerene was evaporated at a temperature of about 600–6808C in an atmosphere of argon gas at a flow rate of 5–15 ml/min. The argon pressure within the chamber was 670–1340 Pa. The


17

thickness of the fullerene thin film deposited on the substrates was 2–3 mm. The response of the sensor to changes in temperature, humidity, and pressure is shown in Fig. 2.1a–c. It was demonstrated that the sensitivities of the sensors were increased by subjecting the fullerene thin film to an oxidation process.

(a)

(b)

Fig. 2.1 (a) Temperaturedependent resistance change of fullerene thin films that are treated at various treatment temperatures: (1) initial sample; (2) 3708C; (3) 3908C. (b) Humidity-dependent resistance change of fullerene thin films: (1) initial sample; (2) treated at 3008C. (c) Pressure-dependent resistance change of fullerene thin films that are treated at 3908C at various testing conditions: (1) 208C; (2) 308C; (3) 708C [40]

(c)

18


2.3.2 Flow Sensors Ghosh et al. [41] have proposed a flow sensor based on SWNTs. The sensor is based on the generation of a current/voltage in a bundle of SWNTs, when the bundle is kept in contact with a flowing fluid. Generation of such current/ voltage was theoretically predicted by Kral and Shapiro [42] earlier. In general, an electric current is generated when the flow of free charge carrier is induced in any material. According to Kral and Shapiro, the electric current generation in CNTs is due to the transfer of momentum from the flowing liquid molecules so as to have a dragging effect on the free charge carriers in the nanotubes. The predicted relationship between the electric current and the fluid flow velocity is linear, which is in sharp contrast to the experimental findings of Ghosh et al. [41]. It was found experimentally that the induced voltage fits logarithmic velocity dependence over nearly six decades. It was observed experimentally that the ionic strength of the flowing liquid significantly affected the induced voltage. For flow velocities of the order of 10–5 m/s, the induced voltage was found to be saturated. The experimental data points fitted empirically to a logarithmic equation. The CNT-based flow sensors presented by Ghosh et al. [41] has great potential to be used in micromachines that work in fluidic environment such as heart pacemakers, which need neither heavy battery packs nor recharging.

2.3.3 Temperature Sensors As temperature changes, CNTs embedded in polymer matrices exhibit a shift in the Raman D* band. It was observed by Wood and Wagner [32] that the wavenumber of D* band increases with decreasing temperatures as CNTs experience compression. Although the proof-of-concept prototype demonstrates the potential of CNT composites for temperature sensing, an indepth study is warranted to understand the dispersion and interconnectivity of the CNT network. In another study, Wong and Li [43] used electrical properties of CNTs and developed MWNT-based thermal sensors. They manipulated bulk MWNTs by AC electrophoresis to form resistive elements between gold microelectrodes and demonstrated that MWNTs can potentially serve as temperature and anemometry sensors. The MWNT sensor chip was put inside an oven after packaging on a PCB for data acquisition. The oven temperature was monitored by a Fluke type-k thermocouple, which was attached on the surface of the PCB. As the temperature inside the oven was varied, change in resistance of MWNT sensors was measured. The representative data set indicated a linear relationship between the resistance and the temperature in the range of 20–608C. The I–V measurements of the devices revealed power consumption in mW range when MWNTs were used in


19

constant current configuration, which indicated that CNTs could be a promising material to fabricate ultra low-power-consumption thermal sensors.

2.3.4 Chemical Sensors Kong et al. [44] reported that the electrical resistance of semiconducting SWNTs dramatically changes when exposed to gaseous molecules such as nitrogen dioxide (NO2), ammonia (NH3), and oxygen (O2). They found that the nanotube sensors are at least an order of magnitude faster than those based on solid-state sensors. In addition to their small size, semiconducting SWNTs operate at room temperature with sensitivity as high as 103. This forms a basis for building semiconducting SWNT-based chemical sensors. However, it was pointed by Modi et al. [45] that the carbon nanotube gas sensors based on electrical conductance changes have certain limitations. The limitations include poor diffusion kinetics, inability to identify gases with low adsorption energies, and low capability to distinguish between gases or gas mixtures. It was also argued that the conductance of CNTs is highly sensitive to changes in moisture, temperature, and gas-flow velocity. Gas ionization sensors, featuring the electrical breakdown of a range of gases and gas mixtures at the tips of CNTs, were proposed by them to overcome these limitations. The cathode used for the purpose was aluminum and the anode was vertically aligned MWNT film (25–30 nm in diameter, 30 mm in length, and 50 nm separation between nanotubes) grown on SiO2 substrate. A glass insulator was used to separate the electrodes. The proposed sensors demonstrated by them were found to have good selectivity and sensitivity and were unaffected by various environmental conditions (moisture, temperature, and gas flow). Chopra et al. [51] developed microwave resonant sensors coated with either SWNTs or MWNTs for detection of ammonia. Comparative experiments revealed that SWNT sensors were more sensitive than the MWNT sensors. This sensor system can be used for applications that prohibit the use of physical connections or require non-destructive testing. In another study, Chopra et al. [65] demonstrated that SWNTs can be used as chemical sensors for detection of other gases in addition to NH3. The sensors developed by them showed sensitivity to CO, N2, He, O2, and Ar. Vapor sensors based on SWNT field effect transistors (FETs) have been developed by Someya et al. [66] for the detection of alcohols. The structure of the FET sensor and the corresponding experimental geometry are schematically shown in Fig. 2.2. The drain current measurements as a function of time are shown in Fig. 2.3. A sharp spike is observed few seconds after the saturated ethanol vapor is delivered to the surface and then the current decreases and reaches a steady value. Staii et al. [69] have proposed chemical sensors based on single-stranded DNA (ss-DNA) as the chemical recognition site and SWNT field effect transistors as the electronic read-out component. These sensors have rapid response and fast recovery times

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Fig. 2.2 Cross-sectional structure of the FET-based sensor and the experimental geometry. Reprinted with kind permission from [66], Someya et al., Nano Lett., 3, 877 (2003). Copyright @ 2003 American Chemical Society

Fig. 2.3 Drain current measurements as a function of time with a source–drain bias of –100 mV and a gate bias of –10 V. Reprinted with kind permission from [66], Someya et al., Nano Lett., 3, 877 (2003). Copyright @ 2003 American Chemical Society

on the scale of seconds and are able to detect a variety of gases. The schematic of the experimental setup and the gases used in the experiment is shown in Fig. 2.4. The change in sensor current upon exposure to different gases is shown in Fig. 2.5. These sensors are self-regenerating: samples maintain a constant response with no need for sensor refreshing for approximately 50 gas exposure cycles. These features can be used for applications ranging from homeland security to disease diagnosis. It was observed by Snow et al. [71] that the capacitance of SWNTs is highly sensitive to a wide range of vapors. Therefore, this property can be utilized to develop fast, low-power sorption-based chemical sensors. In another study, Jang et al. [72] proposed a chemical sensor employing laterally grown MWNTs as the active sensing element and found that the electrical resistance of MWNTs changes upon exposure to air or NH3. Also, as the measurement temperature and gas concentration were increased, fast response time and higher sensitivity were observed. Penza et al. [73] developed surface acoustic wave (SAW) sensors that were coated by CNTs for chemical detection of volatile organic compounds (such as ethanol, ethyl acetate, and toluene in


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Fig. 2.4 (a) Schematic of experimental setup. (b) Gases used in the experiment. Reprinted with kind permission from [69], Staii et al., Nano Lett., 5, 1774 (2005). Copyright @ 2005 American Chemical Society

Fig. 2.5 Change in sensor current upon exposure to different gases. Reprinted with kind permission from [69], Staii et al., Nano Lett., 5, 1774 (2005). Copyright @ 2005 American Chemical Society

nitrogen). These sensors were found to be highly sensitive during experiments. Ong et al. [74] built a gas sensor comprising MWNT-silicon dioxide (SiO2) composite on the principle that the conductivity and permittivity of the composite change with the absorption of different gases in the MWNT-SiO2 layer.

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The sensor developed by them has the advantage of allowing remote monitoring conditions inside the opaque, sealed containers. Szymanska et al. [75] synthesized a supported lipid bilayer membrane (S-BLM) that was modified using C60 for the detection of neutral odorant molecules. C60 displays good electron transfer properties and behaves as moderate electron acceptor molecules. Bilayer lipid membranes were formed on cleaved Teflon-coated stainless steel wire. The structure was immersed in a lipid solution with 2% l--phosphatidylcholine, 0.5% cholesterol in n-decane saturated with C60. The C60-enhanced electrochemical sensor showed a general larger response to neutral smell compounds. Fig. 2.6 shows the schematic of the C60 electrochemical sensor.

2.3.5 Biosensors It has been found that the electrical properties of CNTs are highly sensitive to external charges. Even small amount of electron transfer between CNTs and adsorbates may result in significant change in the conductance of the device. Since biomolecules such as DNA, RNA, and proteins are heavily charged molecules, their adsorption onto the CNT surface can change the electronic properties of CNTs in a similar way as gas molecules. Therefore, CNTs can be potentially used to develop biosensors. An amperometric biosensor was developed by Sotiropoulou and Chaniotakis [102] using CNTs as immobilization matrix. Aligned MWNTs were grown on platinum (Pt) substrate, which served as the transduction platform for signal monitoring. The schematic diagram of the CNT array biosensor is shown in Fig. 2.7. The CNT arrays were purified by

Fig. 2.6 Schematic of electrochemical sensor based on C60-modified s-BLM for detection of neutral odorant compounds[75]


23

Pt O H O O H O O H O O H O O H O

O H

e–

Glucose

O O H O O HH O O H O O H O

Gluconic Acid

O H O O H O O H O O H O O H O

Fig. 2.7 Schematic diagram of the CNT array biosensor. Reprinted with kind permission from [102], Sotiropoulou and Chaniotakis, Anal. Bioanal. Chem., 375, 103 (2003). Copyright @ Springer Science and Business Media (2002)

acid and air treatments. The SEM images of the Pt-aligned CNT arrays are shown in Fig. 2.8. After the enzyme was immobilized, the response and sensitivity of the sensor treated by acid were found to be very high when compared to the air-treated sensor. In another study, a single-strand DNA chain was chemically attached onto the surface of a CNT suspended on gold electrodes [106]. This helped in detection of complementary DNA and/or target DNA chains of specific sequences. To overcome the challenges of tissue penetration and the natural autofluorescent media, Barone et al. [109] developed near-infrared optical sensors based

Fig. 2.8 SEM images of Pt-aligned CNT arrays (a) In original state, (b) after acid treatment, and (c) after air oxidation. Reprinted with kind permission from [102], Sotiropoulou and Chaniotakis, Anal. Bioanal. Chem., 375, 103 (2003). Copyright @ Springer Science and Business Media (2002)

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on SWNTs. The sensors were developed making use of the fact that CNTs fluoresce in a region of the near infrared where human tissue and biological fluids are particularly transparent to their emission. Zhang and Gorski [112] have developed an electrochemical sensing platform based on the integration of redox mediators and CNTs in a polymeric matrix. It was found that the incorporation of CNTs decreased the overpotential for the mediated process by an extra 0.30 V and reduced the response time from 70 to 5 s. This concept can find useful applications in electrochemical devices such as sensors, biosensors, and biological fuel cells and reactors.

2.3.6 Radiation Sensors Ionization chambers are currently the most important dosimeters due to their sensitivity and relatively flat energy response. However, their applications for in vivo dose measurements are limited because of their large size and high bias voltage requirements for achievement of acceptable ionization collection efficiency. To this end, radiation sensors utilizing CNTs have been proposed [119, 120]. The radiation sensor developed by Ma et al. [120] is based on the principle that upon interaction between ionization radiation and the gas in the active volume of the detector chamber, energy is transferred by high-energy particles to the gas molecules. As a result, the gas molecules split into electron and hole pairs. When an external electric field is applied, holes and electrons move to the cathode and anode, respectively, which can be used to quantify the radiation dosage. The sensor was characterized by signal saturation characteristic, signal response linearity, electrode separation distance, and oblique incident beam measurements. Excellent linear responses to exposure and high sensitivity to oblique incident beams were found. Overall, these results indicate that the ionization collection efficiency of CNTs can be utilized to miniaturize the state-of-the-art ionization chambers and lower the bias voltages.

2.4 Conclusions The discovery of fullerene as the third form of pure carbon besides diamond and graphite has started a new field of fullerene science. This important discovery in the laboratory will be felt by the general public as many practical applications of CNTs and fullerenes are developed. In particular, improved sensors with increased sensitivity and selectivity have already been demonstrated by incorporating these nanomaterials. It is anticipated that the intense research on CNT and fullerene-based sensors will have a significant impact on a wide range of industries. Hospitals will be able to provide timely and accurate diagnosis by having biosensors that respond to minute quantity of targeted analytes. Industries will benefit by having miniature sensors that detect pressure, temperature,


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and other physical parameters. Multiple sensors could be deployed simultaneously as companies take advantage of the small size and mass of the sensors. Environmental monitoring such as chemical and gas sensing will be increasingly relevant. CNT and fullerene sensors will play a major role in our society that has an insatiable need for more information. Not only will we have sensors that deliver better performance, but they will be smaller, faster, and more power efficient.

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Chapter 3

Non-carbon Nanotubes: Hydrogen Sensors Based on TiO2 Kristen E. LaFlamme and Craig A. Grimes

3.1 Introduction Sensors for the detection of gases such as oxygen, water vapor, and hydrogen are becoming increasingly important for a number of areas such as manufacturing, environmental monitoring, medicine, and defense/security [19]. Hydrogen sensing in particular is needed for industrial process control, combustion control, and in medical applications where the presence of hydrogen is indicative of certain types of health conditions [18]. Considerable effort has historically been spent in both broad and specific development of sensor technologies. In order to achieve a useful sensing device, it is necessary to simultaneously solve a number of design parameters including but not limited to size, cost, and durability, which are often contradictory in nature. Perhaps a starting point in considering a sensing platform is the transduction mechanism: do we seek, for example, to detect changes in electrical impedance, electrical phase, magnetic properties, frequency, elasticity, or mass? Is the material being used as a sensor selective for the target species or does it require an e-nose approach? Once a target sensor material is identified, operational issues that must be determined include sensitivity, dynamic range, resolution, hysteresis, fatigue, and drift. Materials investigated for sensing utility may possess one or more favorable attributes but be unusable because of a severe limitation in another [19]. Metal oxide materials are widely recognized for their outstanding gas-sensing properties. Capable of operating at elevated temperatures and in harsh environments, they are mechanically robust, relatively inexpensive, and offer exquisite sensing capabilities. Furthermore, it has become evident that introducing nanoscale architectural features onto the surfaces of metal oxides results in superior and unexpected gas-dependent electrical behavior [13]. Titanium oxide (TiO2), or titania, has previously earned much attention for its oxygen-sensing capabilities. With proper manipulation of the microstructure, crystalline phase, and/or addition of impurities or surface functionalization, K.E. LaFlamme Boston University, Boston, MA


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K.E. LaFlamme, C.A. Grimes

this material can also be used for the detection of reducing gases. In this work, the authors describe the development of highly ordered TiO2 nanoarrays for hydrogen sensing. These devices have exhibited unprecedented changes in electrical conductivity in response to hydrogen, representing the largest known change in electrical properties of any material in response to any gas at any temperature. The fabrication of the TiO2 nanotube arrays will be outlined first, followed by a discussion on their operating characteristics, and finally a presentation of the performance of these sensors in specific environmental and medical applications.

3.2 Fabrication of TiO2 Nanotube Arrays Titania nanotube arrays are fabricated using a simple anodization approach. Briefly, titanium foils (typical starting thickness 250 mm) are subjected to potentiostatic anodization in a two-electrode electrochemical cell connected to a dc power supply using a platinum foil counter electrode. A diagram of a typical setup is shown in Fig. 3.1. In combination with HF, KF, or NaF to provide fluoride ions, it is possible to obtain nanotube arrays up to 222 mm in length using a variety of organic electrolytes, including dimethyl sulfoxide (DMSO), formamide (FA), ethylene glycol, and N-methylformamide (NMF). The formation of nanotube arrays in a fluoride-containing electrolyte is the result of several simultaneously occurring processes. During anodization, the nanotube formation occurs as a result of the combined effect of oxide growth at the oxide–Ti interface, the electric field-assisted dissolution at the oxide–electrolyte interface, and the chemical

Fig. 3.1 Anodization setup comprising a glass beaker, an electrolyte, and a dc power supply [21]

3 Non-carbon Nanotubes: Hydrogen Sensors Based on TiO2

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dissolution of the titanium oxide by fluoride ions. Shallow pores are formed at the initial stages of anodization, leaving unanodized metallic portions between the pores. As the pores become deeper, the electric field at these protruded metallic regions increases, enhancing the field-assisted oxide growth and dissolution there. This results in the formation of well-defined inter-pore voids simultaneously with the growth of the pores. Both these processes finally yield a tubular structure. The mechanism of titania nanotube formation by anodization is discussed in detail elsewhere [11]. The chemistry appears quite flexible, with substitution between the fluoride-containing acids and organics generally resulting in nanotube arrays several tens of microns in length. If longer tubes are desired, it is necessary to minimize water content in the anodization bath to less than 5%. Prior to anodization, the foil is cleaned with ethanol. The anodization procedure is carried out at approximately 208C, with the anodization current being monitored using a digital multimeter interfaced with a computer. The resulting structure consists of adjacent, parallel-oriented nanotubes separated from the unanodized foil by a thin oxide layer (barrier layer), the initial thickness of which is approximately equal to the nanotube radius [21]. Physical features of the nanotubes can be controlled by changing the anodization conditions [13, 21]. This ability to tailor nanotube array dimensions has enabled the ability to meaningfully test the gas-sensing properties of the nanotube arrays. The length of the nanotube array is dependent upon the pH, with higher pH values that remain acidic resulting in longer nanotubes. Tube length is also somewhat dependent on anodization voltage. The wall thickness and pore diameter are controlled by the anodization bath temperature and anodization voltage. The scanning electron micrographs in Fig. 3.2 show some examples of nanotubes that were fabricated under different conditions. The as-fabricated, amorphous nanotube arrays demonstrate no hydrogensensing capabilities. Consequently, the nanotube arrays must be annealed at high temperatures ranging from 480 to 5808C for several hours. The annealing process crystallizes the nanotubes, primarily in the anatase phase, but with some rutile crystals on the barrier layer, enhancing the sensitivity of the hydrogen response [10]. Anatase, the polymorph of titania, has been reported to have high sensitivity for reducing gases like hydrogen and carbon monoxide. Thus, as the diffusing hydrogen atoms go to the interstitial sites and as the c/a ratio of anatase is almost four times that of rutile, it appears that the anatase phase accommodates hydrogen easily and hence makes a higher contribution to hydrogen sensitivity. Studies indicated that no significant differences in gas-sensing properties were found as a function of annealing temperature; however, above 5808C, protrusions start to come out through the nanotubes, an effect which spreads with increasing temperature. These protrusions, which are due to oxidation of the titanium substrate, collapse the nanotubes [18].

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Fig. 3.2 Cross-sectional and top view SEM images of TiO2 nanotubes fabricated under different conditions. A–C Formamide-based electrolyte at 35 V for 48 h, tube length 70 mm; DE Dimethyl sulfoxide electrolyte containing 2 vol% HF at 60 V for 70 h, tube length 93 mm; F Ethylene glycol electrolyte containing 0.25 wt% NH4F at 60 V for 48 h, tube length 222 mm;. Magnification: A, 650; B, 55,000; C, 120,000; D, 750; E, 50,000; F, 45,000 [22]


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3.3 Sensor Development and Operating Characteristics 3.3.1 Sensor Design To transform the nanotube arrays into gas sensors, it is necessary to achieve electrical contact with the material so that the resistance or impedance of the material can be measured as a function of gas ambient. This is accomplished by depositing metal contacts onto the array. Typically, platinum electrodes have been used with these types of hydrogen sensors [7, 9, 10, 11, 13, 17, 18, 19, 20, 21, 22]. Upon deposition of the electrodes, metal wires are bonded onto the pads. Using an electronic interface, the changes in electrical resistance in response to variations in hydrogen concentration can be quantified. A diagram of a typical sensor setup can be found in Fig. 3.3. When the sensor comes into contact with hydrogen, the resistance is reduced. For a sensor measurement, a constant current I is applied to the sensor and the resulting dc voltage V is recorded. The sensor resistance R is then calculated by R = V/I. The electrical resistance of the nanotubes is sensitive to hydrogen concentrations ranging from 10 ppm to 4% [17]. The dominant mechanism behind the observed hydrogen-sensitive electrical behavior of the nanotubes appears to be chemisorption of hydrogen. During chemisorption, hydrogen acts as a surface state and a partial charge transfer from hydrogen to the titania conduction band takes place. This creates an electron accumulation layer on the nanotube surface, enhancing its electrical conductance. Upon removal of the hydrogen ambient, electron transfer back to the hydrogen molecule takes place, which subsequently desorbs, restoring the original electrical resistance of the material.

Fig. 3.3 Schematic diagram of a typical setup used for gas-sensing experiments [20]

34


Another factor that may play a role in the hydrogen sensitivity (and selectivity) is the platinum electrodes. At elevated temperatures hydrogen dissociation can occur on platinum surfaces. These dissociated hydrogen atoms may spill onto the nanotube surface in accordance with the well-known spillover mechanism of hydrogen by platinum. Once on the nanotube surface, the hydrogen atoms diffuse into the material, affecting its electrical properties.

3.3.2 Operating Features The nanotube arrays are extremely sensitive in the presence of hydrogen and their response behavior is highly reproducible. Figure 3.4 demonstrates the change in resistance of a 76 nm diameter titania nanotube sensor as the hydrogen concentration was repeatedly cycled at 2908C between a pure nitrogen ambient and a 500 ppm hydrogen in discrete steps of 100 ppm [7, 17, 19]. A variation in resistance of 3 orders of magnitude is observed at only 100 ppm hydrogen. Moreover, the electrical behavior of the sensor is consistent, recovering its original resistance after repeated exposure to hydrogen at different concentrations without hysteresis.

3.3.3 Tunability In these titania nanotube array-based sensors, hydrogen can produce changes in electrical resistance of about 3–8.7 orders of magnitude from baseline dependent on the operating conditions. Figure 3.5 shows the normalized change in electrical conductance of a nanotube array (tube diameter = 76 nm; tube

Fig. 3.4 Measured electrical resistance of a 76 nm titania nanotube hydrogen sensor when exposed to different concentrations of hydrogen at 2908C [19]


35

Fig. 3.5 Normalized change in electrical conductance of a 76 nm diameter TiO2 nanotube array to 1,000 ppm hydrogen at different temperatures. Gg is the conductance in the presence of hydrogen and G0 the base resistance associated with a nitrogen atmosphere [17]

length = 400 nm) as a function of temperature as it is switched from a nitrogen ambient to nitrogen plus 1,000 ppm hydrogen and then back to nitrogen. With reference to this figure, Gg is the conductance in the presence of hydrogen and G0 is the base conductance associated with a nitrogen atmosphere. The magnitude of the conductance variation increases with temperature, starting from a marginal variation at 1808C to a variation of 3 orders of magnitude at approximately 4008C [17]. Physical properties of the nanotubes can also affect their performance. For example, Fig. 3.6 demonstrates the variation in electrical conductance for sensors with nanotube diameters of 76, 53, and 22 nm and wall thicknesses of 27, 17, and 13 nm, respectively [17]. It is clear that the conductance variation is more prominent at smaller nanotube diameters. The sensitivity of the sensors also changes with respect to tube diameter, as represented in Fig. 3.7.

Fig. 3.6 Normalized change in electrical conductance of TiO2 nanotube arrays of 76, 53, and 22 nm diameter to 1,000 ppm hydrogen at 2908C [17]

36


Fig. 3.7 The sensitivity variation of samples with pore diameters of 22, 53, and 76 versus hydrogen concentration [17]

The sensitivity can be calculated as s¼

Ggas G0 G0

(3:1)

where Ggas and G0 represent the conductance of the sensors in hydrogen and at baseline, respectively. Assuming the density of the nanotube is 90% that of single crystal titania, the calculated surface area of the 76 nm diameter nanotube is approximately 19 m2g1 and that of the 22 nm array 38 m2g1. Interestingly, while the reduction of the nanotube diameter from 76 to 22 nm increases the surface area by only a factor of 2, the sensitivity is increased by approximately a factor of 200! This suggests that the nanotube wall thickness and the number of contact points between the nanotubes are additional factors that play a role in determining the nanotube sensitivity. With the reduced wall thicknesses, the space charge layer created by the chemisorption of hydrogen extends throughout the wall, resulting in a significant reduction in resistance consistent with the observed differences in sensitivity between nanotubes with different diameters. Also, as can be seen in Fig. 3.2, the nanotubes are in contact with each other, creating a high resistance path for electrons to travel. Upon the creation of the space charge layer due to hydrogen, these tube-to-tube contact points become highly conducting relative to the rest of the nanotube. For a bulk conductivity constant with nanotube diameter, the greater the number of contact points the greater will be the resistance change upon exposure to hydrogen. Therefore, the smaller diameter tubes, with thinner walls and greater number of contact points will exhibit higher sensitivities than their larger diameter counterparts [17]. On the other hand, sensors with smaller diameter tubes also tend to be more brittle and harder to handle without breakage. Therefore, a balance must be struck between sensitivity and mechanical durability [18]. Tube length is another physical parameter that affects the functionality of these sensors. Figure 3.8A demonstrates the gas-sensing properties of a sensor composed of a nanotube array with tube length of 1 mm. A change in electrical


A

37

B

Fig. 3.8 A Hydrogen gas-sensing properties of a 1 mm long nanotube array; B hydrogen gassensing properties of a 20 mm long nanotube array [22]

resistance of 8.7 orders of magnitude is seen in response to 1,000 ppm hydrogen, while the response time is approximately 20 s. Similar sensitivities and response times are obtained from sensors made of nanotube arrays of that approximate length. On the other hand, Fig. 3.8B shows the change in electrical behavior for a sensor made from 20 mm long nanotube arrays. While the sensor still demonstrates an excellent change in resistance of 3 orders of magnitude in response to 2,000 ppm hydrogen, there is a clear reduction in sensitivity accompanied by longer response and recovery times of nearly 2 h due to the time required for hydrogen to diffuse inside the long pores [22].

3.3.4 Cross-Sensitivity Up to this point, the performance of these sensors has been discussed in the context of a nitrogen ambient. What about other gases? In the real world, sensors are exposed to a number of other gases such as oxygen, carbon dioxide, carbon monoxide, humidity, and organic vapors. Thus, it is important to assess the cross-reactivity of these sensors with other gases. Figure 3.9 shows the variation in resistance of a nanotube array hydrogen sensor (tube diameter = 76 nm; tube length = 400 nm) upon exposure to various gases including carbon dioxide, carbon monoxide, ammonia, and oxygen. As shown in Fig. 3.9A, when the sensor is switched from a pure nitrogen environment to nitrogen plus 10% CO2, there is no detectable change in resistance; this is expected because carbon dioxide is neither a strong reducing agent nor a strong oxidizing gas [21]. Figure 3.9B shows the normalized change in resistance with respect to oxygen, ammonia, and carbon monoxide [18]. The sensitivity of the nanotubes to carbon monoxide and ammonia is negligible compared to that of hydrogen. The

38


A

B

Fig. 3.9 Variation of resistance of a 76 nm diameter titania nanotube array sensor upon exposure to A carbon dioxide [21] and B carbon monoxide, ammonia, and oxygen [19]

resistance of the nanotubes did increase in the presence of oxygen; however, as shown in Fig. 3.10, while oxygen does reduce the sensitivity of the sensor, there is still a clearly detectable hydrogen signal. Furthermore, the sensitivity difference was only about 1 order of magnitude between a pure nitrogen environment and one with 20% oxygen (atmospheric condition). On the other hand, the sensors are significantly affected by moisture. Figure 3.11 plots the change in sensor sensitivity (at 1,000 ppm hydrogen) as a function of relative humidity (RH). Between RH of 3 and 56%, the sensitivity of the hydrogen sensor varies

108 H2 (1%) + 20% O2 H2 (1%)

7

10

H2 (1%) + 15% O2

H2 (1%) + 2% O2

6

Resistance (Ohms)

10

105 104 103 102 101

Air 0

200

400

600

800

1000

1200

1400 1600

Time (Seconds) Fig. 3.10 Electrical behavior of the hydrogen sensor in the presence of increasing concentrations of oxygen [21]


39

Fig. 3.11 The sensitivity of the hydrogen sensor is reduced with increasing humidity [21]

from 2.5 108 to 7 104, a change of more than 3 orders of magnitude. For these reasons, it is important to calibrate the sensor to RH.

3.3.5 Room Temperature Sensing From Fig. 3.5 we have seen that temperature plays a role in the performance of titania nanotube-based hydrogen sensors, with the sensors generally displaying higher sensitivity at higher temperatures. However, elevated operating temperatures are not favorable for many applications, particularly those involving flammable environments and those requiring low-power operation. To realize hydrogen sensing at room temperature, a thin film of palladium (Pd) can be deposited on the surface of the tubes. A typical sensor response upon exposure to 20–1,000 ppm hydrogen gas is shown in Fig. 3.12. In this case, the sensor used had a tube length of approximately 200 nm and a diameter of about 22 nm. As seen in this plot, the initial resistance is of the order of 106 , and falls to below 102 when exposed to 1,000 ppm hydrogen. Typical 90% response times are approximately 15 s. The sensor recovers completely after H2 flow is terminated in each case with no detectable drift [20]. The catalytic properties of palladium for hydrogen dissociation and its hydrogen adsorption–desorption characteristics are well known. Similar to platinum, palladium dissociates hydrogen into atomic form. The activated hydrogen removes adsorbed oxygen and is adsorbed onto the Pd surfaces of

40

K.E. LaFlamme, C.A. Grimes 108

Resistance (Ω)

107

20

40

60

Hydrogen concentration in ppm 80 100 200 400 600

800

1000

6

10

105 104 103 102

air air air

air

1

10 0 100

3

2 10

3

4 10

air 3

6 10

air

air 3

8 10

air 4

1 10

air

1.2 104

air

1.4 104

Time (Sec) Fig. 3.12 Typical sensor response at 258C to different concentrations of hydrogen in the 20–1,000 ppm range [20]

the sensor. Hydrogen adsorption reduces the Pd work function; therefore, the height of the potential barrier at the metal–titania interface is lowered, which in turn reduces the electrical resistance of the sensor. In this manner, Pd enhances the H2 sensitivity of the nanotubes [20].

3.4 Applications of Titania Nanotube Hydrogen Sensors 3.4.1 Self-Cleaning Sensors A critical concern of any sensor platform is the potential for unwanted contamination, or poisoning, which introduces spurious measurements and generally ends the useful lifetime of a sensor. A sensor used in a non-controlled environment faces potential contamination from volatile organic vapors, carbon soot, oil vapors, dust, and pollen to cite just a few examples. While the titania nanotubes that have been described in this chapter are cheap to manufacture and could be used disposably, an important advance in sensor technology would be a sensor able to self-clean, thereby extending its useful lifetime and minimizing the potential for inaccurate measurements. The photocatalytic activity of titania can be used to mitigate the effect of contamination on these sensors by irradiating the surface of the titania surface with light with an energy equal to or greater than the titania band-gap energy (3.0 eV for the rutile phase and 3.2 eV for the anatase phase). Titania is a semiconductor characterized by a filled valence band and an empty conduction band. When a photon with energy h matches or exceeds the band-gap energy, an electron is promoted from the valence band to the conduction band, leaving a hole behind. The valence band holes are powerful oxidants, whereas the conduction band electrons are good reducers. Most organic photodegradation


41

reactions utilize the oxidizing power of electron holes, either directly or indirectly, generally producing CO2 and H2O without the production of potentially toxic side effects [10, 20]. The physical properties of the tubes contribute to the self-cleaning ability. Nanotube wall thickness, shape, surface area, and length are important factors in potentially enhancing the photocatalytic efficiency on which the self-cleaning capabilities discussed here are based. For example, when the feature size of the titania structure is less than 25 nm, the wave function of the charge carriers spreads throughout the structure, and hence the photogenerated electrons and holes are readily accessible to the donors or acceptors on the surface, resulting in a high degree of photocatalysis. Additionally, the crystallinity of the material plays a role: the anatase phase of titania is optimal for photocatalytic activities, while the rutile phase contributes to its hydrogen-sensing capabilities [10, 20]. In one study, Mor et al. demonstrated the ability of titania nanotube-based hydrogen sensors to completely recover their hydrogen-sensing abilities with exposure to UV light after being completely extinguished by immersing the sensor in motor oil [9]. Figure 3.13 shows the real-time electrical resistance of an illustrative hydrogen sensor operated at room temperature in response to different atmospheres, contamination, and UV light exposure. These self-cleaning sensors were tested in a 60 cm3 Plexiglas test chamber, with an opening for introducing the motor oil onto the sensor surface and a quartz window for passing the UV illumination onto the sensor surface. The sensor was exposed to a hydrogen–nitrogen mixture of 1,000 ppm hydrogen. After reaching the saturation resistance, the gas was switched back to air with the sensor returning to its original state (Fig. 3.13A). Prior to sensor contamination (Fig. 3.13A) there was a change in sensor resistance with exposure to 1,000 ppm hydrogen of approximately 175,000%. While in air (Fig. 3.13B), the sensor was then contaminated with an approximately 0.06 mm layer of 10 W–30 (Penzoil) motor oil after which the sensor demonstrated virtually no change in electrical resistance with hydrogen exposure over a period of approximately 10 min (Fig. 3.13B at time 1,600 s). The oil-contaminated region on the sensor (Fig. 3.13B) was uniformly illuminated with UV light in the presence of air. The recovery of the sensor’s hydrogen-sensing capability after sequential UV exposures with durations of approximately 1.3 h can be seen in Fig. 3.13B and C; 3.8 h in Fig. 3.13C; and 4.4 h in Fig. 3.13C and D. As seen from Fig. 3.13D, at the end of the UV-driven sensor-cleaning period, the measured relative change in electrical resistance with exposure to 1,000 ppm hydrogen is 100,000%. The clean and recovered air sensor has similar resistance values with exposure to air; the recovered sensor has a 1,000 ppm hydrogen resistance of approximately 100

compared with the resistance of 60 of the sensor prior to contamination. The photocatalytic properties of the nanotubes can be qualitatively understood by viewing FE-SEM images of a sensor contaminated with stearic acid before and after cleaning. Figure 3.14A shows a 22 nm sensor coated with stearic acid, applied by dripping melted stearic acid onto the sample. As seen in Fig. 3.14B, after 1 h of UV exposure, the sample is essentially clean.

42


A

B Fig. 3.13 A plot of real-time variation of resistance change before, during, and after cleaning the contaminant, motor oil 10 W–30, with UV exposure. The plot, broken into four parts for clarity, shows A The original sensor behavior from time 10 to 1,000 s. B Behavior of the sensor over time 100–6,000 s during which the sensor is contaminated with oil losing its hydrogensensing capabilities and is initially exposed to UV light C The behavior of the sensor from time 5,000 to 45,000 s. At time 7,000 s, the UV is turned off, with the sensor regaining its nominal starting resistance of approximately 100,000 W, at which point it is exposed to 1,000 ppm hydrogen and shows relative change in resistance of approximately 50. The sensor is then again exposed to UV, from roughly time 15,000 to 29,000 s. After this second UV exposure, the sensor is again exposed to 1,000 ppm hydrogen, showing an approximate factor of 500 change in electrical resistance. The sensor is once again exposed to UV from time 36,000 s. D Sensor behavior from time 45,000 to 70,000 s continues with UV exposure of the sensor to time 52,000 s, after which the sensor is repeatedly cycled between air and 1,000 ppm hydrogen showing a relative change in impedance of approximately 1,000. Compared to the hydrogen sensitivity of a non-contaminated sensor, the relative response of the ‘recovered’ sensor is within a factor of 2 [9]


43

C

D Fig. 3.13 (continued)

To more closely observe the recovery of sensor properties with UV exposure time, Mor et al. measured the changes in relative resistance of a sensor in response to incremental UV exposure following contamination with both stearic acid and a 1 mm thick layer of motor oil [10]. Figure 3.15 shows the relative resistance change, Rair/RH, of the sensor with each incremental 1 h UV exposure step; Rair is the resistance of the clean sensor in air, generally equivalent to that of the contaminated sensor, and RH is the steady-state resistance of the sensor exposed to 1,000 ppm hydrogen. The value of Rair/RH for the clean sensor is shown at an arbitrary time, whereas that of the contaminated sensor is shown at

44


Fig. 3.14 FE-SEM top views of the stearic acid-contaminated titania nanotubes A before and B after cleaning using a UV exposure of 1 h [10]

time 0. This plot clearly demonstrates the successive recovery of the Rair/RH value toward that of the clean sensor. Notably, the electrical resistance of the sensor rapidly drops with UV illumination due to photogeneration of charge carriers; yet, after the UV light source is turned off, a relatively long time is required for resistance to be regained. This phenomenon can be easily seen in Fig. 3.13C when the UV illumination is turned off at approximately 7,000 s. Since the UV light source used in these experiments has a shutter that closes when the light source is

Fig. 3.15 A semi-log plot illustrates the stepwise improvement of contaminated sensor. Here the relative change in resistance of a sensor measured with respect to 1,000 ppm of hydrogen (in nitrogen) is shown as a function of duration of UV exposure. The fresh sensor is shown at arbitrary time, while the dirty sensor is at time 0 [10]


45

turned off, and charge carrier lifetimes do not run into the minutes, it is clear that another mechanism must be responsible for the relatively slow recover times. This behavior is explained by the role of oxygen in manipulating the sensor conductivity. On UV illumination, the chemisorbed oxygen must be desorbed, increasing the conductivity. Hence, the conductivity increase upon UV illumination can be attributed to both the photogenerated current and the electrons that are donated by the oxygen. Once the UV illumination is removed, the oxygen is readsorbed and the electrons are extracted from the sensor; however, the process of oxygen readsorption is slow, on the order of several minutes, thus resulting in the increased time for the sensor to recover its original resistance [9, 10]. It is well established that the presence of oxygen and water plays a crucial role in the photocatalytic cleaning of titania, hence the sensor was exposed to air during UV exposure to facilitate removal of the oil from the sensor surface. If the relevant redox potential of the contaminant does not lie within the band gap of titania, the organic contaminants cannot be oxidized by photogenerated electron–hole pairs on the surface of titania. However, because the potential of water and oxygen exists within the band gap of titania, the photogenerated holes in the valence band can oxidize water to produce a highly reactive hydroxyl radical (.OH) and the photogenerated electrons in the conduction band can reduce oxygen to form highly reactive superoxide (O2.) ions (see Equations 3.2–3.4), which then assist in oxidizing the organic species. TiO2 þh ! Hþ þe

(3:2)

H2 O þ Hþ ! OH þ Hþ

(3:3)

O2 þe ! O2 :

(3:4)

Because the utility of a given chemical sensor technology is determined, in part, by how rapidly its properties degrade with non-specific contamination, development of a self-cleaning sensor technology of excellent sensitivity is an exciting prospect. In this section, a room temperature hydrogen sensor that demonstrates over a 170,000% change in electrical resistance upon exposure to 1,000 ppm hydrogen is described. The sensor is able to self-clean photocatalytically with UV exposure, fully recovering its initial properties lost due to contamination.

3.4.2 Hydrogen Sensing for Biomedical Applications: Transcutaneous Sensors In addition to their utility in environmental sensing, industrial process control, and manufacturing, hydrogen sensors also have a place in the biomedical arena. For example, breath hydrogen is a clinically important parameter used as an

46


indicator for such disorders as lactose intolerance [3], fructose maladsorption [12], bacterial growth [15], fibromyalgia [14], and neonatal necrotizing enterocolitis [6]. Transcutaneous gas monitoring has also been used as a means for diagnosing disease as well as monitoring treatment effectiveness. For example, transcutaneous carbon dioxide is routinely monitored for bronchopulmonary dysplasia, apnea, upper airway obstruction, mechanical ventilation, and respiratory problems associated with hypercapnia [1]. Transcutaneous carbon dioxide is also routinely used in neonatal intensive care units [2, 16] or in the preclinical detection of pneumothorax [8]. In a similar vein, Varghese and coworkers developed a sensor to measure hydrogen transcutaneously for the diagnosis of lactose intolerance [21]. Lactose intolerance is caused by the inability to digest significant amounts of lactose, a major sugar in milk. Failure to digest lactose results in fermentation of the lactose in the colon, which subsequently leads to the formation of various gases including hydrogen. The hydrogen is then absorbed from the intestines and carried into the bloodstream. A portion of the hydrogen is released in the skin, thus allowing hydrogen detection with a non-invasive hydrogen sensor [21]. A picture of the transcutaneous sensor is shown in Fig. 3.16. As previously discussed, metal oxide sensors such as the one described in this chapter can respond to some extent to other gases, resulting in unwanted cross-reactivity. Thus, it is necessary to use other sensors and cross-correlate the responses of these different sensors to obtain an absolute measurement of the target gas from within a complex environment. Therefore, a humidity–temperature sensor is incorporated with the transcutaneous hydrogen sensor. In this device, the hydrogen sensor and humidity–temperature sensor are secured in a white

A

B

Fig. 3.16 A Packaging of sensors comprising a hydrogen sensor and a humidity–temperature sensor. B The connector twists onto the adhesive-backed ring. Vent holes (not visible) on the side prevent the accumulation of water vapor and gases in the housing [21]


47

Teflon housing, which screws onto the adhesive-backed ring that is applied to the skin of the patient undergoing the test. The sensors were tested on patients who were referred by their doctors for a breath hydrogen test due to lactose deficiency. The transcutaneous sensor was placed on the forearm of children and adult volunteers, and the sensor documented the transcutaneous excretion of hydrogen. The results demonstrated a good correlation between the transcutaneous sensor readings and the exhaled breath hydrogen levels as measured by a commercial gas chromatography instrument. Furthermore, some of the patients tested had no detectable transcutaneous hydrogen excretion or detectable levels of exhaled breath hydrogen, thus indicating that the sensor does not respond to other volatile molecules that might be excreted through the skin. Figure 3.17 shows an illustrative measurement of transcutaneous hydrogen levels using the described transcutaneous sensor for a lactose-intolerant adult volunteer who drank a small sample of milk; note the sensor measurement is continuous in operation, and the hydrogen levels (ppm) in exhaled breath are measured using a gas chromatograph. Figure 3.18 shows the decrease in the sensor measurement, which indicates an increase in transcutaneous hydrogen, coinciding with an increase in exhaled breath hydrogen concentration. In addition to lactose intolerance, the transcutaneous hydrogen sensor can also be applied to other diseases such as necrotizing enterocolitis (NEC), a devastating disease of uncertain etiology and pathogenesis that causes high levels of mortality and morbidity in about 10% of preterm infants in neonatal

Fig. 3.17 Measurement of transcutaneous hydrogen concentrations using the described sensor (y-axis on left-hand side) and exhaled breath hydrogen levels (y-axis on right-hand side) measured by a gas chromatograph from a lactose-intolerant adult volunteer [21]

48


Fig. 3.18 Correlation between the response of the transcutaneous hydrogen sensor and exhaled hydrogen concentration [21]

intensive care units throughout the world. Almost 20 years ago, several investigators demonstrated that elevated levels of exhaled breath hydrogen gas precede the onset of the clinical and radiographic signs of NEC [4, 6]. From a practical standpoint, it is very difficult to measure gas levels in exhaled breath of preterm infants because of their small tidal volumes and rapid respiratory rates. In neonatal intensive care units, the use of endotracheal tubes, nasal CPAP systems, and nasal cannula flow systems are all barriers to the successful collection of undiluted end expiratory gas samples for gas analysis. Therefore, transcutaneous sensing of hydrogen gas in preterm infants has the potential to facilitate the early identification and treatment of cases of NEC. This would permit the early withdrawal of enteral feeds and the early initiation of systemic antibiotics and supportive therapy with fluids, pressors, and blood products. In addition to saving lives (NEC mortality rate is about 35%), the early detection of NEC may also significantly reduce hospital costs, which was about $95 million per year in the United States in 2002 [5].

3.4.3 Sensor Networks In many instances, practical application of hydrogen sensors would be significantly enhanced by the development of an inexpensive, wide-area sensor network technology incorporating hydrogen sensors capable of real-time in situ detection of 0.1–1,000 ppm H2 gas. As a step toward that ultimate goal, Grimes and coworkers incorporated titania nanotube hydrogen sensors into a wireless


49

sensor network to measure hydrogen concentrations from multiple sources in real time [7]. The hydrogen sensor itself is composed of a highly ordered thin film of titania nanotubes to which platinum electrodes are connected, as described earlier in this chapter. Briefly, the nanotubes are highly sensitive to hydrogen due to the chemisorption of molecular hydrogen, displaying a change in electrical resistance of approximately 3–8.7 orders of magnitude in response to hydrogen gas, depending on the operating conditions such as temperature, physical parameters of the tubes, or environmental conditions. The hydrogen sensors are completely reversible, with a response time of about 5 min and a recovery time of about 30 min. The sensor network consists of an array of nodes distributed throughout the area of interest, with a ‘host’ or ‘parent’ node connected directly to a computer that graphically presents the collected data. The main component of a sensor node is a microcontroller that oversees node operation, including transmission of node sensor information as well as relaying of information from distant nodes toward nodes closer to base. Each node has a RF transceiver integrated with the microcontroller for wireless communication between nodes. Interface circuitry is used to convert the response of the hydrogen sensor, a change in electrical resistance, to a voltage variation so that it can be digitized and processed by the microcontroller. The host node also contains a serial interface to communicate with a computer via RS232 protocol.

3.4.3.1 Design of the Sensor Network Before a sensor node can transmit data it needs to know the identity of its ‘parent,’ or the node it should relay its data to. To achieve this, the computer first instructs the parent node to send a broadcast signal that contains its identity. When a nearby node receives the signal, it remembers the parent node as its parent node and in turn sends its own broadcast signal to other nodes. This process is repeated until all nodes in the network know their parent node identity. The broadcasting operation is important for the nodes in the network to initialize communication links or re-establish communications after some nodes are damaged or removed. When relaying sensor information, all nodes transmit data to their parent nodes. This ensures that the data of all nodes are eventually sent to the host node and downloaded to the host computer. The circuit schematic of the sensor node is shown in Fig. 3.19. The microcontroller (AT90S8535) is connected to a RF transceiver chip (TR1000) via its serial bus. The microcontroller oversees all operations of the node, including acquiring data from the sensor, transmitting sensor data, and relaying the data of other nodes. A temperature sensor is included in each sensor node (AD7418). The resistance of the hydrogen sensor is converted to a voltage by a sensor interface circuit (see Fig. 3.20). The converted voltage is sent to the analog-to-digital converter port (ADC0, pin 40) of the microcontroller to be digitized.

50


Fig. 3.19 Circuit schematic of the sensor node. The ADC0 port (pin 40) of the microcontroller AT90S8535 is connected to the output of the sensor interface circuit [7]

Figure 3.20 shows the schematic diagram of the hydrogen sensor interface circuit. The voltage reference IC6 (ADR380) provides a constant voltage of 2 V across pins 2 and 3. When R5 is set to 10 k , IC6 provides a constant current of 200 mA through the hydrogen sensor. By Ohm’s law, the output voltage of the unity-gain op amp IC7 (AD820) V is proportional to the sensor resistance RS: V¼RS 200A

(3:5)


51

Fig. 3.20 Circuit schematic of the sensor interface. The output (pin 6 of the op amp) is connected to the ADC0 port (pin 40) of the microcontroller AT90S8535 (see Fig. 3.19) [7]

The microcontroller has a 10-bit analog-to-digital converter with a maximum sampling voltage of 3 V. In other words, the sampling voltage from 0 to 3 V will be linearly converted to a digital value of 1 to 1,023, with a voltage resolution of about 3 mV. From Equation 3.5, a 3 mV voltage resolution leads to a resistance resolution of 15 , which cannot accurately represent the resistance variation of the hydrogen sensor when the hydrogen concentration is larger than 100 ppm because the electrical resistance of the sensor becomes less than 60 at 100 ppm. To increase the resolution, another op amp, IC9, with a gain of 15 is used to amplify the output voltage from IC7. The outputs from both IC9 (with a gain of 15) and IC7 (with a gain of 1) are sent to a switch (IC10), which is controlled by a comparator (IC8). When the output of IC7 is between 0 and 200 mV (corresponding to an RS of 0–1 k ), the comparator sends out an output of 1 to pin 1 of the switch so that the switch can choose IC9 as its input. Conversely, when the voltage output of IC7 is larger than 200 mV, the switch will select IC7 as its input. With this design, the sensor interface circuit will generate a voltage of 200 mV–3 V when the sensor resistance is from 1 to 15 k and 0 to 3 V when the sensor resistance is 0–1 . As a result, the resistance resolution increases to 1

when the sensor resistance is less than 1 k . The output of the comparator is also sent to the I/O port of the microcontroller (PA1) and is included as part of the data. When the host computer receives a data package from the sensor node, it will divide measurements by 15 if the value of PA1 is 1. Doing so will ensure that the measured voltage is linearly proportional to the resistance from 0 to 15 k . The operational characteristics of this sensor network were tested using an experimental setup illustrated in Figs. 3.21 and 3.22. The hydrogen sensor itself

52


Fig. 3.21 The experimental setup for testing the sensor response. For elevated temperature measurements only the hydrogen sensor is placed within the test chamber, with an electrical connection passing to the interface circuitry. Using other sensor nodes as data relays the hydrogen measurements are sent to the host node and downloaded to the host PC for graphical presentation [7]

was placed in a test chamber, consisting of an 8 cm diameter and 60 cm long quartz tube, in which the temperature and atmosphere could be controlled (Fig. 3.21). Node 2, which was connected to the hydrogen sensor, transmitted sensor information to Node 3 since it was the only node within transmission range of Node 2. Similarly, Node 4 was the parent node of Node 3 due to its proximity and Node 1 was the parent node of Node 4 (Fig. 3.22). The test

Fig. 3.22 Nominal physical layout of hydrogen sensor network nodes. Node 2 is connected with the hydrogen sensor placed within the test chamber, Nodes 3 and 4 are relays, and Node 1 is the host node; the nodes can of course be re-located within the general area. The described configuration allows Node 3 to relay sensor information for Node 2 and Node 4 for Node 3; for this configuration the communication to Node 2 will be lost if either Node 3 or Node 4 is damaged [7]


53

chamber temperature was held constant at 2908C, and the environment was repeatedly cycled between a nitrogen ambient and nitrogen + 1,000 ppm H2. Every 2 s, Node 2 converted the electrical resistance of the hydrogen sensor to voltage and transmitted the information to Node 1 using Nodes 3 and 4 as relays. The host PC downloaded the measurement information from Node 1 every 6 s. The host computer also instructed the host node to send out a broadcast signal every 5 min to re-establish communication links between nodes. As the sensor is switched from a nitrogen to a nitrogen + 100 ppm hydrogen atmosphere, there is a factor of 200 change in measured voltage, as shown in Fig. 3.23. Figure 3.24A shows the voltage recorded by Node 2 as the concentration of hydrogen is varied from 50 to 104 ppm, and Fig. 3.24B shows the results as the concentration of hydrogen is varied from 50 to 1,000 ppm. The voltage ranges from 0.049 to 2.046 V, linearly corresponding to sensor resitivities of 245–10.23 k . The behavior of the sensor is consistent, recovering its original resistance after repeated exposure to varying hydrogen concentrations. The sensor responds linearly to hydrogen concentrations of approximately 500 ppm, with an exponential response above this value. Figure 3.23 shows a factor of 200 change in measured electrical impedance upon exposure to 100 ppm hydrogen. With respect to the ultimate sensitivity we can expect from titania nanotube hydrogen sensors using resistive bridge

Fig. 3.23 The voltages recorded by the sensor network when the hydrogen concentration is cycled between 0 and 100 ppm [7]

54


Fig. 3.24 A The sensitivity of the sensor toward hydrogen. The response is best fitted with a function V = 11.98 h0.8375, where V is the measured voltage in volts and h is the hydrogen concentration in ppm. Note: 10,000 ppm is equal to 1% hydrogen atmosphere. B The curve fit on the figure is log V = C1h3 + C2h2 + C3h + C4 where V is the measured voltage in Volts, h is hydrogen concentration in ppm and C1 = 1.22 109, C2 = 4.48 106, C3 = 5.04 103, and C4 = 0.271 [7]


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circuits, changes in electrical resistivity of 1% can be readily measured. However, if we consider a 10% change in electrical resistance, and linear scaling between hydrogen concentration and electrical resistance as seen in Fig. 3.24B for low hydrogen concentrations, a hydrogen resolution of 0.05 ppm appears readily achievable. The sensor network is a robust system, which is able to function if some of the nodes are damaged or removed. This was demonstrated by both relocating nodes within the network and removing nodes from the network. For example, consider rearranging Nodes 3 and 4 so that both are within the communication range of Node 2, as shown in Fig. 3.25. At first, Node 2 recognizes Node 3 as its parent since it receives the broadcast signal from Node 3 before Node 4. Node 2 occasionally switches its parent node to Node 4 when the sensor nodes reestablish communications every 5 min. When Node 3 is turned off, the host node stops receiving data from Node 2 since the link is cut off. However, when the host node sends out another broadcast signal, Node 2 re-connects to the network again through Node 4 and the measurement continues. The power of a sensor network is that widespread communication can continue even if one or more of the nodes is removed from the network. In this design, the sensor node can operate for over 100 h on three AA batteries. In the future, the sensor node can become even more energy efficient by self-engaging low-power mode when idle and turning on only when it needs to collect and transmit data. This can extend the lifetime of the sensor node to a few months or even years. Beyond hydrogen monitoring, the sentinel sensor network can also be applied to other applications by integrating the nodes with different types of sensors. For example, with proper integration of the nodes with desired sensors,

Fig. 3.25 This configuration allows both Nodes 3 and 4 to relay sensor data for Node 2. Although shorter in communication range, the network can still function even if Node 3 or Node 4 is damaged [7]

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the network can be used to monitor for chemical or biological events in airports, theaters, and other public venues. The sensor network can also be used for monitoring of pollution levels, be they industrial, medical, or at home.

3.5 Summary This chapter summarizes the fabrication of TiO2 nanotube arrays for hydrogensensing applications via anodization. The nanotube sensors contain both anatase and rutile phases of titania and show appreciable sensitivity to hydrogen at temperatures as low as 258C; furthermore, their response behavior is highly reproducible. The sensitivity of the sensors increases drastically with temperature, showing an increase of nearly 3 orders of magnitude from a starting temperature of 180–4008C. Physical properties of the nanotubes can also affect their performance, with higher hydrogen sensitivity associated with decreased tube diameter and increased tube length. The sensors are highly selective toward hydrogen compared with carbon monoxide, ammonia, and carbon dioxide. While oxygen does reduce the sensitivity of sensor, there is still a clearly detectable hydrogen signal. On the other hand, the sensors are significantly affected by moisture, thus it is important when designing a sensor to calibrate it to RH. Hydrogen sensors such as the ones described here are exceedingly versatile and can be used in many different applications. The sensors are able to selfclean in a photocatalytic manner, which is valuable in a number of environmental and industrial applications where the potential for contamination by vapors, soot, and the like is high. The sensors can also be used in the medical field to detect a variety of ailments such as lactose intolerance or necrotizing enterocolitis. The sensors can also be incorporated into networks for widespread hydrogen monitoring. In conclusion, work so far on the interaction of titania nanotube arrays with hydrogen has revealed an unprecedented gasdependent shift in electrical resistance. Extension of the technology presented here to other metal oxides should enable dramatically improved gas-sensing materials.

References 1. Capovilla J, VanCouwenberghe C, Miller WA (2000) Noninvasive blood gas monitoring. Crit Care Nurs Q 23:79–86 2. Carter BG, Wiwczaruk D, Hochmann M, Osborne A, Henning R (2001) Performance of transcutaneous PCO2 and pulse oximetry monitors in newborns and infants after cardiac surgery. Anaesth Intens Care 29:260–265 3. Chong SKF, Ramadan AB, Livesey E, Wood G (2002) The use of a portable breath hydrogen analyser in screening for lactose intolerance in paediatric patients with chronic abdominal pain or chronic diarrhoea. Gastroenterology 122:M1827 Suppl. 1821 APR 4. Engel RR, Virnig NL (1973) Origin of mural gas in necrotizing enterocolitis. Pediatric Res 7:292A


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5. Garstin WIH, Boston VE (1987) Sequential assay of expired breath hydrogen as a means of predicting necrotizing enterocolitis in susceptible infants. J Pediatr Surg 22:208 6. Godoy G, Truss C, Philips J (1986) Breath hydrogen excretion in infants with necrotizing enterocolitis. J Pediatr Res 20:348A 7. Grimes CA, Ong KG, Varghese OK, Yang X, Mor G, Paulose M, Dickey EC, Ruan C, Pishko MV, Kendig JW, Mason AJ (2003) A Sentinel sensor network for hydrogen sensing. Sensors 3:69–82 8. McIntosh N, Becher JC, Cunningham S, Stenson B, Laing IA, Lyon AJ, Badger P (2000) Clinical diagnosis of pneumothorax is late: use of trend data and decision support might allow preclinical detection. Pediatr Res 48:408–415 9. Mor GK, Carvalho MA, Varghese OK, Pishko MC, Grimes CA (2004) A roomtemperature TiO2-nanotube hydrogen sensor able to self-clean photoactively from environmental contamination. J Mater Res 19:628–634 10. Mor GK, Varghese OK, Paulose M, Grimes CA (2003) A self-cleaning, room-temperature titania-nanotube hydrogen gas sensor. Sens Lett 1:42–46 11. Mor GK, Varghese OK, Paulose M, Mukherjee N, Grimes CA (2003) Fabrication of tapered, conical-shaped titania nanotubes. J Mater Res 18:2588 12. Moukarzel AA, Lesicka H, Ament ME (2002) Irritable bowel syndrome and nonspecific diarrhea in infancy and childhood relationship with juice carbohydrate malabsorption. Clin Pediatr (Phila) 41:145–150 13. Paulose M, Varghese OK, Mor GK, Grimes CA, Ong KG (2006) Unprecedented ultrahigh hydrogen gas sensitivity in undoped titania nanotubes. Nanotechnology 17:398–402 14. Pimentel M, Chow EJ, Lin HC (2000) Comparison of peak breath hydrogen production in patients with irritable bowel syndrome, chronic fatigue syndrome and fibromyalgia. Gastroenterology 118:2141 Part 2141 Suppl 2142 15. Riordan SM, McIver CJ, Duncombe VM, Thomas MC, Bolin TD (2000) Evaluation of the rice breath hydrogen test for small intestinal bacterial overgrowth. Am J Gastroenterol 95:2858–2864 16. Tobias JD, Wilson WR, Jr, Meyer DJ (1999) Transcutaneous monitoring of carbon dioxide tension after cardiothoracic surgery in infants and children. Anesth Analg 88:531–534 17. Varghese OK, Gong D, Paulose M, Ong KG, Dickey EC, Grimes CA (2003) Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure. Adv Mater 15:624–627 18. Varghese OK, Gong D, Paulouse M, Ong KG, Grimes CA (2003) Hydrogen sensing using titania nanotubes. Sens Actuat B 93:338–344 19. Varghese OK, Grimes CA (2003) Metal oxide nanoarchitectures for environmental sensing. J Nanosci Nanotechnol 3:277–293 20. Varghese OK, Mor GK, Grimes CA, Paulose M, Mukherjee N (2004) A titania nanotube-array room-temperature sensor for selective detection of hydrogen at low concentrations. J Nanosci Nanotechnol 4:733–737 21. Varghese OK, Yang X, Kendig J, Paulose M, Zeng K, Palmer C, Ong KG, Grimes CA (2006) A transcutaneous hydrogen sensor: From design to application. Sens Lett 4:120–128 22. Yoriya S, Prakasam HE, Varghese OK, Shankar K, Paulose M, Mor GK, Latempa TA, Grimes CA (2006) Initial studies on the hydrogen gas sensing properties of highly-ordered high aspect ratio TiO2 nanotube-arrays 20 mm to 222 mm in length. Sens Lett 4:334–339

Chapter 4

Alternative Nanostructured Sensors: Nanowires, Nanobelts, and Novel Nanostructures Abhishek Prasad, Samuel Mensah, Zheng Wei Pan, and Yoke Khin Yap

4.1 Introduction State-of-the-art microfabrication techniques have led to smaller and faster computers and electronic devices. The emergence of nanoscale science and engineering (NSE or the so-called nanotechnology) has led toward the fabrication of much smaller and faster devices based on nanostructured materials. One-dimensional (1-D) nanostructures such as carbon nanotubes (CNTs) [1, 2, 3, 4, 5, 6, 7, 8] and boron nitride nanotubes (BNNTs) [9, 10, 11, 12, 13, 14,] have attracted tremendous research interest in the past decade. While the growth of these nanotubes with desired electronic properties is still challenging, scientists and engineers have extended their interest into alternative nanostructures such as nanowires that may offer better chances to achieve controllable electronic properties. These alternative nanostructures did not have the unique closed-shell structures like CNTs and BNNTs. Instead, they are often based on the known semiconductors such as Si, InP, GaP, ZnO, and GaN. These nanostructures usually maintain the crystal structures of their bulk precursors and thus have well-established physical and chemical properties that can be employed for applications such as biological and chemical sensors. In this chapter, we will review the current research status of these alternative nanostructures in various aspects including their synthesis processes, and their applications for sensors. This chapter will cover nanostructures such as nanowires, nanocombs, nanobelts, nanorods, nanoswords, and nanosquids. The overview and appearance of these nanostructures will first be introduced in Section 4.2, followed by their synthesis and fabrication techniques in Section 4.3. Examples of these nanostructures for sensing will be given in Section 4.4.

A. Prasad Department of Physics, Michigan Technological University, Houghton, MI 49931, USA


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4.2 Novel Nanostructures Nanoscale materials offer larger surface area (A) to volume (V) ratios (A/V) than the bulks. Thus nanostructures are believed to have better performances than materials in the micro- or larger scales for sensing applications. The earlier investigations of these nanostructures have been demonstrated on the growth of carbide and superconducting nanorods [15, 16]. Later, efforts have been devoted on the study of semiconducting nanowires such as Si [17], GaN [18], InP [19], and GaP [20]. Interesting ribbon- or beltshaped nanostructures were then demonstrated for a series of metal oxide nanostructures including SnO2 [21], ZnO, In2O3, and CdO [22], which are now generally referred as nanobelts. Among these metal oxides, ZnO appeared to be structurally interesting. Nanostructures of ZnO can appear as nanowires, nanobelts, nanopropellers, nanocombs, nanotubes, nanoswords, nanotripods, nanotetrapods, nanosquids, nanorods, nanotips, etc., as shown in Fig. 4.1 [23, 24, 25, 26].

4.3 Methods of Synthesis and Fabrication In this section we will introduce various growth techniques employed for the synthesis of some alternate nanostructures, including nanowires [17, 18, 19, 20, 27, 28, 29], nanobelts [21, 22, 23, 24, 25, 26, 30, 31,32], nanocombs [23, 24, 25, 26, 33, 34], nanotubes, and nanosquids [23–26]. We have classified these growth techniques into physical vapor deposition (PVD, Section 4.3.1), chemical vapor deposition (CVD, Section 4.3.2), solution-based chemistry (Section 4.3.3), and other synthesis techniques (Section 4.3.4).

4.3.1 Physical Vapor Deposition Physical vapor deposition (PVD) are vapor–solid (VS) deposition techniques that involve the generation of reactant vapors by physical processes such as heats, plasmas, and lasers.

4.3.1.1 Laser-Assisted Catalytic Growth Laser-assisted catalytic growth (LCG) was among the earlier approaches for the growth of semiconducting nanowires [17, 18, 19, 20]. This technique involves the use of a pulsed laser to ablate a target that containing the element(s) desired in the nanowires and the metal catalyst component. In this technique,

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Fig. 4.1 In addition to nanowires, ZnO nanostructures can appear in various morphologies including (left to right, top to bottom) nanobelts, nanopropellers, nanocombs, nanotubes, nanoswords, nanotripods, nanotetrapods, nanosquids, nanorods, and nanotips. Scale bar = 1 mm

the target was kept inside the growth chamber of a tube furnace. The vaporized compounds were then grown into nanowires of Si [17], GaN [18], InP [19], and GaP [20], through the vapor–liquid–solid (VLS) mechanism [35]. This approach was used to prepare bulk quantities of single-crystalline nanowires

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with diameters of as small as 3 nm and lengths as long as 30 mm [17]. This technique was similar to the growth of single-wall carbon nanotubes by laser vaporization [36].

4.3.1.2 Thermal Evaporation Thermal evaporation was one of the most widely used techniques for the growth of alternative nanostructures. In most cases, horizontal furnaces are used with a tubular reaction chamber constructed by an alumina tube or a quartz tube. This was the technique used for the growth of nanobelts of SnO2 [21], ZnO, In2O3, and CdO [22]. For example, the growth of SnO2 nanobelts was conducted in a horizontal tube furnace with an alumina tube chamber. SnO2 powders were used as the source material that was evaporated at 1,3508C under a pressure of 200–300 Torr and Ar gas flow rate of 50 sccm. These vapors were then condensed as belt-like nanostructures in a narrow region downstream where the temperature was 900–9508C. This vapor–solid condensation process did not involve the use of other metal catalysts, although Sn itself could have mediated the formation of SnO2 nanobelts. This approach was used for the growth of interesting diskettes of SnO2 [31] in a horizontal tube furnace as shown in Fig. 4.2. In this case SnO2 powders were vaporized at 1,0508C under a pressure of 500–600 Torr with the flow of Ar gas. SnO2 diskettes were deposited at the low temperature (LT) region (200–4008C). By using the same approach, Ga2O3 nanoribbons and nanosheets were produced by thermal evaporation of GaN powders in a horizontal alumina tube furnace at 1,1008C [30]. These Ga2O3 nanostructures were condensed at 800–8508C under a pressure of 300 Torr created by the flow of Ar gas at a rate of 50 sccm. Various ZnO nanostructures can be produced by thermal evaporation of Zn powders. ZnO nanotetrapods were synthesized in a horizontal tube furnace with a fused-quartz tube chamber at 9008C. A quartz plate held with a few spherical Zn pellets (3 mm in diameter) was inserted into the tube and vaporized in air ambient. After heating for about 2 min, ZnO nanotetrapods were found on the surface of the quartz plate [37]. These ZnO nanotetrapods were used for humidity sensors. On the other hand, ZnO nanobelts can be grown by evaporation of Zn powder at 6008C. Results indicate that the control of gas flow rates and partial pressures of Ar, O2, and Zn vapors are important for the growth of ZnO nanobelts [38]. Finally, the growth of ZnO nanowires in quantity of several grams was demonstrated by heating the mixture of Zn powders and NaCl to 600–7008C in the flow of Ar (25 sccm) and O2 (20 sccm) gases that create a pressure of 2 Torr [39]. About 70–80% of the Zn powders were converted to ZnO nanowires when NaCl was used, otherwise 5–10% without NaCl.


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Fig. 4.2 (Top) Various appearances of SnO2 diskettes (a to e). (Bottom) Schematic diagram for a horizontal tube furnace for the growth of various oxide nanostructures by thermal vaporization. Nanostructures can be condensed at high (H.T.), medium (M.T.), and low temperature (L.T.) regions downstream

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4.3.1.3 Radiofrequency Magnetron Sputtering Vapor–solid growth of nanostructured materials can also be obtained by other physical vapor deposition technique such as radiofrequency (rf) magnetron sputtering. For example, ZnO nanobelts were grown by using rf magnetron sputtering of a ZnO target [40]. These nanobelts were deposited on sapphire substrates without the use of catalyst at relatively low pressure of 40 mTorr and an rf power of 300 W for 60 min. The sputtering gas used in this technique is not indicated in this report. The deposition of these nanobelts was carried out without external heating to the substrates. This approach usually results in the formation of ZnO films but nanobelts were deposited when sapphire substrates were used.

4.3.2 Chemical Vapor Deposition Chemical vapor deposition (CVD) techniques involved chemical decomposition or chemical reaction in the formation of nanostructured materials. These chemical reactions required to either create the growth species or form nanostructures by the use of catalyst through the vapor–liquid–solid (VLS) mechanism. We will discuss two types of CVD techniques, namely thermal CVD and metal–organic chemical vapor deposition (MOCVD).

4.3.2.1 Thermal Chemical Vapor Deposition Various ZnO nanostructures can be grown by carbothermal reduction of ZnO powders either with or without the use of catalyst such as Au [23–26]. In this case, the precursor ZnO and graphite powders were mixed and loaded into a ceramic boat, which was placed in a small quartz reaction tube. This reaction tube was then inserted into a larger quartz tube of a horizontal tube furnace. This approach was sometimes referred as double-tube thermal CVD. Oxidized Si substrates (or other substrates) were usually placed downstream to the source materials at the lower temperature zones. In most cases, these precursor powders were combusted at 1,1008C so that ZnO powders will be reduced into Zn and ZnOx vapors in the presence of graphite powders. These Zn and ZnOx vapors were then condensed as ZnO nanostructures in O2 gas at a pressure of several Torrs. ZnO appeared as nanowires, nanobelts, nanopropellers, nanocombs, nanotubes, nanoswords, nanotripods, nanotetrapods, nanosquids, nanorods, nanotips, etc., as shown in Fig. 4.1 [23, 24, 25,26]. We found that nanorods can be grown without Au catalyst. These ZnO nanorods can be transformed into nanotubes and nanosquids with appropriate cooling during the growth [23, 24, 26]. These nanorods can also be transformed into long nanowires by placing a Au-coated substrate beside the sampling substrates during the growth, a so-called ‘‘side-catalyst’’ approach [25].


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This technique was applied for the growth of ZnO nanocombs by heating the tube furnace at 9008C for 30 min. Argon (100 sccm) and oxygen (1 sccm) were used in this case without Au catalyst [41]. Under this oxygen-deficient ambient, Zn vapors/droplets might be formed that functions as a catalyst for the growth of ZnO nanocombs. These nanocombs were tested as the biosensors for glucose detection. ZnO nanowires can also be grown by using ZnO and graphite powders with germanium (Ge) as the catalyst [29]. In this case, Ge catalysts were supplied in two ways. In one method, GeO2 powders were reduced by the graphite powders together with the ZnO powders. In the other method Ge dots were patterned on the SiO2-coated silicon wafer by photolithography. ZnO nanowires were condensed at the temperature zone of 500–6508C with diameters and lengths of 50–400 nm and 50–200 mm, respectively. It is interesting to note that the Ge catalysts remain at the tips of these nanowires with diameters much larger than those of the nanowires. In addition to ZnO, other nanostructured metal oxides were grown by this vapor–liquid–solid approach. For example, SnO2 nanowires were grown directly on the electrodes of sensors and tested for NO2 gas sensing [42]. The synthesis of these SnO2 nanowires was carried out by thermal evaporation of Sn metal powders at low temperatures of 600–7008C. This was performed in the flows of O2 gas at a flow rate of 10 sccm and growth pressure of 1–10 Torr. Gold films of 0.2–0.3 nm were used as the catalyst for the growth of these nanowires. The diameters of the resulted SnO2 nanowires were ranged from 50 to 100 nm, which was determined by the growth temperatures. The growth of silicon oxide (SiOx) nanowires was demonstrated by a simple thermal CVD approach [28]. In this case, GaN powders were used as the Ga source, i.e., the metal catalyst. The Si sources can be the Si wafers, SiO powders, or silane (SiH4) gas. The GaN powders were placed at the location with temperature around 1,1508C, no matter what Si source was used. This technique was based on the VLS process. In fact, bundles of these SiOx nanowires can be grown from a micrometer-sized Ga droplet. They can assemble into gourdlike, spindle-like, badminton-like, and octopus-like morphologies at different temperature zones [43]. For example, the badminton-like structures can be formed at 980–1,0108C (Fig. 4.3a and b). These SiOx nanowires exhibit branching growth features with a height of 10–15 mm (Fig. 4.3c). Thermal CVD was used for the growth of doped nanostructures. For example, Sb-doped SnO2 nanowires were grown for gas sensor application [44]. In this case, metal Sn and Sb powders mixed in the weight ratio of 10:1 were used as the evaporation sources in a thermal CVD system. Si substrates coated with 5 nm gold were placed downstream in the furnace. The temperature at the center of quartz tube was 9008C, and a constant flow of 1% oxygen and 99% nitrogen was maintained at a flow rate of 5 l/min. The as-grown nanowires have a diameter of 40–100 nm and lengths up to several tens of micrometers. As discussed, CVD has been a very useful and versatile technique. Nanostructures that were initially grown by other techniques have later been

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Fig. 4.3 Badminton-like SiOx nanowire at different magnification (a, b). (c) TEM image of a badminton-like structure

successfully grown by CVD. For example, instead of laser-assisted catalytic growth, silicon nanowires are now commonly grown by thermal CVD [27, 45]. This was carried out using Au nanoparticles as the catalyst and Silane (SiH4, 10% in He) gas as the Si source.

4.3.2.2 Metal–Organic Chemical Vapor Deposition (MOCVD) Thermal CVD techniques discussed so far are not always the suitable approach for the growth of certain nanostructures. For example, GaN nanowires have been grown by laser ablation [18] and thermal CVD [46, 47]. High temperatures were needed to generate Ga vapor source in the thermal CVD technique, and Ni, Fe, and Au were used as the catalysts. The use of a solid Ga is technically


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simple but often leads to nonconstant vapor pressures for continuous growth of GaN nanowires. Metal–organic chemical vapor deposition (MOCVD) was found to be a more versatile technique for the growth of GaN nanowires. The MOCVD technique seems to overcome the vapor pressure problem and yield high-quality GaN nanowires [48]. In the MOCVD approach, trimethylgallium (TMG) and ammonia gas were used as Ga and N precursors. Silicon substrates or c-plane and a-plane sapphire substrates coated with 2–10 nm thin film of Ni, Fe, or Au were used for the growth at 800–1,0008C. The reaction was carried out in an oxygen-free environment at atmospheric pressure. TMG was kept cool in a – 108C temperature bath. Nitrogen was used as a carrier gas and percolated through the TMG precursor and coupled with a second nitrogen line to give a total nitrogen flow rate of 250 sccm. Hydrogen and ammonia gases were supplied at a total flow rate of 155 sccm. The as-grown GaN nanowires were having diameters of 15–100 nm and lengths of 1–5 mm, oriented predominantly along the [210] or [110] direction. MOCVD technique was also used for growing nanostructured IrO2 crystals [49]. The nanostructured crystals were grown on a gold-coated quartz substrate and their gas-sensing properties were studied by quartz crystal microbalance (QCM) technique. The growth of these nanostructures was performed by using (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium, (MeCp)Ir(COD), as the precursor. The quartz substrate temperature (Ts) was kept between 350 and 5008C. High-purity oxygen was used as carrier gas at a flow rate of 100 sccm, which leads to a growth pressure of 7.2 Torr. The temperature of the precursor reservoir (Tp) was varied between 95 and 1058C. Different morphologies, such as nanoblades, layered columns, incomplete nanotubes, and square nanorods, were observed at various combinations of Ts and Tp.

4.3.3 Solution-Based Chemistry In addition to the vapor phase growth techniques discussed so far, several solution phase deposition techniques have been used for the growth of various alternative nanostructures.

4.3.3.1 Hydrothermal Synthesis Tungsten oxide nanowires (WO2.72) were grown by hydrothermal technique and were tested for hydrocarbon sensing [50]. Sensors based on WO2.72 nanowires show high sensitivity for 50–2,000 ppm of LPG (propane–butane mixture) at 2008C as well as relatively short recovery and response times. These WO2.72 nanowires were prepared by solvothermal synthesis. Tungsten hexachloride (1 g) was loaded into a 25 ml autoclave filled with ethanol up to 90% of its volume. Hydrothermal synthesis was carried out at 2008C for 24 h. The product

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obtained by centrifugation was washed with ethanol. The as-grown nanowires were having diameters of 5–15 nm and lengths of 100–200 nm. ZnO nanorods can also be grown by hydrothermal techniques [51]. Zinc and cetyltrimethylammonium bromide (CTAB) were used as the precursors. CTAB (1.5 g) was dissolved in deionized water (35 ml) to form a transparent solution. Then, zinc powders (1.8 g) were added to the above solution under continuous stirring. The resulting suspension was transferred into a teflonlined stainless steel autoclave (volume 40 ml) and sealed tightly. Hydrothermal treatments were carried out at 1828C for 24 h. The autoclave was then cooled down. Next precipitates were collected, washed with deionized water for several times, and dried in air. The lengths of the ZnO nanorods are usually shorter than 1 mm and their diameters ranging from 40 to 80 nm. These nanorods were used for the sensing of various vapors including alcohol, LPG, gasoline, ammonia, etc. Vanadium oxide (V2O5) nanobelts coated with Fe2O3, TiO2, and SnO2 nanoparticles have been prepared by mild hydrothermal reaction [52]. For the growth of the V2O5 nanobelts, nitric acid was added dropwise to a 0.1 M ammonium metavanadate solution until the final pH value of the solution reached about 2–3 under stirring. Solution obtained was transferred to a teflon-lined autoclave and filled with deionized water up to 80% of the total volume. Then the autoclave was kept at 1808C for 24 h. The final product was washed with deionized water and pure alcohol several times to remove any possible remnants. The as-grown nanobelts were tens of micrometers long with smooth surfaces, typically 60–100 nm wide and 10–20 nm thick. These nanobelts were then coated with the oxide nanoparticles for the sensing of alcohol, benzene, cyclohexane, gasoline, ammonia, etc. NiFe2O4 nanospheres, nanocubes, and nanorods were prepared by a hydrothermal method [53]. For synthesizing these NiFe2O4 nanostructures, Ni(NO3)2 6H2O and Fe(NO3)3 9H2O were dissolved in deionized water to form a mixed solution with [Na2+] ¼ 0.10 mol/l and [Fe3+] ¼ 0.20 mol/l. NaOH solution (6.0 mol/l) was added dropwise under stirring into 20.0 ml of the mixed solution until the desired pH value was attained to form an admixture. In the next step, the admixture was transferred to a teflon autoclave (50 ml volume) with a stainless steel shell up to 80% of the total volume. The autoclave was kept at 120–2008C for 24–96 h and then cooled down. The final product was washed with deionized water and alcohol for several times and then dried. The length and diameter of the nanorods were about 1 mm and 30 nm, respectively; the side length of the nanocubes was about 60–100 nm. It was found that sensors based on NiFe2O4 nanorods were relatively sensitive and selective to triethylamine. 4.3.3.2 Hydrolysis The growth of Fe2O3/ZnO core/shell nanorods was reported by a solution phase-controlled hydrolysis process of Zn2+ ions in the presence of Fe2O3


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nanorods [54]. One hundred milligram of Fe2O3 nanorods (100 nm long, 25 nm in diameter) were dispersed in 200 ml deionized water by ultrasonication. Then 20 mg of zinc acetate (Zn(Ac)2 6H2O)) was introduced to the solution, and the suspension was heated in an oil bath at 408C under vigorous stirring. Twenty milliliters of 5% ammonia was then added into the suspension for 30 min, and the reaction was maintained at the temperature for 1 h. The colloidal suspension was then centrifuged and sintered at 4008C for 2 h to obtain Fe2O3/ZnO nanorods. These core/shell structures have diameters of 30 nm. 4.3.3.3 Aqueous Chemical Growth ZnO nanostructures were deposited on glass substrates by the aqueous chemical growth technique at 958C [55]. The growth process involves the use of an equimolar (0.01 M) aqueous solution of Zn (NO3)2 6H2O and C6H12N4 as the precursors. The solution and the substrates were then placed in glass bottles and heated at 958C for 1, 5, 10, and 20 h. After each induction time, the substrates were thoroughly washed with deionized water and dried in air. Flower-like aggregations of ZnO nanorods were deposited in all cases. The diameters of these nanorods are relatively big (500–1,000 nm) and increased with the deposition duration. Their lengths are usually 1 Tbit/in.2 [89] (Fig. 7.4). This potential arises from the highanisotropy (L10) phase of approximately equi-atomic FePt that allows particles of 3 nm diameter to be thermally stable at room temperature. However, in order to create the L10 phase of FePt it is necessary to anneal at temperatures in excess of 5008C, typically for times of 30 min. This gives rise to a number of thermally activated processes including the desired phase transformation, oxidation and particle agglomeration. Coercivities of up to 13 kOe at room temperature were readily obtained, demonstrating that self-assembled nanoparticles do indeed offer significant potential as recording media. However, most applications derived from these materials are still in an early stage of technical development.

7.2.2 Magnetic Nanowires and Films: Fabrication Techniques Experimental techniques to grow functional nanostructured materials are briefly described here. We first consider techniques to prepare 1D nanostructures followed by those currently used in the preparation of 2D nanostructures. Some of the techniques employed for nanowires, also common for films, are described in more detail in the second section. 7.2.2.1 Fabrication of Nanowires The controlled production of magnetic nanowires and their arrays is recently attracting great interest owing to their applications in emerging nanotechnologies

Fig. 7.4 Self-assembled FePt magnetic nanoparticles [83]

FePt nanoparticles

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related with magnetic storage, magneto-transport, controlled opto-magnetic response and, in general, for multifunctional sensor applications [90]. Nanowires can be prepared by means of more or less sophisticated techniques involving ultrahigh vacuum techniques, e.g. e-beam lithography, similar to advanced techniques used for the production of nanostructured magnetic thin films and multilayers. Alternative techniques based on self-assembly and filling of templates are being increasingly used (for example, by electroplating filling of ordered nanopores in alumina membranes) [91, 92]. Lithography Methods Top-down and bottom-up strategies have been used and sometimes combined to obtain 1D functional materials. In top-down approach highly ordered nanolines can be obtained by different lithography methods (e.g. by electron beam lithography, EBL [93] or nanoimprint lithography) which can produce features characteristic in the order down to 25 nm [94]. Lithography involves a number of related processes like resist coating, or exposure [95, 96, 97]. Generally speaking, a uniform layer of resist is first deposited onto the surface of an unpatterned film or substrate. The resist, with thickness typically from a few nanometres to a micrometre, can be deposited by spin coating. Selected areas of the resist are subsequently exposed to a radiation source, often through a mask. Upon sufficient exposure, the polymer chains in the resist are either broken or cross-linked leaving to a positive or negative resist, respectively. As the lithography process transforms a 2D pattern into a 3D structure in the resist and eventually in the unpatterned film, the depth profiles in both materials are very important. By selecting a suitable developer, temperature and developing time, one can obtain different tailored profiles in the resist. The pattern transfer can be realized in two general processes: from the resist to the unpatterned film by etching or by post-deposition onto patterned resist by lift-off or electrodeposition. The lithography resolution limit is ultimately determined by the radiation wavelength. Hence lithography is usually categorized by the radiation source as optical, electron-beam, ion-beam or X-ray lithography. In the optical lithography, generally UV light is used ( ¼ 193 nm for ArF laser). In the electronbeam lithography, an electron-sensitive resist is exposed on an electron beam. This exposure is usually done using the electron source of the scanning electron microscopy (SEM) or the transmission electron microscopy (TEM). This method combined with, for example, electroplating technique enables the fabrication of patterned elements of high aspect ratio as nanowires. In the case of X-ray lithography, the key point is the exposure of a resist to X-ray radiation in a parallel replication process. Similar to the electron-beam lithography, the sample is covered by a resist layer with high sensitivity in the X-ray wavelength zone. Figure 7.5 shows the SEM image of an array of Ni nanowires fabricated using combined electron-beam lithography and electrodeposition techniques [98].

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Fig. 7.5 Ni nanocolumns prepared by combined e-beam lithography and electrodeposition [98]

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It is well known that the costs of lithographic techniques increase when the circuit dimensions are diminished which is related with the wavelengths used to define the features through predetermined masks. The improvement of the masks is one of the most significant innovations introduced to increase the resolution of imprint processes on the silicon wafers. For this reason masks constitute an important device which raises the costs of lithographic processes; therefore, nowadays much effort is centred on the production of maskless lithography and nanoimprint lithography [99, 100]. Lithography-based techniques have been very successful and powerful for the fabrication of a large variety of circuit components at nanometric scale. Nevertheless, owing to the limit of the molecule sizes of photosensitive materials [101] and the focusing lengths of the ion beams [102], lithographic methods fail to produce nanostructures with feature sizes smaller than 20 nm. The main determinant process for a self-assembly process in the relief of a solid surface is the creation of a desirable surface structure on which nanowire growth will take place. In that sense, several top-down techniques like molecular beam epitaxy (MBE), electron beam evaporation (EBE), phase-shift optical lithography and sputtering have been used [103, 104] by which continuous and insulated wires could be obtained. Viernow et al. [105] have obtained linear arrays of CaF2 stripes of around 7 nm by self-assembly on silicon . Au nanowires fabrication has been also reported on a previously prepared Si surface [106]. The theoretical explanation of the obtained results predicts the formation of bands on the metallic surface with gaps similar to those described for bulk semiconductors. Other surface manipulation for nanowire growth is based on the stress sources created by misfit dislocations at an interphase [107]. Preferential orientations can appear on the surface due to electric polarity [108]. In this case, the surfaces serve as template as well, determining anisotropic growth of different material species. The processes can be addressed so that vapour or liquid species surrounding the solid surface could be attached to it to compensate daggling bonds or to minimize the surface stress and therefore give place to thermodynamically stable surface morphology. Recently, Wang et al. [109] taking advantage of these facts have reported interesting results on the fabrication of nanocantilever arrays and a varied group of ZnO 1D nanostructures including wires, belts, springs and helices.

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The development of patterning nanometre scale lines over large areas using nanowires as masks has been recently reported [110]. Core–shell Si–SiO2 nanowires uniaxially compressed on a Langmuir-Blodgett trough were aligned with controlled pitch and then transferred to silicon substrate. Reactive ion etching (RIE) was used to remove the SiO2 shell from the sides and the tops of the nanowires. After the deposition of metals or other materials, the nanowires mask is removed to get parallel lines all over the substrate surface. This constitutes an ingenious nanolithographic method with great potential for the development of integrated nanosystems. Recently the same group using a similar strategy produced assembly of nanowires into integrated device arrays [111]. Based on the advantages offered by this approach such as formation of ordered monolayers over large areas, the facility for transferring the organized monolayers to substrates and the possibility to obtain multilayer by repeating these processes, they have been able to grow aligned nanowires with strict control on several important parameters, as the nanowire pitch and orientation, and the array size. On the other hand, this approach allows one room temperature growth of the arrays and multilayered devices which make it a compatible technique with the required low costs and with the use of flexible substrates for applications in integrated functional nanosystems [112, 113]. Methods based on vapour liquid solid growth (VLS) are commonly used for semiconductor nanowire growth, although laser ablation, chemical vapour deposition (CVD) or template-based synthesis [114, 115, 116, 117] have been also used. Recently, single-electron transistor devices (SET) based on InP have been used which show carrier transport behaviour similar to that shown by carbon nanotubes (CNTs) in this kind of devices [118].

Using Templates and Self-Assembly Self-assembly and filling of templates is a quite versatile technique to fabricate nanoscaled arrays with systematically reproducible properties. A number of novel materials with new and optimized properties can be envisaged, thanks to the capability of manipulating the nanowire nature (i.e. its magnetic properties) and the templates using, i.e., metallic, semiconductor or insulating, which can be tailored by suitable replica–antireplica processes [119]. For example, in the case of composite magnetic materials new phenomena and interactions can be studied in ideally ordered nanomagnets embedded into magnetic/non-magnetic matrices. Low-cost preparation of nanowires is possible with the bottom-up approach by self-assembly of atoms and molecules although it is hard to get them patterned and well arranged [120]. Recently, an excellent review by Y. Xia et al. [121] reports on chemical methods to synthesize 1D nanostructures including wires, belts, rods and tubes. There, molecular wires exhibiting natural growth into 1D are reported with high-anisotropic crystalline structure so that it is not necessary to use surfactant molecules or other elements to co-direct the formation of


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1D structures. The cleanness of this strategy should be useful for nanoelectronic device fabrication. The template based synthesis strategy has been used in combination with self-assembly processes in a number of works to synthesize nanoparticles, nanowires and thin films. The most successful methods to obtain nanowires and nanowire arrays by using the synthesis on templates and self-assembly are (i) self-assembly of one material in the roughness or relief present on the solid surface, (ii) self-organization within the channels and cavities of porous material and (iii) the formation of inorganic mesoscopic material by self-assembled on surfactants organic molecules. The formation of metallic nanowires and nanochannels through guided selfassembly has recently been reported [122]. In this method, the initiation and termination points of the nanostructure are pre-designed so that the pattern evolution is dictated by stress-assisted cracking on a dielectric film attached to a silicon substrate and then their cracks are used like moulds for the nanowires growth by filling them with a desired material. The technique used to produce the dielectric films that later on will be cracked is plasma enhanced chemical vapour deposition (PECVD). The use of DNA as scaffolds for nanostructure fabrication has already been explored [123] in the last years as well and it has been proposed as a way for nanoelectronic devices construction. Particularly, the fabrication of metallic nanowires has been reported based on the self-assembling of complementary DNA used as localized templates on specific surface sites [124, 125]. Anodization techniques are increasingly being used to create ordered nanoporous structures which can lately be used as templates for growing functional nanowires of different species by using mainly self-assembling process and electroplating techniques. Up to now, aluminium constitutes the unique metal which allows one to fabricate highly ordered nanoporous anodic films (NAFs) with hexagonal symmetry in a densely packed array of nanopores [126, 127]. These NAFs have been specially used to fabricate functional nanowire arrays. The template-synthesis strategy for nanofabrication has been treated in details by Hulteen et al. [128] considered one of the pioneer groups in this subject, particularly in the fabrication of functional nanowire arrays. Recently, similar strategies of synthesizing the anodization of titanium foils have been followed to obtain thin films of self-aligned TiO2 nanotube arrays which have also acquired great interest [129, 130]. The anodization and electroplating processes for tailoring functional 1D nanostructures somehow fulfil some of the demanding requirements of new technologies, such as low costs, repeatability and potential compatibility with silicon-installed technologies which make these nanostructure synthesis routes very attractive. Anodization and electrodeposition processes are detailed and largely described in the current literature [131]. In general, by means of these strategies it is possible to take control of (i) ordering degree, i.e. the size of crystalline single domains (up to several square micrometres), (ii) the single 1D structure diameter (from 15 to 200 nm) and length (from tens to thousands of

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nanometres) and (iii) the lattice parameter of ordered arrays (between 65 and 500 nm). One of the most interesting characteristics of NAFs is the parallelism of the pores with a symmetry axis perpendicularly oriented to the substrate surfaces. Taking advantage of this particular property, the electrochemical growth technique, either galvanostatic, potentiostatic or a mixture of both methods, is the most used procedure to obtain functionalized NAFs [132, 133, 134]. Thus, NAFs have become an important material in nanotechnology developments within research fields such as optoelectronic, high-density magnetic storage devices and biotechnology, in which the main attention has been focused on the use of NAFs as templates for self-assembly processes of a wide variety of species [135, 136, 137]. The ordering of a given NAF can be transferred to other media by suitable replica/antireplica processes [119]. Such a technique has been successfully employed to prepare metallic and polymeric membranes or antidot arrays [139]. In addition, hard nanostructures can be obtained which can be used as stamps for reproducing a given ordering in precursor Al disk, an example of which is given in Fig. 7.6. 7.2.2.2 Fabrication of Thin Films The main techniques to prepare thin films and heterostructures are described in this section. Physical Vapour Deposition (PVD) Physical vapour deposition (PVD) is a technique whereby physical processes, such as evaporation, sublimation or ionic impingement on a target, facilitate the transfer of atoms from the source to a substrate. A schematic representation of main PVD techniques is given in Fig. 7.7. Evaporating and sputtering are the

Fig. 7.6 HRSEM image from nanostructured TiN obtained by sputtering and using nanoporous anodic alumina films as substrates [138]

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7 Nanostructured Magnetic Sensors Fig 7.7 Scheme of the different physical vapour deposition, PVD, techniques

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two most widely used PVD methods for depositing films. The principal difference between the chemical vapour deposition (CVD) and the PVD is that in the case of PVD the source target is in solid state and there is no chemical reaction of the evaporated gases, while in the CVD methods the source is in the gaseous state and chemical reactions occur on or very close to the substrate surface. The laser ablation method is a particular case of PVD where a laser beam is used to evaporate the source material. Although pulsed laser method is commonly used, continuously laser beam can also be employed. The amount of material removed by a single laser pulse, just at the surface and below, where the laser is active depends on the optical properties of the material and on the laser wavelength.

Sputtering This is one of the most used techniques along the last decades both in research laboratories and at industrial level for producing a lot of materials with a broad variety of compositions [138, 140, 141]. Sputtering techniques are basically based on the bombardment of a selected material, acting as a target, with ions coming from electric discharge produced in plasma form. The incident ions and the atoms on the surface of the target interact very strongly to produce vapours of the latest. In a vacuum chamber, those vapours can be deposited on the highly clean surface of determined substrates. Argon is the most used discharge gas. The target acts as a cathode and the substrate is located, in general, on a conductor anode. Among others, the following advantageous characteristics should be mentioned: (a) low temperature deposition, (b) growth of a wide variety of materials with very different nature and properties, (c) reproducibility of the target composition on the substrate surfaces with high efficiency and (d) high deposition rate and environment control which allow high quality in the obtained materials. A number of magnetic nanostructures are fabricated by sputtering, with applications profiting from their various magnetic and magneto-optical properties [142].

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Evaporation In this process, thermal energy is supplied to a source from which atoms are evaporated for deposition onto a substrate. The vapour source configuration is intended to concentrate heat near the source material and to avoid heating the surroundings. The source material can be heated by several methods. The simplest way is the Joule heating by a resistance of wire or stripe of refractory metal to which the material to be evaporated is attached. The evaporated atoms travel through reduced background pressure in the evaporation chamber and condense on the substrate surface. Molecular Beam Epitaxy (MBE) This is a technique to fabricate thin films where molecular or atomic beams fall upon substrates at constant temperature in an ultrahigh vacuum (UHV) chamber. It can be considered as a particular case of evaporation, already mentioned. The main advantage of this technique is the possibility to use moderate deposition rates and temperatures allowing, among others, a high spatial control of the impurities and doping. On the other hand, the UHV atmosphere allows one to obtain epitaxial layers with very high purity. In addition, such experimental conditions enable the use of different in situ characterization techniques of the grown materials so that the growth processes can be checked and controlled. By means of this complex and sophisticated technology it is possible to prepare low-dimensional magnetic nanostructures (0D, 1D and 2D) with high homogeneity and control on their chemical composition. In this case, the main characteristic of such structures is their epitaxial nature. Thus, layer-by-layer growth homoepitaxial and heteroepitaxial are possible to be prepared. In the first case, the grown layers have the same lattice parameter as that of the substrate, while in the second one the layers grow on the substrate surface with different lattice parameter. Although MBE was initially used for semiconductor growth in the microelectronic industries, today it is further employed for fabrication of a wide range of functional materials particularly magnetic nanostructures [143, 144, 145]. The high control of the thickness of layers enables the study, for example, of magnetic coupling through non-magnetic layers, or the giant magnetoresistance effect [146]. In Fig. 7.8, an example of a particular multilayered system of magnetic/non-magnetic [(NixFe12x)yAu12y]/Au multilayers is shown [147]. Chemical Vapour Deposition, CVD From the seventies, chemical vapour deposition (CVD) techniques have played a crucial role in microelectronic industries, specially in the fabrication of highly homogeneous thin films. In these techniques, chemical reactions of precursor gases or liquids are caused to produce the formation of pre-designed materials on the surface of determined substrate. By this method it is possible to obtain single and multicomponent materials with high stoichiometric control. Homogeneous


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Fig. 7.8 Antiferromagnetically coupled FeNi/Au multilayer growth by MBE [147]

or heterogeneous reactions can occur in CVD experiments, taking place in the gaseous phase or just in contact with the substrate surface, respectively. The first one can produce the formation of particles with diameters in the nanometric scale, while the second gives rise to the formation of thin films with highly controlled thickness allowing the formation of nanostructured 2D systems. One important shortcoming of traditional CVD techniques is the high temperature required for chemical reactions (typically above 4508C) which restricts the type of substrate that can be used for the growth of specific materials. In the last decades, plasma enhanced chemical vapour deposition (PECVD) has been developed which particularly enables to obtain different thin films with high stoichiometry at lower temperatures than those used in CVD processes where thermal activation is only applied [148, 149, 150].

7.2.3 Characterization of Magnetic Nanostructures 7.2.3.1 Structure Characterization Before magnetic properties of nanostructures are properly determined, their structure characterization is really necessary to be performed particularly since nanoscale dimensions play a determinant role in many of those magnetic

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structures. That has to be performed in many cases through sophisticated techniques [151, 152, 153, 154]. In the following, a brush image of the different methods is illustrated. Most used methods to characterize nanostructured materials are those systematically used for bulk materials. From structural, morphological and composition points of view, due to the size scale diminution, some of those methods have acquired special relevance, such as high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM) and synchrotron radiation-based techniques. Both scanning and transmission microscopy are essentially based on the same physical principles. In the former, the images are mainly formed from secondary electrons generated on the material surfaces, while in the case of transmission the electrons transmitted through the samples are responsible for the formation of images. Another difference between these microscopies is the invasive character of the HRTEM in which it is necessary to use very high accelerating voltage values in addition to the sample slimming. Nevertheless, as a positive characteristic, we should mention that the material structure can be determined by means of the electron diffraction spectra produced from the interaction with the samples. In the last decades high-technology microscopes have been developed with very high resolutions to solve nanometric features in the samples and micro-probes which, based on the X-ray fluorescence phenomena, allow one to determine the elemental chemical composition of the samples. In general, the synchrotron-based techniques needed for a complete characterization (structure and composition) of nanostructured materials are X-ray fluorescence (XRF), X-ray absorption spectroscopy (XAS), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge spectroscopy (XANES) and X-ray diffraction (XRD). A brief description of these techniques is given. X-Ray Diffraction From advanced semiconductor technology to new magnetic materials, crystallography using hard X-ray diffraction techniques at synchrotron radiation facilities plays a key role in our ability to understand and control the material’s properties. The research issues that are addressed by this technique are structural studies of crystalline materials, size determinations, texture identification, crystallographic orientations, phase transitions, residual stress fields or in situ environments. X-Ray Fluorescence It provides one of the simplest, accurate and straightforward analytical methods for the chemical composition determination. It can be used for a wide range of elements providing detection limits well below the ppm level. It is specially recommended when the sub-micrometre scales are too small.


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X-Ray Absorption Spectroscopy It is applied in a wide variety of fields as quantum heterostructures, high-pressure studies, magnetic systems and doping issues in order to investigate geometric and electronic structures. This technique includes two kinds of analyses: (i) extended X-ray absorption fine structure (EXAFS) and (ii) X-ray absorption near-edge structure (XANES). EXAFS analysis is very useful to determine local environments of specific atoms to correlate them with the macroscopic physical properties, while XANES generates determinant information on the dopant charge state. 7.2.3.2 Techniques to Determine Magnetic Properties of Nanostructures In this section we consider some of the most relevant techniques to determine different magnetic properties of interest for magnetic applications and sensors. Hysteresis Properties: Vibrating Sample and SQUID Magnetometers and Kerr Effect The magnetic susceptibility and the reversal magnetization process determine the sensibility and the range of the sensor particularly in the case of the magnetic field sensor, SPM tips and MR-based sensors. The vibrating sample magnetometer, VSM [155, 156], is a conventional and reliable instrument to measure the magnetic moment of any ferromagnetic material. The sample is placed inside a magnet and vibrated perpendicularly to the field direction. The magnetometer is based on the induction law by which the magnetic moment is determined through the change of flux received by a system of coils when the sample vibrates. The signal from a pair of pickup coils is compared with that induced in a pair of reference coils by a permanent magnet. Figure 7.9a shows as an example the hysteresis loop measured with 40 FC

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VSM for a FePt thin film [159]. Some VSM instruments use superconducting magnets creating fields that are able to saturate even hard magnetic materials. Cooling and heating devices can be implemented. The sensitivity is limited by the mechanical noise induced by the vibrator. The superconducting quantum interferometer device, SQUID [157, 158], is a very sensitive magnetometer used to measure extremely small magnetic fields. SQUIDs are based on superconducting coils containing two Josephson junctions, consisting of two weakly coupled superconductors separated by a very thin insulating barrier, where the current flowing across the junctions is sensitive to the magnetic flux. The two superconductors separated can experience tunnelling of Cooper pairs of electrons through the junction. The Cooper pairs on each side of the junction can be represented by a wavefunction similar to a free particle wavefunction. In DC Josephson effect, a current proportional to the phase difference of the wavefunctions can flow in the junction in the absence of a voltage, while in the AC alternative, a Josephson junction oscillates with a characteristic frequency proportional to the voltage across the junction. Since frequencies can be measured with great accuracy, a Josephson junction device has become the standard measure of voltage. The SQUID is sensitive to fields near 1010 G, near the theoretical limit of energy sensitivity given by the uncertainty principle, and has a low l/f noise. The SQUID is more sensitive than regular VSMs although in some cases external perturbations may cause spurious signals. Magnetic nanoparticles exhibit some peculiar properties that can be resolved by sensitive magnetometry as SQUIDs. As an example, Fig. 7.9b depicts the SQUID low-temperature asymmetric hysteresis loops observed for core/shell CoNi nanoparticles. In this case, the shift in the hysteresis loop is determined by the exchange bias effect introduced by the antiferromagnetic shell to the ferromagnetic core. The shell of these nanoparticles, also represented in Fig. 7.3b, represents a very significant fraction of the whole nanoparticle volume. When compared to bulk material, magnetic behaviour of nanomaterials, particularly in the case of magnetic nanoparticles, is characterized by two main features that finally derive from the larger fraction of atoms at the surface compared to those at the volume: an effective decrease of the magnetic moment and the enhancement of the magnetic anisotropy. The first one is ascribed to the existence of a magnetically dead layer at the surface, spin canting or spin-glasslike behaviour of the surface spins. Concerning the second one, the magnetic anisotropy is larger than that of crystalline and shape origin, and it has been shown to vary with the adsorption of different molecules, which means that surface anisotropy is the main source. Therefore, magnetic properties of nanoparticles are highly dependent on particle size, shape and in some cases surface coating [161]. A complex interplay between particle core and coating determines the magnetic properties of the resulting composite. In the case of inorganic coatings, for example, silica coating has been used to tune the magnetic properties of nanoparticles by controlling the distance between particles and therefore dipolar interactions. Gold-coated Co nanoparticles have a lower magnetic anisotropy than uncoated particles, but


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Au-coated Fe enhances the anisotropy probably due to the formation of a Fe–Au alloy. Organic ligands can also modify the anisotropy and magnetic moment of metal atoms located at the surface of the particles. Cobalt nanoparticles show a reduction of magnetic moment and a large anisotropy; however, magnetite shows an enhancement of the saturation magnetization and reduction in anisotropy when coated with oleic acid [162]. Finally, a magnetic coating has a dramatic influence on the magnetic properties. The exchange coupling across the interface between a ferromagnetic core and an antiferromagnetic or ferromagnetic shell or vice versa causes the shift of the hysteresis loop along the field axis [163]. VSM and SQUID magnetometers are macroscopic techniques that supply information about the magnetization process and parameters as saturation magnetization, coercive field and susceptibility, as well as temperature or time dependences of magnetic moment of the nanomaterial as a whole. In turn, the hysteretic magnetic behaviour arising from a tiny depth at the surface of materials, particularly relevant in the case of bidimensional nanostructures, can be determined through magneto-optical methods. They are based on the small rotation of the polarization plane of the light when it is reflected (Kerr effect) or transmitted (Faraday effect) by the magnetic material [164, 165]. Since the light interacts with the surface of the sample within a penetration of up to few tens of nm, the behaviour of the top layer can be studied separately from the whole sample. In addition, the hysteresis loops of the different regions of a patterned media can be characterized independently [134]. Figure 7.10 shows the Kerr effect loop of a FePt nanopatterned thin film irradiated with Cl ions. An important characteristic of the magneto-optical Kerr effect is its time resolution. Magnetization dynamics on the sub-nanosecond scale, and particularly high-speed switching phenomena, can be determined by this technique [166, 167], even for individual single-domain nanomagnets [168].

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Magnetic Imaging A number of techniques are used to gain direct information of the orientation of magnetic moments at the surface of nanostructured samples at micron, sub-micron and nanoscale. Some of them represent the most advanced scanning magnetic sensor devices that will be considered later. Magnetic imaging techniques used to characterize the domain structure of magnetic nanostructures can be sensitive to the stray field, the magnetic polarization or the total flux density. Magneto-optical Effects Since the magnetic material used in the magnetic sensor is not usually transparent, magneto-optical characterization based on Kerr effect [169] is used to observe the domain structure. The magnetic moment distribution at the surface is imaged through the magneto-optic interactions which depend directly on the magnetization. The relative orientation between the magnetization, M, and the polarization, E, vectors determines which component of the magnetization will be visible in a particular magneto-optic image. Figure 7.11 shows schematically the different configurations to observe perpendicular and in-plane magnetization components with regard to the sample surface plane. The polar Kerr effect is used to image perpendicular magnetization. The most common arrangement is the longitudinal effect in which the magnetization lies in the scattering plane of light. A drawback of the Kerr effect technique is its limited resolution reduced to few hundreds of nm. An example of domain structure obtained by Kerr effect is given in Fig. 7.12. Electron Microscopies Higher resolution can be obtained with scanning electron microscopy with polarization analysis (SEMPA). Secondary electrons emitted by a ferromagnetic material are spin-polarized [171]. In particular, the magnetic moment is parallel to the magnetization direction of the origin region. The polarization of secondary electrons can be used for imaging magnetic domains [172]. SEMPA (a)

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Fig. 7.12 Kerr effect image the free layer (Ni81Fe19/ Co90Fe10) of a spin valve showing the effective anisotropy distribution after annealing under magnetic field perpendicular to the magnetic anisotropy axis. Two-axis anisotropy is observed [170]

directly provides an image of the surface magnetization while the finely focused (unpolarized) beam scans over the sample, as shown schematically in Fig. 7.13. SEMPA therefore produces a direct image of the magnitude and the direction of the magnetization in the region probed by the incident electron beam [174]. SEMPA depends on the fact that the polarization of the secondary electrons reflects the net spin density of the material. For the purposes of SEMPA, it is sufficient to treat each component of the vector polarization separately. The polarization along the x-direction is Px ¼ (N" N#)/(N" þ N#), where N" (N#) are the number of electrons with spins parallel (antiparallel) to the x-direction. As an example, Fig. 7.14 shows SEMPA images for a nanoscaled trilayer under indicated conditions. SEMPA has several unique capabilities that set it apart from other magnetic imaging techniques: it measures the magnitude and direction of the magnetization with high spatial resolution (about 10 nm); topographic maps are measured simultaneously; it is a relatively surface-sensitive technique (1 nm). The drawbacks of the technique are the long exposure time, the UHV conditions’ requirement and that it is limited to metallic materials. Alternative electron microscopes are used to characterize the magnetic properties of nanostructures, all of them require UHV working conditions and thus they are less extended for sensor characterization. In the Lorentz methods,

Fig. 7.13 Scheme of SEMPA microscopy [173]

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Fig. 7.14 SEMPA images displaying the magnetization observed in different layers of a Fe/ Cr/Fe trilayer. (a) My magnetization component of a Fe whisker substrate; (b) the same region after deposition of 1 nm thick Cr layer; (c) the same region subsequent addition of 2 nm thick Fe layer. (df) Mx magnetization component for the same conditions as in ac [175]

based on the transmission electron microscope (TEM), electrons in the beam are viewed as particles deflected by the Lorentz force produced by the magnetic field resulting from nearby magnetic material. There are three Lorentz microscopy modes, which are frequently referred to as Fresnel, Foucault and differential phase contrast (DPC) microscopy. In all modes, the deflection of the electron beam is sensed as it travels through a magnetic field. On the other hand, spin-polarized low-energy electron microscope (SPLEEM) is a surface-sensitive technique using very slow electrons (0–20 eV). Its main advantage is its capability to simultaneously record images showing topographic (low-energy electron microscopy, LEEM) and magnetic contrast. SPLEEM images are obtained by subtraction and subsequent normalization of two images taken with antiparallel polarization of the incident electron beam. Magnetic contrast in such images is a consequence of the exchange-scattering asymmetry. Magnetic transmission X-ray microscope (MTXM) [176] is based on the X-ray magnetic circular dichroism effect which occurs in the vicinity of elementspecific inner-core absorption edges. Depending on the relative orientation between magnetization and helicity of the circularly polarized photon, the absorption coefficient varies. Scanning Techniques A new imaging technique has become widely extended, the magnetic force microscope (MFM) [177]. It is based on the magnetic interaction between the stray field of the sample and the magnetization of the MFM tip. The high spatial resolution of this technique [178] is based on the low-dimension tip radius and the short tip–sample working distance. Topographic contribution is avoided from the magnetic signal by a double scan technique: the first scan, usually obtained in dynamic mode [179], relates to the topography while the magnetic signal is recorded during the second scan. In such second scan, the distance between the

7 Nanostructured Magnetic Sensors Fig. 7.15 Scheme of the MFM system. Two images can be obtained simultaneously: the topography and the domain distribution

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tip and the sample is of few tens of nm. When the tip–sample interaction is negligible, the MFM signal corresponds to the distribution of the magnetic poles which concentrates in the centre of the domains or at the domain walls in materials with perpendicular or in-plane anisotropy, respectively. A schematic view of the MFM system is presented in Fig. 7.15, while an example of MFM images is given in Fig. 7.16 for an array of triangular Ni nanostructures. The advantages of this technique are its high resolution, the possibility of obtaining simultaneous topographic and magnetic images, the easy sample preparation, not having restrictions for measuring at ambient conditions and the possibility to apply magnetic fields during the microscope operation, i.e. to study reversal magnetization process [180]. The drawbacks include the reciprocal tip–sample influence and the lack of quantitative information. In some works, the use of novel elements to be employed as MFM probes is proposed, in particular, ferromagnetic nanowires tips could improve the spatial resolution and avoid the always annoying tip-induced changes (see subsequent section) [181]. Spin-polarized scanning tunnelling microscope (SPSTM) can also be used to characterize magnetic nanostructures useful for sensor applications as in the

Fig. 7.16 Topography and MFM signals of an array of triangular Ni nanostructures fabricated by nanolithography [182]

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Fig. 7.17 Map of the dI/dU signal of a single 8 nm high Fe island recorded with a Cr-coated W tip. The vortex domain pattern can be recognized. Arrows illustrate the interpreted orientation of the domains [183]

example given in Fig. 7.17. The current between the tip and the sample in a scanning tunnelling microscope can be used to obtain the magnetic state of the surface. High-resolution images can be obtained with this technique that should be used in UHV and low temperature [183]. Magnetic Resonance Imaging When the nuclear magnetic moment associated with a nuclear spin is placed in an external magnetic field, the different spin states are in different magnetic potential energies. In the presence of the static magnetic field that produces a small spin polarization, a radio-frequency signal of the proper frequency can induce a transition between spin states. This process is called nuclear magnetic resonance (NMR), and its resonant frequencies for each particular substance are directly proportional to the strength of the applied magnetic field, in agreement with the precession Larmor equation. This is used to great advantage in the medical imaging process with micrometre resolution known as magnetic resonance imaging (MRI). MRI has been regarded as a powerful imaging tool with non-invasive nature, high spatial resolution and tomographic capabilities, low signal sensitivity being its major limitation. The image contrast is due to the response of water protons to an external magnetic field. Energy is applied to the protons (in the radio-frequency range), exciting the water protons and when the radio-frequency source is


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removed, water protons relax or return to a state of equilibrium. During relaxation to lower energy states, energy associated with the spin flip is emitted at the resonant frequency resulting in the signal observed in MRI. The difference in relaxation rates and proton densities within the various tissues is responsible for the native soft-tissue contrast. Despite the inherent contrast of MRI, there are situations where contrast agents are required to enhance the relaxation of water protons in specific tissues. Contrast agents are either paramagnetic complexes such as gadolinium chelates (chemicals that control the concentration or effects of metal ions) or superparamagnetic particles of iron oxide with various surface modifications. Today’s NMR microscopy has reached voxel resolution of 3.5 mm3 at 400 MHz, with 30 h acquisition time [184]. Alternative electron spin resonance is used in electron spin resonance (ESR) microscopy [185]. Some theoretical derivations have been shown that ESR imaging methods should achieve voxel resolution better than 1110 mm in several minutes of acquisition (at 35–60 GHz) for bio/liquid samples [186].

Magnetoresistance Magneto-transport effects (Hall effect and magnetoresistance) appear as a consequence of the influence of magnetic field onto electrical current carriers so modifying the resistance of the material. They have been observed and studied traditionally in semiconductors in connection with the development of magnetic field sensors. The study of the magneto-transport properties of ferromagnetic nanostructures is particularly important for their applications, and they supply in addition information about the magnetic microstructure and reversal magnetization process. The present relevance of various magnetotransport effects is related to their wide use in advanced magnetic recording heads. Besides conventional magnetoresistance, MR, effect observed in semiconducting layers, two effects can be distinguished: the anisotropic magnetoresistance (AMR) and the giant magnetoresistance (GMR). The anisotropic magnetoresistance [187] is based on the dependence of the resistance on the angle between the current density and the magnetization. AMR is a relatively weak effect, where about 4% resistance variation can be reached. However, it is enough to use it in reading heads read. The GMR effect appears when two ferromagnetic material layers are separated by a non-magnetic or spacer metallic layer. The discovery, understanding and technological applications of this phenomenon have resulted in the awarding of the Nobel Prize to Fert and Gru¨nberg [188, 189]. If the metallic interlayer is thinner than the spin diffusion length and the ferromagnetic layers are magnetized in different directions, the electrons coming from the source suffer an additional scattering process in the second as schematically shown in Fig. 7.18. This is also observed in multilayer systems consisting of a large number of bilayer systems as that described here.

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No magnetic field

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In spin valves, both ferromagnetic layers have different hysteresis behaviour. One layer is magnetically soft, and consequently sensitive to small applied fields, while the other layer is magnetically harder. As the magnetization of the soft ‘‘free’’ layer changes due to an applied magnetic field, the resistance of the whole structure will vary. The hard magnetic layer is typically pinned or exchange biased by an antiferromagnetic material as shown in Fig. 7.19. An electrical current can flow through local barriers as nanoconstrictions or insulating layers by tunnelling effect (see Fig. 7.20a). A variant of the GMR elements is the tunnelling junction where the two ferromagnets are separated by a thin insulating film. In this case, the electrons travel from one ferromagnet to the other by tunnelling effect through the insulator layer [190]. Higher tunnelling magnetoresistance, TMR, effects can be obtained with this configuration (see Fig. 7.20b). Finally, the giant magneto-impedance (GMI) is being increasingly used in microsensor applications [191]. This effect occurs when a high-frequency

Free layer Spacer layer Pinned layer AF pinning layer

Fig. 7.19 Schematic view of the different nanoscaled layer in a spin valve

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Fig. 7.20 A magnetic nanoconstriction (a) and magnetoresistance effect in a tunnel junction (b)

current flows along a soft magnetic metallic conductor, typically in the form of microwire or thin film. The current actually flows along a thin skin-depth penetration below the surface of the material. Under the action of a stationary magnetic field, the skin-penetration depth increases which induces a decrease in the impedance, both real and imaginary components. The GMI effect is particularly effective in ultrasoft magnetic materials where the action of that stationary field strongly modifies their ac permeability. In the case of magnetic microwires exhibiting bamboo domain pattern, when an alternating current flows through the wire, the induced circular magnetic field displaces the domain walls reversible. As a result strong impedance is measured in the wire that is suppressed by applying a longitudinal field. The working frequencies range from 1 kHz to 1 MHz. Nowadays some sensors based on nanostructures use this GMI effect [192].

7.3 Magnetic Sensors and Applications This section is divided into three main sections attending to the dimensionality of the magnetic nanostructures on which the magnetic sensors and technological applications are in general based. Owing to the above-mentioned large variety of those applications, we focus on each section towards most relevant applications. The first one is devoted to technological applications where magnetic nanoparticles are involved, which are mainly related to magnetobiological and biosensor applications. The second is devoted to magnetic nanowires and their use as magnetic probes in different scanning techniques. Finally, the third related to thin films is mainly devoted to magnetic sensing in connection with magnetic storage of information.

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7.3.1 Biological Applications Based on Magnetic Nanoparticles In molecular biology, a strong interest is being devoted to the use of magnetic nanoparticles as an efficient tool for a fast, efficient and easy biomolecule extraction [193]. Today, these processes can advantageously replace other techniques such as centrifugation, filtration and chromatography. The large surface area per volume of the nanoparticles provides rapid reaction kinetics, while the magnetic separation allows easy and rapid handling, and is extremely gentle to the target, preserving protein nativity and viability of fragile cells. On the other hand, the use of magnetic nanoparticles as contrast agents for nuclear magnetic resonance imaging will be addressed. One of the current challenges in the biomedical sciences is the ultrasensitive imaging of biological targets under non-invasive in vivo conditions, and NMR imaging is a very powerful technique for this purpose [194].

7.3.1.1 Biosensors for Detection and Separation The major application of magnetic nanoparticles nowadays concerns the extraction of biomolecules such as proteins, antibodies and nucleic acids [195, 196], although in addition, they are used for specific bacteria, virus captures and cell recognition [197]. The most important factors are the magnetic particles and the used ligand. Magnetic beads should have high magnetic oxide content for their fast magnetic separation and a good compatibility with other biomolecules such as those enzymes used for nucleic acid amplification. Geometry characteristics, as size and shape, are important parameters to be considered. Some commercial products, such a Dynabeads1, are based on monosized polystyrene beads which have an even dispersion of magnetic nanoparticles inside (Fe2O3 or Fe3O4) coated with a thin polymer shell. A schematic description of the cell separation process is given in Fig. 7.21. A wide range of surface activated and pre-coated Dynabeads1 products are available [198]. For example, by using Dynabeads1 with covalently coupled streptavidin together with a biotinylated probe/ligand, any target molecule can be captured, isolated and further manipulated. This product has also been used for detection of various metastatic cancers in tissues and blood [199]. These commercial products are used on a wide variety of automated devises. Polyamine particles of 1 mm diameter have also been used in the development of an ultrasensitive method for detecting protein analytes [195]. In general, a biosensor is a compact analytical device incorporating a biological-sensitive element associated with a physicochemical transducer. Such transducer systems can be based on electrochemical, piezoelectric or magnetic principles. A schematic representation of the enzyme-linked immunomagnetic electrochemical assay (ELIME) is presented in Fig. 7.22. An immunogenic analyte (bacteria, for example) is sandwiched between an antibody-coated magnetic bead and an antibody-enzyme conjugate. The bead is


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Fig. 7.21 The principle of dynabeads work for cell separation. Dynabeads are added to a starting sample in a tube. The beads bind to the target cells after 15–30 min incubation. A magnet is placed near the tube containing sample and beads. The beads and bound cells migrate to the magnet and the supernatant is removed with a pipette. Dynabeads can be detached from isolated cells if required. Cells isolated with dynabeads can be used in many assays/applications including cell culture, functional and proliferation studies, flow cytometry, molecular studies, cytokine secretion, phenotyping and further sorting

O

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trapped magnetically on the electrode surface, exposed to the enzymatic substrate and the electroactive product is detected electrochemically. This type of assay has been applied for different analytes with very good reproducibility, i.e. 2% standard deviation [200, 201, 202, 203]. For the detection of antigen concentration in biological samples, the electrical properties of an antibody immobilized on a gold electrode can be followed by electrochemical impedance measurements. Beads are modified with streptavidin and the antibody with biotin which have a high affinity for streptavidin. In this way, a small pesticide molecule has been detected with a detection limit of 10 ng/ml [204]. A piezoelectric immunosensor based on a 5 MHz quartz crystal resonator has been used for the detection of biological pathogens such as Salmonella typhinium [205]. Salmonella cells are captured by antibody-coated magnetic beads and then moved magnetically to the sensing quartz. An analyser measured the impedance behaviour of the oscillating quartz crystal exposed to various concentrations of Salmonella (102–108 cells per ml). Enzymes immobilized on magnetic beads can be trapped by magnets and retained on an electrode surface at a specific location in flow analysis devices and further analysed by amperometric, potentiometric or conductimetric measurements. An example of enzyme immobilization strategy is given in Fig. 7.23. For environmental toxicity analysis, detection limits as low as 1011 M for heavy metal ions and 1012 M for organophosphates and carbamates have been achieved [206]. The practice of DNA sequence detection has become more and more ubiquitous in genetics, pathology, criminology, food safety and many other fields. Magnetic nanoparticles, as biomolecule carriers via a suitable immobilization process, offer good potential for sensitive sensors. Nanoparticles, prepared by co-precipitation and then coated with a carboxylic acid containing polymer layer, have been used in electrochemical nucleic acid sensor systems [207]. Nanoparticles prepared by decomposition in organic media have been also modified using a combination of alkylphosphonate surfactants and ethoxylated fatty alcohols. These particles can be selectively hybridized to DNA functionalized gold surfaces and used in biomolecule detection [208].Moreover, if magnetic nanoparticles are provided with a gold coating, the combined benefits of the robust chemistry for gold surfaces and the uniqueness of magnetic nanoparticles could be realized [209].

Fig. 7.23 Preparation of magnetic beads for a tyrosinase-based biosensor


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On the other hand, Reich et al. demonstrated the utility of high aspect ratio magnetic nanowires for cell manipulation [210]. Nanowires were synthesized by electrodeposition of Ni into nanoporous alumina membranes and isolated as a suspension. Finally, novel magnetic and fluorescent nanocomposites prepared by layer-by-layer self-assembly approach have shown to be sensitive probe for the determination of proteins [211]. A genomagnetic electrochemical bioassay (GEME) for the separation of DNA based on the immobilization of DNA on magnetic beads and electrochemical detection has been proposed and successfully used with detection limits of 100 ppb for breast cancer gene [212]. For high-sensitivity detection of protein and DNA, bio-bar code assay appeared in the early 2000s as a promising analytical tool. It relies on a sandwich structure based on specific biological interaction between magnetic beads and nanoparticles (gold and polystyrene) with an immobilized oligonucleotide, which is call bio-bar code complement. Magnetic beads are added to the solution and allow interaction with the biological target to be detected, generally via DNA/DNA interaction or antibody/antigen interaction. Then, nanoparticles are added and interact with the biological target to form a sandwich-like structure. Beads are then separated and the sandwich redispersed in water. The bio-bar code DNA is then dehybridized and captured on a DNA chip and detected. Three main detection methods are used: scanometric detection, fluorescent detection and rolling circle amplification (RCA). Scanometric detection leads to detection as low as 5001021 M (molar), in the case of DNA and 31018 M for protein (PSA). The detection limit of the fluorescent method is of a few hundreds of 1018 M, while for the RCA assay it is of 1 pg/ml for DNA. In the case of the magnetic biosensors, a magnetic field sensor can be used in combination with magnetic nanoparticles, which act as magnetic labels to detect low concentration of the target of interest [213] (see Fig. 7.24). Recently developed giant magnetoresistance (GMR) or tunnelling magnetoresistance (TMR) magnetic sensors have been developed (see a later section for details). Then, the magnetic nanoparticles are specifically attached to the target molecules, and their magnetic stray field is picked up by an embedded magnetoresistive sensor as a change of the electrical resistance. In comparison to previously mentioned methods, i.e. fluorescent, these magnetic biosensors have a number of advantages, including low-molecular detection limits, flexibility and the direct availability of an electronic signal suitable for further automated analysis. This makes them a promising choice for the detection units of future widespread and easy-to-use lab-on-a-chip systems or biochips [214]. Another novel method of detecting either protein binding or DNA hybridization at room temperature has been developed based on magnetic nanoparticles of manganese ferrite and a magnetic tunnel junction-based biosensor situated in orthogonal magnetic fields [215]. Spin valve sensors have been used for single bead detection of 2 mm diameter [216]. These sensors, described in a previous section, consist of an antiferromagnet layer next to a pair of spaced ferromagnetic layers, where the antiferromagnet serves as exchange biaser for

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Fig. 7.24 Some examples of magnetic biosensors developed in different laboratories where magnetic field sensors are used in combination with magnetic nanoparticles

the adjacent magnetic layer, pinning its magnetization direction. The other magnetic layer is free to rotate. This leads to a linear magnetic field dependence of the resistance. Spin valves are sensitive not only to the magnitude but also to the direction of the field in the plane. GMR and spin valves are the most common sensor types. Biosensors based on tunnelling magnetoresistence and giant magneto-impedance are still very new. Biomagnetic sensor can be also based on Brownian relaxation of magnetic nanoparticles suspended in liquids. The characteristic time scale of the Brownian relaxation can be determined directly by ac susceptibility measurements as a function of frequency. This is a consequence of the shift of peak of the imaginary component of the ac susceptibility to lower frequencies upon binding the target molecules to the magnetic nanoparticles. The frequency shift is consistent with an increase of the hydrodynamic radius corresponding to the size of the target molecule [217]. Measurements based on the variation of magnetic permeability of a compound using inductance measurements have been applied for detection and quantification of DNA with detection limits of around 50 mg/ml for plasmid DNA in buffered solutions [218]. For human albumin detection in undiluted urine, a detection limit of 5 mg/ml was obtained [219]. Further work should be focused on optimizing biological molecule immobilization in order to reduce crossreactivity and non-specific adsorption.


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7.3.1.2 Magnetic Nanoparticles as Contrast Agents in MRI Imaging One of the current challenges in the biomedical sciences is the ultrasensitive imaging of biological targets under non-invasive in vivo conditions, and NMR imaging is a very powerful technique for this purpose [220]. The success of the in vivo imaging techniques is highly dependent on the molecular imaging agents, which could lead us to precisely detect early-stage diseases, monitor the responses to drug therapies and track cell migrations. Low-molecular-weight agents are extravascular and distribute with blood flow but provide no cell-specific or process-specific information. They cannot be used for cell tracking. Polysaccharide-coated iron oxide nanoparticles have been largely investigated because of their good inherent signal differentiation and ability to be internalized by cell with phagocytic capacity. Iron oxide nanoparticles have been used as imaging agents in cell migration, gene expression, angiogenesis, apoptosis, cancer detection [221] and also as marker of inflammatory status [222, 223]. Effect of particle size and coating on biodistribution has been explored leading to the conclusion that smaller and neutral particles circulate longer and have a limited uptake by the reticuloendothelial system (RES). The signal enhancement of magnetic resonance imaging, MRI, however is still unsatisfactory compared to that obtained with other imaging modalities. One problem is the lack of a clear relationship between nanoscale material characteristics and MR signal enhancement effects. Magnetic iron oxide nanoparticles used as contrast agents are generally synthesized in water. However, high-quality nanoparticles prepared by decomposition in organic media and possessing a variety of metallic dopants with distinct magnetic spin magnitude have been recently checked for NMR imaging and proposed for ultrasensitive detection of target biological molecules. It was observed that composition controls the magnetic spin magnitude and that this is critical for modulating the spin–spin relaxation processes of protons in the water molecules surrounding the nanoparticles. In addition, there was a marked size dependency of MR signals with a gradual increase in contrast as the size increases. A faster spin–spin relaxation process of water molecules is induced by materials with a larger magnetization. Combining both effects, composition and particle size, manganese ferrite particles, 12 nm in diameter, have been shown to enhance sensitivity for cancer cell detection and to enable in vivo imaging of tumours as small as 50 mg [224]. Bi-functional contrast agents with both optical and magnetic contrast have been demonstrated to serve as good molecular imaging probes for in vitro and in vivo experiences [225]. Dual modality detections can be simultaneously achieved using a single material, i.e. by MRI and fluorescence microscopy. The material consists of CdSe/ZnS core/shell QDs, coated with a pegylated phospholipids and a gadolinium compound. Dye molecules have also been incorporated into a silica shell coating iron oxide nanoparticles [226] and the material has been probed to serve as a superior multifunctional tracking agent.

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MRI also offers the potential of in vivo tracking of cells using innovative approaches for cell labelling and image acquisition. Cell labelled with magnetic nanoparticles can be imaged from weeks to months after intravenous administration or direct injection in order to track migration into the target tissue, for example in cardiovascular diseases [227]. Cell labelling with magnetic nanoparticles is now widely used as a means of in vitro cell separation, to isolate cells of interest, or in vivo for delivering drugs and locally concentrating them at the desired site. If cells are labelled with nanoparticles, they can be located, tracked and recovered and those processes can be followed by imaging techniques such as NMR. Magnetic cell targeting opens up new possibilities for vascular tissue engineering and for delivery localized cell-based therapies [228]. Various organic coatings have been used. Amphiphilic coatings such as polyethylene glycol and dendrimers and also transfection agents such as HIV-derived TAT proteins allow their free passage into cells. Nanoparticles with anionic charge result in non-specific adsorption at the surface and endocytosis into the cell of a large number of particles, i.e. up to more than 3–4 orders of magnitude (see Fig. 7.25). However, there is a certain concern about the effect of intracellular iron oxide nanoparticles on the normal cell behaviour. Some works have shown that nanoparticles and surface coating can have a dramatic effect on cellular morphology, uptake efficiency, cytotoxicity and cell mobility [229]. Moreover, moderate levels of anionic iron oxide nanoparticles modified with dimercaptosuccinic acid (DMSA) adversely affect cell function in growing neurons [230] although DMSA by itself has been demonstrated that in the same concentrations or larger have no measurable effect. More studies on the acute and long-term effects of cellular Fe2O3 internalization are then necessary and warranted. Non-invasive imaging of myocardial macrophage infiltration has been shown to be possible by both fluorescence tomography and magnetic resonance imaging using magnetofluorescent nanoparticles, which are taken up by macrophages in infrarcted myocardium. An increase in magnetic resonance imaging contrast-to-noise ratio, indicative of myocardial probe accumulation, has been observed in the anterolateral walls of the infarcted mice together with significantly greater fluorescence intensity over the heart. The

Fig. 7.25 HeLa cells labelled with anionic magnetic nanoparticles (courtesy of Dr. A. Villanueva and M. Can˜ete, Faculty of Biology,UAM, Madrid)


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uptake of magnetofluorescent nanoparticles by macrophages infiltrating the infarcted myocardium was confirmed by fluorescence microscopy and immunohistochemistry [231]. Other Applications A magnetic hydrogen sensor has been proposed based on the variation of saturation magnetization and remanence of nanoscale antiferromagnetic hematite with hydrogen flow [232]. The study of transparent magnets, among other magnetic composites, is an interesting challenge because of their novel potential applications in magnetooptical switches, modulators, optical circulators, laser isolators, magnetic field and electric current sensors based on Faraday effect. It is important to reduce the size of the magnetic particles in the composite in order to obtain superparamagnetic behaviour allowing the sample to be used as a low magnetic field sensor. Guerrero et al. [233] have observed Faraday rotation in -Fe2O3/SiO2 samples in which the iron oxide particles grew during the formation of the sol–gel silica matrix. Further improvement of this composite was achieved by impregnation of slices of porous Vycor glass (VG) rods with an iron nitrate solution followed by a thermal treatment and a reduction process [234]. The resulting -Fe2O3/Vycor glass composites exhibit remarkable Faraday rotation and can find application in a large range of magneto-optical devices such as a magnetic field sensor. Finally, very recently a magnetic hydrogen sensor has been proposed based on the variation of saturation magnetization and remanence of nanoscale antiferromagnetic hematite with hydrogen flow [235].

7.3.2 Magnetic Nanowires and Sensors for Magnetic Scanning Techniques In this section we review the use of cylindrical nanostructures, such as nanowires and nanotubes, and thin films in magnetic sensors technologies. We particularly focus on those technologies related to magnetic scanning techniques where various kinds of tips with elongated shape and nanowires are employed. Nevertheless, in few examples sensing elements include 2D nanoelements. 7.3.2.1 Sensors Based on Nanowires Grown into Ordered Membranes Earlier in this chapter, the preparation of magnetic nanowires has been described, covering processes such as nanolithography, electroplating or sputtering. Of particular interest are nanowires grown into ordered hexagonal membranes, either polymeric or alumina made. Such nanowires can be removed

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from the template by suitable methods in case sensing requires individual nanowires as for example in scanning techniques which will be considered in more detail in the next section. One of the most common applications of nanowires is in technologies related to magneto-transport properties. In this regard, magnetoresistance, MR, and magneto-impedance, MI, sensors are proposed to detect magnetic fields, where a large change in the impedance is obtained when the susceptibility of the sample changes. An interesting example of this type of sensors is given by Enculescu et al. [236]. who makes use of a nanowire consisting of alternating layers of Co and non-magnetic Cu obtained by electrochemical filling and etched-track nanochannels. The single ion track is prepared by irradiating a 30 mm thick polycarbonate foil with heavy ions and then the damaged trail is transformed into a narrow channel by selective etching, interrupted once the desired size is reached. By changing the potential between anode and cathode during the electroplating process it is possible to force selective growth of Co or Cu, as required. Therefore a pulse voltage leads to the growth of multilayers nanowires. The contacts of the sensor are established by sputtering the top of the membrane with a gold film and mounting it on a circuit board. An application of nanowires for magnetic biosensing has been proposed by Anguelouch et al. [237] that makes use of Ni nanofilament with 170 nm diameter and length ranging from 5 to 30 mm. The nanowires are obtained using template electrodeposition and the GMR sensor consists of two ferromagnetic electrodes separated by a non-magnetic metal. The magnetization direction of the top layer, being fixed, defines the axis of maximum sensitivity, i.e. the sensing direction. The magnetoresistance of the sensor depends therefore on the divergence of the magnetization direction of the bottom layer relative to the top layer. The nanowires are suspended in an aqueous solution and introduced over the sensor chip and the bridge output is recorded, as discrete increase of the output, as the wire settled onto the sensor, as shown in Fig. 7.26. This sensor has the ability to detect a single nanowire touching the GMR sensor, widening its application to liquid flow tracking and biosensing applications where single particle sensitivity is needed. A more exotic application is represented by the use of magnetic nanowires for acoustic sensor [238]. Also in this case, nanowires are grown into nanoporous template such as nuclear track etched, block polymer or anodic aluminium oxide membranes. A combination of anodic aluminium oxide and nanoimprinting is proposed to obtain a nanoporous membrane with large ordered areas. With this technique, the aluminium precursor foil was electrochemically polished prior to imprinting. A Si3N4 stamp was used to transfer the ordered pattern to the Al foil, the average depth of the prepatterned imprint having value of 30 nm, followed by an anodization process. The resultant nanoporous membranes present areas of ordered hexagonally distributed pores, where the imprinting was performed, and disordered porous areas, as shown in Fig. 7.27. To grow the nanowires, a metal electrode was


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Fig. 7.26 Discrete response of a GMR sensor using nanowires in the case of (a) two 5 mm nanowires and (b) one 30 mm approaching the surface of the sensor [237]

deposited on the back of the membranes and Galfenol wires have to be grown by electrochemical route. The magnetic nanowires are exposed by partially removing the alumina membrane via selective acid attack. Acoustic measurements are obtained by bounding the matrix of nanowires to the surface of an integrated circuit die containing a commercial giant magnetoresistive, GMR, element. According to the authors this sensor requires further optimization in terms of reducing the distance between the nanowire arrays and the GMR sensor, so increasing the sensitivity.

Fig. 7.27 SEM micrograph of surface view of anodized pretextured aluminium [238]

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7.3.2.2 Magnetic Sensors Based on Scanning Techniques Scanning techniques are very useful to determine the nanoscale characteristics. Here, we describe some advanced techniques that make use of magnetic nanostructures and associated phenomena which enable the measuring of magnetic fields.

Magnetic Force and Magnetic Resonance Force Microscopies As described in a previous section, magnetic force microscopy, MFM, is a very useful tool to determine the surface magnetic characteristics of a nanostructured material. This microscopy requires of a magnetic tip to sense the stray field created by that surface. Recently, a number of studies have been performed trying to incorporate magnetic nanowires and nanotubes to substitute conventional magnetic tips. For example, Yang et al. [239]. have proposed the use of Ni nanowires as MFM tips. In this case, the nanowires are also obtained by filling the pores of anodized alumina membranes with Ni by electrodeposition. The diameter of the nanowires ranges typically between 20 and 40 nm, their lengths around 500 nm and presents saw tooth morphology. The nanowires are separated from the alumina membrane by attacking the latest with phosphoric acid at room temperature and subsequently dispersed in deionized water. Ni nanowires are then attached to a commercial AFM probe by a dielectrophoresis process, as shown in Fig. 7.28. The assembled tip is straight and aligned along the cone axis of the Si cantilever, having length of around 2 mm and a diameter at the very end of approximately 40 nm. Carbon nanotubes have been shown to work successfully when employed to image magnetic domains [240]. A conventional MFM magnetic probe was prepared using iron-filled multiwall carbon nanotube. The nanotubes are prepared by pyrolysis of ferrocene in a quartz furnace. Their diameters range between 30 and 100 nm while the Fe filling has a mean diameter between 10 and 20 nm and lengths up to several microns. A single nanotube is attached to the cantilever by pinning it with the electron beam of a scanning electron microscope that induces the deposition of carbon-containing contaminations.

Fig. 7.28 SEM images of a Ni bundle attached to a commercial AFM tip [239]


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The magnetic resonance force microscopy (MRFM) [241, 242] actually represents a hybrid of magnetic resonance imaging (MRI) and atomic force microscopy (AFM), which is widely used in nanotechnology and nanoscience. Gene sequencing, drug development, analysis of storage media and many other commercial, medical and industrial applications can benefit from this new imaging technology. The resolution of this technique is so high that the "spin" of a single electron can be detected [243] and high spatial resolution has been obtained [244]. A schematic view of a MRFM is given in Fig. 7.29 [245]. Composite nanowires have been proposed by Barbic [246] for the fabrication of probes for atomic resolution magnetic resonance force microscopy (MRFM). The fabrication technique combines electrochemical deposition of multifunctional metals into nanoporous polycarbonate membranes and chemically selective electroless deposition of optical nanoreflector onto the nanowire. The completed composite nanowire structure contains all the required elements for an ultrahigh sensitivity and resolution MRFM sensor with a magnetic nanowire segment providing atomic resolution magnetic field imaging gradients as well as large force gradients for high sensitivity, and a noble metal enhanced nanowire segment providing efficient scattering cross-section from a sub-wavelength source for optical readout of nanowire vibration. A nonmagnetic/non-plasmonic nanowire segment acts as a cantilever structure for mechanical detection of magnetic resonance. In MRFM, a ferromagnetic tip is brought near the sample being the other end fixed to a cantilever. The cantilever, with very low spring constant, is in a perpendicular configuration to determine high force sensitivity. They stick electrostatically to the surface if used in a parallel configuration. The nuclear spins in the sample are polarized by the inhomogeneous magnetic field. A second oscillating magnetic field is applied by an RF coil, which excites a spin resonance in the atoms of the sample. By slow frequency or amplitude modulation of the RF field, a modulation in the nuclear magnetization of the resonant

Fig. 7.29 Scheme of the magnetic resonance force microscopy. See the coil system that produces the field gradient; the tip is fixed to the end of the cantilever. The changes in the cantilever oscillation due to the magnetic resonance are optically detected [245]

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fraction of the sample occurs, leading to a modulation in the force between the sample and the magnetic tip. This force produces a measurable oscillation in the deflection of the cantilever, which is optically detected. For a Fe tip with a 100 nm radius of curvature, the field gradient at the surface is approximately 110 G/cm. Three-dimensional elementally specific images can be constructed by scanning the magnetic tip in three dimensions and scanning the RF frequency. An example is given in Fig. 7.30. In (a), a schematic view is given of a CaF2 structure used for the imaging test that represents a thin film evaporated onto a template with a focused ion beam. The dimensions, in nanometres, are taken from electron micrographs. A simulated image for the cyclic-CERMIT protocol using a conical tip model is given in (b), while the magnetic resonance image taken at a tip–sample spacing of 45 nm is shown in (c), where the image colours represent the spatially varying mean-square force signal. The resonant field was 2.89 T, with an applied field of 2.83 T. Generally, good correlation with the expected morphology is observed, although the island at the right of the image is only barely visible. This discrepancy is perhaps due to a slight tilt of the sample with respect to the plane of the scan. The data were acquired with a

Fig. 7.30 (a) Schematic view of a CaF2 structure used for the imaging test. (b) Simulated image for the cyclic-CERMIT protocol using a conical tip model. (c) Magnetic resonance image taken at a tip–sample spacing of 45 nm, where the image colours represent the spatially varying meansquare force signal. (d) Line scan showing raw image data taken from the location of the dotted line in c [247]


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measurement bandwidth of 0.44 Hz. In (d), the line scan shows raw image data taken from the location of the dotted line in (c). The 100 nm and 250 nm gaps in the test sample are both resolved with essentially 100% contrast [247]. MicroSQUIDs Superconducting quantum interference devices (SQUIDs) are currently used as ultrasensitive magnetic flux detectors in research and industrial applications. Sensors for detection of magnetic field below 100 pT in the frequency range between 1 and 1 kHz are largely dominated by the technologies of fluxgates and superconducting quantum interference devices (SQUIDs). Unfortunately, these sensors are bulky and expensive. Above 100 pT, small inexpensive solidstate thin-film sensors provide a more attractive alternative. A major challenge is thus to extend the range of thin-film sensors below 100 pT. A promising approach is to develop a magnetically softer sensing layer. Recently, ultrasoft magnetic materials have been developed with coercivity as small as 102 mT in the easy axis magnetization and very large magnetic permeability (105) in the magnetization hard axis direction. The major driving force for this development has been the demand for higher efficiency in electrical transformers, and these soft materials have been developed in bulk form. The hard saturation field of the soft layer in commercial thin-film sensors is typically several tenths of mT which corresponds to susceptibility values of the order of 103. Thus, there is a potential to improve permeability of the order of 103 if the properties of the best soft materials can be integrated in these sensors. Technological improvements require the understanding of dynamical magnetization reversal processes at nanosecond time scales. New strategies are needed to overcome the limitations of current devices. For example, the application of high fields to reverse the magnetization of high-anisotropy nanoparticles as well as the measurement of such reversal process. Thirion et al. [248] have proposed a new method to overcome these limitations. A constant applied field, well below the switching field, combined with a radiofrequency (RF) field pulse can reverse the magnetization of a nanoparticle. The efficiency of this method is demonstrated on a 20 nm diameter cobalt particle by using the microSQUID technique. The miniaturization of these devices has been possible, thanks to the use of the electron-beam lithography. Reducing the size of SQUIDs to the micrometre regime (microSQUIDs) has already given important scientific results in the characterization of magnetic nanoparticles. Figure 7.31 shows a Josephson junction (microbridge) of a microsuperconducting quantum interference device, on which a 20 nm-diameter h.c.p. cobalt particle was placed. The microbridge of the SQUID is used like a strip line. An injected RF supercurrent IRF induces an RF field HRF that is directly coupled to the nanoparticle on the microbridge. In addition, microSQUID can be used as a particular scanning microscopy as reported by Tsuei et al. [249]. In 1994, the use of a high-resolution scanning SQUID microscope made possible the first direct observation of the half-flux

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Fig. 7.31 (a) Scanning electron microscope image of the microbridge junction. The SQUID is patterned from a 20 nm thick niobium film. (b) Schematic of the microbridge junction. The microbridge of the SQUID is used like a strip line. An injected RF supercurrent IRF induces an RF field HRF that is directly coupled to the nanoparticle on the microbridge [248]

quantum effect in YBa2Cu3O7 (YBCO). The presence of a spontaneously generated half-flux quantum in the three-junction ring centred at the tricrystal meeting point and the fact that there is no magnetic flux in the other three rings (as shown in Fig. 7.32) represent the first definitive evidence for dx2y2-wave pairing symmetry in a cuprate superconductor. Magnetoencephalography (MEG) is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as microSQUIDs. It measures the magnetic fields generated by the intercellular currents of neurons in the brain. The magnetic field generated by a single neuron is extremely low in amplitude, and when several thousands of closely packed cells are synchronously active, the resultant extracranial magnetic field takes values of the order of picoTesla.

Fig. 7.32 Micrograph of a scanning microSQUID probe (a) and scanning SQUID image in a YBCO (b) [249]


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The SQUID magnetometer systems with small diameter pickup coils have been developed to measure in vivo MEG on small animals. The diameter of a pickup coil was chosen according to the size of objectives. The system with the pickup coils of 4 mm in diameter was used for MEG measurement of a swine, 5 mm for MEG measurement of a rat and 200 for MCG measurement of a frog [250].

Hall Probe Microscopy Hall micromagnetometry and scanning Hall probe microscopy, SHPM, have been shown to be very sensitive and versatile experimental techniques for detection of extremely small magnetic fields. The traditional materials applied for the fabrication of this kind of sensors are GaAs/AlGaAs heterostructures, with a two-dimensional electron gas (2DEG) buried below the surface due to the large Hall coefficient. However, this material is extremely sensitive to patterning that induces charge depletion that limits the applications of these devices in lateral size and temperature. To overcome such 2DEG limitations, different materials are thus currently investigated to fabricate sub-micron Hall probes. Sandhu et al. [251] have demonstrated that Bi and InSb thin films are practical alternative materials for fabricating sub-micron Hall effect probes of high spatial resolution for room temperature scanning Hall probe microscopy. As mentioned in their work, GaAs/AlGaAs 2D electron gas Hall probes become impractical for sub-micrometric dimensions at room temperature, mainly due to surface depletion effects that limit the Hall driving current and magnetic sensitivity. The authors fabricated the InSb probes using photolithography, while Bi probes are prepared by optical and focused ion-beam lithography. Fabricating Hall magnetometers by means of focused ion-beam milling or focused electron-beam-induced deposition, active areas as small as 104 nm2 can be obtained [252, 253] (see Fig. 7.33) [254]. Three different classes of materials as metallic Au, semi-metallic Bi and doped bulk Si doped GaAs semiconductor were selected. It was found that Au nanoprobes can work from room temperature down to liquid helium temperature with a magnetic flux sensitivity less than 0.21015 Tm2 (10% of 0).

Fig. 7.33 SEM micrographs of Hall devices with an active area of about 500 nm2 fabricated by focused electron-beam-induced deposition [254]

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Fig. 7.34 SHPM images of a YBCO thin film in (a) 0.1 Oe (gray scale spans 0.73 G), (b) 20.1 Oe (gray scale spans 0.8 G), (c) 1.1 Oe (gray scale spans 1.1 G), (d) 20.9 Oe (gray scale spans 1.1 G) [255]

An example of the resolution of the SHPM is shown in Fig. 7.34, where a thin film of YBCO can be observed with precipitates after field cooling to 77 K [255]. The advantages of the use of Hall sensors made of InSb type thin films in the cryogenic range, between 1.8 and 250 K, have been recently analysed [256]. The sensors are grown epitaxially on insulating GaAs substrates, and for specified temperature region, the intrinsic electron concentration is well below 1016 cm3. To achieve magnetic sensors with the temperature coefficient of the magnetic sensitivity as low as 0.001%/deg, the InSb films have to be doped to the effective donor concentration of about 1018 cm3. The investigations have shown that epitaxial n-InSb/GaAs thin-film structures are an excellent material for the preparation of very high quality Hall sensors in cryogenics applications. By changing the film thickness and the technological parameters, such as the doping level, sensors with various adjustable parameters can be manufactured. Their temperature coefficient of the input/output resistance ¼ 104/K in the temperature range from 1.8


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Fig. 7.35 Comparison of sensitivities for various magnetic sensors with different size of sensing element (MFM, microSQUIDs and Hall probes)

to 250 K is smaller than those reported earlier for InSb magnetic sensors [257]. This is very important for the accuracy of measurements and/or magnetic field stabilization. They also show lack of any Shubnicov-de Haas oscillations, both in the Hall voltage and in the resistance. Finally, Fig. 7.35 summarizes the sensitivity for various magnetic field sensors based on MFM, microSQUID and Hall probes of different size.

7.3.3 Magnetic Sensors Based on Bidimensional Magnetic Nanostructures The magnetic quality of thin films is being improved, thanks to the developing of manufacturing techniques and optimization of the magnetic properties through the control of preparation methods. Here, in a first section, we summarize some of the applications currently used in connection to magnetic recording, mainly reading the information. Afterwards, we consider sensors for other applications.

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7.3.3.1 Magnetic Recording and Related Sensors Magnetic recording is the most extended application for the 2D magnetic nanostructures. It essentially involves a recording media where magnetic information is stored, being each bit of information typically a single-domain structure, and the writing and reading processes by which such information is stored and read, respectively. The recording media is made out of a relatively hard magnetic material, and the main tasks in storage process are the cost reduction and development of faster, more compact and less power-consuming memory systems, with greater storage capacity. In a first approach, those benefits are obtained by reducing the size of the basic storage units; hence it is natural to assume that nanotechnology will play a fundamental role in this process. However, it is imperative that the whole system must also include a head for writing and reading the units efficiently, entity to which we will pay attention in this section due to its intrinsic sensing character. The applications of nanomaterials as patterned media for hard disks, miniature magnetic sensors using GMR effect and magnetic memory cells which are being developed are outlined in several text books. Conventional magnetic recording has been based on longitudinal recording media where bits are stored in an in-plane configuration of the planar recording media. The recording media consists of a regular array of isolated singledomain magnetic element, and the requirements for a thin film with high application potential lie in a good response to the recording head (coercivity not too high) and to retaining the magnetization in spite of the magnetostatic field of adjacent bits, stray fields and ambient temperature fluctuations (coercivity not too low). The coercivity of the medium is typically in the range of 500–3000 Oe. A bit ideally should be composed of a single-domain, isolated magnetic particle. In practice, approximately 1000 particles are required to constitute a bit in order to ensure a sharp transition between two information units. The thin-film media also must have sufficient high remanent magnetization and saturation magnetization to be easily detected by the head. Figure 7.36 shows schematically a system for conventional magnetic recording device. The writing process involves passing a current through the coil of the writing head. This current generates a field in the air gap of the U-shaped core and a fringing field in the plane of the tape or disk that extends out of the gap or disk that is moving past it. The fringing field will change the magnetic state of the media, and if the magnetic properties of the media are appropriate, the remanence of the tape in that region will be proportional to the coil current. For digital signals only two remanent states are required for the material and hence the material requirements are not as stringent as for analogue recording, although smaller particle size is desired for high storage capacity and faster access time. The reading process, when carried out with an inductive head, is similar to the writing process; the magnetic field extending out from the tape or disk induces a field in the core of the head that, in turn, generates a voltage in the reading coil. When


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Fig. 7.36 Writing/reading magnetic device that uses a GMR reading head [258]

using a GMR head, the reading process takes place measuring the changes in the resistance of the GMR probe due to the mentioned extending magnetic field out from the bit information in the disk or tape. The areal density of magnetic recording has been increased over the past few years at 60%/year due to the current optimization of successive generations of thin-film heads, miniaturization of the planar write head and improvement of the Co-based in-plane thin-film media. However, the effects associated with the finite grain size of magnetic alloys used for storage limit the miniaturization [259]. A conventional hard disk is constituted by a thin film of granular Co-alloys. The thin films contain exchange decoupled grains, each grain reversing individually its magnetization. Intergranular exchange coupling is suppressed by adding a non-magnetic element (Cr, for example) that segregates at the grain boundaries. In order to increase the storage capacity, it is necessary to reduce the size of the grains and to guarantee the stabilization of those grains. Recording media is reaching nowadays its limits for stored information: from a basic viewpoint it deals with the superparamagnetic limit indicating a saturation in the reduced size of bits (thermal energy overcoming the magnetic anisotropy stored in the volume of the particle). Other technical limitations are related to signal-to-noise ratio, proportional to the number of particles in a bit, the dispersion of magnetic characteristics or protective coating. The new generation of media involves perpendicular storage media where bits are in an out-of-plane configuration of a hard media, while at the bottom a soft layer allows the closing of the magnetic flux. Perpendicular recording enables a significant increase of areal density of information, where the superparamagnetic limit is no more a restriction due to the volume increase of the bit due to the perpendicular dimension. Present density reached at the laboratory is

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Perpendicular Recording

Fig. 7.37 Comparision between longitudinal and perpendicular magnetic recording [260]

around 200 Gb/in.2 Figure 7.37 shows schematically a comparison between longitudinal and perpendicular recording media [260]. Future trends that are currently being investigated are heat-assisted magnetic recording/writing, bit patterning of media and eventually self-organized media technologies [261]. Heat-assisted recording essentially consists of increasing locally the temperature of the bit in order to reduce the field required to print that information. Bit patterning considers the possibility of fabricating single bits consisting of a single magnetic unit (nanowire, nanocolumn or nanoobject in general) that enables an increase of density of storage since neighbouring bits are no more exchange coupled. Self-organized media includes arrays of self-assembled magnetic nanoparticles (i.e. FePt around 3 nm size) which at the moment present some technical difficulties to reach the hard phase and exhibit ordering simultaneously. Alternatively, arrays of magnetic nanowires as bits are obtained by electroplating filling of self-organized pores in non-magnetic media as alumina membranes. Continuous magnetic thin films containing arrays of antidots have been also suggested as innovative recording media. In Fig. 7.38, an array of Ni antidots is shown prepared by sputtering onto a nanoporous alumina membrane: the Ni film exhibits an in-plane uniaxial magnetic anisotropy, and the presence of nanostructured antidots determines arrays of small in-plane single domains or bits of information. These different attempts are being searched nowadays with a final objective of reaching an areal density of information of up to 1 Tbit/in.2


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Fig. 7.38 Ni antidot array as a potential magnetic recording media (50 nm antidot diameter)

In all the cases, magnetic recording essentially involves detecting changes in the direction of magnetization in the storage medium. The corresponding stray fields are read generally by resistance changes in the head as the stored medium goes through them. The most commonly used technology to read the information is based on the magnetoresistance phenomenon observed in thin films. More recently, with the discovery of the giant magnetoresistance, GMR, in multilayers [188], the efficiency of the magnetic recording systems, especially the reading head, was significantly enhanced. Specific deposition technology has been developed for the fabrication of thinfilm magnetoresistive magnetic head readers. Deposition of sensor materials involves the deposition of a stack consisting of a large number of individual layers, whereby the individual layers are extremely thin (1 nm). Thickness control over such depositions can be achieved by a number of methods. Deposition of sensor materials also involves a strict control over the microstructure (grain structure and interfaces). Detailed description of the deposition technology of thin films for magnetic recorder read head can be found elsewhere [262]. Apart from applications in magnetic recording, various commercial magnetoresistive sensors using conventional MR materials, such as NiFe and NiCo, are available on the market, used in rotation, angular and position sensing. For certain uses, the enhancement of the sensor output is required rather than enhance the sensitivity to the magnetic field. In such cases, conventional MR sensors can be replaced by GMR sensors of the superlattice type (not spin valve) which can produce a larger signal than spin valve and conventional MR sensors. They are strongly resistant to electromagnetic interference, and the

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preparation of the superlattice is easier than that of the spin valve, resulting in high production throughput. Therefore, GMR superlattice sensors have many advantages for application to rotation, angular and position sensing in automobiles and aircraft. However, there is the drawback that superlattices may not be durable enough to be used under such severe conditions. On the other hand, for practical applications, durability for a much longer period in air is required. Suzuki et al., 10 years ago, have shown that Co/Cu superlattice GMR sensors passivated from the air by a SiO2 layer show extremely high durability against high temperature air [263]. The exchange bias is one of the phenomena associated with the exchange anisotropy created at the interface between an antiferromagnetic and a ferromagnetic material when the system is cooled through the Neél temperature of the antiferromagnetic material [264, 265]. Possible applications of the exchange bias effect include permanent magnets, magnetic recording media and, which is most interesting for this propose, sensors based on giant magnetoresistance since the exchange bias systems show a reduction of the saturation fields. Spin valves (SVs) constitute a major type of metal-based spintronics which utilizes both the spin and the charge properties of an electron. It has been extensively investigated since the last decade because of its important application in hard disk drives and magnetic random access memories (MRAM). One of the common driven forces in magnetic storage and memory devices is the ever-growing demand of higher storage density. It requires the ever small and in particular more sensitive read sensor. For example, sensors with a feature size of sub-100 nm and sensitivity of more than 15 mV/mm are required for hard disk drives with an areal density of more than 100 Gb/in.2 This means that the SVs must be electrically and magnetically robust; thus the magnetoresistance ratio (MR) of the spin valve should be as large as possible in order to increase the sensitivity. In this sense, nano-oxide layer (NOL) added SVs is very promising in terms of MR ratio because of the enhancement of the specular reflectivity. Li et al. [266] systematically studied the effect of NOL on the electrical and magnetic properties of the SVs. The NOL layer can smooth the surface topography of the interface so that it can suppress the ferromagnetic Neél magnetostatic coupling and, at the same time, enhance the RKKY exchange coupling between the free and pinned layers. About 4.5% of MR ratio has been observed for a particular sensor configuration with laminated CFe/Cu used as the free layer and NOL layer inside the pinned layer. Liu et al. [267] have investigated the effect of Ta buffer layer in IrMn top spin valve. They conclude that, using 3 nm Ta buffer layer, spin valves with high MR (9.24%), high exchange bias field (255 Oe) and low coercivity (2.43 Oe) are obtained. Utilizing these spin valve thin films and standard IC process, mass production of robust GMR sensors could be achieved. Magnetic tunnel junctions (MTJ) [268] have attracted considerable attention in the last years due to their high potential in applications such as magnetic random access memory (MRAM), magnetic read head in hard disks (HD) and


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highly sensitive magnetic sensors [269]. During the last years new advances using MTJ materials have been carried out in order to increase the tunnel magnetoresistance (TMR) ratio [270]. The standard fabrication process for MTJ involves both e-beam and optical lithography. For large-scale production of relatively large devices this process has many advantages. However, on the nanometre scale, difficulties are encountered with the lift-off step of both the insulator and the removing of the resist. Wei et al. [271] have reported a controlled fabrication method for nanoscale double barrier magnetic tunnel junctions (DBMTJs) that showed good characteristic properties with TMR ratio, resistance-area product and switching field. This method consists in the deposition of the MTJ film with double barriers on Si/SiO2 wafer using magnetron sputtering system. A platinum nanoscale pillar was deposited by focused ion beam on the metal stack to act as a patterning mask. UV lithography with Ar-ion etching was used to pattern the top and bottom electrodes of the DBMTJs. A non-volatile magnetic random access memory (MRAM) (see Fig. 7.39) essentially consists of an array of individual magnetic memory cells, each one being a magnetic tunnel junction (MTJ) element and a diode electrically connected in series. Each MTJ is formed by a pinned ferromagnetic layer whose magnetization direction is prevented from rotating by some mechanism like antiferromagnetic coupling due to an antiferromagnetic additional layer, a free ferromagnetic layer whose magnetization direction is free to rotate between states of parallel and antiparallel to the fixed magnetization layer and an insulating tunnel barrier between and in contact with the two ferromagnetic layers. Each memory cell in the array is controlled by only two lines. The write line applies the currents to change the magnetic state of a MTJ by use of the write current inherent magnetic fields to rotate the magnetization of the free layer. The writing lines are used to read the ‘‘information’’ by

Fig. 7.39 Scheme of an array of magnetic memory cells used for MRAM

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(a)

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Fig. 7.40 (a) Scheme of the device. (b) The TMR changes sign with Vg. Linear response resistance R as a function of H at temperature T ¼ 1.85 K for different Vg. The dark grey (light grey) arrow indicates the up (down) magnetic field sweep direction, respectively. Inset: SEM picture of a carbon nanotube (NT) in contact with ferromagnetic PdNi strips

measuring the resistance across the MTJ junction that will depend on the relative magnetization direction between the ferromagnetic layers. Spintronics [272] aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them. The most basic spintronic device is the spin valve [273] formed by two ferromagnetic electrodes separated by a thin tunnelling barrier. As shown in a previous section, in most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction, i.e. the magnetoresistance is positive [188, 189]. However, if the transport of carriers inside the device is spin- or energy dependent, the opposite can occur and the magnetoresistance is negative [274]. Sahoo et al. have tried to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate [275]. They achieve a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes (see Fig. 7.40) connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.

7.3.3.2 Other Sensors Based on Bidimensional Nanostructures Magnetic Computer Sensors for Biomolecules Studies As described in a recent paper [276], NIST researchers found that arrays of ‘‘spin valves’’ switches, commonly used as magnetic sensors in the read heads of high-density disk drives, also show promising use as tools for controlled trapping of single biomolecules. The arrays might be used in chip-scale, low-power


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microfluidic devices for stretching and uncoiling, or capturing and sorting, large numbers of individual biomolecules simultaneously for massively parallel medical and forensic studies, a sort of magnetic random access memory (MRAM) for biosciences. Spin valves are made by stacking thin layers of materials with different magnetic properties. Their net magnetization can be switched ‘‘on’’ and ‘‘off’’ by applying an external magnetic field of sufficient strength to align the electron ‘‘spins’’ in the magnetic layers in the same (on) or opposite (off) directions. NIST researchers made an array of spin valves, each about 1 4 mm in size, patterned on a 200 nm thick silicon nitride membrane in fluid. When the spin valves are turned on, a local magnetic field is created that is stronger near the ends of the magnetic stack below the membrane, a field strong enough to trap nanoscale magnetic particles. The NIST experiments demonstrated that the spin valves not only can trap magnetic particles but also can be used as the pivot point for rotating strands of particles when a rotating magnetic field is applied (see Fig. 7.41). These experimental results, combined with computer modelling, suggest that if biomolecules such as proteins or DNA strands were attached to the magnetic particles, the spin valve array could apply torsional forces strong enough to alter the structure or shape of the biomolecules. The NIST group is now working on a microfluidic chip that will accomplish this electronically, which would be a significant milestone for applications. Parallel processing of single biomolecules would represent a significant advance over existing techniques limited to studying one molecule at a time. Optical tweezers, which use lasers to trap and manipulate biomolecules, tend to

Fig. 7.41 Image (taken from a video) shows a strand of magnetic particles trapped by a ‘‘spin valve’’, highlighted in white and rotated by the application of a rotating magnetic field. The use of spin valve arrays for parallel processing of biological molecules is presently under study

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be slow and limited in force, and the particles need to be micrometre sized or larger. Existing magnetic tweezers can trap smaller particles and apply torque, but typically require permanent immobilization of biomolecules, which is time consuming and prevents subsequent analysis. Sensors Based on Magneto-optical Effects Other kinds of magnetic sensors are those based on magneto-optical effects. Magneto-optical sensors provide the opportunity to combine the advantages of optical methods (i.e. contactless, wide dynamic range, absence of electrical connections) with those of magnetic methods, thus easing the requirements placed at the overall set-up. Magneto-optical devices are mainly used for switching, polarization and amplitude modulation of light. Industrially available magneto-optical sensor is based in the magneto-optical current transformer measuring magnetic field of strong currents by the Faraday rotation in diamagnetic fibres. Particularly, orthoferrites are good candidates for this purpose since they are ferrimagnetic, transparent in the visible and nearinfrared regions of the spectrum, present high domain wall (DW) mobility and high magneto-optical effect which provide rich opportunities for using in sensor application utilizing the modulation of light [277, 278]. The combination of such magneto-optical properties with the low-dimensional effects has been proposed [279], a new magneto-optical surface plasmon resonance (MOSPR) sensor which can improve the sensitivity of the conventional surface plasmon resonance (SPR) sensors (see Fig. 7.42). This MOSPR sensor is based on the combination of the surface plasmon resonance in thin metallic layers and the magneto-optic (MO) activity of ferromagnetic metallic materials. Such device generates a large enhancement of the MO effects closely localized at the surface plasmon resonance. The sensor device uses Co/Au multilayers of nanometric thicknesses as transducers, a prism-coupling configuration and p-polarized light to excite the surface plasmon, rotating magnets or magnetic coils to apply a modulating magnetic field, and detects the

Fig. 7.42 Schematic representation of the surface plasmon resonance system developed by Sensia [280, 281]. In the magneto-optic sensors the gold layer is substituted by a gold matrix with magnetic nanostructures


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magneto-optic effects of the reflected light as a function of the angle of incidence. A large and sharp enhancement of the MO effect is produced at the angle of incidence in which SPR is excited and depends also on the refraction index of the adjacent dielectric medium. The layers are prepared by physical methods as sputtering and molecular beam epitaxy. The experimental characterization of the MOSPR sensor has shown an increase in the limit of detection in a factor of three in changes of refractive index and in the adsorption of biomolecules as compared to the standard SPR sensors. An improvement of the limit of detection up to one order of magnitude can be achieved by an adequate combination of the magnetic metallic layers and by decreasing the noise of the experimental set-up.

Gas and Humidity Sensors Applications Sandu et al. [282] have shown that cobalt–manganese–ferrite thin films appear to be quite suitable for an application as gas sensors. A sensitive layer for gas sensor application requires a high surface activity which is strongly dependent on the magnitude of their surface area and the nature of their porosity. The necessary properties are a low density and a high surface area, which imply a small crystallite size. The effective index N of a guided mode is changed by adsorption and desorption of molecules on the surface of a planar waveguide or inside the volume of the waveguiding film itself. The principle of integrated optical grating couplers and Bragg reflector switches or gas sensors is as follows. An adsorbate increases the effective refractive index N of the guided mode in a planar waveguide. A change in N is very sensitively detected in an input grating coupler as a change in the power of the in-coupled mode and in a Bragg reflector as a change in its transmission or reflectance. Tiefenthaler and Lukosz [283] showed that SiO2/TiO2 waveguides are highly sensitive sensors or switches actuated by adsorption of a few monolayers of water either on the surface or in the micropores of the waveguide itself. Therefore these integrated optical grating devices can be employed as adsorption–desorption actuated directional switches or gas sensors.

7.4 Final Remarks The investigation of nanoparticles, nanowires and thin-film magnetic nanostructures, from their preparation by different routes to their experimental characterization and modelling of magnetic behaviour, and finally to their manipulation at the nanoscale, has become during the last decade one of the hot topics in multidisciplinary research. The manipulation of such nanostructures in a controlled way has enabled the development of a series of applications and particularly of magnetic sensor devices that make use or are based on the

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particular properties of those magnetic nanostructures. Such development has been related to the parallel optimization of experimental equipments to face the real knowledge and even direct observation of the magnetic nanoscale nature. The perspectives of technological use of such magnetic nanostructures in many different fields are really very broad; nevertheless some restrictions are still evident in connection with the ‘‘nanodimensions’’. In the present chapter we have reviewed some of the most relevant and useful aspects of magnetic nanostructures starting by the methods of fabrication and magnetic characterization. Sensing devices and technologies in general make use of the particular shape of magnetic nanostructures. In that way, we could summarize by saying that most outstanding technologies of magnetic nanoparticles are related to biomedical applications as for magnetic resonance imaging, contrast agents or separation media which is at least partially due to their smallest size. In turn, magnetic nanowires are particularly useful in those applications making use of their directional character, for example, as magnetic probes in a number of magnetic imaging devices. Finally, bidimensional magnetic nanostructures are particularly important in magnetic field reading in magnetic recording applications. This is probably the most relevant application of all magnetic nanostructures in connection with the huge funding resources invested to develop novel families of recording media and phenomena ascribed to the nanoscale. In this regard, we believe that the real challenge is in reducing size in a controlled way, and most promising effects are probably connected to new recording media and sensing devices for magnetic storage of information.

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Chapter 8

Encapsulated Probes Michael J. McShane

8.1 Introduction and Rationale The term ‘‘encapsulated probes’’ refers to a class of molecules or multimolecular cocktails that are responsive to their environment – typically to a certain target analyte found in that environment – and have been somehow physically encased within a protective package. These ‘‘probes’’ are often chemical assays that produce an optical change (absorbance or fluorescence) in proportion to the concentration of their target, and the encapsulation matrix serves as a means to protect them from interfering materials in the surrounding environment or to protect the surrounding environment from the probes themselves. In the former case, the matrix must be engineered to allow rapid penetration of the target while excluding interfering species. In the latter case, the probes may contain toxic materials that must be sequestered away from living systems to avoid unintended damage.

8.2 Brief Overview of Optical Probes Three primary classes of probes (receptors) will be considered for this discussion: direct, competitive, and indirect (Fig. 8.1). Direct probes are those that respond directly to analyte through a physicochemical interaction between the target molecule and a probe molecule or material, producing a change in fluorescence or absorption magnitude or wavelength. A large array of ‘‘molecular probes’’ have been developed and commercialized, including molecules responsive to ions, reactive oxygen species, and molecular oxygen, as well as those that selectively partition into and report from lipids and membranes, organelles, and ion channels. Examples of encapsulated probes have recently surfaced as (or within) retail products, including fluorescent polymer beads [1] and fiber optic oxygen sensors [2]. Other examples of research on developing direct probes have M.J. McShane Biomedical Engineering Department, Texas A&M University, College Station, TX 77843, USA

F.J. Arregui (ed.), Sensors Based on Nanostructured Materials, DOI 10.1007/978-0-387-77753-5_8, Ó Springer ScienceþBusiness Media, LLC 2009

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Fig. 8.1 Illustration of generic receptor types (R, E) and typical interactions with analyte (A). For the indirect case, the receptor is an enzyme (E) that catalyzes the conversion of analyte and a co-substrate (C) to products (P1, P1). The concentration of a co-substrate or one of the products that is then directly transduced as an indirect measure of the analyte

been reported for sugars, including both synthetic and engineered protein probes [3, 4, 5, 6, 7, 8, 9, 10]. Competitive assays rely on competition between the target and an analogous ligand for binding sites on the probe; thus, these assays are bimolecular and require free dissociation of target and ligand from the probe according to binding kinetics [8, 11, 12]. Transduction in competitive systems generally involves a change in the optical signal due to dissociation of a labeled ligand from the labeled receptor. A common approach to this is based on resonance energy transfer (RET) [8, 13, 14], which exhibits a distance-dependent efficiency in transfer between two molecules with complementary spectral properties and proper dipole alignments [15, 16]. To satisfy the conditions for RET, the emission spectrum of one fluorophore, termed the ‘‘donor’’, must overlap with the excitation spectrum of the second fluorophore, termed the ‘‘acceptor’’, such that the donor-excited state is nonradiatively transferred to the acceptor, resulting in photon emission. RET efficiency (E) is highly dependent on the distance (r) between the donor and the acceptor, following E ¼ R60 R60 þ r6 , where R0 represents the Forster distance, the distance at which 50% efficiency is obtained ¨ (typically in the range of 50A) [15]. RET-based transduction in competitive probes relies simply on the decreasing energy transfer between the fluorescent

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competitive ligand and the labeled receptor, as the ligand is displaced from the receptor when in the presence of the target analyte [17, 18]. It is noteworthy that sensors based on competitive binding are research-level systems that have not found broad use outside the labs of the developers. A number of successful examples for glucose sensing have been demonstrated based on this concept. Finally, indirect probes rely on a cascade of effects to transduce the interactions; typically, a probe for one parameter (e.g., pH) is used to respond to pH changes induced by a chemical reaction between a second probe molecule (e.g., enzyme) and the target (e.g., the enzymatic substrate). This approach is useful when sufficiently selective and sensitive direct probes are unavailable. An example that illustrates the progression from protein-based molecular recognition agents to encapsulated probes can be seen in the development of fluorescence-based glucose sensors. In the late 1980s, it was reported that glucose-binding events triggered measurable changes in the intrinsic flavine fluorescence of glucose oxidase (GOx) from Aspergillus niger [19]; as a result, it was proposed that the enzyme may be used as a ‘‘probe’’ in a quantitative assay for glucose. By labeling the enzyme with a fluorescein derivative, this intrinsic fluorescent response was later converted to a single-molecule RET system, with energy transfer occurring between the flavine group and fluorescein derivative; the fluorescence spectrum was observed to shift in proportion to glucose concentration [20]. Later studies investigated reagentless sensors using the deactivated form of the enzyme (apoenzyme) [21], where apo-GOx was labeled with the environmentally sensitive fluorophore 8-anilino-1-anpthalene sulfonic acid (ANS). This work showed that apo-GOx retained its high selectivity to glucose and in many ways paved the way for advances in the development of a new biosensor genre [22, 23, 24, 25]. Similar work was performed using yeast hexokinase (HEX), which exhibits a decrease in intrinsic tryptophan fluorescence during exposure to glucose [26, 27]. It was later determined that HEX-based transduction schemes are vulnerable to static quenching when exposed to serum, suggesting that a separation from the biological environment – an encapsulation of the probe – is necessary for in vivo applications [28]. Following this notion, HEX was immobilized in a sol–gel matrix, resulting in enhanced quenching resistance while retaining glucose sensitivity [29]. Thus, the encapsulation of the probes met the requirements for sensing applications: the matrix allowed permeation of the target molecules while providing a stable framework for the probe molecules that protected from the environment.

8.3 Toxicity of Probe Materials It is noteworthy that many small molecules and nanomaterials possess significant toxicity [30, 31, 32, 33, 34, 35]. Therefore, packaging must be given due consideration for nanostructured sensors that may be in some form or fashion exposed to living systems. In particular, optical sensors as described above typically

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contain small-molecule fluorescent probes, protein-based receptors, and/or nanophotonic elements such as quantum dots or metal nanoparticles. When recognizing the potential risks associated with exposure, and identifying requirements for encapsulation strategies (beyond the functional aspects of the immobilization scheme), it is important to note that the materials used must be sufficiently biocompatible to avoid severe acute or sustained long-term host response to the system for the expected route of exposure. For aerosolized or surface-adherent materials, the respiratory tract, eyes, and skin are potentially affected sites; the gastrointestinal tract could receive exposure to materials ingested via contamination of the food or water supply. For sensors intended for implantation, the site of implantation will determine specific requirements. In general, an acute inflammatory response to the implant is expected due to local tissue trauma during the procedure, though the steady-state response should be limited to minimal fibrous tissue formation, and activation of the immune response by antibodies recognizing foreign material must be avoided. A key point related to nanostructured sensors, however, is that the response to the foreign material is determined by the surface properties [36] and, so long as the contents are not released or degraded over time, a stable interface will be formed between the implant surface and the host. Thus, toxic sensor components may potentially be employed in systems with high exposure risk, as long as the encapsulation adequately separates the dangerous entities from the living system. The purity of the materials used in sensor fabrication can be a key factor in determining biocompatibility.

8.4 Immobilization Requirements When developing nanostructured sensors based on a combination of carrier materials and exogenous indicator materials such as fluorimetric or colorimetric probes, the primary goals involve entrapment of the assay chemistry in a way that physically restricts the transduction molecules yet allows sufficient transport of the target analyte [37]. Recent research from our group has reported advances in these areas, particularly in co-immobilizing enzymes and dyes or competitive-binding reagents within microcapsules [38, 39, 40, 41, 42, 43, 44 ,45, 46, 47, 48, 49, 50, 51, 52, 53, 54]; while the focus of our investigations has been on noninvasive glucose biosensing via fluorescence spectroscopy, the findings are more generally applicable to encapsulation of probes in nanostructured materials. In this context, it is especially noteworthy that use of certain transduction schemes requires dramatically different and more complex encapsulation strategies. Take, for example, competitive-binding reactions, which are common to many bioassays. These require free movement of two molecules, which must be able to associate and dissociate quickly in accordance with binding kinetics and the relative affinity to the target analyte; and this must be maintained within a restricted and protected interaction volume.


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The permeability of the encapsulating matrix is also a key parameter influencing the performance of encapsulated probes. In direct and competitive systems, permeability to the analyte affects response time; in addition, the partitioning of the analyte into the matrix can affect the relative forward and reverse response times. In indirect sensing schemes, however, mass transport considerations can become much more crucial in controlling both the response time and the magnitude of the signal generated; in fact, the type of encapsulating materials can ultimately dictate whether or not a particular design is successful. A prime example of this can be seen for enzyme-based glucose sensors, which operate on the principle of glucose-limited oxygen consumption within the local environment of the sensor. As glucose diffuses into the sensor, oxygen is consumed and rapidly replaced, allowing indirect glucose monitoring through an oxygen reporter. To achieve this fundamental condition, the relative diffusion rate of oxygen into the sensor must be greater than that of glucose; therefore, the local oxygen levels within the sensor are in excess and the sensor is operating in a glucose-limited regime. As bulk glucose levels are increased, the rate of glucose delivery into the sensor begins to eclipse the rate of oxygen replenishment, preventing the relative recovery of oxygen levels within the sensor. In this state of operation, the sensor response begins to deviate from linearity, thus implicating the onset of oxygen-limited glucose consumption. Further increases in glucose levels elicit no sensor response, due to complete internal depletion of oxygen levels (oxygen is readily consumed upon diffusion into the sensor). This condition is elicited by the surplus of local glucose within the sensor and signifies absolute oxygen-limited catalysis and response saturation. Therefore, the mass transport properties of the encapsulating materials effectively control the operational range of enzymatic-based encapsulated probes.

8.5 Encapsulation Strategies A number of different approaches to encapsulation of functional probes have been developed, ranging from ‘‘brute force’’ methods relying on filling of prefabricated containers to self-assembled, spontaneously loaded micro/nanocapsules produced and filled in situ. An example of the former, demonstrated by pioneering work in translating wet reagents into reusable semi-solid-state components, involved the filling of hollow fiber dialysis membranes [11, 55, 56]. These relatively large structures are ideal for interfacing with the tip of optical fibers, but are disadvantageous for reasons of susceptibility to fouling and poor mechanical stability. Standalone materials such as polymeric microspheres are preferable due to the large surface area to volume ratio, which maximizes transport, and low absolute area, which enhances mechanical integrity. Polymer matrices are good encapsulation solutions when the entrapped molecules need not be mobile; in contrast, liposomes, polymersomes, and other microcapsules are superior encapsulation of competitive assays when free association and dissociation of

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reagents and target are required. Furthermore, nanocomposite materials are often preferred, as the precise control over transport properties offers solution to a number of existing problems in creating nanostructured sensors based on encapsulated optical sensing probes. The following sections will focus on encapsulation strategies based on hollow vesicles and nanoengineered composites that enable tunable sensor responses.

8.6 Progress and Opportunities for Encapsulated Probes 8.6.1 Liposomes Liposomes are phospholipid vesicles that contain a hydrophilic interior surrounded by a protective outer phospholipid shell (Fig. 8.2); many types have been prepared from a wide array of natural and synthetic materials. These nanostructured systems have been considered for applications such as drug delivery and bioreactors as well as biosensors, and are particularly attractive for biomedical applications because of their chemical similarity to the cell membrane. For sensors, the aqueous interior of the liposome provides a hydrophilic environment that is preferred for many probes and ensures high permeability for hydrophilic species. To achieve sufficient stability for use in sensing applications, however, phospholipid vesicles typically must be crosslinked through use of polymerizable phospholipids, incorporation of hydrophilic polymers, or polymerization of monomer units incorporated into the bilayer. Encapsulation of fluorescent probes with liposomes has led to demonstration of sensors for molecular oxygen [57], acetylthiocholine chloride [58], calcium [59], and pH [60]. It is noteworthy that phospholipid bilayers are attractive as biocompatible coatings, and therefore have been studied as potential solutions to overcome immunogenicity or toxicity issues for other materials used to encapsulate optical probes. As an example, fluorescent microspheres intended as intracellular monitors for pH, O2, and Cl– have been coated with phospholipids to increase biocompatibility, providing a protective barrier between the probe chemistry and the cell. It is also noteworthy that polymersomes – synthetic vesicles comprising self-assembling block copolymers – are newer materials that

Fig. 8.2 Schematic of enzyme and probe encapsulation within liposomes. (Reproduced with permission) [58]


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are similar in structure to liposomes and are generally more versatile [61, 62, 63, 64]; they have not yet been used for sensing purposes, but are useful for encapsulation and therefore may impact sensor architectures in the near future.

8.6.2 PEBBLEs (Probes Encapsulated by Biologically Localized Embedding) PEBBLEs are a class of polymeric or organically modified silica particles (20–500 nm) originally developed as a means of protecting sensing chemistry from the ‘‘harsh’’ internal environment of the intracellular space (Fig. 8.3) [65, 66]. Sensing PEBBLEs have been demonstrated for many relevant analytes, including molecular oxygen [67] as well as univalent (K+) and divalent (Ca2+, Zn2+, Mg2+) cations. A key advantage shown in this work was the ability to assemble sensors targeting ‘‘difficult’’ analytes by co-localizing multiple chemistries (e.g., probe + ionophore) within the confined volume of the sensor. Nanoscale glucose-sensing particles have also been reported, utilizing a combination of GOx and O2 indicators [68].

8.6.3 Polyelectrolyte Multilayers Layer-by-layer self-assembly (LbL) of multilayer films [69] has been demonstrated to be a practical and versatile approach to surface modification with nanocomposite materials [70]. LbL has recently been reviewed extensively [71, 72], and therefore will only be introduced here in the context of constructing sensors. The LbL process is general, relying primarily on the attractive force between oppositely charged molecules; following polyion adsorption to an oppositely charged surface, the terminal charge is reversed after every

Fig. 8.3 Various embodiments of PEBBLE sensors. (Reproduced with permission from [37])

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Fig. 8.4 Schematic illustration of layer-by-layer nanofabrication process, applied to (top) planar surfaces and (bottom) colloidal templates. (Reproduced with permission from [71])

subsequent layer deposition (see Fig. 8.4). The resulting films have a thickness on the order of a few nanometers per layer deposited, with the exact thickness depending on the adsorption material and reaction parameters such as pH and ionic strength. Materials employed in the assembly process cover a wide variety of synthetic and natural materials [73, 74], typically charged polymers (‘‘polyelectrolytes’’ or ‘‘polyions’’) or proteins, but also including small molecules such as dyes and even inorganic nanoparticles [75, 76, 77]. In addition to assembly of films on bulk planar templates (1D) or cylindrical (2D) templates, the LbL procedure has also been employed for the modification of three-dimensional surfaces by using charged substrates with micrometer and nanometer dimensions [78, 79, 80]. Specifically, objects such as microspheres, nanoparticles, nanotubes, and platelets [81, 82] have been extensively studied because of the wide availability and attractive surface properties of spherical particles. Functional nanocomposite films containing encapsulated probes can be deposited on these carriers, which are attractive for controlling


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biocatalytic reactions and other interfacial phenomena due to the high surfaceto-volume ratio [40, 80, 83, 84, 85, 86, 87]. Relating to the focus of this chapter, a number of different LbL structures containing sensing probes embedded within a multilayer have been reported, and several different processes for encapsulation have been employed, as illustrated in Fig. 8.5. It is also noteworthy that sensing films can be prototyped by constructing LbL films with sensing materials on macrotemplates and then, once the desired

Fig. 8.5 Schemes for encapsulation of optical molecular probes in multilayer films: (top) direct electrostatic assembly of anionic (left) and cationic (right) indicators; (middle) covalent attachment of indicators to charged polymers, followed by incorporation via electrostatic adsorption; and (bottom) post-assembly indicator immobilized by electrostatics (left) and precipitation (right)

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Fig. 8.6 Representative templates used for LbL assembly of fluorescent sensing films: (a) glass slides, (b) optical fibers, and (c) latex microspheres. (Reproduced from [38])

(a)

(b)

(c)

behavior has been achieved, the same process parameters may be used to deposit the same materials onto micro/nanotemplates with confidence that very similar sensing behavior will be observed [38]. This has been demonstrated using glass slides, optical fibers, and nanoparticles. These findings point to the versatility of the approach, as well as its ‘‘portability’’ to the point that functional films can be assembled on nearly any surface using the same process (Fig. 8.6).

8.6.4 Multilayer Capsules In the past decade, the multilayer assembly process applied to colloids has been used as the first step in a technique for fabrication of hollow micro/nanocapsules [88]. Following deposition of multilayer coatings onto the templates, the core material is removed using organic solvents or chemical etching to arrive at hollow capsules (Fig. 8.7) [78, 89, 90, 91]. The versatility in construction of these tiny capsules and control over their properties make them attractive for use in sensor applications, especially those in which encapsulation of active molecules and control over transport properties are critical to proper function. Microcapsules based on the layer-by-layer assembly technique have been demonstrated as effective systems for encapsulation in many different potential applications, with a heavy emphasis on drug loading and delivery [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107]. Early investigations of the properties of polymer capsules revealed that permeability depends directly on the wall composition, and some architectures allow for dynamic control over permeability to open and close pores [108, 109, 110, 111]. For example, pHinduced formation of pores ( 100 nm) was observed for a combination of

Fig. 8.7 Illustration of multilayer capsule fabrication process using sacrificial templates to create hollow microcapsules. (Reproduced with permission from [71])


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strong and weak polyelectrolytes [112]. Furthermore, changes in solvents have also been shown to induce encapsulation [113, 114]. A more recent development has demonstrated permanent changes in capsule walls’ permeability by employing a photosensitive cationic resin (diazoresin, DAR) [115, 116, 117] to enable photo-induced crosslinking via UV irradiation; such capsules can be used providing permanent and stable encapsulation of macromolecules that permeate the capsules prior to photocrosslinking [48]. In principle, similar strategies based on chemical, thermal, or other crosslinking strategies may also be employed [106, 108, 109, 118–134], though it is desirable to limit crosslinking to the walls and avoid undesired crosslinking of assay components; hence, the DAR-based approach is advantageous. Based on these findings, it has been proposed that such capsules have attractive properties for sensing applications, where reagent chemistry may be packaged within the capsule interior. A number of embodiments for chemical sensing and biosensing can be envisioned, as illustrated in Fig. 8.8. This approach, involving entrapment of assay molecules in a hollow capsule, is particularly attractive for competitive assays, which require mobility of competitive ligands when they are displaced from the receptor. A prime example of this can be seen in the recent literature on glucose-sensing ‘‘smart tattoo’’ systems [135], which can be seen as generic prototypes for similar chemical using optical probes encapsulated in multilayer capsules.

Polyanion Polycation

Catalytic Biosensor

Dye-filled Capsules= Chemical Sensors Reference Dye Indicator

Competitive-binding Biosensor

Enzyme 5µm

Labeled Protein Labeled Ligand

FITC

Transmission

TRITC

Overlay

Fig. 8.8 General description of microcapsule-based sensors. Top left: polyelectrolyte capsule, hollow or filled with polymer matrix; Top right: capsule filled with indicator and reference dye for ratiometric monitoring of a target analyte; Bottom left: enzyme-based microcapsule sensor containing enzyme and dyes; Bottom right: microcapsule sensor employing competitivebinding FRET assay, comprising glucose-binding protein labeled with acceptor and glucose analog labeled with donor. (Reproduced with permission from [52])

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Following demonstration of glucose sensitivity in solution phase, RET-based competitive assay components (apo-glucose oxidase and dextran, labeled with complementary donor–acceptor pair) were encapsulated within a hollow polymeric shell [44, 51]. These polyelectrolyte microcapsules, templated on 5 mm MnCO3 cores with walls comprising multilayer nanofilms including a photosensitive diazoresin, were suspended in a solution of the RET assay and then irradiated with UV light. A key advantage of this approach is the absence of polymer or other matrix in the capsule interior, maintaining a nonrestrictive environment for the molecular components of the competitive-binding assay to easily move relative to one another. The hollow shells act in a similar way to a dialysis membrane, as a semipermeable barrier that allows glucose to diffuse in and out while retaining the sensing components inside. This expected behavior was verified via fluorescence spectroscopy, whereby intensity ratio measurements performed at varying glucose levels revealed a completely reversible response to glucose, matching the observations for the solution-phase assay [44, 51].

8.6.5 Enzymatic Sensors In addition to the obvious encapsulation possibilities afforded by capsules constructed via nanoassembly methods, multilayer nanofilms have also been used to directly entrap sensing probes, or as designed transport-controlling materials deposited on top of prefabricated sensing materials with encapsulated probes. Layer-by-layer construction of enzyme multilayers has been demonstrated for many different catalytic proteins with relevance to sensors; for example, multilayer enzyme nanoreactors can be coupled with multilayers entrapping chemical nanosensors for pH, oxygen, etc., resulting in an exquisitely simple stratified film comprising reactive and monitoring regions. Alternatively, the sensing chemistry can be integrated within the matrix of the colloidal particles, and multilayers deposited on the surface aid in stability, interface interactions with the environment, and transport control. For example, enzymes and fluorescent probes for pH have been combined in calciumcrosslinked alginate microspheres via emulsion and in organically modified silica (ormosil) particles via charge-enhanced absorption (described below). In these cases, a nanofilm coating is easy to apply, serving as a simple and effective barrier to loss of encapsulated macromolecules as well as a diffusion barrier to enable control of relative flux rates for enzymatic substrates [47]. A number of different material combinations are being compared to assess potential for stable immobilization of active enzyme as well as control of relative diffusion of enzymatic substrates; glucose and oxygen have been studied extensively due to the interest in developing glucose-sensing particles [45, 46]. Diffusion-loaded capsules and emulsion-based systems have advantages in stability and ease of use and appear to be the most viable solutions for encapsulation of competitive assays. However, these approaches have limitations


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in the concentrations of encapsulated materials that can be achieved. Higher encapsulation concentration is more critical to construction of enzyme-based systems, where high enzyme concentrations are required to maintain diffusionlimited behavior. Furthermore, more loaded enzyme allows longer operational lifetimes as the enzyme becomes deactivated through spontaneous or other pathways. One possibility to increase loading capacity is the use of electrostatic interactions between the matrix of a particle and the desired loading material; this has been successfully demonstrated to result in high concentrations of oppositely charged molecules from the surrounding solution. These examples constitute only a limited view of many possibilities for microcapsule-based sensor construction; they show promise for building stable systems with entrapped glucose-sensing chemistry and therefore provide sufficient basis for discussion of the different sensing systems that can be achieved using them. Enzymatic sensing typically relies upon monitoring either a product or cosubstrate of a specific reaction between the enzyme and the target. A common example is the oxidation of glucose driven by glucose oxidase, described as Gox

follows: glucose þ O2 þ H2 O ! gluconic acid þ H2 O2 [136]. This reaction is discussed in more detail in a subsequent section; here, it is sufficient to note that oxygen is a co-substrate with glucose, and both hydrogen peroxide and gluconic acid are produced. Fluorescence monitoring of oxygen, discussed below, is typically preferred, as it provides an indirect measure of glucose level. Alternatively, pH or peroxide could similarly be used. Drawbacks specific to using enzymes include (1) changes in activity over time, leading to drifting calibration curves; (2) dependence on local oxygen levels; and (3) consumption of analyte and co-substrates, accompanied by production of by-products. Even if these issues can be overcome, simply creating a system where diffusion and reaction are sufficiently balanced for a sensitive response is a difficult task. Because layer-by-layer self-assembly allows deposition of ultrathin polyelectrolyte multilayer films on the surface of colloidal templates, such a nanofilm coating may be used to perform several functions: (1) provide a diffusion barrier to inhibit leaching of encapsulated material out of the spheres, (2) provide a transport barrier to slow inward diffusion of substrates, allowing control over the response of the sensor, and (3) introduce an internal intensity reference complementary to the oxygen-sensitive fluorophore by the use of polyelectrolyte–fluorophore conjugates, allowing ratiometric measurements. Figure 8.8 contains an illustration and image of a prototype smart tattoo microsphere using this approach; this is further elaborated in the following sections. Oxygen is one of the best known collisional quenchers of fluorescence; therefore, many fluorophores exhibit, to some degree, oxygen sensitivity. In collisional quenching, the quencher contacts the fluorophores while in the excited state, returning the fluorophore to ground state without photon emission [15]. The process of collisional quenching is typically characterized by the Stern–Volmer equation F0 =F ¼ 0 = ¼ 1 þ kq 0 ½Q ¼ 1 þ KD ½Q. In this equation, F0 and F are the fluorescence intensities in the absence and presence of the quencher, 0

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and are the lifetimes of the fluorophore in the absence and presence of the quencher, kq is the biomolecular quenching constant, and [Q] is the concentration of the quencher (in this case, molecular oxygen). The Stern–Volmer quenching constant is KD and is calculated as the product of kq and 0. Metal porphyrin compounds are highly sensitive to oxygen; two of the more popular examples of these materials are the Pt(II) and Pd(II) octaethylporphines, both of which possess lifetimes on the order of 100 ms, exhibiting excitation peaks at ca. 375, 500, and 535 nm and emission at 640–660 nm [137]. Pt(II) complexes are highly sensitive to low oxygen levels; however, these complexes are poorly soluble in water, making aqueous application difficult. To overcome this difficulty, surface adsorption to a carrier, usually silicon, organic glassy, cellulose derivatives, or fluoropolymers is common practice; however, the immobilization matrix has been shown to significantly affect O2 sensitivity [138, 139], and it is understood that the difference in reported sensitivities for various immobilization media is due to the solubility and diffusivity of oxygen in the host matrix [137]. Thus, the encapsulation of the probe is a key factor in affecting sensor performance. Given this observation, the highest O2 sensitivities with Pt(II) complexes have been reported with Pt(II) porphyrin adsorption onto silicacontaining matrices [67, 137]. It is hypothesized that oxygen molecules adsorb to the surface substrate then rapidly diffuse across the surface; interestingly, silica-containing materials exhibit high oxygen-binding affinities and surface diffusion rates [137, 138]. While the layer-by-layer self-assembly process coupled with other chemical production techniques such as emulsification enables the construction of a wide variety of microspheres from a seemingly infinite selection of materials, creating useful devices that function well at the microscale demands careful consideration of the system of interest. In the case of microsphere glucose sensors employing enzymes to drive a reaction that will be monitored with an oxygen indicator, this requires a balance between reaction (consumption) and diffusion (supply) of the co-substrates glucose and oxygen. This balance must be engineered to arrive at a measurable signal change for the expected glucose concentrations. A mathematical model has been used to predict the expected behavior of a microsphere system containing homogeneously distributed enzyme and a nanoscale polymer coating, using glucose sensing as a model application. The model requires solving a set of coupled partial differential equations that describe the reaction and diffusion kinetics for the system. A detailed explanation of the model is provided elsewhere [140], and the reader is encouraged to review details of these previous theoretical and experimental investigations to fully understand the implications for enzymatic sensing at the microscale. Here, the major conclusions are summarized to describe the expected functional properties of enzyme-encapsulating microspheres with nanofilm coatings. Some key variables for these nanostructured enzymatic sensors are the composition, density, and thickness of the coating. It is well established that the membrane applied to enzyme-based sensors is critical in determining response


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sensitivity, range, and time [19, 141, 142, 143, 144, 145]. When working at the microscale, practical realization of transport control requires (1) a deposition method amenable to efficient coating and (2) a highly precise means to control substrate transport, either through precise depositions or by having a wide variety of candidate materials. Multilayer films meet both of these criteria. Transport of various molecular species through polyelectrolyte multilayers has been studied, with very interesting results [146, 147, 148, 149, 150, 151]. In one example, glucose diffusivity through multilayer films of only 22–40 nm thickness was decreased by four orders of magnitude when compared to the glucose diffusivity in water [146]. Since multilayers are easily scaled up, it is easy to see that varying nanofilm thickness through simply adjusting the amount of bilayers adsorbed onto the surface enables precise control over molecular flux rates. For the example of the glucose sensor (Fig. 8.9), glucose relative to oxygen is modulated with film thickness, thereby adjusting the average steady-state oxygen levels observed within the sensor and altering the effective sensitivity of the encapsulated probes. More convincingly, excellent agreement between simulated response and experimental response was observed, validating the predictions of the model [152]. It is noteworthy that, while these comments relate to enzymatic sensors in which oxygen was monitored, a similar approach could be taken for microspheres monitoring pH [153], or other local variables, that are changed as a result of reactions occurring within the confined environment of the microsphere/capsule. These theoretical predictions were followed with experimental work to investigate the potential for using nanofilm coatings to control transport into microsphere sensors. Sensor particles were prepared by immobilizing Pt(II) octaethylporphine (PtOEP), a phosphorescent dye readily quenched by molecular oxygen, into hybrid silicate microspheres, followed by loading and subsequent covalent immobilization of glucose oxidase (GOx) and reference dye, rhodamine B (RITC). Multilayer nanofilms were subsequently assembled on

Fig. 8.9 Predicted average oxygen concentration at steady-state within enzymeloaded alginate particles with varying glucose levels and nanofilm thicknesses. (Reproduced from [152])

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the surfaces of the particles to provide critical control of glucose transport into the particle. The final particles were characterized as a population to determine the optical response to changes in glucose concentration. The sensors were observed to respond rapidly (90 s) and were fully reversible. The sensitivity to glucose, defined as the change in intensity ratio per unit concentration over the linear response range, was determined to be 4.16 0.57%/mg dl–1, over a range of 2–120 mg/dl [154]. The ability to controlling sensor response by adjusting the nanofilm properties was further investigated. Specifically, alterations in the surface-adsorbed polyelectrolyte nanofilms were employed to modulate the relative fluxes of glucose and oxygen into the sensor, allowing the analytical range and sensitivity to be tuned. It was found that the nanofilm thickness, ionic strength of assembly conditions, and outermost constituents played key roles in determining substrate flux. In general, increasing film thickness through additional cycles of adsorption resulted in consistently decreased glucose flux, correspondingly decreasing sensitivity and increasing range. It was shown that the sensor response may be customized to cover the hypo- (0–80 mg/dl), normo- (80–120 mg/dl), and hyperglycemic levels (>120 mg/dl) from a single batch of particles through appropriate selection of coating structure and assembly conditions. These findings demonstrate the key influence of the encapsulation system on sensor response.

8.7 Summary and Conclusions This chapter highlighted the key requirements and progress in nanostructured sensors based on encapsulated probes. For the limited examples available, a review has revealed positive outcomes in the use of nanomaterials to entrap probes for stability, protection, and even to control nanoscale diffusion in order to adjust sensor properties. With the proliferation of new materials and nanofabrication methods, it is likely that the future will see more common research into these areas, and development of standard approaches and products based on probes encapsulated in nanostructured materials. Acknowledgments This work was funded in part by NIH (R01 EB000739), NSF (0210298 and 0640037), and the Louisiana Board of Regents (LEQSF(2001-04)-RD-A-18). The majority of the work and illustrations cited are the result of dissertation research conducted by J. Quincy Brown, Rohit Srivastava, Erich Stein, Patrick Grant, Swetha Chinnayelka, Suman Nayak, and Saurabh Singh and the postdoctoral research work of Huiguang Zhu and Jinshu Mao.

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Chapter 9

Optical Fiber Sensors Based on Nanostructured Coatings Francisco J. Arregui, Ignacio R. Matias, Javier Goicoechea, and Ignacio Del Villar

9.1 Introduction Optical fiber sensors have been developed from the late 1960s, when optical fiber was proposed as a practical medium for communication [1, 2]. Since then, a great effort has been dedicated for the design and development of optical fiber sensors. In fact, the use of this technology to fabricate sensors is very attractive because optical fibers make possible large sensor data capacities over long distances (kilometers). This implies that the sensing head can be very far from the electronic unit that processes the information. In addition to this, the optical fiber is made of dielectric materials which make possible to incorporate these devices in circumstances where high electromagnetic fields are applied, such as in medical magnetic resonance or in situations with high radiation doses [3]. Besides, optical fibers are made of biocompatible materials. Therefore, this technology is very suitable to develop biomedical instrumentation. Other advantages with respect to conventional sensors are that several sensors can be multiplexed in the same optical fiber or even distributed sensing along a fiber cable can be achieved [1, 2, 4]. Basically, a standard communications optical fiber is a cylindrical dielectric waveguide which consists of a core surrounded by a cladding. The fiber can transmit light along its axis due to the phenomenon of total internal reflection, which promotes the confinement of the optical signal in the core. This total internal reflection happens because the refractive index of the core is greater than that of the cladding. From an electromagnetic waveguide perspective it can be analyzed and seen that the light energy in the fiber is not completely confined to the core. Instead, a significant fraction of the energy travels in the cladding as an evanescent wave. Some optical fiber sensors are fabricated by coating the optical fiber core with a sensing cladding. This sensitive cladding is easy to excite by means of the evanescent field [5]. Thus, the interaction between light and matter is governing the sensing mechanisms of these devices. F.J. Arregui Electric and Electronic Engineering Department, Universidad Pu´blica de Navarra, Edificio de los Tejos, Campus Arrosadı´ a, 31006 Pamplona, Navarra, Spain


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Therefore, these sensors are based on physical phenomena like optical absorption, fluorescence or scattering. In fact, the penetration depth of the evanescent field, dp, which is defined as the distance where the evanescent field is reduced to 1/e of its interface value, can be calculated from the solution of Maxwell’s equations [5]: dp ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2p n1 sin2 n22

(9:1)

In this equation is the free-space wavelength of incident light, n1 and n2 are the refractive indices of the core and the sensing cladding of the fiber, respectively, and is the angle of the incident light at the interface between both optical media. Considering that the refractive index of the core, fused silica, is around 1.5 at these wavelengths and that the sensing coatings used have refractive indices similar to the core, for instance 1.49, the penetration depth of the evanescent field is of the order of magnitude of one wavelength. Since the wavelength range of standard optical fibers goes from the visible to the near infrared band, typically until what is called the U band, 1,675 nm, this implies that for devices with these characteristics, sensing coatings with thickness greater than 2 mm could be considered as an infinitum media and the phenomena to be studied would be restricted to this supposition. The assumption of an infinitum medium is valid when the sensing coatings used in optical sensors are fabricated by means of standard procedures that only allow the synthesis and arrangement of sensitive coatings on the micrometer scale. On the contrary, new phenomena have to be considered when the coatings are under the micron scale. Fortunately, with the appearance of techniques that permit the development of nanostructured coatings, it is possible to fabricate devices that take advantage of the new observed phenomena. These nanostructured coatings have opened the door to new sensors that have been studied and presented in the last years. This chapter intends to show a collection of some of them.

9.2 Methods of Fabrication of Nanostructured Films on Optical Fibers: the Layer-by-Layer Technique The fabrication of proper sensing films is a complex discipline that requires a deep multidisciplinary study which has to take into account multiple variables in order to achieve the optimal performances with respect to sensitivity, response time, working range, hysteresis or cross-sensitivity of the sensing devices. In addition to this, if the sensing devices are based on optical fibers, the special geometry of the fiber devices requires also the ability of depositing not only on flat surfaces but on cylindrical or conical substrates as well. Classic deposition techniques such as physical vapor deposition or spin coating are intended usually for flat semiconductor substrates and cannot deposit uniform

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films easily on complex geometries. In fact, there are only three different techniques that have been successfully used for the deposition of uniform coatings onto the cylindrical, and sometimes conical, shape of optical fibers such as the dip-coating (DC) technique, the Langmuir–Blodgett (LB) technique and the layer-by-layer electrostatic self-assembly (LbL) technique. The DC technique is usually associated to sol-gel or hydrogel coatings and, although suitable for cylindrical shapes, it is not useful for controlling the thickness of the coatings on the nanometer scale. On the contrary, the LB and LbL techniques can be used for the fabrication of nanostructured films. Unfortunately, the LB technique is limited to very specific molecules with combinations of lipophilic and hydrophilic parts [6]. On the other hand, the LbL process has been successfully probed as a useful tool for the fabrication of nanostructured materials that include many diverse species, as was anticipated in Chapter 8, such as colorimetric dyes, fluorescent indicators, inorganic semiconductors, conducting polymers, ceramics, metals, quantum dots, enzymes, antibodies or even DNA strands [7–14]. Since the LbL technique can summarize by itself the different devices fabricated by means of nanostructured sensing films, this chapter will mainly focus on the LbL technique. The subsequent adsorption of oppositely charged colloids, the LbL technique, was suggested for the first time by R. Iler in the mid-1960s [14]. Unfortunately, nobody followed this line of research until almost 40 years later when the technique was rediscovered by G. Decher and coworkers [9], and extended to the layering of polyelectrolytes and many other systems [7–14]. In the last years the number of works on this topic has increased exponentially and some reviews permit to understand the current state of the art [15–17]. Authors have referred to this technique in different ways: ionic self-assembly monolayer (ISAM) process, electrostatic self-assembly (ESA), layer-by-layer process (LbL). Henceforward we will refer to this technique as the LbL method. Basically, the LbL method is based on the electrostatic attraction between oppositely charged polyelectrolytes in each monolayer deposited, and involves several steps. The LbL film deposition method is described schematically in Fig. 9.1. First, a substrate (in this case the optical fiber) is cleaned and treated to create a charged surface. Then, the substrate is exposed to a solution of a polyion of opposite charge for a short time (minutes) and by adsorption a monolayer of polyions is formed on the surface. In this way, the substrate is alternately dipped into solutions of cationic and anionic polymers (or appropriately charged inorganic clusters) to create a multilayer thin film, a polyanion–polycation multilayer. The molecular species of the anionic and the cationic components and the long-range physical order of the layers determine the resulting coating properties. It is important to notice that the polyanions and polycations overlap each other at the molecular level and this produces a homogeneous optical material [18–20]. The pair of one anionic monolayer and one cationic monolayer will be called bilayer henceforward. The composition and thickness of an individual bilayer can be controlled by adjusting the deposition parameters. Moreover, these coatings can be formed in many different substrates; for instance, metals, plastics, ceramics and semiconductors. Additional details

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POSITIVE

CHARGED

CHARGED

MONOLAYER

MONOLAYER

CHARGED SUBSTRATE

−

−

−

−

−

−

−

−

− − − − − − − − − − −

−

−

POLYANION DIPPING & WASH

−− −

−

POLYCATION

−

−

−

− −

− − − − − − − − − −

DIPPING & WASH

−− −

− − −

−

REPEAT

Fig. 9.1 LbL schematic for construction of multilayer assemblies by consecutive adsorption of anionic and cationic molecule-based polyelectrolytes. After the clean substrate is treated to obtain an appropriate charged surface (positive in the figure) the substrate is immersed in solutions containing the anionic and cationic polyelectrolytes. Thus, the coating is built up monolayer by monolayer. At each dipping step, the surface charge gets reversed, allowing the adsorption of each monolayer. The symbols are idealized and are not intended to represent exactly the conformation of the polyelectrolyte chains

concerning the deposition process and its possible applications can be found in the literature [7–20] and also in the chapters about ‘‘Encapsulated probes sensors’’ and ‘‘Nanostructured flexible materials’’ of this book. Taking advantage of the versatility of the LbL method for the synthesis of materials, this technique has been applied to fabricate different optical fiber devices such as Fabry–Perot interferometers on the nanometer scale, in-fiber optical gratings and different types of sensors, such as humidity, harmful gas, pH, hydrogen peroxide or glucose sensors, just to mention a few. Some of these devices will be described in the following paragraphs. For a given combination of cationic and anionic materials, the LbL technique also permits the possibility of tuning some of the deposition parameters such as the concentration of the solutions, their temperature, ionic strength or pH. This will help to tailor the properties of the fabricated films. For instance, by means of changing the pH of the polyelectrolytes solutions involved in the deposition process, it is possible to fabricate bilayers whose thickness, roughness or even the amount of the active material (a colorimetric or fluorescent indicator) can be modified. This can be clearly appreciated in Fig. 9.2 where two atomic force microscopy (AFM) images of the surface of [PAH+NR/PAA]50 coatings fabricated at two different pH values, 5.5 and 7, are shown. PAH stands for the cationic poly(allylamine hydrochloride), while PAA means the anionic poly(acrylic acid). NR is an anionic pH indicator, Neutral Red, whose


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Fig. 9.2 AFM images of [PAH+NR/PAA]50 coatings built up at pH 5.5 (left) and pH 7 (right)

color changes from red (acid form) to yellow (basic form) and 50 is the number of bilayers. An AFM Digital Instruments NanoScope IIIa multimode in tapping mode was used in order to measure the thickness of the coatings by making a scratch in the films with a razor blade and then measuring the depth of the line. The roughness of the films was also measured with the AFM in tapping mode, with silicon cantilever (RTESP, Veeco) in a region near the central zone of the sample to avoid border effects. From Fig. 9.2 it is possible to observe the great difference in roughness with values of the rms roughness (Rq) of 45.3 and 1.38 nm for the samples fabricated at pH 5.5 and 7, respectively. The thickness of these two samples was also very different, 35.2 and 10.6 nm per bilayer. This is only an example of the tunability of the coatings just by changing the pH of the solutions used for the fabrication of the films. For this particular combination of materials, [PAH+NR/PAA], the evolution of thickness and roughness of the coatings when the pH of the polyelectrolyte solutions is varied in the fabrication process is plotted in Fig. 9.3. This information is very useful to determine the optimal conditions of thickness or roughness for the final sensitive films.

9.3 Types of Devices and Sensing Mechanisms 9.3.1 Interferometric Cavities: NanoFabry–Perots The simplest structure that can be formed using nanostructured coatings on optical fibers is by depositing a coating on the cleaved or polished end of an optical fiber. The final parallel layer structure of the coating forms an interferometric cavity on the optical wavelength scale and the behavior of this structure is the same as that of a nano Fabry–Perot cavity, as shown in Fig. 9.4. In this interferometric cavity the mirrors are formed by the refractive index differences between the different optical materials: n1, n2 and n3 are the refractive indices of the optical fiber, the sensing coating and the surrounding

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Fig. 9.3 Left: the bars show the bilayer thickness as the pH of the polyelectrolyte solutions was varied. Right: the dotted line is the square roughness of the surface of [PAH+NR/PAA]25 coatings. Graph from [21]

Fig. 9.4 Schematic of a nanoFabry–Perot formed by the nanostructured coating at the end of an optical fiber

medium, respectively. The reflectance of each mirror will be approximately that determined by Fresnel’s law for the case of normal incidence: R1 ¼

R2 ¼

ðn1 n2 Þ2 ðn1 þ n2 Þ2 ðn2 n3 Þ2 ðn2 þ n3 Þ2

;

(9:2)

(9:3)

where R1 is the reflection coefficient at the first interface (optical fiber–coating) and R2 is the reflection coefficient at the second interface (coating–air). Usually,


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these coatings incorporate some indicator and are optically denser than the core. Then the refractive index of the coating is higher than core refractive index. Then, n2>n1> n3, and for this case we can determine an expression for the reflected optical power by following the procedure of Lee and Taylor et al. to obtain the reflected optical power of a Fabry–Perot resonator [22]. Following this procedure, the phase shift in reflector 1 is p rad, because n1n3. The successive reflections in the two mirrors are indicated in Fig. 9.5. InpFig. ffiffiffiffiffiffiffi 9.5, E0 is the field amplitude of the incident lightwave on this nanocavity; E0 T12 is the field amplitudepof lightwave when it passes from ffiffiffiffiffiffiffithe transmitted d j=2 the fiber to the coating; E0 T12 e e is the field amplitude of the pffiffiffiffiffiffiffi lightwave after traveling a distance d in the coating; T12 is the transmission coefficient in the first mirror (from the fiber to the coating, from media 1 to pffiffiffiffiffiffiffi media 2); is the absorption coefficient of the coating; and T21 is the transmission coefficient from media 2 to media 1. The value is the round-trip phase shift and assuming that is the wavelength in the free space then is defined by ¼

4 p n2 d

(9:4)

Therefore, the reflected field amplitude ER is the addition of all the reflected terms, 0 pffiffiffiffiffiffi

1

R1 þ pffiffiffiffiffiffi pffiffiffiffiffiffiffi B þpffiffiffiffiffiffiffi C T12 R2 T21 e2d ej þ C (9:5) ER ¼ E0 B p ffiffiffiffiffiffiffi p ffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffiffi @ þ T12 ffiffiffiffiffi A R2 R1 R2 T21 e4d ej2 þ pffiffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffiffi 6d j3 þ T12 R2 R1 R2 R1 R2 T21 e e þ

coating n2

optical fiber n1 E0

E 0 ⋅ T1

E 0 ⋅ T1 ⋅ e − α d ⋅ e

E 0 ⋅ R1 ⋅ e −π

E 0 ⋅ R 2 ⋅ T1 ⋅ e − 2αd ⋅ e E 0 ⋅ R 2 ⋅ T1 ⋅ R 1 ⋅ R 2 ⋅ e − 4αd ⋅ e

surrounding medium n3 − jφ 2

E 0 ⋅ T1 ⋅ T2 ⋅ e − αd ⋅ e

− jφ 2

− jφ E 0 ⋅ T1 ⋅ R 2 ⋅ R 1 ⋅ T2 ⋅ e − 3αd ⋅ e

− j2φ

. . .

. . . d

Fig. 9.5 Optical reflections in the nanoFabry–Perot

− j⋅ 3 φ 2

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Assuming that the transmission coefficients are similar then

pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi pffiffiffiffiffiffi T12 ffi T21 ¼ T1 ,

0 pffiffiffiffiffiffi 1 R1 þ p ffiffiffiffiffi ffi B þ R T e2d ej þ C 2 1 B C ER ¼ E0 B pffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4d j2 C @ þ R2 T 1 R1 R2 e e þ A pffiffiffiffiffiffi þ R2 T1 R1 R2 e6d ej3 þ

(9:6)

therefore, 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n X pffiffiffiffiffiffi pffiffiffiffiffiffi ER ¼ E0 R1 þ R2 T1 e2d ej R1 R2 e2d ej

(9:7)

n¼0

In addition, it is clear that pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R1 R2 e2d ej 51

(9:8)

Then, the next sum is the sum of a geometric series with a common ratio lower than 1, hence we can write 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n X R1 R2 e2d ej ¼ n¼0

1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2d j 1 R1 R2 e e

(9:9)

Since it is possible to define Ai based on the equation, Ai þ Ti þ Ri ¼ 1

(9:10)

(where Ai would be associated with the losses due to scattering), it is possible to simplify the expression of the reflected field amplitude:

1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 1 R1 R2 e2d ej pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi R1 þ R1 R2 e2d ej þ R2 T e2d ej pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼E0 ¼ (9:11) 1 R1 R2 e2d ej pffiffiffiffiffiffi pffiffiffiffiffiffi R1 þ R2 ð1 A1 Þ e2d ej pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼E0 1 R1 R2 e2d ej

pffiffiffiffiffiffi pffiffiffiffiffiffi ER ¼E0 R1 þ R2 T1 e2d ej

Since what is experimentally measured is the optical power instead of the field amplitude, it is more interesting to know the expression of the reflected optical power that will be defined as


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1 IR ¼ ER ER ¼ 2

pffiffiffiffiffiffi pffiffiffiffiffiffi 1 R1 þ R2 ð1A1 Þe2d ej pffiffiffiffiffiffiffiffiffiffiffiffiffi 2d j ¼ E20 2 1 R1 R2 e e pffiffiffiffiffiffi pffiffiffiffiffiffi (9:12) R1 þ R2 ð1A1 Þe2d ej pffiffiffiffiffiffiffiffiffiffiffiffiffi 2d j ¼ 1 R1 R2 e e pffiffiffiffiffiffiffiffiffiffiffiffiffi 2 4d 1 R1 þR2 ð1A1 Þ e 2 R1 R2 ð1A1 Þe2d cos pffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ E20 2 1þR1 R2 e4d 2 R1 R2 e2d cos

Since the incident optical power is I0 ¼

1 1 E0 E0 ¼ E20 2 2

(9:13)

then, the ratio between reflected optical power and incident optical power will be finally, RFP ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi IR R1 þ R2 ð1 A1 Þ2 e4d 2 R1 R2 ð1 A1 Þ e2d cos pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ I0 1 þ R1 R2 e4d 2 R1 R2 e2d cos

(9:14)

This theoretical model can predict the reflected optical power from the cavity depending on the thickness of the coating [18–20, 23]. Similar curves can be also obtained experimentally just by using a very simple optical arrangement, see Fig. 9.6. In this experimental setup a low coherence light source, like a LED which usually has a coherence length of some tens of microns, can be used. This is possible because, in order to observe the interferometric behavior of the cavity, it is necessary to use light sources with coherence lengths longer than the round-trip optical path length. Since the cavities studied here are under the micron thickness the utilization of inexpensive LEDs or even incandescent white light sources can be used for monitoring this phenomenon. An example of one of these curves is plotted in Fig. 9.7. nanoFabry-Perot

light source coupler detector

index matching gel

Fig. 9.6 Basic experimental setup for monitoring the reflected optical power from the nanoFabry–Perot

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7 6 5 4 3 2 1 0 0

50

100 150 Number of bilayers

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Fig. 9.7 Experimental and theoretical results of the optical power reflected by the nanointerferometric cavity as the number of bilayers increases, the theoretical results have been obtained using the model described earlier. Reproduced with permission from [18]

The curves obtained here, see Fig. 9.7, are similar to a dumped sine where the dumping factor of this curve would be determined by Ai and parameters that have to be estimated and adjusted experimentally: Ai depends on the scattering of the coating, therefore it depends on the roughness of the coating and this value is affected by the fabrication conditions as was shown in Fig. 9.3, is also determined by the fabrication conditions. But perhaps the more interesting thing is that the oscillation of these curves, the position in number of bilayers of peaks and valleys, defines the optical thickness, the product of the refractive index multiplied by the thickness of the coating. This is because the maximum reflected power will happen when the round-trip phase shift is an integer multiple of p. Then, from Equation (9.4), the optical thickness n2_d can be calculated. Since it is easy to have an estimation of the refractive index of a material with an error lower than 2% (i.e., by ellipsometry), then the thickness of these coatings could be deduced. For instance, usually most of the polymeric coatings have refractive indices around 1.5–1.8 and it is easy to obtain a measurement of the refractive index of these materials with a precision of 2 decimals or higher by using different methods, therefore, after obtaining the experimental curves of the reflected optical power it is easy to determine the approximate thickness of the nanoFabry–Perot; more details can be found in [18–20, 23]. These nanoFabry–Perots can be very useful for sensing. The classical optical fiber Fabry–Perot etalon operates based on the variation of the length of the cavity, d, to produce a phase shift, and thus, a change in the intensity of reflected or transmitted optical power. This can be possible if the sensing coating is made


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with materials that swell, for instance hydrogels. In addition, this new Fabry– Perot etalon can take advantage of changes in the reflection of the external mirror at the endface of the optical fiber, R2, that can produce a change in the reflected optical power. For instance, changes in R2 with the humidity could be achieved because the coating material is hygroscopic. Other sensing mechanism of the nanoFabry–Perots could be based on the incorporation of dyes or indicators sensitive to a target, for example, pH. Some of these applications will be shown later.

9.3.2 Microgratings Since the combination of polyanions and polycations overlap each other at the molecular level and this produces a homogeneous optical material with a given refractive index A, then with other polyanions and polycations it is possible to build stacks of materials with different refractive indices, A, B, C, etc. Following this philosophy, a stack of multiple quarter-wavelength-thickness films with alternating high and low refractive indices can be deposited on the end of an optical fiber to form a grating, see Fig. 9.8. This structure materializes a micrograting whose spectrum changes when the number of quarter-wavelengththickness films in the stack increases. The more quarter-wavelength-thickness films are built on the stack the narrower are the resonance bands of the resultant optical filter. See Fig. 9.9. where the optical response of two of these microgratings is plotted. The spectral response of these microgratings to the sensing target is such that it provides some wavelengths where the reflected optical power does not change when the sensing target varies and that could be taken as reference wavelengths for normalizing the signal, in a similar way to the isosbestic points in pH sensors [24, 25].

9.3.3 Coatings on Conical Surfaces Sometimes, especially when we are dealing with fluorescent indicators, it is very interesting to have surfaces that minimize the reflected optical power from the

Fig. 9.8 Schematic of the micrograting fabricated by means of a stack of quarter-wavelengththickness films. The higher refractive index material stacks are noted as ‘‘H’’, and the lower refractive index material stacks as ‘‘L’’

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5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 –0.5

HLHLH H

1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650

Wavelength (nm)

Fig. 9.9 Spectral response of microgratings fabricated with the same materials and experimentally adjusted for achieving a maximum reflectance at 1,310 nm (left) or at 1,550 nm (right). These gratings consist of an alternating stack of 5 quarter-wavelength-thickness films of materials with high refractive index (H) and low refractive index (L). In the figure, the H curve is the almost flat spectrum of a quarter-wavelength-thickness coating (a nanoFabry–Perot) and HLHLH is the spectrum of the micrograting. The fabrication procedure and materials employed are described in [24, 25].

excitation light and maximize the fluorescence from the indicator. This can be achieved with different approaches. On one hand, perpendicular cuts at the end of the fiber made easy the reproducibility of the devices because they can be performed using a fiber cleaver with negligible human participation. However, this perpendicular cut provides less surface area for deposition than an oblique cut at the end of the fiber (a slanted endface) or a tapered end surface. Even worse, a perpendicular cut is the optimum configuration for a maximum reflection, thus, most of the excitation light sent through the fiber to the fluorescence coating is reflected back masking the fluorescence spectrum and making more difficult the study of the devices’ properties. On the other hand, the conical shape at the tapered end maximizes the depositing surface and minimizes the reflected signal which makes it an optimum candidate for the fabrication of sensors based on fluorescence [26], see Fig. 9.10. The difficulty of depositing materials on a conical surface by classic deposition techniques is easily overcome by the LbL technique as has been reported in the literature [27, 28]. In addition to this, biconically tapered optical fibers of single-mode optical fibers, see Fig. 9.11, are very sensitive to changes of the surrounding refractive index and the wavelength of the input light and can be used for evanescent fieldbased sensing schemes. In this configuration, when a fiber is tapered, the core/ cladding interface is redefined in such a way that the single-mode fiber in the

Fig. 9.10 Schematic of an optical fiber tapered end coated with a LbL sensing coating


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Fig. 9.11 Schematic of a biconically tapered optical fiber sensor with a LbL sensing overlay

central region of the taper acts as a multimode fiber and the light becomes guided through the cladding of the fiber, which plays the role of the new core, and the new cladding is the surrounding external medium. Theoretical aspects of this phenomenon have been exhaustively studied in the literature [29–32]. Classical deposition techniques such as physical vapor deposition (PVD) cannot deposit azimuthally symmetrical palladium, other techniques such as sol-gel dip coating do not allow the overlay thickness to be tuned in the nanometer scale. By means of the LbL, the idea here is that a nanostructured film can be deposited on the thinner part of the biconically tapered optical fiber with the possibility of controlling its thickness in real time and in a nanometer scale, providing the possibility to stop the nanodeposition process when the optimum working point is reached [33, 34, 35].

9.3.4 Coatings onto Long-Period Gratings Long-period gratings (LPG) consist of a periodic index modulation of the refractive index of the core of a single-mode fiber, with a much longer period than the more known fiber Bragg gratings (FBG). If FBGs have periods typically on the sub-micron scale, LPGs have periods in the range 100–1,000 mm. This grating induces attenuation bands in the transmission spectrum based on the coupling between the core mode and the copropagating cladding modes. Consequently, the influence of the surrounding medium on the properties of LPGs is more important than in FBGs, where there is a contrapropagative coupling between core modes. In this way LPGs have extended its applications to both optical communications and sensor fields [36]. The sensing mechanism is based on the spectral shift of the attenuation bands which are generated due to the coupling of light from the core mode to the cladding modes. Since the conditions of this coupling can be affected by the temperature, strain, curvature and, what is more relevant in this chapter, the refractive index of the surrounding medium, then temperature, strain, curvature and refractometric sensors based on LPGs can be fabricated [37–40]. In fact, LPGs exhibit a high

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sensitivity for refractive indices similar or slightly lower than the one of the fiber, usually around 1.45 at 1,550 nm, and show negligible sensitivity to refractive indices higher than 1.48 [41]. This means that it is possible to coat a LPG with a sensitive material that changes its refractive index with a target parameter (gas, chemical specie, biological agent). Once again, if the coating is thin enough some very interesting properties appear. For instance, the central wavelengths and the minimum transmission of the LPG attenuation bands can exhibit a dependence on both the thickness and the refractive index of an overlay material, even when the overlay has a refractive index higher than that of the cladding [41, 42]. Moreover, the sensitivity of these devices can be enhanced by adjusting the exact thickness of the coatings [43, 44] or by combining a two-overlay structure where the sensitivity can be improved by a factor of 70 [45, 46]. James et al. have also proposed the utilization of cascaded LPGs [47]. These concatenated LPGs act as a Mach–Zehnder interferometer: light coupled into the cladding by the first LPG is re-coupled into the core by the second LPG, where it interferes with the light that propagated in the core, producing interference fringes within the attenuation bands. Very recently, the selective removal of the LbL coating of Fig. 9.12 to form a periodic coating onto the LPG has also been proposed as a potential device for sensing applications [48].

9.3.5 Coatings on Hollow Core Fibers Very recently, it has been proposed a structure for evanescent field sensing which consists of a short segment of hollow core fiber (HCF) spliced between two standard multimode fibers (MMFs) [35, 49]. For the sake of simplicity, it is designated MMF-HCF-MMF (MHM) throughout the text (see Fig. 9.13). Early works reported about a MHM structure which was used for the surrounding medium

LbL overlay cladding core nh

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output light nl

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Transmission (dB)

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Fig. 9.12 Schematic of a sensor based on a nanostructured overlay onto a long-period grating (LPG). In the graph the input light from a broadband light source is represented with a flat spectrum (left) and the output light from the LPG has attenuation bands inherent to the LPG


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Fig. 9.13 Schematic of a MHM structure with a sensing coating in the inner part of the hollow core fiber

fabrication of strain sensors based on in-line fiber etalons; these interferometric cavities were formed by the segment of HCF spliced between two sections of multimode fiber [50]. In the cited works, light is confined in the air core of the HCF. In contrast, the aim here is to spread out the light into the silica cladding of the HCF in order to achieve a higher evanescent field ratio with respect to the total transmitted optical power. This evanescent field intensity will be modulated as a function of the coating deposited onto the external part of such a HCF segment. The HCF-based structure used is schematically shown in Fig. 9.13. It consists of one short segment (10–20 mm) of HCF spliced between two standard MMFs. The jacket of the HCF has previously been removed. If the HCF and the MMF are spliced together using the appropriate electric arc conditions, the HCF collapses, forming a tapered solid fiber in the interface between both fibers (see photograph in Fig. 9.14). In these devices, the light that is guided into the core of the lead-in MMF can be coupled to the cladding of the HCF due to the tapered region instead of being confined in the air core. When the light reaches the lead-out MMF, it is coupled into the silica core again. Because the light is guided by the silica cladding in the HCF region, these devices can be used as evanescent field sensors that are sensitive to any coating deposited onto this region. Plus, using the LbL technique it is possible to adjust the thickness of the coating to its optimum value for sensing [35, 49]. (a)

(b) (d)

(c)

(e)

Fig. 9.14 (a) Collapsed end of the HCF spliced to the lead-in MMF section. (b) Collapsed end of the HCF spliced to the lead-out MMF section. (c–e) Pictures of the light projected by different parts of the MHM on one screen, using one green laser at 530 mm. (c) Light projected by the lead-in MMF. (d) Light projected by the HCF. (e) Light leaving the lead-out MMF

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9.4 Sensing Applications After reviewing the different structures that have been fabricated by LbL assembly on optical fibers, we will summarize the sensors that have been reported so far using these structures.

9.4.1 Humidity A humidity sensor based on a nanoFabry–Perot, see Fig. 9.4, was the first optical fiber sensor reported using the LbL technique [20]. The sensing mechanism of this sensor approach relies on the change in the reflected optical power of the specific coating materials, used to form the interferometric cavity of the nanoFabry–Perot, that is induced by humidity or a target chemical. That is, some molecules of the target can be trapped at the surface of the sensing coating, altering the optical reflectivity. For instance, in the case of relative humidity measurements, it is clear that, for a given relative humidity value, the choice of either a hydrophilic or hydrophobic sensing material will determine the size of the water drops trapped on the surface due to the respective affinity or repulsion of the water to the sensing material. In fact, choosing properly the polyelectrolytes that form the nanostructured coating, for instance a given combination of hydrophilic and hydrophobic materials, it is possible to fabricate humidity sensors with a very fast response, shorter than a tenth of a second. Because the resulting nanostructured coatings have mixed properties of the hydrophilic and hydrophobic materials, these films are sensitive to humidity with little water absorption. Due to this the response time is so fast that these sensors can be used even for human breathing monitoring [19, 20, 51], see Fig. 9.15. In addition, as can be seen in Fig. 9.16, the sensors showed a repetitive response. Another device for simultaneously measuring the humidity and temperature is based on a fiber Bragg grating (FBG) cascaded with a low-finesse Fabry–Perot [52]. The sensing scheme is depicted in Fig. 9.17. Light from a broadband light source is launched to the combined FBG and Fabry–Perot fiber sensor head through a 3-dB optical fiber coupler. The FBG element reflects optical power with maximum reflection coefficient centered at the Bragg wavelength. This wavelength will shift due to changes in temperature, and the measurement of this shift in wavelength can be used to give a reading of the corresponding change in temperature at the physical location of the sensor head. The FBG also behaves as a stop-band optical filter, so optical wavelengths outside of the Bragg grating window are transmitted with negligible attenuation. These transmitted wavelengths thus pass through the Bragg grating and reach the humidity sensor. For these wavelengths, reflection at the Fabry–Perot changes depending on the humidity. In this way, the reflected optical power again passes through the Bragg grating, this time in the opposite direction with respect to that of the incident light. Because


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RH (climatic chamber) OFHS

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Fig. 9.15 Response to human breathing of an optical fiber humidity sensor based on a LbL nanoFabry–Perot formed. The materials used were Poly R-478 and poly(diallyldimethyl ammonium chloride). Reprinted with permission from [20]

–48 –49 300

Fig. 9.16 Response of the optical fiber humidity sensor (OFHS) to changes of relative humidity (RH) at a constant temperature of 258C, inside a climatic chamber for 96 hours of continuous monitoring. Reprinted with permission from [20]

the optical power spectrum of this reflected signal matches the transmission window of the Bragg grating, the optical power transmitted at the Bragg wavelength is not altered by the presence of the grating. Therefore, by measuring the reflected optical power within this unaltered transmission window of the Bragg

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optical fiber

d

BRAGG GRATING

WHITE LIGHT SOURCE

HUMIDITY SENSOR

TRANSMISSION

OPTICAL POWER

INCIDENT LIGHT

WAVELENGTH

OPTICAL FIBER

OPTICAL FIBER REFLECTION

TEMPERATURE

REFLECTION

WAVELENGTH

%RH WAVELENGTH

WAVELENGTH

REFLECTED LIGHT

Fig. 9.17 Schematic of the sensing system of the dual temperature–humidity sensor. Reprinted with permission from [52] # 2002 IEEE

grating, it is possible to measure intensity changes corresponding to variations in the humidity. In order to corroborate this, the concatenated FBG and nanoFabry–Perot sensors described above were evaluated in a climatic chamber. The results obtained after a test cycle involving a constant temperature of 258C with relative humidity values of 11, 33, 52, 68, 85 and 97% are plotted in Fig. 9.18. In the region labeled ‘‘Zone A’’, where the six curves overlap, it is noticeable that the reflected optical power at the Bragg wavelength was not affected by humidity changes. In contrast, in the region labeled as ‘‘Zone B’’, the reflected optical power is observed to change as a function of the relative humidity. More details about this dual sensor are in [52].

9.4.2 Temperature In [53] the deposition of CdTe Quantum Dots (QD) on tapered ends of optical fibers has been reported. A very interesting property of QDs is that they can be excited in a broad range of wavelengths and, at the same time, have a narrow emission spectrum. Moreover, the center wavelength of the emission peak depends on the geometrical size of the QD; therefore, the emission wavelength can be tuned by changing the size of the nanocrystals that gives a large choice of emission wavelengths. In addition, QDs offer an exceptional photostability and

9 Optical Fiber Sensors Based on Nanostructured Coatings 16

293 Temperature = 25ºC


Zone A

14 12 10 8 6 Zone B

4 RH = 11% 2 0 RH = 97% –2 1540

1545

1550

1555

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Wavelength (nm)

Fig. 9.18 Spectral response of the dual temperature–humidity sensor with a coating of 350-nm thickness at a constant temperature of 258C and different humidity rates. Reprinted with permission from [52] # 2002 IEEE

have a high quantum yield compared to regular fluorescent dyes used for sensing applications: higher than 50%. In [53] QDs of different sizes, 4 and 5 nm, with a maximum fluorescence around 570 or 620 nm, respectively, were successfully tested for temperature monitoring, and showed a linear and reversible variation of the emission wavelength for a temperature range from 30 to 1008C, with a sensitivity of 0.2 nm/8C. One of the main drawbacks of this sensor is the photobleaching effect of the QDs due to photooxidation. In order to minimize this effect, a good strategy could be the encapsulation of the sensing coatings. In [54] the fabrication of a QD nanofilm inside a hollow core fiber (HCF) has been reported. This portion of HCF with an inner sensing coating is then spliced between two multimode fibers as was plotted in Fig. 9.13. This way the material is protected against the environmental conditions and is still able to sense temperature changes since it is conducted through the thin glass cylinder of the HCF segment (50 mm of diameter). Using this self-encapsulated structure, photobleaching behavior is enhanced: if previous devices with no encapsulation showed an intensity decay of 80% after 1.5 hours of continuous illumination, with this new device a decrease of only 6.3% of the fluorescence emission intensity after 4 hours of continuous illumination has been reported.

9.4.3 Gas and Volatile Organic Compounds An example of an optical fiber sensor for the detection of volatile organic compounds (VOCs), such as acetone, dichloromethane or ethanol, has been reported based on the deposition of Al2O3 and polymer ultra-thin films on the

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ends of optical fibers forming a nanoFabry–Perot [55]. These sensors are designed to operate at the standard communications wavelengths with no cross-sensitivity to temperature from at least 10 to 708C. A humidity crosssensitivity of less than 1.4% from 11 to 85% of relative humidity was achieved and this cross-sensitivity was still negligible after 1 year of the fabrication. A similar approach which consists of a nanoFabry–Perot of polymeric films doped with a vapochromic material, Au2Ag2(C6F5)4(C6H5N)2, has been reported for the detection of some VOCs such as methanol, ethanol and isopropanol. This material changes its color when exposed to some organic vapors, recovering its original state when vapors get disappeared [56]. A more complex scheme for the detection of dichloromethane is based on a multilayered dielectric stacks on the ends of optical fibers (microgratings, see Fig. 9.8). This technique allows the selection of the sensing wavelength, as well as the reference wavelength, prior to sensor fabrication. The sensors exhibit negligible cross-sensitivity with temperature between 10 and 708C, a fast response time (less than 2 s), and negligible hysteresis [24, 25]. As is usual in generic VOC sensors, the main drawback of all these sensors is the low selectivity to a single compound. In a different approach, multilayer fluorescent films containing ruthenium complexes were also deposited on optical fibers and additional layers of fluorescein 5(6)-isothiocyanate (FITC) were also deposited to serve as an internal reference and allow ratiometric measurements of dissolved oxygen in aqueous solutions [57].

9.4.4 pH and Chemical Species Different techniques have been studied for pH sensing. The simplest ones are based on the colorimetric behavior of a pH indicator embedded in a polymeric matrix [21, 58, 59]. Very recently, it has been discovered that the utilization of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), two of the most used polymers in LbL coatings, under certain circumstances can lead to the fabrication of polymeric films with a high swelling behavior. Moreover, devices which incorporated a pH indicator, for instance Neutral Red (NR), in the (PAH/PAA) multilayer structure were sensitive to pH changes with a high non-linear response. In [60] the polymeric matrix without the indicator, the (PAH/PAA) multilayer structure, was tested against pH variations and showed a high sensitivity to pH changes mainly due to swelling. Therefore, this matrix is not the optimum solution for the fabrication of optical fiber pH sensors based on colorimetric reagents because the swelling behavior of the polymeric matrix can affect the optical response of the nanoFabry–Perot and mask the changes in color due to pH. In order to solve this, the substitution of PAH by some pH indicator, i.e., (NR/PAA) films, has been reported as a successful strategy for the fabrication of pH sensors with a response curve more similar to the optical


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response of the sensing pH indicator. Besides, these last devices showed a fast response time (shorter than 1 s), a good resolution of 0.03 pH units, negligible drift in 30 min of continuous monitoring and a low cross-sensitivity to temperature[60]. Sometimes, conversely, it is possible to take advantage of the swelling behavior of the (PAH/PAA) matrix. In fact, these coatings can be deposited onto long-period gratings, see Fig. 9.12. Since these coatings swell depending on pH, the refractive index and the thickness of the coatings change with pH. Therefore, the wavelengths of the attenuation bands of the LPG will shift with pH [61]. Other schemes, which can overcome the possible pH-dependent behavior of the polymeric matrix due to swelling, are based on the incorporation of fluorescent dyes. Among them, 8-hydroxypyrene-1, 3, 6-trisulfonic acid trisodium salt (HPTS), a well-studied pH fluorescent dye can serve as an optimum candidate for pH monitoring. The continuous illumination of the fluorescent dye can provoke photobleaching which can be minimized with the incorporation of antifading agents in the nanostructured coatings. In fact, an improvement has been achieved from the original photobleaching rate of 58% of the non-optimized devices after only one hour and half under continuous illumination to a decrease of only 4.7% after 72 hours using the same continuous excitation source in the case of the optimized sensors, as shown in Fig. 9.19 [28]. In these fluorescent pH sensors fabricated by the LbL method the response time of the devices can be limited sometimes due to the hydrophobic nature of the polymeric matrix. In [27] the response time is minimized by modifying the hydrophilic properties of the sensitive nanocoatings, this strategy helps the diffusion of water molecules and ions through the multilayer structure. In this work, LbL multilayer polymeric pH-sensitive and non-sensitive coatings were successfully

Fig. 9.19 Lifetime enhancement of fluorescence-based optical fiber pH sensors. In this case HPTS was used as active pH-sensitive fluorophore, embedded into a LbL polymeric nanostructured coating. (a) The response of a (PAH/PAA+HPTS)10 multilayer coating shows a very high photobleaching rate. (b) The incorporation of the antifading agent DABCO into the LbL structure significantly improves the sensor lifetime, stabilizing the fluorescent signal. Reprinted from [28] with permission from Elsevier

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built up onto both glass slides and tapered optical fiber ends. Two different structures were used to compare the results: first, fabricating only the sensitive coating and second, adding a high hydrophilic coating prior to the sensitive one. The hydrophilicity of two different structures, S1 (sensitive coating without a previous hydrophilic matrix) and S2 (sensitive coating with the previous hydrophilic matrix) were studied by means of water contact angle measurements and the results obtained were 508 and 108, respectively, as can be seen in Fig. 9.20. Using this alternative construction technique, the rise time response was five times minimized from 15 to 3 min between pH 3 and 7 and fall time response was three times minimized from 3 to 1 min between pH 7 and 3. In general rise time responses were considerably reduced to less than a fourth while fall time response reductions had less significance. In [62] a LPG, see Fig. 9.12, is coated with a LbL film sensitive to copper. The sensing coating is formed by incorporating the reagent Cibacron Blue with generation 4-poly(amidoamine) dendrimer. In that work, it is reported that a detection of 1.3 mg Cu2+ L-1 is observed when six bilayers comprised the coating. In addition, a stable response is achieved with 0.6 mg L-1 in less than 1 min and when 0.1 M HCl was used as the rinsing solution, this LPG sensor was reversible and the signal response to similar concentrations of Cu2+ reproducible [62]. The immobilizing of a redox indicator, Prussian Blue, in a nanoFabry–Perot makes possible the detection of H2O2 in the range from 10-6 to 10-3 M [63]. For higher values, the sensor saturated. The recovery of oxidized Prussian Blue was successful after immersion in a reductive agent such as ascorbic acid. In this device, the reflected power depends highly on the pH, as is usual in sensors based on redox indicators, but the slope of the change in the reflected power

Fig. 9.20 Contact angle measurements for S1 (structure with a sensitive coating deposited) and S2 (structure with a hydrophilic coating deposited before the sensitive one) devices. Reprinted from [27]


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produced by different concentrations of H2O2 presents a low dependence on the pH in the range between 6 and 7.4. The range of pH values where the sensor detects hydrogen peroxide is located between 4 and 7.4. In addition to this, the sensor is immune to interferences of a large number of products like chloride, bromide, iodide, oxalate, thiocyanate, tartrate, urea, sulfate, phosphate and glucose except for ascorbic acid, which is the reductive agent of the sensor.

9.4.5 Biological Recognition An approach presented in [64] is based on the immobilization by LbL of singlestranded capture DNA on the surface of a silica optical fiber tip. When there is a presence of the target (complementary) DNA sequence in the tested sample, the optical thickness of the fiber probe increases. This is mainly due to the change in density of the DNA monolayer. In this study, the change in optical thickness is measured by a multicavity Fabry–Perot interferometer formed by a short piece of hollow fiber sandwiched between two pieces of optical fiber. The interference of light reflected from the three fiber endfaces results in periodic oscillations in the reflection spectrum. This method can be very appealing because in contrast to other direct DNA methods, this does not require labels as indicators [64]. Based on the same sensing scheme of the multicavity Fabry–Perot, an immunosensor was proposed based on the immobilization of proteins such as immunoglobulin G (IgG) [65]. A similar approach that enhances the response of the device above described combines the utilization of microgaps in optical fiber and thin films. The microgaps are fabricated by splicing an etched fiber (the chemical etching can be achieved using hydrofluoric acid) with a cleaved end of another fiber. Then, a microgap is generated inside the fiber. After a short length of fiber (some tens of microns) the process is repeated and low-finesse FP cavities are formed between the microgap reflectors and the cleaved fiber end. This structure is a multicavity sensor which uses a cavity as temperature sensor and the other cavity for measuring the changes in thickness of the thin film. A temperature resolution of 0.18C has been reported with the microgap sensor. In this way it can be used for measuring the changes in thickness of a nanostructured LbL film deposited at the end of the fiber with an error of less than 0.2 nm from 0 to 1008C [66]. A device which takes advantage of the shift observed in the resonance peak of a surface plasmon resonance (SPR) sensor due to changes in refractive index of external medium is reported in [67]. In the device, the deposition of thin films or particles of silver or gold (usually less than 50 nm thick) combined with fluorescent anti-immunoglobulin G (IgG) antibody for the detection of IgG is necessary. A similar approach is proposed in [68] for the detection of nitric oxide using extracellular enzymes like lignin peroxidase. Very recently the detection of anti-gliadin antibodies has also been reported by means of the deposition of gliadin LbL films onto biconically tapered optical

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fibers (Fig. 9.12); this device could be very useful as a diagnostic test for the detection of celiac disease [69].

9.5 Conclusions This chapter summarizes the applications of the layer-by-layer electrostatic selfassembly method for the fabrication of optical fiber sensors using novel sensing architectures. The suitability of this procedure to complex geometries makes this technique an optimum tool for the deposition of nanostructured sensing coatings on optical fiber devices. Humidity, temperature, oxygen, VOCs, pH, H2O2, copper or glucose optical fiber sensors have been fabricated by means of the LbL method. The possibility of incorporating proteins, enzymes or antibodies as well makes this technique especially useful for the fabrication of biosensors for biological recognition. Acknowledgments This work was funded in part by the Spanish Ministry of Education and Science-FEDER TEC2006-12170/MIC Research Grant and Government of Navarre-FEDER Euroinnova Research Grants.

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Chapter 10

Nanostructured Flexible Materials: Metal RubberTM Strain Sensors Christelle Jullian, Jennifer Lalli, Bradley Davis, and Richard Claus

10.1 Introduction Strain sensors are fundamental building blocks in measurement of materials and structures. Conventional foil strain gages are based on macroscopic principles of bulk material deformation due to stress, and changes in the electrical resistance of deformed bulk metal geometries. Nanostructured strain sensors that operate based on very different physical principles may be envisioned. This chapter discusses such nanostructured strain sensor devices based on self-assembled Metal RubberTM materials. The first part of the chapter reviews the background on self-assembly processing. The second part of the chapter discusses Metal RubberTM manufacturing and Metal RubberTM strain sensor operation.

10.2 Molecular-Level Self-Assembly Processing: Long-Range Ordered Langmuir–Blodgett (LB) Films and Self-Assembled Monolayers (SAMs) More than eight decades ago, Langmuir published the first theory supporting that a monomolecular film can be created and self-assembled on solid surfaces [1]. However, it was only in the 1980s that the field of self-assembly started to grow exponentially, partly motivated by the need of materials and devices miniaturized and controlled at the molecular scale. Multiple monolayers of long-chain amphiphilic oil molecules were first realized by Langmuir and Blodgett based on the following theory. A sufficiently small amount of olive oil molecules dropped on clean water should maximize spreading resulting in one monolayer of oil at the surface of water [1]. Calcium ions from talc were used as a witness of the oil layer. By measuring the area of the oil layer, and knowing the volume of oil that was dropped, the layer was C. Jullian Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061, USA

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estimated to be one molecule thick. Furthermore, the orientation and packing of the oil molecules was demonstrated. The carboxylic acid end (COO) was found to be bound to calcium ions in the water, while the hydrocarbon end (CH3) was facing up toward the air. The spreading of oil was hypothesized to be due to the presence of ‘‘active groups’’ in the oil molecule that combined with water via ‘‘secondary valence’’. Later, the transfer of a calcium stearate ((C17H35COO)2Ca) monolayer from the air/water interface to a glass substrate was demonstrated with conservation of the order and packing that existed on the water [2]. The clean hydrophilic glass was placed in the water subphase. When rose through the floating molecules, the first layer was applied. The possibility of depositing successive monolayers was then demonstrated. Following the first monolayer applied during an upward motion of the substrate, alternating layers oriented in opposite directions were successively applied as the substrate was moving downward and upward, creating a Langmuir–Blodgett (LB) film. The film’s alternating structure was deduced from the change in the wettability of the film’s outermost layer, given by the measurement of the surface’s contact angle at different stages of the process. Surface wettability is readily noticeable as the substrate emerges dry when the hydrophobic moieties are the outer moieties, whereas it emerges wet when the hydrophilic moieties are the outer moieties. The LB deposition method has given rise to self-assembled monolayers (SAMs), which are highly ordered and crystalline thin films. SAMs are very often used to mimic lipid biomembranes, but have found some applications in molecular electronics and biotechnology devices such as protein-based devices and sensors of interest here.

10.2.1 Surfactants and Floating Monolayers Approximately 50 years separated the first transferred LB film to the extensive use of this technique for very different applications in various research fields. Surfactant molecules are also known as amphiphiles because they are usually comprised of two moieties, one hydrophobic and one hydrophilic [3]. While hydrocarbon or fluorocarbon chains constitute hydrophobic segments, charged or polar groups (NH3+, PO4, COOH, OH) and even small oligomers such as hexa(ethylene glycol) are common hydrophilic moieties. Thus, by definition, surfactants are specifically comprised of an active hydrophilic head and a hydrophobic tail. The head undergoes chemisorption via the creation of a chemical bond with a liquid or solid interface, which may involve electrostatic and van der Waals interactions. The tail forms intermolecular van der Waals interactions, with neighboring tails helping the packing and ordering of the floating surfactant. Because van der Waals interactions are short-range interactions, high packing of the molecules present at the surface and thus long-range molecular order, can only be achieved if the molecules are close enough to allow molecular tails

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interactions. This is characterized by the surface pressure, introduced in the next paragraph. Depending on the polarity of the tail, long-range electrostatic interactions may also arise between neighboring tails. Finally, the third part of amphiphile molecules is the end group. It is usually a methyl (CH3) or hydroxyl (OH) group. The physicochemical properties of amphiphiles determine their orientation and architecture at an interface or in aqueous solutions [3]. Shorter hydrophobic tails may result in more soluble amphiphiles, which may associate as micelles in aqueous solution as opposed to longer hydrophobic tails which could give rise to floating monolayers.

10.2.2 Surface Pressure One of the critical parameters in building stable LB films is the bath surface pressure which is measured as a function of the water area per molecule at a given temperature [4]. Thus, a plot of surface pressure represents various phase transitions between solid, liquid, and gaseous states of the floating monolayer. A sufficiently high surface pressure means that the floating monolayer is cohesive and highly crystalline. This highly ordered monolayer may then be transferred to a substrate conserving the same order, provided the surface pressure remains constant. In some cases molecular reorientation occurs during monolayer transfer to a substrate. Constant surface pressure can be achieved by physical motion of the bath barrier during transfer of the ordered floating monolayer to the substrate. Variable surface pressure translates into variable amphiphile orientations at the liquid interface as shown in Fig. 10.1. As a result, film defects such as boundaries between solid, liquid, and gas phases, boundaries between crystalline and collapsed areas, and pinholes generally result from lower surface pressures [4]. This translates into thickness and refractive index nonuniformity, as well as rigid, amorphous, and unstable structures. As mentioned previously, this ‘‘ideal’’ surface pressure depends on the chemical and physical structures of the amphiphile molecules and is often obtained empirically. Water is a highly ordered liquid via hydrogen bonds and thus has a fairly high surface tension (73 mN/m at 208C) [4]. Surfactants usually lower the water

Fig. 10.1 Illustration of a Langmuir isotherm showing the crystalline, liquid, and gas phases, with their corresponding orientations at the liquid interface

Surface pressure

solid

••••••••

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•• • •••• •

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•

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•

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surface tension. The surface pressure is simply the difference between , the surface tension of pure water, and 0, the water surface tension with the floating monolayer. is given as Y

¼ 0

Controlling the surface pressure of the floating monolayer allows control of the state of the monolayer at the water interface. Increased surface pressure corresponds to increased packing, order, and crystallinity of the floating monolayer. As a result, as the surface pressure increases, the number of molecules per unit water surface increases, and the monolayer state at the interface may change from gas-, to liquid-, to solid-like structures. Disordered floating molecules refer to a large water surface available to floating amphiphiles. By decreasing the water area, the molecules are physically moved closer together until crystalline-like order is achieve d.

10.2.3 Monolayer Transfer on Solid Surfaces The deposition of a monolayer on a substrate is obtained by immersing the substrate down through the monolayer or/and pulled up through the monolayer [4]. When a hydrophobic substrate is immersed in the liquid phase through the monolayer on the liquid interface, the hydrophobic tails of the monolayer spontaneously adsorb on the substrate. Conversely, when a hydrophilic substrate is pulled up through the monolayer on the liquid interface, the hydrophilic heads of the monolayer adsorb on the hydrophilic substrate. Depending on the sequence chosen, various multilayer structures can be achieved [3, 4]. A head-to-tail or X-type structure is created by depositing a first monolayer where the tails are adsorbed on the substrate and the heads are sticking out, and subsequently depositing a second monolayer where the tails adsorb on the previously deposited monolayer heads. Varying the substrate hydrophilicity/ hydrophobicity and the dipping up/down sequence, tail-to-head, head-to-head, or tail-to-tail structures may also be created. The design of the LB film deposition apparatus (troughs) has been significantly improved over the years. Automated systems improving deposition speed and film quality and reducing vibrations with shock absorbers are in wide use today. A wide range of temperatures for the water bath is available. Troughs comprised of two water baths and two rotating arms are now common, decreasing bath contamination and allowing multi-functional film fabrication. Deionized and bacteria-filtered water is used to ensure maximum purity. These have led to improved structural quality and crystallinity of LB films. The deposition of monolayers on a substrate via self-assembly can therefore create long-range ordered structures. Although the nature of both the substrate and the amphiphilic molecules may differ, this particular adsorption mechanism is referred to as spontaneous chemisorption via the creation of a chemical


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bond between surface-active amphiphile heads and a surface-active substrate. The most common chemical bonds include covalent, ionic, dipolar, and hydrogen bonds. The LB plate substrate is usually made of glass, quartz, silicon, platinum, or mica. These allow for self-assembly of alcohols, carboxylic and fatty acids, phospholipids, and amines via van der Waals or ionic interactions, as well as self-assembly of hydrophobic tails on hydrophobic substrates via hydrophobic interactions. The water pH, ionic strength, and temperature significantly affect the rate at which the floating monolayer can be transferred to a substrate.

10.2.4 From LB Films to SAMs Although self-assembled monolayers and multi-monolayers were first realized about 85 years ago, the creation of films with specified physical, mechanical, and thermal properties remains a challenge. One strategy has been to use amphiphilic polymers with hydrophilic groups present along the chain as opposed to monomers [4]. This has allowed to expend the range of materials which can be deposited with the LB technique. Reinforcement of the multilayer mechanical and thermal properties has been achieved by taking advantage of cross-linkable groups [4] and introducing additional interactions such as electrostatic interactions between amphiphiles and proteins [5]. The LB process is also limited by the deposition rate, and the substrate size and topology which can be coated. Finally, while the LB technique offers fabrication of tailored molecular architectures, it is restricted to amphiphilic molecules with appropriate physicochemical properties which make them insoluble, i.e., floating. The obtained long-range ordered structures often comport defects. Self-assembled monolayers (SAMs) carry the same high order and crystallinity as LB films do, but are generally processed by immersing a substrate in a dilute solution containing amphiphile molecules. Because the deposition process occurs in solution, single SAM with the amphiphile surfaceactive heads chemisorbed onto the surface-active substrate limits the number of feasible film architectures. Consequently, van der Waals interactions usually control the adsorption process, inducing long-range order. While the LB technique allows for multi-monolayers deposition, a SAM traditionally refers to a single, ultrathin, and crystalline monolayer. In fact, most SAMs are formed from stabilizing alkyl CH2 chains, allowing for crystalline packing of the amphiphiles. This concept allows for some freedom in designing the amphiphile molecule. For instance, Prime and Whitesides have synthesized mixed SAMs comprised of highly crystalline hydrophobic alkyl moieties and highly amorphous hydrophilic oligo(ethylene glycol) moieties [6]. Because adsorption occurs in solution, soluble amphiphiles are used, which opens up new architectural possibilities such as utilizing micelles. Unlike LB films, SAM deposition is controlled by the diffusion of amphiphiles to the substrate which has the potential to provide longer-range order with very

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a) X X

X

b) X

X

X X

X

Si S

S

S

S

S

S

O

X

Si O

O

Au

X

Si O

O

Si O

O O

O

Si

Fig. 10.2 (a) Organothiol SAMs on gold substrate showing the 308 tilt angle with the surface normal and the long-range order, and (b) organosilicon SAMs on hydroxylated silicon substrate. The Xs represent the end groups attached to the tails, which are most of time hydroxyl (OH) or methyl (CH3) groups

few defects. Often, amphiphiles comprising thiol (SH) end-functionalities are used to deposit organothiol monolayers on gold substrates due to high bond stability [7]. The deposition process is known to first occur via chemisorption of the amphiphilic active heads onto the substrate which follows a first-order Langmuir isotherm. There is a linear relationship between the SAM adsorption rate and the number of unoccupied gold atoms. In addition, lateral rearrangement of the SAMs occurs in order to satisfy van der Waals attraction forces between neighboring chains and consequently leads to long-range order. Van der Waals interactions are maximized by a 308 tilt of the alkyl chains (from the gold surface normal) arising from the hexagonal structure of S–Au bonds [8]. Organosilicon SAMs deposited on hydroxylated substrates (OH functionalized) is another well-known and stable system based on Si–O covalent bonds. Organosilicon SAMs are characterized by very strong adhesion to substrates due to the high binding energy of siloxane Si–O–Si bonds, but are not as ordered as organothiol-based SAMs. Figure 10.2. represents organothiol and organosilicon SAM structures.

10.3 Layer by Layer: Toward Shorter-Range Ordered Structures 10.3.1 Electrostatic-Based Self-Assembly In contrast to LB films and SAMs, layer-by-layer (LBL) self-assembly allows for the formation of multilayers that may exhibit short-range order as opposed to long-range order seen with LB films and SAMs. This results from the common deposition of high-molecular-weight flexible polyelectrolytes, which render LBL films more amorphous in nature with a fuzzy internal structure. However, this more amorphous structure has proven to be appropriate for many applications such as organic light-emitting diodes and photovoltaics [8] due to the absence of


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defects which are readily introduced using LB films and SAMs. Two- and threedimensional self-assembled structures have been demonstrated. In 1992, Decher and coworkers introduced this novel technique to deposit polyelectrolyte thin films on charged substrates based on electrostatic attraction [8]. Cationic and anionic polyelectrolytes in solution at relatively high concentrations were thus successively adsorbed on an oppositely charged surface, inducing a reversal of the surface charge. A rinsing step in between deposition of each cationic and anionic layer is necessary in order to remove loosely attached ionic residues and prevent solution contamination. The formation of multilayer assemblies was thus achieved by Decher and coworkers. Up to 100 layers were deposited, and the linearity of the process was demonstrated using UV/Vis spectroscopy and small angle X-ray scattering, techniques still used today. The film thickness was found to increase linearly with the number of bilayers. The LBL technique was in fact first experienced with the alternate deposition of charged inorganic colloids by Iler in 1966 [9]. Later, magnetic particles [10], polyelectrolyte polymers [8], functionalized fullerenes [11], charged DNA [12], proteins [13], gold nanoparticles [14, 15], and clay platelets [16] were selfassembled on substrates based on electrostatic interactions. The LBL deposition technique is therefore appropriate for various types of nanoparticles in addition to polyelectrolytes, which makes it considerably more versatile than LB films and SAMs. Additionally, the adsorption of higher molecular weight molecules makes the LBL technique appropriate for building supramolecular self-assembled systems. Kunitake demonstrated the fabrication of ordered multilayer free-standing films, which opened up new perspectives [17]. Casting of dispersed bilayers and polymers was found to conserve the multilayerarchitecture, and therefore, ordered free-standing composite films were demonstrated. The tailoring of molecular design and interactions translated the multilayer molecular order into macroscopically ordered materials. Multilayer composite cast films made from dispersions of amphiphiles, polymers, proteins, quantum-sized metallic particles (CdS), and metal oxide (Al2O3) have been fabricated [17]. LBL films can be highly uniform due to the conformal nature of the LBL electrostatic self-assembly process. The driving force is the electrostatic attraction between oppositely charged or polar molecules on a given substrate. Adsorption of charged or polar molecules on a surface induces a change in free energy at the interface. This change in free energy is believed to be associated with the desolvated substrate, the displacement of counterions at the interface, and the electrostatic attraction between the substrate and the charged adsorbent [18]. Indeed, the formation of LBL self-assembled architectures represents a thermodynamic equilibrium (minimum free energy) that is mainly governed by a change in entropy and a relatively small change in enthalpy. The change in enthalpy depends on the interactions between adjacent bilayer molecules and is supposed to be relatively small compared with covalent bond formation for example. The change in entropy occurs by bringing molecules close to each other, which were originally somewhat far apart. This depends on

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the solute concentration and creates a decrease in translational entropy, while binding these molecules to the substrate induces a small decrease in conformational entropy. Note that the change in entropy is generally smaller for the LB technique than for the LBL process, because the LB technique is only affected by a relatively small decrease in conformational entropy since the molecules are already ordered at the liquid–gas interface. This entropy loss is outweighed by a large entropy gain due to the liberation of solvent and counterions from the interface, which drives the adsorption. Because of the change in entropy, kinetic controls the adsorption of LBL molecular layers.

10.3.2 Factors Influencing Adsorption and Structure As in the LB process, the solution pH, temperature, and concentration are known to affect the LBL film structure and deposition rate. This is due to the fact that LBL film deposition is controlled kinetically due to a great change in entropy during molecule adsorption. The influence of adsorption time and ionic strength of the solutions on the film growth and thermal properties has been studied [19]. Polyelectrolyte multilayers were found to have low roughness compared to bare glass, and increasing salt concentration or ionic strength led to thicker layers. Increased salt concentration results in increased electrostatic shielding between polyelectrolytes in solution. This implies that polyelectrolytes in high-ionic-strength solvents form denser structures with loops as opposed to more loose structures when no or less salt is added [18]. In turn, polyelectrolytes adsorb on the surface conserving the same dense structure which leads to more polyelectrolyte mass per unit area, i.e., thicker layers. Increased ionic strength also shields the attractive forces that exist between oppositely charged surface and polyelectrolyte molecules. Layer thickness and roughness have been found to increase nonlinearly with salt concentration. This means that there should be a critical salt concentration that can be added without causing too many defects or too much roughness. Additionally, polyelectrolytes often contain weak ionic groups such as amine and carboxyl groups whose charges strongly depend on the aqueous solution pH, i.e., it depends on whether the pH is above or below the polyelectrolyte isoelectric point. At a pH close to the isoelectric point, the polyelectrolyte will be uncharged and will adsorb as a relatively thick layer due to water becoming a poorer solvent and due to formation of loops. However, at a pH either well above or below the isoelectric point, the polyelectrolyte will be fully charged, and will adsorb as a thinner layer due to stronger interactions between the surface and the corresponding polyelectrolyte. Polyelectrolyte concentrations also affect the deposited layer structure. Higher concentrations may result in larger surface charge reversal, adsorption time reduction, and thicker layer due to chain contraction and loop formation. Drying steps in between layer deposition may also affect layer thickness due to swelling decrease, especially for hygroscopic molecules.


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As demonstrated by Schmitt and others later, the internal structure of these polyelectrolyte films has often been studied with X-ray and neutron reflectivity and was found to be characterized by a fuzzy internal structure [20]. Although these polyelectrolyte films have a well-defined supramolecular internal structure; bilayer interpenetration is known to occur. Schmitt reported that about 50% of a given layer was in fact overlapping with neighboring layers, the layers close to the substrate being thinner than those closer to the film/air interface. Anionic and cationic layers have been found to have different thicknesses, again depending on electrostatic interaction shielding. Inorganic counterions, such as Na+ and Cl, from the solutions may also be adsorbed into the multilayers.

10.3.3 LBL Advantages As an alternative to LB films, the LBL process is simpler because the adsorption occurs spontaneously in solution and is kinetically controlled. Therefore, adsorption rates and film buildup are generally faster for the LBL process, usually on the order of several minutes per layer, although it has been shown to be on the order of hours for gold colloids [8]. Generally, LBL involves stronger intermolecular interactions such as long-range electrostatic interactions, leading to stable films and defect-free when amorphous polyelectrolyte layers are used. There is no restriction on substrate size and geometry. The increased range of materials that may be deposited using the LBL versus the LB method is appropriate for multiple applications ranging from electronic to photonic and biomedical devices. For instance, the use of proteins and DNA allowed the fabrication of coatings and devices useful for biomedical applications. The use of conducting polymers and magnetic nanoparticles shows promises for electronic and photonic devices. While LB films are mostly used as an interface in devices and to mimic biological membranes, LBL multilayers have had impact in various fields of research.

10.4 Metal RubberTM Manufacturing Molecular-level self-assembly processing can be used to form thin multilayer, multi-constituent coatings on the surfaces of a variety of substrate materials. For use as sensors, however, the mechanical, thermal, chemical, and electrical properties of the substrate may interfere with sensor transduction coefficient, minimum detectable signal, dynamic range, noise, and interference. Free-standing rather than rigidly attached materials may be self-assembled using similar processes and offer improved performance for sensing applications. Free-standing selfassembled materials may be formed by first coating a substrate surface with a chemical release layer and then alternately self-assembling anionic and cationic molecules until the desired thickness is obtained. The release layer may be etched away using an appropriate solvent, and a free-standing piece of material results.

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Important to this manufacturing process are (1) the selection of release layer, (2) the time required to fabricate a coating of desired thickness, and (3) the resulting properties of the material formed. The release layer material must be capable of being etched using a solvent other than the one used during the selfassembly process itself. For example, release materials that can be etched in concentrated acids but not water would be appropriate for aqueous solutionbased self-assembly. The release layer needs to be applied uniformly over the substrate, although the choice of the substrate material is relatively unimportant. A low-cost material with good surface uniformity and the ability to withstand the chemical etchant used to dissolve the release layer is usually chosen. The substrate material itself may be discarded after manufacturing, or cleaned and re-used multiple times. The immersion time required for each individual monolayer and thus for the entire Metal RubberTM material is also important. This is because a sufficient number of bilayers need to be deposited in order to make a thick enough piece of material so that it is mechanically robust. Long immersion times per deposited bilayer mean that the total manufacturing time may be as long as several days. Typically, materials several hundred microns thick are formed, and production times on the order of one full day are used. The deposition time for a single monolayer obeys a normal S-curve. During the first part of the deposition, initiation of molecular coverage of the surface occurs, and the effective rate of molecules deposited per unit time is low. During the middle part of the deposition, the number of molecules deposited per unit time increases to a maximum rate. This rate decreases as saturation is achieved, and the number of additional molecules deposited to the layer is small. The key to reducing the width of this S-curve, and thus minimizing the amount of time required for the formation of a single monolayer, is to reduce the time needed for surface coverage initiation. This is a function of the molecules to be deposited, the property of the substrate, solution concentration, and others. A reduction of deposition to less than 1 min is required in order to synthesize practical materials in a reasonable time period. Another important aspect of Metal RubberTM material production is the use ofan automated manufacturing system rather than the hand dipping of substrates. Automated substrate dipping allows exact control over substrate submersion times, precise counting of the number of bilayers deposited, and continuous operation for longer than a normal work day.

10.5 Metal RubberTM Material Properties The combined mechanical, electrical, and other constitutive properties of Metal RubberTM fabricated by the above process are different from those of other materials. For sensor applications, one combination of properties that is interesting is mechanical modulus and electrical conductivity. This combination is


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achieved by alternately self-assembling monolayers of metal nanoclusters with polymers. The mechanical modulus of the Metal RubberTM material may be controlled by varying the polymer molecules used to form every other monolayer. Since any polymer molecule that can be made water soluble, or soluble in a solvent other than water, can be self-assembled by this process, the possible range of achievable modulus properties is large. In general, modulus is controlled by the nature of the particular polymer and its molecular weight. Modulus values from approximately 0.1 MPa to 1 GPa have been obtained using different polymers as the base for Metal RubberTM, and different associated processing conditions. Electrical conductivity is achieved by incorporating electrically conducting metal nanoclusters and polymers as the opposite monolayers in each bilayer of Metal RubberTM material. Because the metal nanoclusters do not form the entire material but only fraction of it, conductivity does not occur in Metal RubberTM the same way that it does in solid metal conductors. Conductivity in solid metals is usually modeled as the transport of negatively charged electrons through a lattice of positively charged atomic nuclei. The nuclei are located at lattice points that are regularly spaced in three dimensions according to the specific crystal structure of the metal. A potential difference or voltage V applied between two points separated by a distance d in such a metal produces an electric field E, with an amplitude equal to V/d, and a vector direction oriented between the points where the potential is applied. The free electrons experience a force F with a magnitude given by F = qE, where q is the charge on the electron and E is the magnitude of the electric field, and move and accelerate in response to the force. The electrons accelerate through the lattice, collide with the fixed lattice nuclei, and are scattered from their original trajectory. Once scattered, they are again accelerated by the field, and again collide. The average electron speed in the direction of the field is defined as the drift velocity. As the temperature of the metal increases, the vibrational energy of the lattice increases, the fixed lattice points present larger physical targets to the moving electrons, and the drift velocity decreases. Since electrical current is proportional to the product of charge and velocity, this means that at higher temperature, current through the material decreases. Electrical resistance is proportional to the voltage V divided by the current, so as temperature increases, the resistance of normal metals increases. The mechanism for electrical conductivity of Metal RubberTM is very different from that for bulk solid metals. Metal RubberTM can be modeled as a polymer matrix that contains a certain volume percentage of electrically conducting inclusions. Conventional polymer blends – different from Metal RubberTM but similar in constituents – that contain conducting metal particles become conductive through their bulk when the volume percentage of such clusters in the material is increased above the percolation limit. At percolation, there are just enough particles to form a continuous interconnected path through the material and allow electron transport from one side of the material to the other. The onset of percolation in such materials is different for

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conducting inclusions of different shapes. For spherical inclusions, typically tens of volume percent of filler is required. Conductivity in Metal RubberTM is conceptually similar to this behavior with several important differences. First, the volume percentage of metal nanoclusters required to achieve conductivity percolation has been observed to be less than 0.01%, or more than three orders of magnitude smaller than that needed in particle-filled polymer blends. Second, the approximately 10 nm diameter of the nanoclusters incorporated into Metal RubberTM is smaller than the particle size in most particle-filled systems. This may lead to conductivity effects that are controlled by the quantum confinement properties of the clusters and not the bulk transport properties of micron-sized particles. The size uniformity of the particles may also have an influence; the metal nanoclusters used to form Metal RubberTM are approximately monodisperse rather than randomly sized. Third, the spatial uniformity of materials fabricated by the ESA deposition process suggests that percolation may occur for a small increase in the local volume percentage of nanoclusters. Conductivity in Metal RubberTM may be modeled as being due to electron hopping or quantum mechanical tunneling between adjacent nanoclusters rather than electron transport between larger metal particles at positions where they physically touch. This suggests that the uniformity of (1) the molecular weight of the polymers, (2) the dimensions and shapes of the nanoclusters, and (3) the thicknesses of each of the monolayers in the total nanocomposite material is important to electron transfer characteristics.

10.6 Metal RubberTM Strain Sensors Metal RubberTM materials having the combined elastomeric and electrical conductivity properties described above can be used as strain sensors. The function of the sensors is to respond to mechanical force and produce a change in electrical resistance which can be measured using conventional instrumentation methods. The transfer function, F, of Metal RubberTM as a sensor element can be expressed as F¼

@R @e @R ¼ ; @e @ @

or the change in the electrical resistance of the sensor element that occurs as a function of applied stress. There are two parts to this function, namely the change in resistance that occurs due to strain or elongation of the material, and the change in strain or displacement of the material that occurs due to stress. The latter quantity is the slope of the strain/stress curve, or the inverse of the elastic modulus of the material, 1/E, in units of 1/Pa, or m2/N. A low modulus for Metal RubberTM sensor materials is desirable because proportional decreases in modulus produce equally proportional increases in sensor sensitivity. Thus,


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Metal RubberTM materials with modulus on the order of 0.1 MPa should have a sensitivity to strain that is many orders of magnitude larger than similar materials with a modulus of 1 GPa, assuming that all other material properties are identical. The first term in the transfer function expression is related to how the individual nanoclusters physically separate within the material due to strain and how that separation affects the hopping or tunneling of electrons between all of the nanoclusters and the resulting resistance of the material. This is complicated due to the multilayered self-assembly geometry of the Metal RubberTM material and Poisson effects that couple multiple geometrical displacement effects in the material. The layer-by-layer geometry of Metal RubberTM inherently leads to anisotropic properties. Without special design, the electrical conductivity of the material is higher in the two-dimensional plane than through the thickness. This suggests that the uniformity of nanocluster separation in the plane of each of the deposited monolayers is perhaps more regular than the mean separation distance between clusters deposited in alternating cluster monolayers. When strained axially in a direction in the two-dimensional plane of the sheet material, the nanoclusters separate and electrical resistance increases. Figure 10.3 shows a typical result of a piece of Metal RubberTM material being strained axially and the measurement of the end-to-end resistance of the material measured as strain is increased. Metal RubberTM strain sensor materials have been strained to more than 1,000% without failure and cycled at 10% strain to more than 100,000 cycles without fatigue failure. ‘‘Metal RubberTM’’ represents a manufacturing technology

Resistance ( Ω) 90

Stress (psi)

500

70

50

0

2

4

6 Strain %

Fig. 10.3 Resistance versus strain response of Metal RubberTM material

8

30 10

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and not a single material having a single set of material properties. Thus, by changing the properties of the Metal RubberTM material, the transduction properties of Metal RubberTM sensors may be affected. Specifically, the hysteresis properties may be reduced by adjusting the viscoelastic properties of the polymer used during LBL production.

10.7 Summary This chapter has summarized self-assembly processes, the use of such processes to produce Metal RubberTM nanocomposite materials, and the use of Metal RubberTM for the sensing of mechanical strain.

References 1. Langmuir, I., The constitution and fundamental properties of solids and liquids. II. Liquids. J. Am. Chem. Soc., 1917. 39: pp. 1848–1906. 2. Langmuir, I., The mechanism of the surface phenomena of flotation. Trans. Faraday Soc., 1920. 15: pp. 62–74. 3. K. S. V. Instruments LTD, Langmuir and Langmuir-Blodgett films, Application Note #117. 4. Hann, R. A., Molecular Structure Monolayer Properties, in Langmuir-Blodgett Films, ed. G. Roberts, 1990, Plenum Press. 5. Hamachi, I.; Noda, S.; Kunitake, T., Layered arrangement of oriented myoglobins in cast films of a phosphate bilayer membrane. J. Am. Chem. Soc., 1990. 112: pp. 6744–6745. 6. Prime, K. L.; Whitesides, G. M., Adsorption of proteins onto surfaces containing endattached oligo(ethylene oxide): A model system using self-assembled monolayers. J. Am. Chem. Soc., 1993. 115: pp. 10714–10721. 7. Ventra, M. D.; Evoy, S.; Heflin, J. R. Jr., Introduction to Nanoscale Science and Technology, ed. M. D. Ventra; S. Evoy; J. R. Heflin Jr., 2004, Springer. 8. Decher, G.; Hong, J. D.; Schmitt, J., Buildup of ultrathin multilayer films by self-assembly process: III. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces. Thin Solid Films, 1992. 210–211: pp. 831–835. 9. Iler, R. K., Multilayers of colloidal particles. J. Colloid Inter. Sci , 1966. 21: pp. 569–594. 10. Liu, Y.; Claus, R. O., Layer-by-layer electrostatic self-assembly of nanoscale Fe3O4 particles and polyimide precursor on silicon and silica surfaces. Appl. Phys. Lett., 1997. 71: pp. 2265–2267. 11. Wang, Y.-X.; Du, W.; Spillman, W. B., Jr.; Claus, R. O., Biocompatible thin film coatings fabricated using the electrostatic self-assembly process. Proc. SPIE, 2001: 4265: pp. 142–151. 12. Lvov, Y.; Decher, G.; Sukhorukov, G., Assembly of thin films by means of successive deposition of alternate layers of DNA and poly(allylamine). Macromolecules, 1993. 26: pp. 5396–5399. 13. Keller, S. W.; Kim, H.-N.; Mallouk, T. E., Layer-by-layer assembly of intercalation compounds and heterostructures on surfaces: Toward molecular ‘‘Beaker’’ epitaxy. J. Am. Chem. Soc., 1994. 116: pp. 8817–8818. 14. Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E., Electron transfer in selfassembled inorganic polyelectrolyte/metal nanoparticle heterostructures. J. Am. Chem. Soc., 1996. 118: pp. 7640–7641.


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15. Liu, Y.; Wang, Y.; Y.-X.; Claus, R. O., Layer-by-layer ionic self-assembly of Au colloids into multilayer thin-films with bulk metal conductivity. Chem. Phys. Lett., 1998. 298: pp. 315–319. 16. Eckle, M.; Decher, G., Tuning the performance of layer-by-layer assembled by controlling the position of isolating clay barrier sheets. Nano Lett., 2001. 1: pp. 45–49. 17. Kunitake, T., Anisotropic incorporation of functional molecules and synthesis of lowdimensional clusters in cast multibilayer films. Mol. Cryst. Liq. Cryst., 1994. 255: pp. 7–16. 18. Malmsten, M., Biopolymers at Interfaces, ed. M. Malmsten, 2003, Marcel Dekker, Inc. 19. Lvov, Y.; Decher, G.; Mohwald, H., Assembly, structural characterization, and thermal behavior of layer-by-layer deposited ultrathin films of poly(vinyl sulfate) and poly(allylamine). Langmuir, 1993. 9: pp. 481–486. 20. Schmitt, J.; Grunewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Losche, M., Internal structure of layer-by-layer adsorbed polyelectrolyte films: A neutron and X-ray reflectivity study. Macromolecules, 1993. 26: pp. 7058–7063.

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