81068754 chemical sensors fundamentals of sensing materials vol 3 polymers and other materials

Momentum Press is proud to bring you Chemical Sensors Volume 3, the newest addition to The Sensors Tech-nology Series, edited by Joe Watson. In this third Volume, Polymers and Other Materials, new applications for chemical sensing, using materials developments in polymers, calixarenes, biological and biomimetic systems, novel semiconductors, and ionic conductors, are fully explored. This book will make it clear that chemical sensors based on these materials comprise a large part of the chemical sensors market. Inside, you will find background and guidance on: Anoverviewofpolymersusedinchemicalsensingmaterialsincludingwhattheyare,howtheyaremade,

and how their functionality can be designed and enhanced for sensing applications Molecular-Imprintinganewapproachfordesigningpolymer-basedsensors New developments in calixarene-based materials for chemical sensors including their synthesis,

characterization and properties and use for such things as probes for sensing and extraction, sensing ions and molecules, and use as ditopic molecular receptors

Advancesinbiologicalandbiomimeticsystems,includingpolymer-biomoleculeassemblies,membranes,and nanobiosensors

Newsemiconductingsensingmaterialsusingdiamond,galliumnitrideandIII-nitrides,andnano-scalesemiconducting structures

Auniqueanalysison the limitsof sensingmaterialsandhowtobestchoose therightmaterial foraparticular sensing objective

Chemical sensors are integral to the automation of a myriad industrial processes, as well as everyday moni-toring of such activities as public safety, testing and monitoring, medical therapeutics, and many more. This massive reference work will cover all major categories of both the materials used for chemical sensors and their applications. This is THE reference work on sensors used for chemical detection and analysis.TheChemicalSensorsreferencesbookswillspan6volumesandcover in-depthdetailsonbothmateri-

als used for chemical sensors and their applications, with volumes 1 through 3 exploring the materials used forchemicalsensorstheirproperties,theirbehavior,theircomposition,andeventheirmanufacturingandfabrication.Volumes4 through6willexplore thegreatvarietyofapplications forchemical sensorsfrommanufacturing and industry to biomedical uses.

AbouT The ediTorGhenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and Devices in 1976 and his Habilitate Degree (Dr. Sci.) in Physics and Mathematics of Semiconductors and Dielectrics in 1990. He was for many years the leader in the Gas Sensor Group at the Technical University of Moldova. He is currently a research professor at Gwangju Institute of Science and Technology, in Gwangju, RepublicofKorea.Dr.Korotcenkov is theauthoroffivepreviousbooksandhasauthoredover180peer-reviewedpapers.Hisresearchhasreceivednumerousawardsandhonors,includingtheAwardoftheSupremeCouncilofScienceandAdvancedTechnologyoftheRepublicofMoldova.

ISBN: 978-1-60650-230-3

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CHEMICAL SENSORSFUNDAMENTALS OF SENSING MATERIALSVOLUME 3: POLYMERS AND OTHER MATERIALS

CHEMICAL SENSORSFUNDAMENTALS OF SENSING MATERIALSVOLUME 3: POLYMERS AND OTHER MATERIALS

EDITED BYGHENADII KOROTCENKOV

GWANGJU INSTITUTE OF SCIENCE AND TECHNOLOGYGWANGJU, REPUBLIC OF KOREA

MOMENTUM PRESS, LLC, NEW YORK

Chemical Sensors: Fundamentals of Sensing Materials. Volume 3: Polymers and Other MaterialsCopyright Momentum Press, LLC, 2010

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any meanselectronic, mechanical, photocopy, recording or any otherexcept for brief quotations, not to exceed 400 words, without the prior permission of the publisher.

First published in 2010 byMomentum Press, LLC222 East 46th Street, New York, NY 10017www.momentumpress.net

ISBN-13: 978-1-60650-230-3 (hard back, case bound)ISBN-10: 1-60650-2309-1 (hard back, case bound)ISBN-13: 978-1-60650-232-7 (e-book)ISBN-10: 1-60650-232-8 (e-book)DOI forthcoming

Cover design by Jonathan PennellInterior design by Derryfi eld Publishing, LLC

First Edition: December 2010

10 9 8 7 6 5 4 3 2 1

Printed in the United States of America

vCONTENTS

PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS xi

PREFACE TO VOLUME 3: POLYMERS AND OTHER MATERIALS xiii

ABOUT THE EDITOR xv

CONTRIBUTORS xvii

1 POLYMERS IN CHEMICAL SENSORS 1B. Adhikari P. Kar

1 Introduction 12 What Are Polymers? 33 Parameters of Polymers Promising for Chemical Sensor Application 44 Synthesis of Polymers 75 Deposition of Polymers 96 Functionalization of Polymers 13

6.1 Structure Modifi cation 136.2 Surface Modifi cation 146.3 Composition Modifi cation 15

7 Polymers in Chemical Sensors 167.1 Optical and Fiber Optic Polymer-Based Sensors 207.2 Conductometric Gas Sensors 277.3 SAW and QCM Polymer-Based Sensors 407.4 Electrochemical Polymer-Based Sensors 457.5 Chemically Sensitive FET-Based Sensors 57

8 Outlook 619 Acknowledgments 61References 62

vi CONTENTS

2 MOLECULAR IMPRINTING (TEMPLATING)A PROMISING APPROACH FOR DESIGN OF POLYMER-BASED CHEMICAL SENSORS 77

G. Korotcenkov B. K. Cho

1 Introduction 772 General Principles of Molecular Imprinting (Templating) 793 Methods of Imprinting (Templating) 80

3.1 In-Block Imprinted Polymers 803.2 In Situ Imprinted Polymers 823.3 Polymer-Imprinted Beads 82

4 Components of Imprinting Technology 834.1 Target Molecules 834.2 Th e Imprinting Matrix 854.3 Cross-Linkers 864.4 Solvents (Porogens) 884.5 Initiators 89

5 MIP Preparation Methods 896 Combination of MIPs and Monomolecular Host Molecules 917 Control of the Imprinting Eff ect 928 Application of Imprinting Polymers in Chemical Sensors 92

8.1 Advantages of MIP-Based Chemical Sensors 928.2 Detection Principles Used in MIP Chemical Sensors 938.3 Interfacing the MIP with the Transducer 998.4 Factors Controlling the Sensing Characteristics of MIPs-Based Chemical Sensors 1018.5 Micro- and Nanofabricated MIPs 103

9 Outlook 10610 Acknowledgments 108References 109

3 CALIXARENE-BASED MATERIALS FOR CHEMICAL SENSORS 117H. M. ChawlaN. PantS. KumarD. StC. BlackN. Kumar

1 Introduction 117

CONTENTS vii

2 Molecular Receptors and Generation of Signal for Sensing Target Species 1193 Calixarenes and Th iacalixarenes 1204 Synthesis of Calix[n]arenes 123

4.1 Base-Catalyzed Condensation Reactions 1234.2 Acid-Catalyzed Condensation Reactions 124

5 Synthesis of Th iacalix[n]arenes (Sulfur-Bridged Calixarenes) 1246 Physical Properties of Calixarenes and Tetrathiacalixarenes 127

6.1 Melting points 1276.2 Solubilities and pKa Values 127

7 Spectral Properties and Characterization of Calixarenes 1287.1 Infrared Spectra 1287.2 Ultraviolet Spectra 1287.3 NMR Spectra 129

8 Conformational Structures of Calixarenes and Th iacalixarenes 1299 Conformational Characterization of Calix[n]arenes 13210 Calixarenes as Materials for Chemical Sensors 13211 Calixarene-Based Materials for Recognition of Alkali and Alkaline Earth

Metal Ions 13312 Calixarene-Based Materials for Recognition of Transition and Heavy-

Metal Ions 13713 Calixarene-Based Materials as Dual Probes for Sensing and Extraction 13914 Calixarene-Based Materials for Sensing Lanthanides and Actinides 13915 Sensor Materials Based on Polymeric Calixarenes 14316 Naked-Eye Sensing: Calixarene-Based Chromogenic Materials for Sensing

Ions and Molecules 14317 Calixarene-Based Electroactive Sensing Materials 15618 Calixarene-Based Materials for Sensing Anions 161

18.1 Calixarene-Based Electron-Defi cient or Positively Charged Anion Receptors 16118.2 Calixarene-Based Neutral Anion Receptors 16518.3 Calixarene-Based Ditopic Molecular Receptors 175

19 Calixarene-Based Sensor Materials for Neutral Molecules and Biological Amines 176

20 Calixarene-Based Materials for Gas Sensors 17921 Outlook 181

viii CONTENTS

22 Acknowledgments 181References 182

4 BIOLOGICAL AND BIOMIMETIC SYSTEMS IN CHEMICAL SENSORS 201R. JelinekS. Kolusheva

1 Introduction 2012 Polymers and Polymer/Biomolecule Assemblies 202

2.1 Conductive Polymers 2022.2 Luminescent Conjugated Polymers 205

3 Membranes in Chemical Sensors 2103.1 Chemical Membranes 2103.2 Biological Membranes 213

4 Biomimetic Systems for Molecular and Ionic Recognition 2194.1 Biological Receptors and Channels 2194.2 Synthetic Receptors 2214.3 Biomimetic Enzyme-Based Sensors 2234.4 Nanobiosensors 2254.5 Other Biomimetic Sensors 232

5 Monolayers and Films 2425.1 Self-Assembled Monolayers 2425.2 Langmuir-Blodgett Films 244

6 Challenges and Limitations of Biosensors 2477 Conclusions and Outlook 248References 248

5 NOVEL SEMICONDUCTOR MATERIALS FOR THE DEVELOPMENT OF CHEMICAL SENSORS 263N. Chaniotakis N. Sofi kiti V. Vamvakaki

1 Introduction 2632 Th e Silicon EraClassical Semiconductors in Chemical Sensing 2653 Fundamentals of Sensor Development 2694 Surface Chemistry of Semiconductors in Chemical Sensing 2695 Band Gap Th eory and Its Relationship to Sensor Design 2716 Pinning of the Surface Fermi Level 272

CONTENTS ix

7 New Semiconductor Substrates 2737.1 Diamond 2747.2 Silicon Carbide 2777.3 Gallium Nitride and III-Nitrides 279

8 Nanosemiconductor Structures in Chemical Sensors 2819 Forecasting the Future 283References 284

6 ION CONDUCTORS AND THEIR APPLICATIONS IN CHEMICAL SENSORS 291R. V. KumarC. Schwandt

1 Introduction 2911.1 Solid Electrolytes 2921.2 Chemical Sensors 293

2 Ionic Conduction in Solids 2943 Oxygen IonConducting Solid Electrolytes 296

3.1 Zirconia-Based Solid Electrolytes 2973.2 Defect Chemistry of Stabilized Zirconia 3043.3 Preparation of Stabilized Zirconia 3053.4 Oxygen Sensors Based on Stabilized Zirconia 308

4 Proton-Conducting Solid Electrolytes 3194.1 High-Temperature Proton-Conducting Solid Electrolytes 3194.2 Defect Chemistry of Substituted Perovskites 3204.3 Preparation of Substituted Perovskites 3234.4 Hydrogen Sensors Based on Substituted Perovskites 3244.5 Low-Temperature Proton-Conducting Solid Electrolytes 330

5 Metal IonConducting Solid Electrolytes 3325.1 Defect Chemistry and Preparation of -Aluminas 3325.2 Sensors Based on -Aluminas 337

6 Outlook and Future Trends 343References 344

7 SENSOR MATERIALS: SELECTION GUIDE 351G. Korotcenkov

1 Acceptable Materials for Chemical Sensors 3512 Which Metal Oxides Are Better for Gas Sensors? 358

x CONTENTS

3 Choosing a Polymer for a Chemical Sensor Application 3624 Technological Limitations in Sensing Material Applications 3625 Future Trends 3636 Toward a Th eory of Chemical Sensors 3687 Summary 3708 Acknowledgments 370References 370

INDEX 375

xi

PREFACE TO CHEMICAL SENSORS:

FUNDAMENTALS OF SENSING MATERIALS

Sensing materials play a key role in the successful implementation of chemical and biological sen-sors. Th e multidimensional nature of the interactions between function and composition, preparation method, and end-use conditions of sensing materials often makes their rational design for real-world applications very challenging.

Th e world of sensing materials is very broad. Practically all well-known materials could be used for the elaboration of chemical sensors. Th erefore, in this series we have tried to include the widest pos-sible number of materials for these purposes and to evaluate their real advantages and shortcomings. Our main idea was to create a really useful encyclopedia or handbook of chemical sensing materials, which could combine in compact editions the basic principles of chemical sensing, the main properties of sensing materials, the particulars of their synthesis and deposition, and their present or potential ap-plications in chemical sensors. Th us, most of the materials used in chemical sensors are considered in the various chapters of these volumes.

It is necessary to note that, notwithstanding the wide interest and use of chemical sensors, at the time the idea to develop these volumes was conceived, there was no recent comprehensive review or any general summing up of the fundamentals of sensing materials Th e majority of books published in the fi eld of chemical sensors were dedicated mainly to analysis of particular types of devices. Th is three-volume review series is therefore timely.

Th is series, Chemical Sensors: Fundamentals of Sensing Materials, off ers the most recent advances in all key aspects of development and applications of various materials for design of chemical sensors. Regarding the division of this series into three parts, our choice was to devote the fi rst volume to the fundamentals of chemical sensing materials and processes and to devote the second and third volumes to properties and applications of individual types of sensing materials. Th is explains why, in Volume 1: General Approaches, we provide a brief description of chemical sensors, and then detailed discussion of desired properties for sensing materials, followed by chapters devoted to methods of synthesis, deposi-tion, and modifi cation of sensing materials. Th e fi rst volume also provides general background informa-tion about processes that participate in chemical sensing. Th us the aim of this volume, although not ex-haustive, is to provide basic knowledge about sensing materials, technologies used for their preparation, and then a general overview of their application in the development of chemical sensors.

xii PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS

Considering the importance of nanostructured materials for further development of chemical sen-sors, we have selected and collected information about those materials in Volume 2: Nanostructured Materials. In this volume, materials such as one-dimension metal oxide nanostructures, carbon nano-tubes, fullerenes, metal nanoparticles, and nanoclusters are considered. Nanocomposites, porous semi-conductors, ordered mesoporous materials, and zeolites also are among materials of this type.

Volume 3: Polymers and Other Materials, is a compilation of review chapters detailing applications of chemical sensor materials such as polymers, calixarenes, biological and biomimetric systems, novel semiconductor materials, and ionic conductors. Chemical sensors based on these materials comprise a large part of the chemical sensors market.

Of course, not all materials are covered equally. In many cases, the level of detailed elaboration was determined by their signifi cance and interest shown in that class of materials for chemical sensor design.

While the title of this series suggests that the work is aimed mainly at materials scientists, this is not so. Many of those who should fi nd this book useful will be chemists, physicists, or engineers who are dealing with chemical sensors, analytical chemistry, metal oxides, polymers, and other materials and devices. In fact, some readers may have only a superfi cial background in chemistry and physics. Th ese volumes are addressed to the rapidly growing number of active practitioners and those who are interested in starting research in the fi eld of materials for chemical sensors and biosensors, directors of industrial and government research centers, laboratory supervisors and managers, students and lecturers.

We believe that this series will be of interest to readers because of its several innovative aspects. First, it provides a detailed description and analysis of strategies for setting up successful processes for screen-ing sensing materials for chemical sensors. Second, it summarizes the advances and the remaining chal-lenges, and then goes on to suggest opportunities for research on chemical sensors based on polymeric, inorganic, and biological sensing materials. Th ird, it provides insight into how to improve the effi ciency of chemical sensing through optimization of sensing material parameters, including composition, struc-ture, electrophysical, chemical, electronic, and catalytic properties.

We express our gratitude to the contributing authors for their eff orts in preparing their chapters. We also express our gratitude to Momentum Press for giving us the opportunity to publish this series. We especially thank Joel Stein at Momentum Press for his patience during the development of this project and for encouraging us during the various stages of preparation.

Ghenadii Korotcenkov

xiii

Th is volume covers a variety of topics in the rapidly developing fi eld of chemical sensors. Th e purpose of this volume is to explain and illustrate the use of multifunctional materials such as polymers, calixarenes, ion conductors, biological systems, and novel semiconductors in chemical sensors. Th ese materials diff er fundamentally from standard metal oxides and metals, so their application provides opportunities to de-sign sensors based on entirely diff erent mechanisms of sensing. As a result, new trends in the elaboration of sensors with diff erent functional attributes and for building instruments with previously unavailable capabilities demanded by new applications have opened up. Th erefore, this book is intended to be a pri-mary source for both fundamental and practical information related to these multifunctional materials, which will be necessary for future development.

Th is volume comprises seven chapters written by active researchers who are well-known experts in their fi elds and who have made signifi cant contributions to the fi eld over the past several years. Th us, this book presents the most recent advances in all the key aspects of development and application of polymers and other multifunctional materials for chemical and biological analysis. Th e chapters in this book have been written to give the reader the big picture, from the design phase to implementation of chemical sensors of various types. Every chapter addresses the particulars of multifunctional materials synthesis and characterization. In every chapter you will also fi nd descriptions of a very wide range of devices that may be designed using such multifunctional materials.. Th ese chapters thus highlight the materials, the physics, the devices, and even key fabrication issues. We hope that the information pre-sented in this volume will help the reader understand the details of sensing material design for specifi c applications and establish quantitative structurefunction relationships.

Th e intended audience is scientists, researchers, and engineers in industries and research laborato-ries. With its many references to the vast resources of recently published literature on the subject, this book serves as a signifi cant and insightful source of valuable information pertaining to the ongoing scientifi c debates, the current state of understanding, and future directions. Students will also fi nd the book to be very useful in their research and understanding of chemical sensors and multifunctional materials. Th e structure of this book off ers a basis for a high-throughput instrumentation course at the

PREFACE TO VOLUME 3: POLYMERS AND

OTHER MATERIALS

xiv PREFACE TO VOLUME 3: POLYMERS & OTHER MATERIALS

advanced undergraduate or graduate level. As such, it should be very useful to university post-docs and professors as well.

Ghenadii Korotcenkov

xv

ABOUT THE EDITOR

Ghenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and Devices in 1976, and his Habilitate Degree (Dr.Sci.) in Physics and Mathematics of Semiconductors and Dielectrics in 1990. For a long time he was a leader of the scientifi c Gas Sensor Group and manager of various national and international scientifi c and engineering projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova. Currently, he is a research professor at Gwangju Institute of Science and Technology, Gwangju, Republic of Korea.

Specialists from the former Soviet Union know G. Korotcenkovs research results in the study of Schottky barriers, MOS structures, native oxides, and photoreceivers based on Group IIIV compounds very well. His current research interests include materials science and surface science, focused on metal oxides and solid-state gas sensor design. He is the author of fi ve books and special publications, nine invited review papers, several book chapters, and more than 180 peer-reviewed articles. He holds 16 patents. He has presented more than 200 reports at national and international conferences. His articles are cited more than 150 times per year. His research activities have been honored by the Award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004), Th e Prize of the Presidents of Academies of Sciences of Ukraine, Belarus and Moldova (2003), the Senior Research Excellence Award of Technical University of Moldova (2001, 2003, 2005), a Fellowship from the International Research Exchange Board (1998), and the National Youth Prize of the Republic of Moldova (1980), among others.

xvii

Basudam Adhikari (Chapter 1)Materials Science CentreIndian Institute of TechnologyKharagpur 721302, India

David St. Clair Black (Chapter 3)School of Chemistry University of New South Wales Sydney 2052, New South Wales, Australia

Nikos Chaniotakis (Chapter 5)Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of CreteVoutes 71003 Iraklion, Crete, Greece

Har Mohindra Chawla (Chapter 3)Department of ChemistryIndian Institute of Technology DelhiNew Delhi 110016, India

Beongki Cho (Chapter 2)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of Korea

Raz Jelinek (Chapter 4)Department of Chemistry and Ilse Katz Institute for Nanotechnology Ben Gurion UniversityBeer Sheva 84105, Israel

CONTRIBUTORS

xviii CONTRIBUTORS

Pradip Kar (Chapter 1)Polymer Engineering DepartmentBirla Institute of Technology, MesraRanchi 835215, India

Sofi ya Kolusheva (Chapter 4)Department of Chemistry and Ilse Katz Institute for NanotechnologyBen Gurion UniversityBeer Sheva 84105, Israel

Ghenadii Korotcenkov (Chapters 2 and 7)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of KoreaandTechnical University of MoldovaChisinau, Republic of Moldova

Naresh Kumar (Chapter 3)School of ChemistryUniversity of New South Wales Sydney 2052, New South Wales, Australia

Ramachandran Vasant Kumar (Chapter 6)Department of Materials Science and MetallurgyUniversity of CambridgeCambridge CB2 3QZ, United Kingdom

S. Kumar (Chapter 3)Department of ChemistryIndian Institute of Technology Delhi New Delhi 110016, India

Nalin Pant (Chapter 3)Department of ChemistryIndian Institute of Technology DelhiNew Delhi 110016, India

Carsten Schwandt (Chapter 6)Department of Materials Science and MetallurgyUniversity of Cambridge Cambridge CB2 3QZ, United Kingdom

CONTRIBUTORS xix

Nikoletta Sofi kiti (Chapter 5) Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of CreteVoutes 71003 Iraklion, Crete, Greece

Vicky Vamvakaki (Chapter 5) Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of Crete,Voutes 71003 Iraklion, Crete, Greece

1CHAPTER 1

POLYMERS IN CHEMICAL SENSORS

B. Adhikari P. Kar

1. INTRODUCTION

Plants and animals have built-in natural sensor devices targeted either for detection of external agencies in their surroundings or for performing some specifi c function. Plants have electromagnetic detectors for sensing external attacks. Animals have devices for sensing through fi ve diff erent sense organsthe tongue, skin, eye, ear, and nose. Th ey use these organs to perform normal activities as well as to re-main away from unfavorable situations. To help them remain away from deadly poisonous chemical substances, for instance, animals are alerted by smell and by irritation of the eyes and skin. Both the feeling of irritation and detection of abnormal odors occur via their built-in sensor system. Th ese sensor systems, formed through biosynthesis from organic molecules, are embedded in an aqueous environ-ment containing soluble electrolytic salts. Th us the sensing network in living systems is organic as well as polymeric in nature.

Gathering knowledge by studying biological sensing systems and their functioning can help in mim-icking such sensor systems using synthetic macromolecules. Such sensor networks consist of a chemical detection or recognition element, a transducing element, and a signal-processing element, which appear to be extremely complex even though their function and response appear to be very simple and spon-taneous (see Figure 1.1). Th e fi rst element is a chemical detection element of some sensing material, which interacts with the environment and generates a response. Th e second element is a transducer, which reads the response from the chemical detection element and converts it into an interpretable and quantifi able term for the third element, the signal processor. Th e chemical detection element is the

2 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS

heart of the sensor system and can be considered the primary part of the sensor. Th e response, recovery, selectivity, and sensitivity of a chemical sensor depend on the chemical detection element used. Sensor technology depends on progress in materials science and technology for this chemical detection element layer. Th e choice of a particular interactive material is based on the sensitivity, selectivity, reliability, and the reversibility of the related sensing mechanisms. Various chemical detection materials are available, including metal and metal oxide semiconductors, solid electrolytes, insulators, catalytic materials, poly-mers, composites, and others.

Exposure to toxic and hazardous chemicals may cause serious problems to mammals, such as irrita-tion, vomiting, suff ocation, or illness; the chemical may even be deadly poisonous. Th us, as a measure of protection to living bodies, it is necessary to detect hazardous and toxic chemicals using artifi cial sen-sor devices. Toward that goal, sensing of toxic and hazardous chemicals is an emerging fi eld, in which modern research tries to develop more effi cient sensor devices than those in living bodies.

Th e chemical sensors fi eld is one of the fastest-growing areas in both research and commercial ap-plication. During the last 25 years, global research and development in the fi eld of sensors has expanded exponentially in terms of fi nancial investment, published literature, and number of active researchers. Most of the research work in this fi eld is concentrated toward reducing the size of sensors and enabling identifi cation and quantifi cation of multiple species. Since easy handling, quick response, good revers-ibility and reproducibility, sensitivity, and selectivity are qualities expected of an excellent sensor, there is a need for further research.

Exploring the present state of the art of materials used in chemical sensors is our primary goal in this chapter, with special reference to the role of organic polymers as chemical detecting elements in sensors. Since chemical sensors need to either detect or estimate chemical analytes, the sensing element should be selective as well as interactive with the analyte. In terms of general operating principles, there are three major categories of chemical sensors: electrochemical, optical, and mass sensors. Th erefore, the detecting element, which is the key element of a sensor device, must respond to electrochemical or optical stimu-lation or undergo structural changes due to a change in its mass as a result of absorption of a minute quantity of an analyte. From this point of view, polymers represent a class of highly tailorable materials, which have already been qualifi ed as detecting elements that respond at ambient temperature to chemical or electrochemical stimulation, to optical stimulation, and that express piezoelectric behavior after ab-sorbing a small mass. In addition, polymers are easy to synthesize and process to develop a suitable device for sensor application. Th e properties of polymers, such as their polyelectrolytic nature, intrinsic con-ductivity, electrochromism, etc., have made them todays materials of choice in modern sensor devices, gradually replacing the metal oxides and inorganic semiconductors that earlier dominated the fi eld.

Analyte or substrate

Chemical detection element (polymer)

Transducer Signal processor

Figure 1.1. Schematic representation of simplifi ed sensor setup.

POLYMERS IN CHEMICAL SENSORS 3

2. WHAT ARE POLYMERS?

In contrast to discrete small molecular compounds, polymers are macromolecules. Except for a few, polymers are organic macromolecules made of carbon and hydrogen atoms in major percentage with some heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, halogens, etc., as minor constituents. A polymer molecule is formed by the repetitive union of a large number of reactive small molecules in a regular sequence (see Figure 1.2). Th e repeated unit in the backbone of the polymer molecule is known as the mer unit, and the reactive small molecule from which the polymer is formed is called the monomer. Th e simplest example of a polymer is polyethylene, in which the ethylene moiety is the mer unit.

Figure 1.2. Repeating unit structures of some common conducting polymers.

n

N

H

n

S

R

n

Nn

HPolyacetylenePolypyrrole Poly(3alkylthiophene)Polyaniline

Sn

O

n

n

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PolythiophenePolyfuran Polyphenylene Polyphenylenesulfide

n

S

n

S

n

PolyphenylenevinylenePolythienylenevinylene Polyisothianapthene

n

NH

n

n PolyazulenePolycarbazolePolyfluorene


As a class, polymers are unique over other materials with respect to their tailorability and broad range of properties as well as versatility. In terms of size and molecular weight, in general, polymers are more than a million times bigger than small molecular compounds. Properties of polymers, in general, depend on their chemical composition, molecular structure, molecular weight, molecular-weight distri-bution, and morphology. Morphologically, polymers are quasi-crystalline in nature, having small crys-tallites dispersed in an amorphous matrix in which a single molecule may extend from one amorphous region to a distant amorphous region while passing through several crystallite regions. Both the bulk and the surface of a polymer sample may or may not contain active functional groups which can respond to a stimulus in chemical sensing. Although primary covalent bonds predominate in polymers, secondary bonding infl uences both their processing and their functional performance. Due consideration should be given to the role of secondary bonding on the interaction of a polymer with the analyte during the sensing function. Extensive secondary bonding interaction can be a major cause of insolubility of a poly-mer in solvents, which may restrict its processability to produce a suitable device for chemical sensing.

Interpretation of sensing response and recovery is easier if it can be correlated with chemical bond-ing in the polymer. On the other hand, organic macromolecules are known which can exhibit ionic or electronic conduction properties. Th ese are polymers that have ionizable functional groups, viz., poly-electrolytes, and extended -electron conjugation, viz., intrinsically conducting polymers. In general, polymers are electrically insulating in nature due to the nonavailability of free electrons, since the four valence electrons of carbon are fully saturated. However, some organic polymer molecules are semicon-ducting in nature, by virtue of their extended -electron conjugation along the backbone chain of the macromolecule. Th e ability of conducting polymers to conduct electricity depends on the alternating double bondsingle bond structure in the polymer backbone, coupled with the formation of some charged centers on the chain by partial oxidation. Th e introduction of such extra charges on the poly-mer by doping (in analogy to inorganic semiconductors), alters the conductivity of such polymers from almost insulators to something approaching a metallic conductor. Due to the more reactive nature of the ionic groups in polyelectrolytes and -electron conjugation in conducting polymers, these are the important sensing sites for corresponding analyte compounds.

3. PARAMETERS OF POLYMERS PROMISING FOR CHEMICAL SENSOR APPLICATION

It has been pointed out that built-in sensing devices in biological systems are polymeric in nature, al-though they are complex. Th e actual structure and chemistry of the polymers in the biological sensing devices of various sense organs are not known. However, by virtue of their light weight, ease of synthesis, good processability, stability, nonhazardous nature, and low cost, polymers off er a lot of advantages as sensor materials over other materials in the majority of sensor technologies. Th e sensing ability of many synthetic polymers in various artifi cial sensor devices has been established by their excellent tailorability and easy processability to form very-thin-layer devices.

Polymers have high tailorability of their molecular structure and composition for better processabil-ity or to improve specifi c behavior in the bulk material or on its surface. As a result, a broad spectrum of properties can be easily obtained with polymers. In addition, the ease of creation of new functional groups


on the polymer backbone, their ability to respond to redox systems, their charge-carrying capability, and their ability to respond to optical stimulation are other advantages of using them in sensor devices. Due to the fl exible nature of the polymer chains, compact and miniaturized sensor arrays can be easily fabricated. Unlike metal and metal-oxide semiconductor materials, there is no need for special clean room, high temperature, or special high-cost process techniques to fabricate sensor devices using polymers.

Good selective sensing of a specifi c analyte in a mixture is also possible with a polymer because of its high structural tailorability. Polymers have operational/functional advantages at ambient temperature. Th e suitability of a polymer in a sensor device for use at a particular temperature may be suggested by the glass transition temperature (Tg) of the polymer. A polymer in a sensor device may provide proper function if the temperature of the sensing measurement is kept close to its Tg but well below its melting temperature (Tm).

As a basic principle, a polymer in a sensor device can function by absorption/adsorption of an analyte in the form of gas or liquid, reversibly or irreversibly, followed by interaction and change in properties of the polymer, which is transduced electronically, electrochemically, or optically. Processing of the transduced signal provides the sensor output. Large numbers of articles and reviews have been published on chemical sensors, in which many polymers have been utilized to fabricate sensor devices (Armstrong and Horvai 1990; Bidan 1992; Adhikari and Majumdar 2004; Persaud 2005). Although few such polymer-based sensor technologies have been commercialized as yet, many are in the explora-tion stage, and understanding the structure and properties of suitable polymers for use in sensor devices may lead to more advanced devices for chemical sensing.

Polymers have unique characteristics that have been proved to infl uence the operating parameters of sensor devices. Properties of polymers that infl uence the operating parameters of sensors can be physicochemical, electrical (conductivity, resistivity), chemical, optical (photo- and electrolumines-cence, optoelectronic), redox, hydrophobic/hydrophilic, piezoelectric/pyroelectric, etc. Being organic in nature, polymers provide an inherent affi nity to chemical species that need to be detected or estimated. Comparing the solubility parameters of both the polymer as sensing element and the analyte to be sensed can lead to correlation of this affi nity. Because of its long backbone chain and fl exible nature, a polymer can accommodate a large quantity of a foreign substance in the form of fi ne particle (as fi ller) or fi ber or even highly viscous liquid. Th us, a polymer in its solid state contains ample free volume in the bulk. Depending on its affi nity toward a foreign agent, the polymer can hold it for quite a long period. Th is can happen in two ways: by simple physical entrapment provided the foreign agent is compatible with the host polymer; or by chemical bonding between them. Th e latter is well known to provide co-valent immobilization of the foreign agent as the recognition element. A polymer to be used as either a solid carrier or directly as the sensing element can be best selected based on the solubility parameters of both the polymer and the analyte chemical compound.

Another important parameter to be considered in selecting a polymer for sensor application is diff u-sion. Better sensing response in terms of sensitivity and sensor recovery can be obtained from a polymer-based sensor if the polymer off ers a lower diff usion barrier to the analyte compound. Th is way, short response time as well as short recovery time with good sensor repeatability can be available from a par-ticular polymeranalyte system. Exploring the nature of interactions between polymer and analyte can also help in a major way to obtain a good sensing device. Such interactions can be judged by looking at the chemical structure of the polymer chain and its pendant groups vis--vis the chemical structure of the


incoming analyte compound. Adsorption/desorption, stability, morphological features (crystalline/amor-phous), surface area, and the population of the active sensing site also infl uence sensing characteristics.

Th e stability of the polymers in the sensor device is another issue to be considered before selecting a polymer (Durst et al. 1997). In polymers, some percentages of heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, and halogens are present together with a long chain of carbon and hydrogen. Th e carboncarbon or carbonhydrogen bonds are comparatively more stable than the bonds between carbon and the heteroatoms. A polymer that has very good electrical conductivity may not have good stability in the ambient environment, or its stability may decrease when it is in contact with the analyte compound. Th is occurs because the unsaturated bonds in conducting polymers are often very reactive when exposed to environmental agents such as oxygen or moisture. Apart from the inherent stability of the polymers, the stability of some foreign chemical compounds, which are used as dopants in conduct-ing polymers, are also important. Th ese dopants may not be very stable within the polymer matrix and may also react with environmental agents. Th e polymer selected should maintain its intrinsic sensing characteristics across a wide temperature range. It must possess adequate mechanical strength to sustain handling and other stresses. Stability and degradation of the polymer is also important when it may be exposed to chemical environments during sensing. Th e polymer should be resistant to degradation or dissolution in organic solvents but should interact with the reactive sensing element. Th e permeability of the polymer fi lm used as the detecting layer is also important, because this property aff ects the transport properties of the other components. Th erefore, criteria for polymers to be used in chemical sensors need attention, and some information about the stability and degradation of such polymers is essential.

Experimental and theoretical work on polymeric and supramolecular compounds helps in designing highly selective chemical sensors. Diff erent transducer principles are used for sensing molecules in air and water by monitoring changes in mass, temperature, capacitance, and thickness. Such changes in physical and chemical properties are monitored using resonator, calorimetric, impedance, or fi ber optic sensors. Supramolecular chemistry aims at the preparation of molecules with specifi c binding sites inside their cavities. Self-organized layers with a well-defi ned architecture make it possible to design highly spe-cifi c chemical sensors utilizing very fast adsorption processes between recognition sites at the surface of monolayers and molecules to be detected (Schierbaum 1992). Permselective polymeric membranes, used to separate diff erent gaseous and liquid constituents, can also be used in analytical chemistry and the fi eld of sensors. Practical examples include membranes in gas sensors that have improved selectivity to certain gas constituents, membranes in ion-selective electrodes and ion-sensitive fi eld-eff ect transistors (ISFETs), and diff usion membranes in amperometric electrochemical cells. Polyethylene, polytetrafl uoro ethylene (PTFE), polyfl uoroethylene-propylene (PFEP), cellulose acetate, silicone rubber, plasticized polyvinyl chloride (PVC), polydimethylsiloxane (PDMS), and other polymers are used as membranes.

Development of miniaturized sensor arrays using nanostructured polymers and composites has come to the forefront of chemical sensor research in order to obtain specifi c selectivity of one particular chemical analyte in a mixture. By virtue of their inherently fl exible nature, the backbones of polymer chains can undergo conformational change to provide reversible adsorption/desorption in sensing re-sponse, or they can undergo oxidation/reduction when exposed to an analyte with an attendant change in electrical conductivity as a sensing signal. Th e major functions of polymers in sensor devices are to serve as a solid support for a chemical recognition element, as a selective agent for a specifi c analyte, or as the recognition element itself.


4. SYNTHESIS OF POLYMERS

Using some processing and fabrication techniques, pure polymers of adequate molecular weight and properties can be employed in sensor devices. In many cases, however, sensor device fabrication is dif-fi cult because many solubility and processing techniques are not compatible with all polymers. Th ese polymers either possess in-built sensing sites or are modifi ed to attach sensing functionalities. Th is indi-cates that some polymers have inherent properties that off er sensing function, whereas other polymers are used as carriers of sensing elements. So, depending on the need for a specifi c sensing function, a polymer should be selected before attempting its synthesis. Most carrier polymers are electrical insula-tors in which sensing elements are either physically immobilized or chemically attached. In these cases the polymer of choice should be procured from market or synthesized as per the requirement.

Polymers used in sensor devices are either formed in situ or processed from a fully grown polymer prepared by conventional techniques. Polymers are synthesized from their respective monomers either by condensation of reactive functional groups in the monomer or by addition chain reaction through olefi nic unsaturation in the monomer followed by isolation and purifi cation. Flory (1953) described briefl y these two types of polymerization, chain-growth polymerization and step-growth polymeriza-tion, by which most of the polymer molecule may be built up. Th e procedures generally followed are bulk, solution, suspension, emulsion, or interfacial polymerization (Flory 1953; Odian 2004).

In chain-growth polymerization, an initiator reacts with a monomer molecule to create a reactive site, and the reactive site then reacts with successive monomer molecules to yield the polymer. A ho-mopolymer that forms via chain-growth polymerization usually forms from one monomer; copolymers result from chain-growth polymerization of two or more monomers with the same type of reactive functional site (olefi nic unsaturation). Th is type of polymerization is popular for the monomers that have double bonds, which can act as the reactive functional site during polymerization. Th e formation of polyethylene from ethylene in the presence of an initiator is an example of chain-growth polymer-ization. Other examples are styrene, butadiene, propylene, vinylene monomer, and acryl monomer. Initiators that are commonly used for polymerization include peroxide (ROOR), azo compounds (RN=NR), and redox compounds [FeSO4, FeCl3, K2S2O8, (NH4)2S2O8, etc.] by thermal or photo-chemical pathways (Odian 2004).

Step-growth polymerization begins when one monomer with two reactive functional groups reacts with another monomer containing functional groups of another type such that a small by-product molecule leaves the chain. Polymerization usually proceeds by reactions between two diff erent reactive functional groups, e.g., hydroxyl and carboxyl groups, isocyanate and hydroxyl groups, amine and acid groups, etc. So, according to the pair of functional groups in the monomers, a number of diff erent chemical reactions may be used to synthesize polymeric materials by step polymerization, e.g., esterifi ca-tion, amidation, the formation of urethanes, aromatic substitution, etc. (Odian 2004).

Th e synthesis of condensation polymers by ring-opening polymerization is also possible. For the general principle behind the synthesis of polymers, readers are referred to standard textbooks (Flory 1953; Odian 2004).

Intrinsically conducting polymers are widely used in various electrochemical sensing devices. Presently used conducting polymers for these applications are mostly polyheterocycles such as poly-pyrrole, polythiophene, polyfuran, polyisothionapthalene, polyindole, polyaniline, polycarbazole, etc.,


and polyaromatics such as polyazulene, poly-p-phenylene (PPP), poly p-phenylene vinylene (PPV), polypyrene, etc. (see Figure 1.2) (Gurunathan et al. 1999). Th ose conducting polymers are usually syn-thesized by one of two popular methods, chemical or electrochemical oxidation of the corresponding monomers. Th iophene, furan, carbazole, aniline, indole, azulene, and their derivatives are the major monomers used for synthesis of conducting polymers. Synthesis of conducting polymers by oxidative coupling polymerization is a very easy and simple method and, therefore, this method is suitable for producing bulk quantities of polymer in a conducting oxidized state with associated counterions from the polymerization medium. Th e procedure involves simple mixing of monomer and oxidant in aqueous or organic protonic acid solution. Commonly used oxidants are ammonium persulfate, ferric chloride, hydrogen peroxide, potassium dichromate, cerium sulfate, etc.

Th e majority of the redox polymers are synthesized by chemical polymerization. Th e oxidative polymerization of inexpensive, simple aromatic benzenoid or nonbenzenoid (mostly amines, e.g., ani-line, o-phenylenediamine), and heterocyclic compounds (e.g., pyrroles, thiophenes, indoles, azines, etc.) is of greatest interest (MacDiarmid and Epstein 1989; Syed and Dinesan 1991; Martin et al. 1993). Polythiophene and its derivatives are synthesized via oxidative coupling reactions (McCullough 1998). Quite a large number of published reports are available in the literature on the chemical synthesis by oxi-dative coupling reaction of aniline, pyrrole, thiophene, etc. A comprehensive picture may be obtained from some review reports (Toshima and Hara 1995; Feast et al. 1996; Smith 1998).

Diff erent electrochemical principles are followed to synthesize intrinsically conducting polymers, viz., galvanostatic, potentiostatic, cyclic voltammetry, and other potentiodynamic methods (Toshima and Hara 1995; Smith 1998; Feast et al. 1996). Th ese techniques utilize a three-electrode system: a working electrode, a counter electrode, and a reference electrode. During electrochemical synthesis, the conducting polymers are electrochemically deposited on the working electrode, which is made of ma-terials such as platinum, stainless steel, gold, indium tin oxide (ITO), or glass. Th e polymer-deposited electrodes are either used directly or the polymers deposited on the electrode surface are peeled off as self-standing fi lms for a particular application.

In electrochemical polymerization, the electrochemically active groups are either built into the poly-mer structure or are added as a pendant group. Th ese groups can also be incorporated into the polymer during polymerization, or attached to the polymer network in an additional step after the coating proce-dure (postcoating functionalization) to obtain polymer fi lm electrodes (Bruce 1995; Inzelt 2000). Sato et al. (1986) showed that the electrochemical polymerization of long-chain alkyl-substituted thiophene and pyrrole yields highly conducting fi lms, some of which are soluble in common organic solvents in their conducting state. Although chemical oxidation of pyrrole by Fe(ClO4)3 leads to conducting polypyrrole (MacDiarmid and Epstein 1989; Syed and Dinesan 1991; Martin et al. 1993), electrochemical polymer-ization is a preferred technique for polymer fi lm electrodes, thin-layer sensors, in microtechnology, etc., be-cause the control of potential is a prerequisite for polymer fi lm deposition on the anode during synthesis.

Th e oxidation state of the polymer can be varied electrochemically by cycling the potential between oxidized, conducting, and the neutral, insulating state, or by using suitable redox compounds. Varying the composition of the polymerization medium leads to a change in the conductivity of the polymer. For example, increasing the pH of the polymerization medium (MacDiarmid and Epstein 1989; Paul et al. 1985) or including an electron-donor molecule (e.g., NH3) in the gas phase decreases the conductivity of polyaniline (PANI) or polypyrrole (PPy) fi lms (Miasik et al. 1986; Pei and Inganas 1993).


Many conducting polymers and their derivatives are usually synthesized through well-known chem-ical routes rather than by oxidative polymerization. Th ese chemical routes include various well-known reactions, such as the Wittig reaction (Diaz et al. 1979), the Heck reaction (Kobayashi et al. 1985), and Gilch polymerization (Malhotra et al. 1986).

Plasma polymerization or electropolymerization of monomers are preferred when the polymers are diffi cult to process because of insolubility or infusibility. Plasma-polymerized thin fi lms from various monomer precursors have been prepared by employing radio-frequency plasma polymerization tech-niques (Saravanan et al. 2004). Th e apparatus consisted of a 50-cm glass tube of 8 cm diameter, with provision for charging monomer vapors by evacuation. A schematic of the experimental setup is shown in Figure 1.3. Chemically and ultrasonically cleaned glass substrates were placed inside the glass tube exactly under the space separated by the aluminum foil electrodes, which were capacitively coupled and wrapped around the glass tube, separated by a distance of 5 cm.

5. DEPOSITION OF POLYMERS

Th e deposition of polymer as a sensing element on an electrode surface is a very tricky process, since the sensor sensitivity depends on the thickness, chemical composition, crystallinity, conductivity of the coated polymer, etc. Th e present state of the art of polymer-coated electrode preparation is based on ex-tensive research and development, and numerous research articles are available in various sensor-related journals and books. In a chemical sensor, a properly functionalized polymer acts as a chemoreceptor, which is a selective receiving site for analyte recognition and reaction. In the case of a biologically de-rived receptor, the more specifi c term biochemical receptor or bioreceptor may be used. Th e bioreceptor might be an enzyme, tissue organelles, antigen/antibodies, etc. Th ese biorecognition elements might be immobilized in a polymer by covalent attachment, physical entrapment, or cross-linking.

Electrodes are usually fabricated by chemical modifi cation or by deposition of polymer on the elec-trode surface by one of the following techniques.

Figure 1.3. Schematic plasma polymerization experimental setup. A gaseous monomer or monomer vapor such as acetylene or aniline and a gaseous dopant such as iodine, chlorine, or HCl gas are used within the chamber. (Reproduced with permission from Saravanan et al. 2004. Copyright 2004 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.)


1. Th e chemisorption-adsorption technique utilizes valence forces of the same kind as those operat-ing in the formation of chemical compounds, where the chemical polymeric fi lm is strongly and, ideally, irreversibly adsorbed (chemisorbed) onto the electrode surface to provide a monolayer fi lm (Gold et al. 1987). Th is modifi cation creates substrate-coupled self-assembled monolayers (SAMs) in which uncorrelated molecules spontaneously chemisorb at specifi c sites on the surface of the electrode to form a superlattice (Allara 1995).

2. Covalent attachment of one to several monomolecular layers of the chemical modifi er to the elec-trode surface involves some combination of chemisorption and low solubility in the contacting solution, or physical anchoring on a porous electrode.

3. Formation of a polymer fi lm using a chemical modifi er is also done. Th e polymer fi lm can be organic, organometallic, or inorganic as long as it contains the desired chemical modifi er. Other forms of possible modifi cation are substrate-decoupled SAMs, in which adsorbate molecules are arranged on the electrode surface independent of any substrate structure (Allara 1995).

In order to get a polymer-coated ion-selective electrode, two conditions must be met. First, the cations must be freely mobile within the polymer matrix, which can be achieved by saturating the poly-mer with a solvent. Th e solvent helps to dissociate the cations associated with the pendant acidic groups and allows them to move within the polymer. Second, the membrane surface must be made conductive by depositing a thin metal electrode on both surfaces of the membrane. Typically, the ion-exchange property of the polymer is exploited to facilitate metal deposition. Th e polymer surface needs to be pretreated by sand blasting, hydrating, and cleaning with a strong acid, thus ensuring that the polymer is fully saturated with protons. Th e membrane is then placed in an aqueous solution containing ions of the metal to be plated. Th ese ions are allowed to exchange with the protons in the polymer for a predetermined amount of time and are then reduced to their neutral state at the surface of the polymer by a reducing agent (typically NaBH4 or LiBH4) in the outer solution. In this solvated and electroded form, a Nafi on membrane can be made to bend toward the anode side when a small voltage (15 V) is applied across its thickness, thus making it a soft, distributed actuator. Membranes in this form can also be used as distributed sensors. Several researchers have shown that the transient voltage generated across the membrane is correlated to the quasi-static displacement of the membrane (Sadeghipour et al. 1992; Shahinpoor et al. 1998; Keshavarzi et al. 1999).

Th e active polymer layer may be in free-standing fi lm form or on a suitable substrate and forms the heart of the sensor. Polymer fi lm or polymer coated on a substrate can be obtained using the fol-lowing methods.

1. Dip coating. Th is process consists of immersing the electrode material in a solution of the polymer in a suitable solvent for a suffi cient period to allow spontaneous fi lm formation onto the substrate by adsorption. Th e fi lm thickness may be controlled by adjusting the polymer solution viscosity and the speed of withdrawing the electrode from the solution, followed by solvent evaporation to form the polymer fi lm on the electrode.

2. Solvent evaporation. Applying a drop of polymer solution of the required consistency on to the electrode surface, followed by solvent evaporation, creates a fi lm. Th e quality and thickness of a polymer fi lm formed by this manual technique depends on the personal skill of the research


worker, but the method is advantageous because the thickness of the polymer-coated fi lm on the electrode can be known from the original concentration of polymer solution and droplet volume. Th is is the oldest and still popular method for free-standing polymer fi lm casting.

3. Spin coating. In spin coating or spin casting, a dilute polymer solution droplet is placed on the surface of a rotating electrode. Because of the rotation, excess solution is spun off the surface and a very thin polymer fi lm is formed. Following the same procedure, multiple layers can be formed until the desired thickness is obtained. Th is procedure provides pinhole-free thin fi lms.

4. Layer-by-layer (LBL) self-assembly. A composite of two polymeric electrolytes on some suitable substrate can be fabricated (Ram et al. 2005a; Nohria et al. 2006). In this method the sub-strate is alternatively immersed into a polymeric anion solution and a polymeric cation solution. Insoluble doped conducting polymers, e.g., polyaniline with positive charge on the backbone, can be deposited on a polymeric anion layer. Th e thickness of the LBL fi lm depends on the num-ber of times the process is repeated (Nohria et al. 2006).

5. Langmuir-Blodgett fi lm casting. In Langmuir-Blodgett (LB) fi lm casting, polymer molecules with hydrophilic heads and hydrophobic tails are spread on a water surface and then the molecules are compressed using a barrier to align the molecules (see Figure 1.4). Here, the polymer itself contains hydrophobic and hydrophilic groups within the backbone, or monomers having such groups are polymerized, which causes the polymer to orient itself during fi lm casting. A single-layer fi lm is cast on a substrate which is drawn up from beneath the water surface. Generally, LB fi lms form by balancing the interactions at the polymerwater, airwater, and polymerair interfaces (Wegner and Remmers 1995). Th e resulting fi lm is very well ordered, single-layered, and in the range of molecular thickness.

6. Electrochemical polymerization. A monomer solution is oxidized or reduced to an activated form that leads to a polymer fi lm formed directly on the electrode surface. Th is procedure results in few pinholes, since polymerization would be accentuated at exposed (pinhole) sites at the electrode surface. Unless the polymer fi lm itself is redox-active, electrode passivation occurs and further fi lm growth is prevented.

Presently, polymers are being used in layer or fi lm form as an active part of inorganic solid-state sensing devices. Polymers can be easily deposited on various substrates by simple techniques. Easy

Water

Substrate (hydrophobic) Polymer

layer

Barrier

Polymer layer in substrate

Figure 1.4. LB fi lm deposition mechanism.


fi lm casting is also possible for fabrication of diff erent sensor systems. In the case of electrode preparation by direct electrochemical deposition of the conducting polymer, knowledge of the kinetics of the electrodeposition process is also of utmost importance in order to obtain proper sensing function of the electrode.

Along with the above-mentioned infl uencing parameters on polymer growth during electrode-position, the eff ect of the electrode material and its surface properties need attention. For exam-ple, in the autocatalytic oxidation of aniline over a platinum electrode, specifi c interactions and wetting may determine the nucleation and dimensionality of the growth process. Two or more stages of the polymerization process have been distinguished in the case of PANI: a compact layer (200 nm) formed on the electrode surface via a potential-independent nucleation and a two-dimensional (2-D, lateral) growth of PANI islands followed by 1-D growth of the polymer chain with continuous branching in the advanced stage leading to an open structure (Bade et al. 1992; Cruz and Ticianelli 1997). It was established that the formation of the aniline-based polymer involves two electrons in the formation of one monomermonomer bond (Linford 1987; Lyons 1994, 1996). Th e growth rate is proportional, except for the early induction period, to the 0.5 power with respect to the fi lm volume, and it is fi rst-order with respect to aniline concentration (Stilwell and Park 1988). To avoid hydrolytic degradation of the oxidized PANI (pernigraniline form), the positive potential limit of cycling can be decreased after 210 cycles because of the autocatalytic nature of the electropolymerization (Horanyi and Inzelt 1989; Stilwell and Park 1989). In addition to the head-to-tail coupling, formation of p-aminodiphenylamine by tail-to-tail dimerization (benzidine) also occurs as a minor intermediate, as evidenced by the rate constant of dimerization for radicalcation coupling to produce the former product, which is about 2.5 times higher than that for the tail-to-tail dimer (Yang and Bard 1992). Generally, a mixed material is deposited on the surface, containing electrochemically active and conducting as well as inactive and insulating parts, if the polymerization conditions are not carefully opti-mized (Otero and Rodriguez 1994). Since polythiophene does not adhere on a Au or Ti surface, it prefers electrochemical formation and precipitation from the medium. Th erefore, a suitable approach is the deposition of a thin polypyrrole layer on Ti or Au that ensures the deposition of polythiophene on these substrates (e.g. Ti, Au) (Gratzl et al. 1990).

7. Deposition by radio-frequency polymerization of a suitable monomer. In this method, a monomer va-por is exposed to a radio-frequency (RF) plasma discharge to form a thin polymer fi lm on the elec-trode surface. Chemical damage of the polymer fi lm producing unknown functionalities and struc-tural modifi cations due to high energetics of the RF discharge is a disadvantage of this technique.

8. Polymer deposition followed by cross-linking. In this technique, chemical components of a polymer fi lm are bonded with the electrode to impart stability, decreased permeability, or altered electron-transport characteristics to the polymer fi lm. Cross-linked fi lms are often formed by polymeriza-tion of bifunctional and polyfunctional monomers or by chemical, electrochemical, photolytic, radiolytic, or thermal activation.

9. Pellet preparation. In this technique the well-dried polymer powder is made into a thin pellet of particular thickness using a steel die in a hydraulic press under a certain constant pressure. Th e pellet preparation technique is very useful for polymers which are not soluble in organic solvents, such as conducting polymersespecially, doped forms of conducting polymers.


6. FUNCTIONALIZATION OF POLYMERS

6.1. STRUCTURE MODIFICATION

Th e polymer fi lm may also be functionalized subsequent to fi lm application, because immobilizing the fi lm is easier than working with monolayers, such modifi ed fi lms are usually more stable, and stronger electrochemical responses can be available from multiple layers of redox sites. Some questions remain, however, as to how the electrochemical reactions of multimolecular layers of electroactive sites in a polymer matrix occur, for example, the mass-transport and electron-transfer processes by which the multilayers exchange electrons with the electrode and with reactive species such as molecules and ions in the contacting solution (Murray 1984; Murray et al. 1987; Inzelt 1994). Lack of suffi cient know-ledge about the structure and properties of polymer fi lms, as well as morphological changes arising out of various chemical, electrochemical, and physical processes during use, may lead to uncertainty in the ultimate sensing performance. Electrocatalysis on a modifi ed electrode is usually an electron-transfer reaction between the electrode and some solution substrate, which, when mediated by an immobilized redox couple, proceeds at a lower overpotential than would otherwise occur at the bare electrode (Durst et al. 1997).

After about four decades of research on conducting polymers and their utilization in sensor devices, there is suffi cient scope and many avenues for enhancing the conductivity as well as improving sensing ability. From the literature on polymer-based sensor research, it has become apparent that effi ciency in sensor output depends on parameters such as polymer structure, composition, morphology, and active functional sites. Th erefore, to achieve a specifi c sensor device, it is usually necessary to modify the struc-ture of the conducting polymer and perhaps substitute for one of the common conducting polymers.

Not only the sensor output but also the stability of the polymer in the device needs special atten-tion. Polymers having excellent sensing characteristics have been found to be environmentally unstable. Processing and fabrication of the synthesized conducting polymers into a suitable device is a challeng-ing problem today. Many researchers have tried to solve this problem by chemical substitution in the monomer stage or by changing the polymerization conditions.

Although both electrically insulating as well as conducting polymers have been successfully utilized in the fabrication of chemical sensor devices, conducting polymers have clear advantages in sensing performance over insulating polymers. By virtue of tailor-made characteristics, structural as well as functional modifi cation of conventional conducting polymers can add a new dimension to polymers as materials for chemical sensors.

Some of the common modifi cation strategies being practiced for conducting polymers intended for use in sensor devices are described in the following paragraphs, together with some research outcomes.

1. Chemical group substitution on monomers and polymers. Appropriate substitutions in the monomer can improve the stability in air of the electrochemically produced polymers. Although prepared under identical electrochemical conditions, the substitution of a methyl group, e.g., poly(3-methyl thiophene) has shown better air stability than that of polythiophene (Tourillon and Garnies 1982; Bryce et al. 1987). In addition to its increased stability in air, poly(3-methyl thio-phene) has also shown improved conductivity over that of the parent polythiophene (Waltman


et al. 1983). Unfortunately, such an increase in electrical conductivity has not been observed in a pyrrole system after alkyl-group substitution. Th e methyl-substituted polypyrroles had lower conductivities than the parent polypyrrole (Diaz et al. 1982). It appears that a balance between electronic and steric eff ects in substituted polymers of fi ve-membered heterocycles might account for either more or less conductivity than in the parent polymers (Waltman 1984; Waltman and Diaz 1985).

Several polyaniline derivatives have been developed by polymerizing various alkyl- and alkoxy-substituted polyanilines with the objective of improving conductivity as well as achieving bet-ter-processable conducting polyanilines. Common derivatives have included ortho- and meta- substituted anilines with NH2 (Gouette and Leclerc 1995; Levin et al. 2005), OH (Rivas et al. 2002; Salavagione et al. 2004; Levin et al. 2005; Kar et al. 2008) OCH3 (Dao et al. 1989; Park et al. 1989, Cataldo and Maltese 2002), CH3 (Leclerc et al. 1989; Falcou et al. 2005), SO3H (Yue and Epstein 1990; Kuo et al. 1998), Cl, F, I (Kang et al. 1990), CH2CH3 (Dao et al. 1989; Leclerc et al. 1989; Yue and Epstein 1990), etc., giving rise to similar products. A few of these derivatives had improved processability, but very little or no increase in electrical conductiv-ity was achieved.

2. Copolymerization. Electrochemically synthesized conducting polymers suff er from the drawback of poor mechanical strength, which restricts their use in commercial products. Since copoly-merization is one of the most eff ective methods for improving the mechanical strength of brittle polymers, this approach may be followed for conducting polymers without compromising their conductivity. In this respect, copolymerization of polyheterocycles with poly(p-phenylene sul-fi de) (PPS) can lead to better mechanical strength, less pronounced O2 sensitivity, and increased conductivity of the copolymer by doping of PPS. However, copolymerization by the electro-chemical technique is a challenging task due to the diff erent electrochemical oxidation potentials of individual monomers. Graft copolymerization and blending are other tailoring approaches to improve atmospheric stability and mechanical strength of conducting polyheterocycles.

6.2. SURFACE MODIFICATION

A wide variety of modifi cations to the surface morphology of polymers is possible for suiting specifi c sensor architecture, e.g., lithographic and soft lithographic techniques, replication from masters, pattern formation using self-assembly and controlled deposition, nanomachining, emulsion templating, and wetting-assisted templating (Xia et al. 1999; Hamley 2003; Helt et al. 2004; Drain and Batteas 2004; Barbetta et al. 2005). Surface modifi cation is also possible using plasma polymerization techniques. For example, one existing plasma polymerization setup (see Figure 1.3) was slightly modifi ed for the surface modifi cation of inorganic materials by organic coating (Sunny et al. 2006). Th e controlled fl ow of monomer vapor in the radio-frequency plasma polymerization chamber creates a thin deposition on the existing ceramic or semiconductor surface. Th e monomer is plasma-polymerized inside the chamber due to the RF source and is deposited on substrate surfaces placed inside the bottom of the discharge tube. In this method, conducting polymers such as polyaniline, polypyrrole, and polythiophene can be deposited on the surface of conventional semiconducting materials.


Coating of polymers is also possible by thermal spraying of polymer powders onto a wide variety of materials. In this method, polymer powder is injected into a heat source (hot fl ame or plasma) and transported to a preheated substrate. Th is is an eff ective method of producing protective barrier coat-ings. Polymers that have been sprayed include polyethylene, polymethyl methacrylate, polyether ether ketone, polyphenylene sulfi de, nylon, phenolic, epoxy, Tefzel (modifi ed ethylene-tetrafl uoroethylene polymer), and postconsumer commingled polymers. Th e thickness of the coating depends on the num-ber of repeated passes of the spray gun across the substrate. However, polymers with large particle size or higher molecular weight may form more heterogeneous microstructures within the coating, creating voids, trapped gasses, unmelted particles, splats, and pyrolized material.

Direct surface modifi cation of polymer substrates such as the polyimide Nafi on, a sulfonated ionic fl uoropolymer, which is commonly used in polymer sensor-actuator devices, can be done by the plasma technique. Here the polymer surface is treated with a suitable gas plasma to obtain reactive functional groups on the surface of the treated polymer. For example, Kapton, a polyimide, was treated in argon plasma for 2 min at 35 W and 0.2 torr, and Nafi on was treated in oxygen plasma for 10 min at 50 W power and 0.2 torr (Supriya and Claus 2004). Th e plasma treatment activates the polymer surface to fa-cilitate the deposition of other polymers having active functionality, or the polymer can be used directly as the detecting layer for a chemical sensing device. Other important surface modifi cation methods include grafting, surface coupling reactions, electron beam, glow discharge, corona discharge, UV treat-ment, etc. (Uyama et al. 1998).

6.3. COMPOSITION MODIFICATION

It has been established that -electron conjugation along the backbone of a polymer chain is one of the criteria for a polymer to exhibit good electrical (conducting and semiconducting) behavior. Additionally, it is also known that the presence of heteroatoms in polymers can lead to improved electrical (conduct-ing/semiconducting) performance. We have already mentioned that, for chemical sensor detection by the electrochemical principle, it is important to have inherent electrical properties which can alter dur-ing analyte interaction with the polymer detection element. As is known, the electrical properties de-pend on the mobility of free or loosely bounded electrons.

For polymers to be used as detection elements, the inherent electrical conduction may be obtained in two ways: by metal or metal compound dispersion within the polymer matrix; or by -electron con-jugation along the polymer chain. Th e free electrons or charges on the metal are responsible for the elec-trical conductivity of the metal or metal compound dispersed polymer detection system. In this system the polymer does not play a direct role in the alteration of electrical properties. However, the polymer can increase the performance of metal or metal compounds during sensing by chemical interaction of some particular groups. On the other hand, a particular group of polymers may aid interaction with the detection element. Since the discovery of intrinsically conducting polymers (ICPs), which show inher-ent electrical properties through the movement of a loosely bounded -electron cloud, these have been used successfully as chemical detection elements in electrochemical transducer systems.

Metal or metal compounds can also be dispersed within the ICP matrix to increase the sensing per-formance of the polymer. One of the easiest routes to composition modifi cation is composite formation.


Gurunathan and Trivedi (2000) incorporated TiO2 in polyaniline by chemical and electrochemical tech-niques to study the eff ect of photoconducting inorganic semiconductors on thermal stability and appli-cation of a new composite.

Th e structure and conductivity of the polymer can also be altered by further chemical reactions. Active sensing polymers are used in sheet or fi lm form as an integral part of inorganic solid-state de-vices. Conducting polymer composite sheets/fi lms contain conductive fi ller loaded in an electrically insulating polymer matrix. Th e change in resistivity as a function of fi ller concentration in such com-posites can be understood according to the percolation concept, which describes conduction through electrically conducting paths between two fi ller particles. Th e number of these paths dramatically decreases below a critical volume fraction of fi ller (Lundberg and Sundqvist 1986). Common fi ller ma-terials are metals (Cu, Pd, Au, Pt), carbon black, and semiconducting metal oxides (V2O3, TiO2, etc.). Th e insulating matrix polymers have included polyethylene, polyimides, polyesters, poly(vinyl acetate) (PVAc), PTFE, polyurethane, poly(vinyl alcohol) (PVA), epoxies, acrylics, poly(methyl methacrylate) (PMMA), and others. Th ese composites have been used successfully in positive temperature coeffi cient (PTC) thermistors and in piezoresistive pressure, tactile, humidity, and gas sensors (Lundberg and Sundqvist 1986).

Th e ionic conductivity of polyelectrolytes is modulated by several environmental parameters, which is the basis of their application in sensors. Th e conductivity of polyelectrolyte fi lms can be increased by increasing the number of ionic carriers through addition of ions from the environment and modify-ing the degree of dissociation of the polymer electrolyte (Sadaoka et al. 1986; Sakai et al. 1989). Ion-conducting polymers, viz., Nafi on, polyhydroxyethyl methacrylate (polyHEMA) and its copolymers, are widely used as solid electrolytes in electrochemical cells for the detection of various gases or ionic components. Alkali saltpolyether complexes, such as polypropylene oxide (PPO) and polyethylene ox-ide (PEO) with LiClO4, LiCl, LiCF3SO3, LiSCN (Watanabe et al. 1985; Sadaoka et al. 1986), polysty-rene sulphonate, and quaternized polyvinyl pyridine (PVPy), were used in impedance-type or semicon-ductor-based humidity sensors. Th e structures of -electron conjugated conducting polymers, such as polyacetylene, polyaromatics, and polyheterocycles contain a one-dimensional organic backbone, which enables formation of a superorbital for electronic conduction. Conduction through these polymers takes place by charge hopping both along the polymer chains and also between the macromolecules that make up individual fi bers as well as between the fi bers themselves. However, in the neutral (undoped) state, these materials behave like semiconductors. Th e electronic conductivity in these materials is obtained by doping, i.e., by injecting electrons or holes into the superorbital (Bidan 1992).

7. POLYMERS IN CHEMICAL SENSORS

As we have indicated, many types of chemical sensors have been designed for diff erent analytical tasks, such as detection of hazardous gas concentration, control of groundwater pollution, medical analysis and diagnostics, industrial quality and process controls, environmental safety, and so on. Some toxic chemicals and their common sources are listed in Table 1.1. All these various sensor types work by dif-ferent principles and require sensing materials with diff erent properties. However, polymers, due to their unique physical and chemical properties, can be used in all types of chemical sensors (see Table 1.2).

CAT

EGO

RY

TO

XIC

CH

EMIC

ALS

C

OM

MO

N SO

UR

CES

Toxi

c in

dust

rial

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n Pe

troch

emic

al in

dustr

ies a

nd m

otor

fuel

sch

emic

als

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, h

eavy

met

als a

nd th

eir o

xide

s, et

c.

M

ethy

l eth

yl k

eton

e, a

ceto

ne, m

etha

nol,

etha

nol,

chlo

rofo

rm, c

arbo

n te

trac

hlor

ide,

So

lven

ts an

d cl

eani

ng a

gent

s use

d in

te

trah

ydro

fura

n, is

opro

pano

l, m

ethy

l chl

orid

e, tr

ichl

oroe

thyl

ene,

ben

zene

, tol

uene

, ch

emic

al in

dustr

ies

xy

lene

, am

mon

ia, i

norg

anic

aci

ds, h

ydro

gen

sulfi

de, o

xide

s of s

ulfu

r and

nitr

ogen

,

isocy

anid

es, h

alog

ens,

etc.

Fr

eon-

22, c

hlor

ofl u

oroc

arbo

ns, m

ethy

l chl

orid

e, m

etha

nol,

etc.

Re

frige

rant

s and

coo

ling

syste

ms

V

inyl

chl

orid

e, m

ethy

l eth

yl k

eton

e, a

ceto

ne, m

etha

nol,

etha

nol,

chlo

rofo

rm, c

arbo

n Po

lym

er, r

ubbe

r, an

d te

xtile

indu

strie

s

tetr

achl

orid

e, te

trah

ydro

fura

n, is

opro

pano

l, m

ethy

l chl

orid

e, tr

ichl

oroe

thyl

ene,

ben

zene

,

tolu

ene,

xyl

ene,

inor

gani

c ac

ids,

etc.

Toxi

c ho

useh

old

Met

hane

, ker

osen

e, p

heno

l, cr

esol

, ino

rgan

ic a

cids

, sod

ium

hyd

roxi

de, b

leac

h,

Hou

se c

lean

ing

chem

ical

s iso

prop

anol

, 2-b

utox

yeth

anol

, nap

htha

lene

, etc

.

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n K

itche

ns

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, e

tc.

M

orph

olin

e, m

etha

nol,

etha

nol,

carb

on d

ioxi

de, c

arbo

n m

onox

ide,

etc

. M

edic

inal

and

hum

an h

abits

Wat

er-p

ollu

ting

C

arbo

n m

onox

ide,

car

bon

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, e

tc.

Diss

olve

d ga

ses i

n w

ater

chem

ical

s

Amm

onia

, ino

rgan

ic a

cids

, hyd

roge

n su

lfi de

, oxi

des o

f sul

fur a

nd n

itrog

en, i

socy

anid

es,

Rai

nwat

er

halo

gens

, etc

.

Am

mon

ia, i

norg

anic

aci

ds, h

ydro

gen

sulfi

de, o

xide

s of s

ulfu

r and

nitr

ogen

, iso

cyan

ides

, G

roun

dwat

er

halo

gens

, ars

enid

es, t

oxic

met

als,

etc.

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n M

arin

e po

llutio

n

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, p

etro

leum

pro

duct

s, he

avy

met

als a

nd th

eir

ox

ides

, etc

.

Che

mic

al

Chl

orin

e, c

hlor

opic

rin, h

ydro

gen

cyan

ide,

ars

ines

, psy

chot

omim

etic

age

nts,

toxi

ns,

Che

mic

al g

as w

eapo

nsw

eapo

ns

carb

on m

onox

ide,

car

bon

diox

ide,

pho

sgen

e, m

usta

rd g

as, t

ear g

as, l

ewisi

te, G

-ser

ies

ne

rve

agen

ts, V

-ser

ies n

erve

age

nts,

etc.

M

ercu

ry fu

lmin

ate,

lead

styp

hnae

, lea

d az

ide,

dyn

amite

, TN

T, R

DX

, PET

N, H

MX

, Ex

plos

ives

am

mon

ium

nitr

ate,

etc

.

Tabl

e 1.

1. S

ourc

es o

f tox

ic a

nd h

azar

dous

che

mic

als

in th

e en

viro

nmen

t

PO

LYM

ER

USE

S SP

ECIA

L FE

ATU

RES

R

EFER

ENC

ES

Cop

olym

ers o

f pol

y (E

DM

A-co

-MAA

) D

etec

tion

of te

rpen

e in

Pi

ezoe

lect

ric se

nsor

coa

ted

with

Pe

rciv

al e

t al.

2001

at

mos

pher

e m

olec

ular

impr

inte

d po

lym

er

Poly

ethy

lmet

hacr

ylat

e, c

hlor

inat

ed p

olyi

sopr

ene,

Id

entifi

cat

ion

of g

ases

and

Po

lym

erc

arbo

n bl

ack

com

posit

e fi l

m

Zee

and

Judy

200

1po

lypr

opyl

ene

(isot

actic

, chl

orin

ated

), sty

rene

/ ga

s mix

ture

sbu

tadi

ene,

ABA

blo

ck c

opol

ymer

, sty

rene

/et

hyle

ne/b

utyl

ene

ABA

bloc

k co

poly

mer

, po

lyep

ichl

oroh

ydrin

Nafi

on

Det

ectio

n of

eth

anol

gas

Fu

el c

ell w

ith p

olym

er e

lect

roly

te

Kim

et a

l. 20

00

conc

entr

atio

n m

embr

ane

Poly

anili

ne (P

ANI)

, PAN

Iac

etic

aci

d m

ixed

D

etec

tion

of N

O2

Laye

rs o

f pol

ymer

fi lm

s for

med

by

Xie

et a

l. 20

02fi l

m, P

ANI

poly

styre

nesu

lfoni

c ac

id (P

SSA)

Lang

mui

r-Bl

odge

tt (L

B) a

ndco

mpo

site

fi lm

self-

asse

mbl

y te

chni

ques

Poly

[3-(

buty

lthio

)thio

phen

e]

Gas

sens

or

Film

s of p

olym

er p

repa

red

via

LB

Rella

et a

l. 20

00

de

posit

ion

and

casti

ng

Poly

viny

l chl

orid

e (P

VC

) D

etec

tion

of N

O2 i

n ai

r So

lid p

olym

er e

lect

rode

of 1

0% P

VC

H

rnci

rova

et a

l. 20

00

Poly

pyrr

ole

nano

com

posit

e D

etec

tion

of C

O2,

N2,

CH

4 N

anoc

ompo

site

of ir

on o

xide

Su

ri et

al.

2002

ga

ses a

t var

ying

pre

ssur

es

poly

pyrr

ole

prep

81068754 chemical sensors fundamentals of sensing materials vol 3 polymers and other materials

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