chemical sensors: comprehensive sensor technologies, vol. 4: solid state sensors

Momentum Press is proud to bring you Chemical Sensors: Comprehensive Sensors Technologies: Volume 4: Solid-State Devices, the newest addition to The Sensors Technology Series, edited by Joe Watson. In this new volume, the reader will find descriptions of solid-state sensors such as conductometric or resistive gas sensors, Schottky-, FET-, capacitance-, and pyroelectric-type chemical sensors. Also included will be new applications with inte-grated chemical sensors. Inside, you will find background and guidance on:• Sensingandsamplingstrategies• Work-function-basedGasSensors• CalorimetricSensors,includingcatalyticbeaddevices,thin-filmandMEMSdevices,andsignalanalysis

and operating modes• NeweffortsonMicrocantilever-basedchemicalsensors• QuartzCrystalMicrobalancesensorapplications• SurfaceAcousticWavesensorsforchemicalapplications

Chemicalsensorsareintegraltotheautomationofamyriadindustrialprocesses,aswellaseverydaymoni-toring of such activities as public safety, testing and monitoring, medical therapeutics, and many more. TheChemicalSensorsreferencesbookswillspan6volumesandcoverin-depthdetailsonbothmaterials

used for chemical sensors and their applications, with volumes 1 through 3 exploring the materials used for chemical sensors — their properties, their behavior, their composition, and even their manufacturing and fabrication.Volumes4through6willexplorethegreatvarietyofapplicationsforchemicalsensors—frommanufacturing and industry to biomedical uses.

ABOUT THE EDITORGhenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and De-vices in1976andhisHabilitateDegree(Dr.Sci.) inPhysicsandMathematicsofSemiconductorsandDi-electricsin1990.HewasformanyyearstheleaderintheGasSensorGroupattheTechnicalUniversityofMoldova.HeiscurrentlyaresearchprofessoratGwangjuInstituteofScienceandTechnology,inGwangju,Republic of Korea. Dr. Korotcenkov is the author of five previous books and has authored over 180 peer-reviewedpapers.Hisresearchhasreceivednumerousawardsandhonors,includingtheAwardoftheSupremeCouncilofScienceandAdvancedTechnologyoftheRepublicofMoldova.

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CHEMICAL SENSORS VOLUME 4: SOLID-STATE DEVICESEdited by Ghenadii Korotcenkov, Ph.D., Dr. Sci.

A volume in the Sensors Technology SeriesEdited by Joe WatsonPublished by Momentum Press®

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Solid-State Devices

CHEMICAL SENSORSCOMPREHENSIVE SENSORS TECHNOLOGIESVOLUME 4: SOLID-STATE DEVICES

CHEMICAL SENSORSCOMPREHENSIVE SENSORS TECHNOLOGIESVOLUME 4:SOLID-STATE DEVICES

EDITED BYGHENADII KOROTCENKOV

GWANGJU INSTITUTE OF SCIENCE AND TECHNOLOGYGWANGJU, REPUBLIC OF KOREA

MOMENTUM PRESS, LLC, NEW YORK

Chemical Sensors: Comprehensive Sensors Technologies. Volume 4: Solid-State DevicesCopyright © Momentum Press®, LLC, 2011

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

First published in 2011 byMomentum Press®, LLC222 East 46th Street, New York, NY 10017www.momentumpress.net

ISBN-13: 978-1-60650-233-4 (hard back, case bound)ISBN-10: 1-60650-233-6 (hard back, case bound)ISBN-13: 978-1-60650-235-8 (e-book)ISBN-10: 1-60650-235-2 (e-book)DOI forthcoming

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

First Edition: March 2011

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v

CONTENTS

PREFACE TO CHEMICAL SENSORS: COMPREHENSIVE SENSORS TECHNOLOGIES xv

PREFACE TO VOLUME 4: SOLID-STATE DEVICES xix

ABOUT THE EDITOR xxi

CONTRIBUTORS xxiii

1 INTRODUCTION TO CHEMICAL SENSOR TECHNOLOGIES 1G. KorotcenkovB. K. Cho

1 Defi nitions and Classifi cations 12 A Brief History of Chemical Sensors 73 Motivations for Design of Chemical Sensors 94 What Determines Success in Chemical Sensor Design? 155 Materials for Chemical Sensors 17

5.1 Metal Oxides 175.2 Polymers 195.3 New Trends in Sensing Materials 21

6 Some Useful Defi nitions 297 Acknowledgments 34References 34

2 SENSING AND SAMPLING STRATEGIES 39M. Z. AtashbarS. Krishnamurthy

1 Introduction 392 Sensing Parameters 39

2.1 Sensitivity 40

vi CONTENTS

2.2 Selectivity 402.3 Response and Recovery Rates 412.4 Saturation 422.5 Resolution 422.6 Noise 42

3 Sensor Fundamentals 434 Sensor Test Methods 435 Sensor Calibration 446 Repeatability and Stability of Sensors 447 Signal Sampling and Data Processing 458 Signal Processing for Single Sensors 489 Signal Processing in a Multisensor Environment 49References 51

3 CONDUCTOMETRIC METAL OXIDE GAS SENSORS: PRINCIPLES OF OPERATION

AND APPROACHES TO FABRICATION 53G. KorotcenkovV. Sysoev

1 Introduction 532 Fundamentals of Gas Sensing Eff ects in Metal Oxide–Based Sensors:

Main Principles of Metal Oxide Gas Sensor Operation 542.1 Bulk-Conduction Model: Solid Electrolyte–Based Conductometric

Gas Sensors (High-Temperature Operation) 542.2 Ionsorption Model (Chemiresistors, Low-Temperature Operation) 552.3 Requirements for Metal Oxides to be Used at Low and High

Temperatures 652.4 Advantages and Disadvantages of Low- and High-Temperature

Operation of Gas Sensors 663 Metal Oxides Employed in Conductometric Gas Sensors 67

3.1 High-Temperature Sensors (Solid Electrolyte–Based Sensors) 673.2 Low-Temperature Sensors (Chemiresistors) 70

4 Approaches to Gas Sensor Fabrication 774.1 Ceramic Sensors 784.2 Planar Sensors 814.3 Features of Th ick-Film Technology 864.4 Th in-Film Technology 904.5 Kinetics of Metal Oxide Gas Sensor Response: Processes Controlling

the Rate of Sensor Response 93

CONTENTS vii

4.6 Th e Role of Th ermal Treatments in Gas Sensor Fabrication 964.7 Material Requirements for Packaging of Gas Sensors 98

5 Other Approaches to the Design of Conductometric Gas Sensors 995.1 Conductometric Sensors Based on 1-D Nanostructures 995.2 One-Electrode Gas Sensors 105

6 Miniaturization and Microfabrication 1186.1 Microfabrication 1186.2 Integrated Conductometric Gas Sensors 1266.3 Advantages and Disadvantages of Microfabrication 128

7 Approaches to Optimization (Improvement) of Conductometric Gas Sensor Parameters 129

7.1 Structure Control 1297.2 Bulk Doping and Surface Modifi cation 1397.3 Engineering Approaches to Improving Sensitivity 1447.4 Approaches for Improving Gas Sensor Selectivity 1477.5 Approaches to Optimizing the Rate of Sensor Response 1537.6 Stability 154

8 Sensor Manufacturers 1609 Outlook for the Future 161References 161

4 WORK FUNCTION–BASED GAS SENSORS: SCHOTTKY- AND FET-BASED DEVICES 187C. SenftP. IskraI. EiseleW. Hansch

1 Introduction 1872 Th eoretical Background: Gas Adsorption and Work Function Change 189

2.1 Gas Adsorption on Solid Surfaces 1892.2 Th e Work Function 191

3 Transducers 1923.1 Basic Principles of Gas-Sensing Devices 1923.2 Transducers for Interfacial Work Function Changes 1933.3 Transducers for Surface Work Function Changes 1973.4 Transducers for High-Temperature Operation 203

4 Application Example: Th e Temperature-Controlled Phase-Transition FET (TPT-FET) 211

4.1 Introduction 2114.2 Th e Sensing Eff ect 213

viii CONTENTS

4.3 Fabrication of the Sensor 2144.4 Work Function Change at Constant Temperature 2144.5 Temperature Dependence of the Work Function Change 2184.6 Operation of Temperature-Controlled Sensors 2194.7 Response Time 2214.8 Th e TPT-FET: A Benchmark 222

5 Summary 223

References 223

5 CAPACITANCE-TYPE CHEMICAL SENSORS 229S. ChatzandroulisV. TsoutiI. RaptisD. Goustouridis

1 Introduction 229

2 Permittivity Sensors 2302.1 Parallel-Plate Sensors 2312.2 Interdigitated Electrode Sensors 2322.3 Capacitance-Type Chemical Sensors on Flexible Substrates 2372.4 Materials Used in Capacitance-Type Chemical Sensors 238

3 Bimorph Capacitance-Type Chemical Sensors 2443.1 Parameters for Eff ective Chemical Sensing 2453.2 Cantilever Bimorph Sensors 2473.3 Membrane Bimorph Sensors 249

4 Outlook for Capacitance-Type Chemical Sensors 252

References 255

6 GAS SENSORS USING PYROELECTRIC AND THERMOELECTRIC EFFECTS 261W. ShinM. NishiboriI. Matsubara

1 Fundamentals of Sensor Operation 2611.1 Pyroelectricity 2611.2 Th ermoelectricity 262

2 Materials of Th ermal Energy Conversion 2622.1 Pyroelectric Materials 2632.2 Th ermoelectric Materials 264

CONTENTS ix

3 Fabrication and Packaging 2663.1 Transducer Films 2663.2 Catalyst Deposition 2673.3 Membrane Structure by Bulk Wet Etching 268

4 Parameters 2694.1 Transducer Performance 2694.2 Detectivity of Pyroelectric Devices 2704.3 Seebeck Coeffi cients of Th ermoelectric Devices 2704.4 Catalyst Performance 2714.5 Catalyst Parameters 272

5 Approaches to Optimization of Sensor Parameters 2725.1 Pyroelectric Sensors 2725.2 AC Pyroelectric Sensors 2735.3 Th ermoelectric Sensors 2745.4 Long-Term Stability of the Ceramic Catalyst 2775.5 Gas Selectivity 279

6 Fields of Application and Market for Sensors 280

7 Summary 283

References 283

7 CALORIMETRIC SENSORS 287R. E. Cavicchi

1 Introduction 2872 Fundamentals of Calorimetric Sensor Operation 2883 Catalytic Bead Devices 2924 Th in-Film and MEMS Devices 2925 Materials 2996 Poisoning 3017 Signal Analysis and Operating Modes 3038 Packaging 3059 Applications 309

9.1 Gas Detection 3099.2 Biological Applications 3129.3 Safety 313

10 Conclusions 315References 315

x CONTENTS

8 MICROCANTILEVER-BASED CHEMICAL SENSORS 321S. K. VashistG. Korotcenkov

1 Introduction 321

2 Microcantilevers and Th eir Modes of Operation 3222.1 Operating Modes for Cantilever Mass Sensors 323

3 Microcantilever Defl ection Detection Methods 3243.1 Optical Method 3253.2 Piezoresistive Method 3263.3 Capacitive Method 3273.4 Piezoelectric Method 3283.5 Interferometry Method 3283.6 Optical Diff raction Grating Method 3283.7 Charge-Coupled Device Detection Method 328

4 Resonant Operating Mode 3294.1 Mechanical Properties of Microcantilevers 3294.2 Mass Resolution Limitations 3304.3 Infl uence of Surrounding Conditions 331

5 Bending Behavior of Microcantilevers 332

6 Excitation Techniques 3347 Fabrication of Microcantilevers 335

7.1 Silicon-Based Microcantilevers 3377.2 Polymer-Based Microcantilevers 340

8 Surface Functionalization 3428.1 General Strategy 3428.2 Sorption-Induced Eff ects and Th eir Infl uence on Cantilever Operation 3438.3 Functionalization Methods 3458.4 Immobilization of Bioreceptors 348

9 Microcantilever-Based Sensors 34910 Applications of Cantilever-Based Chemical Sensors 351

10.1 Gas Sensing 35110.2 Detection of Herbicides 35510.3 Detection of Metal Ions 35510.4 Humidity Sensing 35610.5 Detection of Volatile Organic Compounds 35610.6 Detection of Tributyrin 35910.7 Monitoring of Missile Storage and Maintenance Needs 35910.8 pH Sensing 359

CONTENTS xi

11 Biosensing Applications 36011.1 Detection of DNA 36011.2 Detection of Prostate-Specifi c Antigen 36111.3 Detection of Hydrogen Peroxide 36111.4 Detection of Myoglobin 36211.5 Detection of Lipoproteins 36211.6 Detection of Glucose 362

12 Ultrasensitive Nanocantilevers 36313 An Electronic Nose Based on a Micromechanical Cantilever Array 36414 Commercial Status 36715 Outlook and Future Trends 367

References 368

9 THE QUARTZ CRYSTAL MICROBALANCE 377M. VoinovaM. Jonson

1 Introduction 3771.1 Short History of the QCM Approach 3781.2 Principle of Biosensoring 380

2 Piezoelectric Materials 3812.1 Piezoelectric Materials Other Th an Quartz 3812.2 Quartz as a Piezoelectric Material 384

3 Basics of QCM Operation 3863.1 Principles of the QCM 3863.2 Construction and Stability of the QCM 3873.3 Confi guration of QCM Electrodes 3913.4 Miniaturization and Integration in Arrays; the MQCM 3943.5 QCM-D Technique 3953.6 Some Disadvantages of the QCM 397

4 Th eoretical Analysis of the QCM Response 3974.1 Physical Analysis of the Propagation of Transverse Shear Waves in a

Loaded Quartz Resonator 3974.2 Equivalent Circuit Models 4034.3 Simultaneous Viscoelastic and Liquid Loading of the Quartz

Resonator: Th ree Models 4064.4 Newtonian Liquid–Loaded Quartz Resonator 4074.5 Th in Viscoelastic Layer Loading 4094.6 Mass Loading of the Quartz Resonator: A Short Summary 410

xii CONTENTS

5 Applications of QCM-Based Sensors 4105.1 Gas Sensors 4105.2 QCM Coatings: Basic Requirements for Sensing Layer 4145.3 Liquid-Phase Measurements; Electrochemical QCM 4155.4 Nanotribology Challenges 4165.5 QCM Biosensors: Selected Examples 421

6 Outlook 4287 Quartz Crystal and Quartz Crystal Microbalance Companies 4298 Nomenclature 4299 Acknowledgments 43110 Recommendations for Further Reading 431References 431

10 SURFACE ACOUSTIC WAVE SENSORS FOR CHEMICAL APPLICATIONS 447Adeel AfzalFranz L. Dickert

1 Introduction 447

2 State-of-the-Art SAW Sensors 4482.1 Principle—Th e Piezoelectric Eff ect 4482.2 Piezoelectric Materials 4482.3 Design—Interdigital Transducers 4492.4 Fabrication—Photolithography 4502.5 Sensor Eff ect—Th e Sauerbrey Equation 4512.6 SAW Sensors in Liquids 4552.7 SAW Sensor Characteristics 456

3 Coating Materials 4573.1 Nonselective Polymers 4573.2 Conductive Polymers 4583.3 Host–Guest Chemistry 4593.4 Imprinting 4603.5 Self-Assembled Monolayers 4623.6 Other Coating Materials 463

4 Applications 4644.1 Gases 4644.2 Organic Vapors 4664.3 Liquids 471

5 Comparing Surface and Bulk Acoustic Wave Devices 471

CONTENTS xiii

6 Th e Market for SAW Sensors 4717 Abilities and Limitations of SAW Sensors 4738 Outlook 475References 475

11 INTEGRATED CHEMICAL SENSORS 485M. E. ZaghloulI. Voiculescu

1 Introduction 4852 CMOS Technology 486

2.1 Overview of CMOS Technology 4862.2 Micromachining CMOS 4872.3 Gas Sensor Fabricated Using Industrial CMOS Technology

Developed at Eth Zürich 4892.4 Electrochemical Sensors Fabricated Using Standard CMOS

Technology 4923 NIST CMOS MEMS Technology 494

3.1 Pecularities of NIST CMOS MEMS Technology 4953.2 Conductometric Chemical Gas Sensor Using NIST CMOS MEMS

Technology 4974 CMU CMOS MEMS Technology 498

4.1 CMU CMOS MEMS Fabrication Technology Overview 5004.2 Resonant Microbeam Gas Sensor Fabricated Using CMU CMOS

MEMS Technology 5035 Jazz SiGe BiCMOS Technology 504

5.1 Jazz SiGe BiCMOS Technology Overview 5055.2 Gas Sensor Fabricated Using Jazz SiGe BiCMOS Technology 505

6 Other Types of MEMS Gas Sensors Integrated in CMOS Technology 5076.1 SAW Resonator as Chemical Gas Sensor Integrated in Industrial CMOS Technology 5086.2 Optical Chemical Sensors in CMOS Technology 509

7 Conclusions 511References 511

INDEX 515

xv

PREFACE TO CHEMICAL SENSORS:

COMPREHENSIVE SENSORS TECHNOLOGIES

In spite of their century-long history, chemical sensors appeared on the commercial market only 50 years ago. In recent years, however, the fi eld of chemical sensors has broadened and expanded greatly. At present, chemical sensors are being used in medicine, agriculture, industry, transport, environmental control, and other fi elds. However, the process of developing new sensors as well as improving older types of chemical sensors is still ongoing. New technologies and the toughening of ecological stan-dards require more sensitive instruments with faster response times, better selectivity, and improved stability. Th e second half of this six-volume series on chemical sensors, devoted to comprehensive sensor technologies, describes these developments and the new processes and applications. Th ese volumes are intended to be a primary source for both fundamental and practical information about where sensor technologies are now and where they are headed for the future. We are sure that Volumes 4–6 in this series will be a useful addition to the fi rst three volumes, on fundamentals of sensing materials, in which various sensing materials that can be used in chemical sensors are discussed in detail. Analysis of chemi-cal sensor design, fabrication, and functioning requires other approaches to description in comparison with materials science problems, and therefore we decided that consideration of materials and devices should be carried out separately. From our point of view, dividing the series into two parts as we have done results in more logical narration and more utility for readers who are interested in diff erent aspects of chemical sensor design.

In this series we provide readers with a thorough understanding of the concepts behind chemical sensors, presenting the information necessary to develop such sensors, covering all aspects including fundamental theories, fabrication, functionalization, characterization, and real-world applications, so as to enable them to pursue their research and development requirements. Th erefore, we hope that this series will help readers understand the present status of chemical sensors and will also act as an introduc-tion, which may encourage further study, as well as an estimate of the roles that chemical sensors may play in the future.

Chemical Sensors: Comprehensive Sensor Technologies is a three-volume series, comprising Volumes 4, 5, and 6 in our series, Chemical Sensors. Volume 4 deals with solid-state devices, Volume 5 with electro-chemical and optical sensors, and Volume 6 with applications of chemical sensors. Th e chapters in-cluded in the volumes consist of review and overview papers written by experts in the fi eld. Th e authors

xvi PREFACE TO CHEMICAL SENSORS: COMPREHENSIVE SENSORS TECHNOLOGIES

of each of the chapters were chosen very carefully and are all well known throughout the world in their fi elds of study. Th erefore, these books provide an up-to-date account of the present status of chemical sensors, from fundamental science and processing to applications.

Specifi cally, Volume 4 includes descriptions of solid-state sensors such as conductometric or resis-tive gas sensors, Schottky-, FET-, capacitance-, and pyroelectric-type chemical sensors. Pellistors, mass- sensitive, and acoustic wave sensors are described as well. Integrated chemical sensors are also discussed in Volume 4. Volume 5 provides information related to electrochemical and optical sensors. Fundamentals of operation, methods of fabrication, and operating characteristics of electrochemical gas sensors, solid electrolyte–based gas sensors, ion-selective electrodes, CHEMFETs, and diff erent types of optical, fi ber optical, and chemoluminescence chemical sensors are discussed. Volume 6 is dedicated to detailed ex-amination of opportunities for applications of chemical sensors in various areas of our lives, including medicine, industry, environmental control, agriculture, and transportation. It is the editor’s wish that theses volume will provide the reader with a detailed understanding of the many applications of chemi-cal sensors in both today’s world and that of the future. In these chapters one can also fi nd descriptions of architecture and fundamentals of “electronic noses” and “electronic tongues,” principles of wireless chemical sensor design, and possibilities for remote chemical sensing for atmospheric monitoring.

In this three-volume series, the authors present sensors that utilize various sensing materials and phenomena. Th e terminology and concepts associated with sensors are presented, including some of the relevant physical and chemical phenomena applied in the sensor signal transduction system. As is well known, chemical sensing is multidisciplinary by nature. Th e role of sensing materials in such phenomena is also detailed.

We need to note that the number of disciplines involved in the research and design of chemical sen-sors has increased dramatically. New knowledge and approaches are needed to achieve miniaturization, lower power consumption, and the ability to operate in complex environments for more selective, sensi-tive, and rapid determination of chemical and biological species. Compact analytical systems that have a sensor as one of the system components are becoming more important than individual sensors. Th us, in addition to traditional sensor approaches, a variety of new themes have been introduced to achieve the attractive goal of analyzing chemical species on the micro and nano scales. Th erefore, throughout these books, numerous strategies for the fabrication and characterization of sensing materials and sensing structures which are employed in sensing applications are provided, and current approaches for chemical sensing are described.

Th is series can be utilized as a text for researchers and engineers as well as graduate students who are either entering the fi eld for the fi rst time, or who are already conducting research in these areas but are willing to extend their knowledge of the fi eld of chemical sensors. We hope that these volumes will also be of interest to undergraduate students in chemical engineering, electronics, environmental control, and medicine. Th ese books have been written in a way that fi nal-year and graduate university students in the fi elds of chemistry, physics, electronics, biology, biotechnology, mechanics, and bioengineering can easily comprehend. We believe that practicing engineers or project managers which would like to use chemical sensors but don’t know how to do so, and how to select optimal chemical sensors for specifi c applications, also will fi nd useful information.

It is necessary here to comment briefl y on the coverage of the literature. During our work on this series we tried to cover the fi eld more or less completely. However, we need to acknowledge that an

PREFACE TO CHEMICAL SENSORS: COMPREHENSIVE SENSORS TECHNOLOGIES xvii

appreciable number of relevant papers may remain unknown to the authors. Regarding these, the editors and contributing authors express regret, not only to the authors of such works, but also to the readers of our books.

Finally, we wish to thank all those who participated in the preparation of this series, including the contributing authors and copyright owners in Europe, the United States, Asia, and the rest of the world. We also wish to express our gratitude to the staff of Momentum Press, and in particular Joel Stein, for his kind assistance in bringing these volumes to fruition.

Ghenadii Korotcenkov

xix

PREFACE TO VOLUME 4: SOLID-STATE DEVICES

Th e fi eld of solid-state chemical sensor design is a research fi eld of increasing interest as a result of the demands for reliable, inexpensive, and portable systems for environmental monitoring, assessing indoor air quality, food quality control, military, and many other applications. Solid-state chemical sensors, because they can be microfabricated using modern technologies of mass production, may be able to realize those requirements in practice. Solid-state sensor technology has advanced remarkably during the past few decades and is rapidly becoming an essential technology. As a result, many solid-state chemical sensors are now commercially available, and researchers are working to develop next-generation solid-state sensors that have all the necessary requirements, including small size, low production costs, and low power consumption.

Th e goal of this volume is to provide a critical assessment of the new trends in the fi eld of solid-state chemical sensors, by describing the working principle and the applications related to the diff er-ent types of solid-state sensors. In this volume the reader will fi nd detailed descriptions of solid-state chemical sensors such as conductometric gas sensors, Schottky, FET, and work-function chemical sen-sors. Capacitance, pyroelectric, calorimetric, mass-sensitive, and acoustic wave chemical sensors are also analyzed in detail. Reduction in size from bulk to polycrystalline and nanostructured sensing materials, from micro- to nano-sized transducers, while promising high sensitivity, small size, high speed, low cost, and increased selectivity, requires new design considerations that should consider factors such as integra-tion with other devices and device lifetime. At present, microfabrication has reached a stage of serious application and is accepted as a good alternative to classical “macroscopic” technologies. Th erefore, approaches to microfabrication of chemical sensors and to integration are discussed in this volume as well. Future trends in solid-state sensors design are also described. So, the volume gives a survey of the latest state of technology and contributes to preparing the ground for future achievements in both solid-state sensor research and development of solid-state sensor technologies.

We believe that this book covers all topics of solid-state chemical sensors from fundamentals of operation and construction of devices to optimal materials for those sensors and approaches to achieving better operating characteristics. Th e reader will fi nd a strong emphasis not only on “what” and “how,” but also on “why.” Specialists and newcomers to the fi eld will fi nd this book easy to use. Each chapter has its own introduction and list of references, in order to make it accessible to any reader notwithstanding his or her background in related subjects. Since the last decade has seen an enormous amount of activity

xx PREFACE TO VOLUME 4: SOLID-STATE DEVICES

in the fi eld of solid-state sensor systems, this book represents a valuable and accessible guide and refer-ence for researchers with up-to-date examples and state-of-the-art results.

We need to admit that a number of edited surveys and monographs related to solid-state chemical sensors have been published during recent decades. However, the present volume analyzes this fi eld of science and technology both fully and in detail. In addition, the majority of published books were writ-ten more than 10 years ago, which is a long period of time for such a rapidly developing fi eld as chemical sensors. Since then many new technologies and new ideas have appeared and been realized.

Th is book is intended for scientists, engineers, and manufacturers involved in the development, design, and application of solid-state chemical sensors. Undergraduate and graduate students can use this book to extend their knowledge in the fi eld of chemical sensors.

Ghenadii Korotcenkov

xxi

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. Korotcenkov’s research results in the study of Schottky barriers, MOS structures, native oxides, and photoreceivers based on Group III–V com-pounds 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 eight books and special publica-tions, 11 review papers, 10 book chapters, and more than 180 peer-reviewed articles. He holds 18 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.

xxiii

Adeel Afzal (Chapter 10)Department of Analytical ChemistryUniversity of ViennaVienna A-1090, Austria

Massood Zandi Atashbar (Chapter 2)Department of Electrical and Computer EngineeringWestern Michigan UniversityKalamazoo, Michigan 49008-5066, USA

Richard E. Cavicchi (Chapter 7)Chemical Science and Technology LaboratoryNational Institute of TechnologyGaithersburg, Maryland 20899, USA

Stavros Chatzandroulis (Chapter 5)Institute of Microelectronics NCSR “Demokritos”Aghia Paraskevi 15310, Greece

Beongki Cho (Chapter 1)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of Korea

Franz L. Dickert (Chapter 10)Department of Analytical ChemistryUniversity of ViennaVienna A-1090, Austria

Ignaz Eisele (Chapter 4)Fraunhofer-Gesellschaft MunichMunich 80686, Germany

CONTRIBUTORS

xxiv CONTRIBUTORS

Dimitrios Goustouridis (Chapter 5)Institute of MicroelectronicsNCSR DemokritosAghia Paraskevi 15310, Greece

Walter Hansch (Chapter 4)University of the Federal Armed Forces of GermanyNeubiberg 85579, Germany

Mats Jonson (Chapter 9)Department of Physics University of GothenburgSE-412 96 Goteborg, SwedenandDivision of Quantum Phases and DevicesSchool of PhysicsKonkuk UniversitySeoul 143-701, Republic of KoreaandSUPA, Department of Physics Heriot-Watt UniversityEdinburgh EH14 4AS, Scotland, UK

Peter Iskra (Chapter 4)University of the Federal Armed Forces of GermanyNeubiberg 85579, Germany

Ghenadii Korotcenkov (Chapters 1, 3, and 8)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of KoreaandTechnical University of MoldovaChisinau, Republic of Moldova

Sridevi Krishnamurthy (Chapter 2)Department of Electrical and Computer EngineeringWestern Michigan UniversityKalamazoo, Michigan 49008-5066, USA

Ichiro Matsubara (Chapter 6)National Institute of Advanced Industrial Science and Technology (AIST)Nagoya 463-8560, Japan

CONTRIBUTORS xxv

Maiko Nishibori (Chapter 6)National Institute of Advanced Industrial Science and Technology (AIST)Nagoya 463-8560, Japan

Ioannis Raptis (Chapter 5) Institute of MicroelectronicsNCSR DemokritosAghia Paraskevi 15310, Greece

Christoph Senft (Chapter 4)University of the Federal Armed Forces of GermanyNeubiberg 85579, Germany

Woosuck Shin (Chapter 6)Advanced Manufacturing Research InstituteAIST Shimo-shidamiMoriyama-ku, Nagoya 463-8560, Japan

Victor Sysoev (Chapter 3)Department of Physics Saratov State Technical University Saratov 410054, Russia

Vasiliki Tsouti (Chapter 5)Department of Applied SciencesNational Technical University of AthensZografou 15780, GreeceandInstitute of MicroelectronicsNCSR DemokritosAghia Paraskevi 15310, Greece

Sandeep Kumar Vashist (Chapter 8)Centre for Bioanalytical SciencesNational Centre for Sensor Research Dublin City UniversityDublin 9, IrelandandNational University of SingaporeSingapore 117580

xxvi CONTRIBUTORS

Ioana Voiculescu (Chapter 11)Mechanical Engineering DepartmentCity College of New YorkNew York, New York 10031, USA

Marina Voinova (Chapter 9)BionanoSystems LaboratoryDepartment of Applied Physics Chalmers University of Technology Göteborg S-412 96, Sweden

Mona E. Zaghloul (Chapter 11)Electrical Engineering and Computer Science DepartmentGeorge Washington UniversityWashington, DC 20052, USA

1

CHAPTER 1

INTRODUCTION TO CHEMICAL SENSOR TECHNOLOGIES

G. Korotcenkov B. K. Cho

1. DEFINITIONS AND CLASSIFICATIONS

Th e Oxford English Dictionary defi nes a sensor as “a device which detects or measures some condition or property, and records, indicates, or otherwise responds to the information received.” Accordingly, sensors have the function of converting a stimulus into a measurable signal (Figure 1.1). Th e stimulus can be a physical stimulus such as temperature, pressure, or acceleration; alternatively, the stimulus can be a concentration of specifi c chemical or biological materials. While the measured signal is typically electrical, it may alternatively be pneumatic, hydraulic, or optical. Sensors are based on a broad range of underlying physical operating principles. One can usually separate three basic components of a sensor: (1) a sensor element; (2) sensor packaging and connections; and (3) sensor signal processing hardware. While they can be used in all three phases of matter, sensors for gas and liquid environments are the most common (Shieh et al. 2001).

Generally, sensors can be divided into three major categories: (1) physical sensors; (2) chemical sensors; and (3) biosensors. In this volume we consider chemical sensors. Chemical sensors are widely used in many diff erent applications, and chemical sensing technology has become a basic enabling technology in many fi elds (see Table 1.1). As will be shown here, interest in chemical sensors has grown as production applications, such as intelligent manufacturing processing, have proliferated. Sensors are also of great importance in safety-related areas, with applications ranging from assessing the integrity of

2 CHEMICAL SENSORS: TECHNOLOGIES. VOLUME 4: SOLID-STATE DEVICES

Figure 1.1. (a) Basic functioning of a sensor. (b) Evolution of the stimulus (sensor input) p over time. A stimulus is applied at time t1 (response process) and removed at time t2 (recovery process). (c) Evolution of sensor response x = f(t) over time. The changes in the stimulus determine correlated changes in the sensor response (sensor output). Changes in the sensor response need some time before an equilibrated sensor response value is achieved during response and recovery processes (response and recovery times, correspondingly).

MARKET/APPLICATION EXAMPLES OF DETECTED CHEMICAL COMPOUNDS AND CLASSES

Automotive O2, H2, CO, NOx, HCsIndoor air quality CO, CH4, humidity, CO2, VOCsFood Bacteria, biologicals, chemicals, fungal toxins, humidity, pH, CO2Agriculture NH3, amines, humidity, CO2, pesticides, herbicides Medical O2, glucose, urea, CO2, pH, Na+, K+, Ca2+, Cl−, biomolecules, H2S, infectious disease, ketones, anesthesia gasesWater treatment pH, Cl2, CO2, O2, O3, H2SEnvironmental SOx, CO2, NOx, HCs, NH3, H2S, pH, heavy metal ionsIndustrial safety Indoor air quality, toxic gases, combustible gases, O2Utilities (gas, electric) O2, CO, HCs, NOx, SOx, CO2Petrochemical HCs, conventional pollutantsSteel O2, H2, CO, conventional pollutantsMilitary Agents, explosives, propellantsAerospace H2, O2, CO2, humidity

HCs, hydrocarbons; VOCs, volatile organic compounds.Source: Reprinted with permission from Stetter et al., 2003. Copyright 2003 ECS.

Table 1.1. Example applications and markets for chemical sensors

INTRODUCTION TO CHEMICAL SENSOR TECHNOLOGIES 3

aircraft to monitoring the environment for hazardous chemicals. Chemical sensors have proven to be useful in many other domestic and military applications as well, such as the remote sensing of various chemical vapors and chemical warfare agents.

Th e literature provides many diff erent defi nitions of chemical sensors (IEC-draft 65/84 1982; Madou and Morrison 1989; Janata 1989; Moseley and Tofi eld 1989; Hulanicki et al. 1991; Stetter et al. 2003). Most often, chemical sensors are considered to be devices that are designed to detect and quantify a specifi c chemical analyte or event. For example, according to the International Union of Pure and Applied Chemistry (IUPAC), a chemical sensor is a device that can detect and signal the presence and/or quantify the concentration of a family of chemicals or a specifi c chemical exposed to the sensor. According to Hulanicki et al. (1991), a chemical sensor is a device that transforms chemical information, ranging from the concentration of a specifi c sample component to total composition analysis, into an analytically useful signal. Such chemical information may originate from a chemical reaction of the ana-lyte or from a physical property of the system under investigation. Göpel and Schierbaum (1991), defi ne chemical sensors as devices which convert a chemical state into an electrical signal. Th is simple defi nition is probably the one that best fi ts the purposes of the present work. Chemical state here must be understood to mean diff erent concentrations or partial pressures of molecules or ions in a gas, liquid, or solid phase. Unless otherwise specifi ed, it is usually assumed that these chemical sensors are just the primary link of the measuring chain—in other words, an interface between the chemical world and the electronics.

Chemical sensors diff er from physical sensors, which measure physical parameters. It is necessary to note that chemical sensors are more complex extensions of physical sensors. However, in many cases the transducer technologies developed and commercialized for physical sensors are the basis for chemi-cal sensors and biosensors. A chemical sensor is an essential component of an analyzer. In addition to the sensor, the analyzer may contain devices that perform the following functions: sampling, sample transport, signal processing, and data processing. An analyzer may be an essential part of an automated system. Th e analyzer, which works according to a time-dependent sampling plan, acts as a monitor.

Th e distinction between chemical sensors and biosensors is more complex. Many authors attempt to defi ne a sensor based on the nature of the analyte detected. In this text, we distinguish between chem-ical sensors and biosensors according to the nature of their reactive surface. By this defi nition, biosensors are devices which contain a biomolecule (such as an enzyme, antibody, or receptor) or a cell as the active detection component. Again, the nature of the analyte and the reaction which leads to detection are not limited in this defi nition.

Some typical properties associated with chemical sensors, according to Stetter and Penrose (2002), are as follows:

• A sensitive layer is in chemical contact with the analyte. • A change in the chemistry of the sensitive layer (a reaction) is produced after exposure to the analyte. • Th e sensitive layer is on a platform that allows transduction of the change to electric signals. • Th e sensors are physically “small.” • Th ey operate in real time. • Th ey do not necessarily measure a single or simple physical or chemical property. • Th ey are typically less expensive and more convenient than an equivalent instrument for the same

chemical measurements.


It follows from these defi nitions that every chemical sensor can be divided into two domains: the physical transducer and the chemical interface layer (receptor) (see Figure 1.2). Some sensors may in-clude a separator which is, for example, a membrane. At the chemical interface, the analyte interacts chemically with a surface, producing a change in physical/chemical properties. Within the receptor part of a sensor, the chemical information is transformed into a form of energy that may be measured by the transducer. Receptor layers can respond selectively to particular substances or to a group of substances. Th e term molecular recognition is used to describe this behavior. Th e transducer part of a sensor is a device capable of transforming the energy carrying the chemical information about the sample into a useful analytical signal. Th e transducer itself does not show selectivity.

Th e receptor part of chemical sensors is based on the following principles (Hulanicki et al. 1991):

• A physical basis in which no chemical reaction takes place (Optical receptors, for example, are based on measurement of absorbance, refractive index, conductivity, temperature, or mass change.)

• A chemical basis in which a chemical reaction with participation of the analyte gives rise to the analytical signal

• A biochemical basis in which a biochemical process is the source of the analytical signal

Note that in some cases it is not possible to determine unequivocally whether a sensor operates on a chemical or on a physical principle. Th is is the case, for example, when the signal is due to an adsorp-tion process.

Th e types of reactions that may occur at the surface of chemical sensors are summarized in Table 1.2. Th is classifi cation is important because chemical parameters (such as the type of chemical reaction, the equilibrium constant, and kinetic parameters) will determine a sensor’s performance, including sen-sitivity and selectivity (Stetter and Penrose 2002).

Among the processes of interaction, those most important for chemical sensors are adsorption (chemisorption), ion exchange, and liquid–liquid extraction (partition equilibrium). Th ese phenomena occur primarily at the interface between the analyte and the receptor surface, where both are in an equilibrium state. Instead of equilibrium, a chemical reaction may also become the source of information. We fi nd this, for example, in receptors in which a catalyst accelerates the rate of an analyte reaction so much that the heat released by the reaction creates a temperature change that can be transduced into an electrical signal (Grendler 2007).

Transducers transfer the results of interactions between the chemical analyte and sensing materials into a measurable signal; some examples are described in Table 1.3. Th e change in the transducer due to

Figure 1.2. Schematic presentation of a chemical sensor. (Idea from Rahman et al. 2008.)


the active surface event is expressed as a specifi c signal which may record changes in impedance, voltage, light intensity, refl ectance, weight, color, or temperature. Th at signal is then detected, amplifi ed, and processed by the electronics/software module.

Taking into account transduction mechanisms, we can distinguish fi ve general categories of sensors: (1) electrochemical sensors; (2) mass sensitive sensors; (3) calorimetric sensors; (4) magnetic sensors; and (5) optical sensors (see Figure 1.3).

Th ese sensors are diff erentiated on the basis of their underlying physical principles and operat-ing mechanisms. For example, electrochemical sensors include sensors that detect signal changes (e.g., resistance) caused by an electrical current having passed through electrodes due to interaction with

TRANSDUCTION MECHANISM OUTPUT CHANGE EXAMPLES

Electrochemical Current, voltage, capacitance/ Gas sensors, ISEs, FETs, CHEMFETs, impedance conductimeters, pH sensors, interdigitated electrode capacitorsPhotometric (radiant) Light intensity, color, or Optical sensors, fi ber optic sensors, emission spectra infrared sensors, ellipsometry, internal refl ectometry, laser light scattering, surface plasmon resonanceAcoustical/mechanical Weight, amplitude, phase or SAW devices, QCM-based sensors, frequency, size, shape, position cantileversMagnetic Field strength, fi eld detection BiosensorsCalorimetric (thermal) Temperature, heat fl ow, heat Th ermoelectric sensors, pyroelectric content sensors, pellistors

FETs, fi eld-eff ect transistors; ISEs, ion selective electrodes; SAW, surface acoustic wave; QCM, quartz crystal microbalance.

CHEMICAL REACTION AT THE INTERFACE EXAMPLE

Adsorption A[gas] + S[surface site] = AS[surface]; Kads

Physisorption Vads = Ae−dG/kT

Chemisorption Partitioning K = C

m/C

s

Acid–base HA + KOH = H2O + K

+ + A

− ; K

a or K

b Precipitation Ag

+[aq] + NaCl[aq] = AgCl[s] + Na

+[aq]; K

sp Ion exchange H

+[aq] + Na

+[surface] = H

+[surface] + Na

+[aq]; K

i

Oxidation/reduction CO + ½O2 = CO

2; K

r × n

Source: Data from Stetter and Penrose 2002.

Table 1.2. Possible reactions at the chemical interface of chemical sensors

Table 1.3. Major sensor transducer technologies


chemicals. Such eff ects may be stimulated electrically or they may result from a spontaneous interaction at the zero-current condition.

Mass-sensitive sensors rely on disturbances and changes to the mass of the sensor surface during inter-action with chemicals. Mass-sensitive devices transform the mass change at a specially modifi ed surface into a change in some property of the support material. Th e mass change is caused by accumulation of the analyte during interaction with the sensing layer.

Magnetic sensors are based on the change of paramagnetic properties of the analyte being analyzed. Th ese are represented by certain types of oxygen monitors. (We will not examine magnetic sensors.)

Optical sensors transform changes in optical phenomena that result from an interaction of the ana-lyte with the receptor part. Th is category may be further subdivided according to the type of optical properties relevant to a particular sensor, for example, absorbance, refl ectance, refractive index; opto-thermal eff ect, or light scattering. More generally, optical sensors detect changes in visible light or other electromagnetic waves during interactions with chemicals. Th e use of optical fi bers in various confi gura-tions has enabled applications of optical sensors.

Of course, sensors may exhibit characteristics that fall into more than one of these fi ve broad cat-egories. For example, some mass sensors may rely on electrical excitation or optical settings. Even so, our fi ve categories of sensors are suffi ciently distinct for the purposes of this review.

We need to point out that our classifi cation represents but one of the possible alternatives. One alter-native scheme classifi es sensors, not according to the primary eff ect, but by the method used for measuring

CHEMICAL SENSORS

Electro-Chemical

Optical

Solid-state (electronic)

Mass-sensitive

Calorimetric

Potentiometric

Culonometric

Conductometric

Surface Plasmon

Amperometric

Chemi-luminescence

Fluorescence

Spectro-photometric

Fiber-optic

Quartz crystal microbalance

Cantilever

Catalytic (Pellistors)

Surface acoustic wave

FET

Work function

Schottky diodes

Conductometric

CHEMFET Thermo-electric

Ion-selective electrodes Capacitance

Pyroelectric

Thermal conductivity

Photo-acoustic

Magnetic

Figure 1.3. Types of chemical sensors which can be fabricated and applied.


that eff ect (an example is provided by the so-called catalytic devices, in which the heat eff ect evolved in the primary process is measured by the change in the conductivity of a thermistor). Another alternative scheme classifi es sensors according to the application to detect or determine a given analyte (examples are sensors for pH, for metal ions, or for determining oxygen or other gases). Yet another basis for the classi-fi cation of chemical sensors might be by the mode of application (for example, sensors intended for use in vivo, sensors for process monitoring, and so on). Of course, it is possible to use various classifi cations as long as they are based on clearly defi ned and logically arranged principles (Hulanicki et al. 1991).

Finally, we note that there is no single sensing class or sensing technology that can eff ectively detect everything of possible interest in every possible environment. As we will show, every type of sensor has both advantages and disadvantages.

2. A BRIEF HISTORY OF CHEMICAL SENSORS

Chemical sensors are relatively new measurement devices. Until about 45 years ago, the glass pH elec-trode could be considered the only portable chemical sensor suffi ciently reliable for measuring a chemi-cal parameter (Schultz and Taylor 1996; Vlasov and Legin 1998). Even that sensor, which has been under continuous development since it invention in 1922 (Table 1.4), needs to be recalibrated on a daily basis and is limited to measurements in solutions or on wet surfaces.

Other sensing technologies based on oxidation–reduction reactions at electrodes were extensively pursued in the 1940s and 1950s and provided analytical methods for the detection of metallic ions and some organic compounds. Th e fi rst application of these electrochemical techniques to make a sensor was

(continued on following page)

Table 1.4. Historical landmarks in the development of chemical sensors

YEAR DEVELOPER EVENT

1904 Kohlraush F. and Nernst W. Th ermal conductivity–based gas sensor1909 Haber F. and Klemensiewicz Z. Development of glass electrode1922 Hughes W.S. First glass pH electrode1925 Kerridge P.T. First blood pH electrode1931 Kautsky H. and Hirsch A. First oxygen sensor (non–fi ber optic)1936 Beckman Corp. Commercial production of pH meter1937 Kolthoff J.M. Crystalline “electrode”1937 Nikolsky B.P. Crystalline membrane1953 Brattain W. and Bardeen J. Gas adsorption infl uence on semiconductor conductance1954 Clark L.C. Jr. Invention of the oxygen electrode1954 Stow R.W. and Randall B.F. Invention of the pCO2 electrode1959 Baker A. Catalytic bead gas sensor (pellistor)1961 Pungor E. Heterogeneous solid ion-selective electrode1962 Clark L.C. Jr. and Lyons C. First amperometric biosensor 1962 Seiyama T. and Taguchi N. Semiconductor (ZnO) gas sensor1963 Goto K. and St. Pierre G.R. Solid electrolyte gas sensor


YEAR DEVELOPER EVENT

1964 King W.H. Jr. Coated piezoelectric quartz crystals as sensors for water, hydrocarbons, polar molecules, and hydrogen sulfi de1966 Simon W. Liquid ion-selective electrode with neutral carrier1967 Ross J.W. Ion-exchange membrane1969 Guilbault G.G. and Montalvo J.G. First potentiometric biosensor1969 Baker C.T. and Trachtenberg I. Chalcogenide glass membrane for ion-selective electrode1970 Bergveld P. IS-FET1970 Taguchi N. SnO2-based gas sensor1972 Shone C. Piezoelectric biosensor1974 Hesse H.H. First fi ber optic sensor1974 Chiang C.K. et al. First polymer-based conductometric gas sensor1975 Lundstrom I. et al. Gas-FET1975 Lubbers D.W. and Opitz N. Invention of the pCO2/pO2 opt(r)ode1976 Schoen F.J. ImmunoFET1976 Matsusite Electric Industrial Co. Commercialization of ceramic humidity sensor1978 Freeman T.M. and Seitz M.R. First chemiluminescence sensors1979 Wohltjen H. and Dessey R. Surface acoustic wave (SAW) sensors for gases1980 Peterson J.I. et al. Fiber optic pH sensor for in vivo blood gases1982 Schultz J.S. et al. Fiber-optic–based biosensor for glucose1982 Persaud K.C. and Dodd G. “Electronic nose”1984 Hall J.P. et al. Th ermoelectric (pyroelectric eff ect) gas sensor1984 Peterson J.I. et al. Fiber optic oxygen in vivo sensor1985 Mc Aleer J.F. et al. Th ermoelectric (Seebeck eff ect) gas sensor1986 Th orn EMI Microsensor First commercial production of IS-FETs1989 Wenzel S.W. and White R.M. Flexural plate wave (FPW) or Lamb wave sensor 1992 Toko T. et al. “Taste” sensor 1993 Abraham M.H. et al. Fullerenes application in gas sensors (SAW, QCM)1995 Klimant I. et al. Microsensors1995 Vlasov Y. et al. “Electronic tongue”1996 Wang L.G. et al. Fullerenes application in electrochemical sensors1996 Dresselhaus M.S. et al. Carbon nanotubes in sensor applications1997 Russell S.D. et al. Monolithic integrated chemical sensor2002 Comini E. et al. 1-D metal oxide–based conductometric gas sensor2008 Javey A. et al. First integrated nanowire sensor circuit

Source: Data from Schultz and Taylor 1996; Vlasov et al. 2005; http://en.wikipedia.org; etc.

Table 1.4. (continued)

for the measurement of oxygen content in tissues and physiological fl uids (Clark 1956). Ion-selective electrodes have provided new measurement capabilities, but they face the same limitations as the pH electrode (and are less selective). Th e key concept for the adaptation of those electroanalytical techniques was Clark’s idea of encapsulating the electrodes and supporting chemical components by a semiperme-able membrane. Th e membrane allows the analyte to diff use freely within the sensor without the loss


of critical components. Th e concept of interposing membrane layers between the solution and the elec-trode also provided the basis for the fi rst biosensor, invented by Clark and Lyons (1962).

Th e fi rst resistive gas sensor using an oxide semiconductor was reported in 1962 by Seiyama et al. and Taguchi. As is well known, the oxygen sensor which uses a stabilized zirconia electrolyte has been most important for the control of automobile emissions. Probably the fi rst attempt to apply an electro-chemical cell with a solid electrolyte to the gas phase was made in 1963 by Goto and St. Pierre. A thin-fi lm conductometric sensor was fi rst introduced in the early 1970s. Perhaps the most important promise of these thin-fi lm sensors for the development of viable chemical microsystems is their compatibility with processes for fabricating standard integrated circuits. Th e early 1970s also saw the development of the ion-selective fi eld-eff ect transistor (ISFET). An ISFET is simply a metal–oxide–semiconductor fi eld-eff ect transwistor (MOSFET) without a gate. Th e oxide layer of the FET is replaced by an insulating, chemically sensitive membrane. Charges from sensitive chemicals accumulate on top of this insulating membrane and are amplifi ed through the operation of the FET. Modifi cations and hybrids of the ChemFET and the ISFET (such as the surface-accessible FET, or SAFET, and the suspended-gate FET, or SGFET) were also introduced into research in the 1970s.

Humidity sensors have also been used popularly for various aspects of domestic life ranging from air conditioning to the protection of electronic instruments from condensation. Unlike classic hydrometers, the history of such humidity sensors is rather short. In 1976, Matsusita Electric Industrial Co. commercialized electronic ovens equipped with ceramic humidity sensors for automated cooking. Notable among recent achievements in the fi eld of chemical sensors are the contributions of Persaud and Dodd (1992), Toko et al. (1992), and Vlasov et al. (1995), which resulted in the creation of the “electronic nose,” the “taste” sensor, and the “electronic tongue” (Vlasov et al. 2005). Sensor technol-ogy has been further advanced by new discoveries in one-dimensional materials, conductive polymers, nanocomposites, and many other sensing materials. We hope that these advances will result in the com-mercialization of a variety of chemical sensors in coming years.

3. MOTIVATIONS FOR DESIGN OF CHEMICAL SENSORS

Many industries have a need to use and analyze process gases (see Table 1.5). Th e breadth of such uses can be illustrated by listing the key market sectors that either produce, use, or analyze process gases, namely (Cleaver 2001):

• Chemical and petrochemical • Environmental, including both ambient air and stack emissions monitoring • Scientifi c and engineering research organizations, including universities and national

laboratories • Medical institutions, including hospitals • Food and drinks processing, which use gases such as nitrogen to enhance the shelf life of products

by reducing oxidation and use carbon dioxide to carbonate soft drinks and alcoholic beverages • Microelectronics, including semiconductor manufacturing and telecommunications • Agriculture


• Fabrication industries, including the motor, ship, and aircraft industries • Power generation, particularly the nuclear industry, which uses, for example, advanced gas

reactors.

Chemical sensors may be used to detect toxic gases in monitoring and alarm systems (such as the detection of harmful gases in the workplace), or to characterize the aroma of a food as part of a quality control process. As chemical sensor technology advances in the next decade, such sensors will become routine on chemical and food processing lines, and more sophisticated sensors will be incorporated into automated process monitoring and control systems. One can estimate how many gases it is necessary to control from the list of hazardous gases typically monitored in semiconductor facilities (see Table 1.6).

Th e results presented in Table 1.7 indicate how important it is to control the presence of toxic gases in the atmosphere. Even at very low concentration, toxic gases are already very dangerous to human

MARKET APPLICATIONS COMPETITIVE ADVANTAGES

Medical/clinical Diagnostics; point-of-care patient monitoring; drug monitoring; artifi cial organs and prostheses; new drug discoveryProcessing/industrial Process monitoring and control; quality control; workplace monitoring; waste stream monitoring; leakage alarmsEnvironmental Detection/monitoring of pollutants, toxic chemicals, waste water, and waste streams Household application Intelligent refrigerator or oven; fi re alarm; natural gas heating; air quality controlLifestyle Detection of bad breath; blood alcoholTraffi c City traffi c control and management; air quality monitoring in tunnels or underground parking garagesAgriculture/veterinary Plant/animal diagnostics; soil and water testing; meat/poultry inspection; waste/sewage monitoringDefense/military Detection of chemical, biological, and toxin warfare agents; treaty verifi cationRobotics/computers Robotic controls; hybrid silicone/ organic computing components

Source: Data from Taylor 1996.

Table 1.5. Current and potential applications for chemical sensors and biosensors

(A) Real-time analysis, immediate/continuous monitoring, high specifi city and selectivity(B) Diverse applications, taste, smell, and other sensory capabilities, combined detection and alarm capabilities, ease of use(C) Portability, cost eff ectiveness, designed to be “smart” sensors, low energy demands and low heat generation, analog/digital computing capabilities


HAZARDOUS GAS FORMULA PEL (ppm)a

Ammonia NH3 35Arsine AsH3 0.05Boron dichloride BCl3 5Boron trifl uoride BF3 1.5Carbon monoxide CO 35Chlorine Cl2 0.5Chlorine trifl uoride ClF3 0.1Diborane B2H6 0.1Dichlorosilane SiH2Cl2 5Germane GeH4 0.2Hydrogen H2 500Hydrogen bromide NBr 3Hydrogen chloride HCl 5Hydrogen fl uoride HF 3Hydrogen sulfi de H2S 10Hydrogen selenide H2Se 0.05Phosgene COCl2 0.1Phospine PH3 0.3Silane SiH4 5

aPEL, permissible exposure limit—concentration deemed safe for 8-h time-weighted average.Source: Data from White 2000.

Table 1.6. Hazardous gases typically monitored in the semiconductor industry

GAS LONG-TERM EXPOSURE LIMIT, 8 H SHORT-TERM EXPOSURE LIMIT, 10 MIN

(ppm) (ppm)

H2S 10 15CO 50 300NOX 3 5SO2 2 5PH3 — 0.3CH3OH 200 250Cl2 0.5 1NH3 25 35HCl — 5

Source: Data from www.inspectapedia.com.

Table 1.7. Long- and short-term exposure limits of some typical toxic gases


health. Th at means that the portability and real-time output of chemical sensors and biosensors will be key advantages to their success in environmental applications.

In agriculture, chemical sensors can be used for animal and plant disease diagnostics, detection of contaminants such as pesticides, drugs, and pathogens in milk, meat, and other foods, and in the determination of product quality, such as the ripeness and fl avors of fruits and vegetables in the fi eld (Taylor 1996). Pharmaceutics is other important fi eld for chemical sensor applications. Drug discovery is critical in the development and commercialization of new pharmaceuticals, providing the basis for new drugs and new therapies. Emerging requirements for sensors in the medical and health care sector are being driven by the movement toward the continuous (and ideally non- to minimally invasive) monitoring of patients (see Tables 1.8 and 1.9). Under these conditions, feedback on the state of the patient and the results of therapy can be obtained without the delays associated with conventional intermittent measurement and the use of central laboratories for chemical and biochemical analysis (Moskalenko et al. 1996; Kharitonov and Barnes 2000; Cao and Duan 2006).

Th e primary market needs can be categorized as economic, regulatory, and unique government requirements. Economic motivations for improved sensor materials and chemical sensing technology include reducing the cost of product manufacture, increasing a product’s functionality at low additional cost, and improving the quality of the product. Th ese motivations also improve product competitive-ness. For example, the quality, safety, and comfort of automobiles have been greatly enhanced by the many sensors incorporated into modern vehicles. An equally important economic driver is the develop-ment and incorporation of sensors into products that aid in extending the useful lifetimes of vehicles.

CLINICAL CONDITION ANALYTE AND BREATH MARKERS

Diabetes mellitus Glucose, ketones, K+, insulin, lactate, pH, vapor of acetone

Vital function monitoring in pH, K+, electrolytes (unclassifi ed), gases (NOx, O2, halothane,

intensive care/anaesthetics/prolonged CO2, etc.), Na+ glucose, hemoglobin, osmolality, lactatesurgery

Infl ammation and oxidative stress NO, CO

Asthma NO, CO, H2O2, isoprostanes, nitrite/nitrateCystic fi brosis

Pulmonary allograft dysfuction NOLung cancer

Chronic bronchitis/chest medicine O2, CO2, pH

Bronchial epithelial infection NO2

Respiratory monitoring CO2/O2 ratio

Renal failure/monitoring, dialysis Urea, creatinine, K+, atrial natriuretic peptide, pH

Source: Data from Marczin et al. 2005; Cao and Duan 2006; etc.

Table 1.8. Principal clinical conditions for which in vivo chemical sensors are considered to be helpful


Examples include sensors to monitor the integrity of motor oil in an engine, allowing a user to change the engine oil only when it is necessary due to lubricant degradation. Other vehicle sensors might be used in lieu of more expensive externally applied inspection procedures.

Chemical sensors have been essential in satisfying a profusion of government-mandated regulatory requirements, including such applications as measuring chemical effl uent from factories and exhaust gases from automobiles (see Table 1.10). Water quality is also one of the objectives for control (see Table 1.11). In sum, chemical sensors have signifi cant economic impacts and eff ects on the quality of life.

Government agencies have many singular needs for chemical sensors across a wide range of ap-plications. Th e military has been on the leading edge of applying sensor technologies to improve its

GAS PHYSIOLOGICAL RANGE IN HUMAN BREATH

Ethane, pentane 1–11 ppbIsoprene 12–1,000 ppbMethanol 160–2,000 ppbCO 0.4–2.0 ppmCO (smoker) 1–20 ppmNO 1–9 ppb (lower respiratory tract) 200–1,000 ppb (upper respiratory tract) 1,000–30,000 ppb (nasal level)Ethanol 13–1,000 ppbMethane 4–20 ppmCO2 2,500–3,000 ppmAmmonia/urea 130–2,400 ppb

Source: Data from Moskalenko et al. 1996; Smith et al. 1999; etc.

Table 1.9. Traces of gases in human breath: physiological ranges

Oxides of sulfur and other sulfur compounds (SO2, etc.)Oxides of nitrogen and other nitrogen compounds (NO, NO2, peroxyacetyl nitrate, NH3, etc.)Oxides of carbon (CO, CO2)Oxidizing agents (O3, Cl2, etc.)Organic compounds and partial oxidation compounds (rubber and plastics, organic solvents, chlorinated hydrocarbons and fl uorocarbons, volatile organic compounds, CH4, etc.)Metals, metalloids, and their compounds (metal carbonyls, Hg, Cd, Cr, Zn, Cu, organometallic compounds, alkali metals and their oxides, and alkaline earth metals and their oxidesAsbestos (suspended particulate matter and fi bers), glass fi bers and mineral fi bersHalogens and their compounds (As, etc.)Phosphorus and its compounds (P, P2O5, etc.)Particulate matter

Table 1.10. Air pollutants: prescribed substances


PARAMETER UNITS MAXIMUM CONCENTRATION OR VALUE

Hydrogen ion pH value 5.5–9.5Sulfate (SO4) mg/1 250Magnesium (Mg) mg/1 50Sodium (Na) mg/1 150Potassium (K) mg/1 12Dry residues mg/1 1500Nitrate (NO3) mg/1 50Nitrite (NO2) mg/1 0.1Ammonium (ammonium and ammonia ions (NH4) mg/1 0.5Kjeldahl nitrogen (N) mg/1 1Oxidizability (permanganate value) (O2) mg/1 5Dissolved or emulsifi ed hydrocarbons μg/1 10Phenols (C6H5OH) μg/1 0.5Surfactants (as lauryl sulfate) μg/1 200Aluminum (Al) μg/1 200Iron (Fe) μg/1 200Manganese (Mn) μg/1 50Copper (Cu) μg/1 3000Zinc (Zn) μg/1 5000Phosphorus (P) μg/1 2200Fluoride (F) μg/1 1500Silver (Ag) μg/1 10

Source: Data from Gangolli 1999; Ho et al. 2005.

Table 1.11. Prescribed substances under the EC drinking water directive

operational capability. Sensors have been used to develop necessary information about the target, and once the weapons are launched, they are guided to the target in real time by other types of on-board sen-sors. A great demand for new sensors stems from the National Aeronautics and Space Administration’s Earth Orbiting Satellite program, which attempts to monitor changes in the chemical composition and temperature of the earth’s atmosphere. In this case, new materials and new sensor technologies will be re-quired to provide sensors that possess the needed sensitivity in the important spectral regions. Reductions in the size of military forces and the resulting closings of military bases have led to a demand for sensors capable of monitoring the clean-up and disposal of numerous toxic organic compounds, chemical war-fare agents, and obsolete munitions. Sensors will also be required for online control in the manufacture of low-volume specialty components or ultra-high-performance military aircraft. Without sensor-based control for these specialized needs, the unit costs of production will very likely be prohibitive.

Th e required sensitivity of chemical sensors is indicated in the results presented in Table 1.12. Interestingly, the detection threshold of diff erent fl avor compounds in water varies considerably,

with a grapefruit compound having an astonishingly low threshold value of 2 parts in 1014. Th at means


that chemical sensors must be extremely sensitive in order to recognize odors as well as the human sensory system does.

4. WHAT DETERMINES SUCCESS IN CHEMICAL SENSOR DESIGN?

Despite the large number of chemical sensors already on the market, selection and design of a suitable sensor for a new application is a diffi cult task for the design engineer. Careful selection of the sensing material, the surface platform, and the sampling system is very important because those decisions can determine the specifi city, sensitivity, response time, and stability of the fi nal device. Selective func-tionalization of the sensor is also critical to achieving the required operating parameters. Th erefore, in designing a chemical sensor, developers have to answer the following questions: (1) Does the application require high sensitivity or a broader range of detection? (2) Can the application’s needs be met by careful choice of the operating parameters of the sensors, or will a combination of technologies be needed to sort out the contributions of various similar analytes? (3) Does the application’s operating environment require special materials or fabrication procedures?

Designing a new sensor requires good understanding of the functioning of various chemical sen-sors, their design, features of exploitation, and modern trends in their development. Multidisciplinary knowledge and very careful multidisciplinary approaches are also needed to achieve good results (see Figure 1.4).

Among the sciences needed for the design of chemical sensors, knowledge of materials science is necessary for elaboration of eff ective technologies. It is required for fabricating appropriate coatings and membranes, synthesis or deposition of sensitive materials, as well as for conducting surface functionaliza-tion and materials modifi cation. In addition, chemical sciences such as interfacial and interphase chem-istry, electrochemistry, molecular chemistry, analytical chemistry, and so on, are necessary for precise understanding of the processes which are the basis on which sensors function. Without knowledge of the biological sciences, including microbiology and molecular biology, protein chemistry, immunology/enzymology, bioprocessing, etc., it is impossible to understand the main principles of biosensor function and design. Th e physical sciences provide spectroscopic detection methods (optical, mass, etc.), as well as knowledge necessary for understanding the mechanisms of interaction at gas–solid or liquid–solid interfaces which determine the sensor’s signal.

COMPOUND ODOR TYPE THRESHOLD

Ether Ether 5.8 mg/mlLimonene Lemon 0.1 mg/mlBenzaldehyde Bitter almond 3.0 μg/mlButyric acid Rancid butter 9.0 μg/mlCitral Lemon 3.0 ng/mlMusk xylene Musky 0.8 pg/ml

Source: Data from Gardner and Dartlett 1999.

Table 1.12. Detection thresholds of human nose for some typical odorants in air


Th ere is also no doubt that the role of engineers in this process is as important as the role of scien-tists. Certainly, during the initial stages of research, interest is generally focused on exploring and prov-ing the principles by which a new technology can be applied to measure a chemical substance. However, at the following stages of elaboration the impact of engineers is signifi cantly increased, especially when they are close to completing a design (Figure 1.5). Sometimes, design of a device that will be suitable for commercial marketing requires resolving a number of complex engineering problems that do not neces-sarily rely on elaboration of basic principles. Modern progress in the fi eld of chemical sensors in many aspects has been determined by the successes of engineers working in such areas as microfabrication, microfl uidics, and signal processing.

Physical Sciences

Biological Sciences

Chemical Sciences

Material Sciences

Electrical Engineering

Sensing Mechanism

Signal Processing Engineering

Sensor Device

Production Engineering

Design Engineering

Figure 1.4. Scheme illustrating the correlation of different sciences in chemical sensor design.

Sensing

Mechanism

Sensor Devices

Smart Sensors

Sensor Systems

Measurement and Control Systems

Figure 1.5. From idea to devices—from science to engineering.


Designing versatile chemical sensors suitable for measurements in any environment and under any conditions is an even harder task. Th e environment determines appropriate materials and the transducing principle that can be utilized in a sensor. Th us the very fi rst step in sensor design is to determine the required working conditions, measurement range, response time, resolution of measure-ments, lifetime, etc. Expected temperature changes, possible chemical interferences, and electromag-netic interactions should also be considered, as well as the size of the sensor, its weight, cost, safety of operation, and reliability.

In addition, commercialization of designed sensors actually infl uences the process of their elabora-tion. Every step of development must pass a diffi cult set of criteria and questions. Th ese questions are aimed at measuring present and future technical and market performance for the sensor and include (Taylor 1996):

1. How specifi c is the sensor? Can it match competing, nonsensor assays? 2. How sensitive is it? Can it meet the needs of the market? 3. How reliable and accurate is it? Can it be manufactured with high reproducibility? 4. How stable is it? 5. What is its shelf life? 6. What restrictions must be placed on storage (e.g., refrigeration, desiccation, etc.)? 7. How much will it cost? 8. Who owns it? Is the technology protected? 9. What is (are) its target market(s)?

It seems that there are many “ready” solutions for given problems, and therefore the careful study of results of previous research may reveal shortages that should still be solved in the future.

5. MATERIALS FOR CHEMICAL SENSORS

Research on fundamentals as reported in Volumes 1–3 of this series has shown that practically all known categories of materials could be used for chemical sensor elaboration. Some could be used as sensitive elements; others could be used as construction materials, or in membranes and fi lters. Some could be used to create contacts, and some (as additives) to promote optimization of working characteristics. At present, however, the materials that are most commonly used as sensing materials are metal oxides and polymers, which can be referred to as traditional or conventional sensing materials.

5.1. METAL OXIDES

Metal oxides are the class of materials which have seen the widest application in chemical sensors (Park and Akbar 2003; Korotcenkov 2007). As can be seen in Table 1.13 and was shown in Volume 1 of this series, they can be used in every type of chemical sensor. For example, in conductometric sensors,


SENSOR TYPE AND

(SENSOR ELEMENTS) DETECTED GAS METAL OXIDES PREFERRED FOR APPLICATION

Chemiresistor Reducing gases (CO, H2, CH4) SnO2; CTO; Ga2O3; In2O3

(semiconductor) Oxidizing gases (O3, NOx, Cl2) In2O3; WO3; ZnO; TiO2

H2S, SO2 SnO2/CuO; SnO2/Ag2O NH3 WO3; MoO3; In2O3

CO2 SnO2/La2O3; Al2O3/V2O5

Alcohol La2O3/In2O3; La2O3/SnO2; In2O3/Fe2O3

Oxygen Ga2O3, SrTiO3, SrTiFeO3; TiO2; Nb2O5; ZnO Humidity In2O3/SiO2; TiO2/MgCr2O4; SrTiO3; LaFeO3

Electrochemical Oxygen ZrO2:Y; Bi2O3/MoO3

(amperometric) H2 Sb2O5; BaCeO3; ZrO2:YSurface acoustic wave Humidity; NO2; H2; ethanol; O3 ZnO; InOx; LiNbO3; SiO2; WO3

Quartz crystal balance Hg vapor; NH3, NOx, SOx, H2S SiO2

Work function (RT) CH4, CO, Cl2 NiO; Fe2O3; Co3O4

Capacitance H2; NH3; C2H5OH (Pd, Pt, Ir)/SiO2

Humidity Al2O3

CO2 CuO/BaTiO3; CeO2/BaCO2/CuO; Co3O4/BaTiO3; NiO/BaTiO3

NOx CoO/In2O3; NiO/ZnOPelistor Combustible gases and vapors Al2O3; SiO2

Pyroelectric H2; CH4 ZnO; LiTaO3; LiTiO3

Heterostructural CO ZnO/Zn2SnO4; SnO2/TiO2; SnO2/Zn2SnO4

H2S ZnO/CuO; SnO2/CuO/SnO2

Schottky diode H2 ZnO; TiO2

Optochemical H2, CO, alcohol WO3; Mn2O3; Co3O4; NiO; CuO(fi ber optic) Surface plasmon NO2; H2S; NH3 Ta2O5; SiOxNy; TiO2

resonanceCataluminescence Organic vapors MgO; TiO2; Y2O3; LaCoO3:Sr Ethanol ZnO

Table 1.13. Metal oxides preferred for applications in various types of chemical sensors

semiconducting metal oxides are typically used as gas-sensing materials that change their electrical resis-tance upon exposure to oxidizing or reducing gases.

Specifi c properties of the metal oxides, such as the wide variety of materials with diff erent electro-physical, optical, and chemical characteristics, their high thermal and temporal stability, and their ability to function in harsh environments, make metal oxides very suitable materials for use in chemical sen-sors. Further, their role in chemical sensors is not limited to being used just as sensing materials. Th ey


are also being used successfully as substrates, electrodes, promoters, and structure modifi ers, and as membranes and fi lters (Table 1.14).

On the other hand, low selectivity is the main shortcoming of sensors based on metal oxides.

5.2. POLYMERS

Polymers constitute another class of materials which are also very promising for application in chemical sensors (Skotheim et al. 1998; Dai et al. 2002; Th omas et al. 2006; Rahman et al. 2008). Th ese materi-als, discussed in detail in Volume 3 of this series, are inexpensive to manufacture and can be prepared as fi lms, which is very important for device application. Moreover, conducting polymers can be deposited over defi ned areas of electrodes. In general, polymers could be used in all types of sensors (chemiresis-tors, work function, optical, mass-sensitive, electrochemical, CHEMFETs, capacitance) as long as they can function at room termperature. Th e great advantage of conducting polymer–based sensors over other available technologies is that the conducting polymers have the potential for improved response properties and are sensitive to small perturbations. Earlier inert polymers were used only to provide mechanical strength to membranes; however, conducting polymers improve the sensitivity of the sensors due to their electrical conductivity or charge-transport properties. Conducting polymers are also known for their ability to be compatible with biological molecules in neutral aqueous solutions (Chaubey and Malhotra 2002). Moreover, the polymer itself can be modifi ed to bind biomolecules to a biosensor (Mulchandani and Wang 1996).

A large number of chemical sensors use polymers because they off er great design fl exibility (McQuade et al. 2000; Gerard et al. 2002). Th e fl exibility of polymers’ properties, however, is attained at the expense of doping and the introduction of functional additives. Th e primary dopants (anions)

FUNCTION TYPE OF SENSOR EXAMPLES

Sensing layer Conductometric, SAW, QCM, SnO2, In2O3, WO3, TiO2, ZrO2, etc. optical, calorimetricSubstrate Conductometric Al2O3, SiO2, BeO; LiNbO3; ZrO2:Y SAW, QCM ZnOElectrode Electrochemical In2O3, ZnO, NiO/Ni; PdO/Pd; Al2O3/Pt; MgAl2O3/Pt; Au/Ga2O3

Membrane, fi lter Electrochemical, gas sensors Al2O3, SiO2, zeolitesFiber Fiber optic SiO2, GeO2–Sb2O3

Promoter Conductometric, SAW, QCM, PdO; RhO; Ag2O; CuO; Fe2O3; P2O5; Co3O4; optical, calorimetric NiO; MnOStructure modifi er Conductometric, calorimetric Al2O3; SiO2; CaO; MgO; BaO; Y2O3; La2O3; (stabilizer) Ta2O5; CeO2

QCM, quartz crystal microbalance; SAW, surface acoustic wave.

Table 1.14. Functions of metal oxides in chemical sensors


introduced during chemical or electrochemical polymerization maintain charge neutrality and gener-ally increase the electrical conductivity. Doping generates charge carriers in the polymer chain through chemical modifi cation of the polymer structure and involves charge exchange between the polymer and the dopant species. Since every repeat unit is a potential redox site, conjugated polymers can be doped n-type (reduced) or p-type (oxidized) to a relatively high density of charge carriers (Persaud 2005). Th e neutral chain can be partially oxidized or reduced, i.e., “doped,” by inducing an excess or defi ciency of electrons into the polymer lattice. Th is eff ect can be reversed by “de-doping” through the removal or chemical compensation of charge carriers. Th e nature of the anion also strongly infl uences the morphol-ogy of the polymer (Pron and Rannou 2002). In addition, anions can serve as specifi c binding sites for interaction of the conducting polymer with the analyte gas (Cabala et al. 1997).

Conducting polymers can be used as the selective layer in a sensor or as the transducer itself. Polymers can form selective layers in which the interaction between the analyte and the sensing matrix generates a primary change in a physical parameter in the transduction mechanism. Th ey are relatively open materials that allow ingress of gases into their interior. Th us, for example, a change of conduc-tance of a conducting polymer upon exposure to a gas is the sensing mechanism in a chemiresistor. Common classes of organic conducting polymers acceptable for conductometric gas sensor application include poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(terthiophene)s, poly(aniline)s, poly(fl uorine)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynaphthalenes, poly(p-phenylene sulfi de), poly(para-phenylene vinylene)s, etc. (Nalwa 1997). Polymers can also form diff erent membranes, provid-ing selectivity of chemical reactions in electrochemical sensors. On the other hand, their performance in devices that form circuit elements such as transistors has been less than ideal (Dimitrakopoulos and Malenfant 2002; Dimitrakopoulos and Mascaro 2001).

Janata and Josowicz (2003) noted that the abundant literature dealing with various applications of polymers can be divided into two groups: polymers in electronic, optoelectronic and electromechanical devices (Angelopoulos 2001) on the one hand, and polymers in chemical sensors based on electronic, optical, or mechanical mechanism (Osada and De Rossi 2000). Acceptable operation of the fi rst group relies on their chemical stability in the ambient environment, whereas the sensor applications take advantage of the physical changes that take place in the polymers when they are exposed to diff erent chemicals. Th is property has its origin in the molecular and macroscopic structure of polymers. Th us the tunability is an important bonus of polymers that aids the preparation of a variety of sensing layers, but it can be troublesome where performance independent of ambient factors is desired. Because they can be readily incorporated into microfabricated structures, conducting polymers are inherently compatible with solid-state integrated chemical sensors. Consequently, they have considerable potential for fabrica-tion of multisensing arrays, an aspect that makes sensors based on conducting polymers particularly suitable for commercialization (Janata and Huber 1985).

According to Persaud (2005), polymer-based sensors have the following advantages:

1. Th e sensors have rapid adsorption and desorption kinetics at room temperature. 2. Th e sensor elements feature low power consumption (of the order of microwatts), because no

heater element is required. 3. Th e polymer structure can be correlated to specifi city toward particular classes of chemical

compounds.


4. Th e sensors are resilient to poisoning by compounds that would normally inactivate some inor-ganic semiconductor-type sensors.

It is apparent that polymers display promising analytical results. However, there are still concerns that many of them are unable to achieve the desired selectivity and sensitivity in real samples. Some polymers undergo degradation or an irreversible swelling process when exposed to certain environments, while others adsorb and respond to water vapor (Pejcic et al. 2007).

5.3. NEW TRENDS IN SENSING MATERIALS

As we mentioned earlier, there are no ideal sensors which meet all requirements. Nor are there any ideal sensor materials. Th at is why research is continually being conducted to search for new sensing materials having new properties which might be used in elaboration of chemical sensors with new and unusual functional characteristics. Detailed fundamental information about these materials is provided in Volume 2 of this series.

5.3.1. Fullerenes: Present and Future

Fullerenes are closed-cage carbon molecules containing pentagonal and hexagonal rings arranged in such a way that they have the formula C20+m, with m being an integer number (Dresselhaus et al. 1996; Mauter and Elimelech 2008) (see Figure 1.6). Th ey are the third allotropic form of carbon, the others being graphite and diamond. Fullerenes comprise a wide range of isomers and homologous series, from the most studied C60 or C70 to the so-called higher fullerenes, C240, C540, and C720. Th e fi rst of these compounds were discovered in 1985 through spectrometric measurements on interstellar dust, and their structure was confi rmed later in the laboratory (Kroto et al. 1985). Kroto, Smalley, and Curl received the Nobel Prize in 1996 for their work.

Figure 1.6. Schematic view of fullerenes. (Reprinted from http://commons.wikipedia.org.)


Physicists, chemists, and material scientists or engineers, among others, have found unusual po-tential in these new spherical carbon structures for use as superconductor materials, sources of new compounds, self-assembling nanostructures, and several optical devices (Dresselhaus et al. 1996). Th is initial attention led to an increasing number of investigations that showed the special properties of fullerenes, some of which might lead to practical applications (Mauter and Elimelech 2008). Although a wide range of uses has been explored and several applications developed, fullerenes are not so far fulfi ll-ing their initial spectacular promise (Baena et al. 2002). Research on the application of fullerenes has proved to be slower than expected, but it must not be considered unsuccessful when one considers the great advances in the knowledge of the physical and chemical characteristics of fullerenes. Th anks to the additional information obtained during recent years, they have been found to be really useful in several fi elds, especially in solid-state applications.

According to Baena et al. (2002), in analytical chemistry, fullerenes can be approached from two diff erent points of view. Th e fi rst sees fullerenes as analytes, which involves their determination in vari-ous samples such as biological tissues. Th e second sees fullerenes as analytical tools, including their use as chromatographic stationary phases, as electrochemical sensors based on their activity as electron mediators, and in the exploitation of their unique superfi cial characteristics as sorbent materials in continuous-fl ow systems.

Initially, to establish the analytical features of fullerenes as sensors, adsorption studies were carried out on organic molecules bound onto fullerenes. For this purpose, the adsorption of gases and organic vapors was studied with fullerene-coated devices sensitive enough to detect changes in mass or pressure related to the adsorption of gas molecules to the fullerene layer—such as surface acoustic wave (SAW) sensors or quartz microbalances (QMBs). Th rough these fi rst investigations, the retention of certain monomeric gas molecules was demonstrated, and consideration was given to the possible use of C60 fi lms as analytical sensors for volatile polar gases such as NH3 (Synowczyk and Heinze 1993). Gas ad-sorption onto the fullerene fi lm reduces the fi lm resistance, resulting in a charge transfer to the electronic system. Sensitivity levels of a few milligrams per liter of NH3 in air were achieved, but there were still some problems, such as the lack of selectivity versus other gas vapors (which were also adsorbed, leading to the same electrical signal), response times of the order of seconds, the infl uence of humidity level on the calibration, or instability of the sensor when exposed to air several times. Nevertheless, the use of fullerenes as modifi ers was found to be a promising research topic in several fi elds, especially as coatings in QMBs and SAW sensores, since it is well known that the presence of fullerene improves the electro-chemical characteristics of the fi lm or membrane by reducing the resistivity.

In recent decades, various reusable and sensitive piezoelectric (PZ) quartz-crystal membrane sensors have been developed to detect organic/inorganic vapors and organic/inorganic biological species in solu-tions. Fullerene C60, and fullerene derivatives, among others, were synthesized and applied as coating materials on quartz crystals of PZ crystal sensors (Chao and Shin 1998). Th us, chemisorption on C60 fullerene was observed for amines, diamines, dithiols, dienes, and alkynes, and only physical adsorption was found for carboxylic acids, aldehydes, alcohols, ketones, alkenes, and alkanes. Th is seems to imply that the nucleophilic addition to fullerene by polar electron-donor groups, as in amines and thiols, is easier than electrophilic addition. Furthermore, diamines and dithiols showed greater interactions than those for the monodentate form, behavior attributed to the formation of stable cyclic compounds be-tween fullerene and the bidentate ligand.


In independent studies, fullerene has been widely used as an electron mediator in electrodes, since the incorporation of C60 signifi cantly reduces the electrical resistance of the coating membrane. By way of example, an iodide-sensitive sensor was reported (Wang et al. 1996) in which the bilayer lipid mem-brane supported on a copper wire—which acted as a modifi ed electrode—also contained C60 fullerene. Th e resulting electrode was further used in a three-electrode system for the determination of iodide in solution, obtaining a detection limit of 10 nM.

Recently, the optical properties of C60 have also been applied to the development of a sensitive oxygen-sensing system based on the quenching of the photo-excited triplet state of the fullerene mol-ecules (Bouchtalla et al. 2002; Nagl et al. 2007). Although the amperometric oxygen electrode has been the most popular sensing system for this element, the instability of the electrode surface itself, and in the oxygen diff usion barrier, demand a practical alternative. For this, much attention has been given to optical sensing systems based on luminescence quenching of an indicator [organic dye, polyamide hy-drazide (PAH)–transition metal complex]. Th e C60 fullerene can also be used as indicator, because it can easily form thermally stable fi lms with polymers, such as polystyrene, and possesses useful electronic and photochemical properties, such as a fairly long lifetime for the photo-excited triplet state (100 s). Th is lifetime is eff ectively quenched by oxygen, and decreases with increasing oxygen concentration. By using time-resolved spectroscopy with laser-fl ash photolysis, a highly sensitive oxygen sensor is obtained.

Probably the unavailability and high cost of fullerenes have deterred their use in analytical chem-istry. Now, many fi rms supply fullerenes at reasonable prices. Th erefore, it is optimistically forecase (Baena et al. 2002) that the advantages of fullerenes as sorbent materials, chromatographic stationary phases, and active microzones in sensors, based on their unique characteristics, will be consolidated and extended in the near future. One of the foreseeable trends is the use of synthetic fullerene derivatives that exhibit better properties than the original fullerenes. Th e introduction of radicals into the fuller-ene spheres can lead to an increase in the reversible sorption of organic molecules, as well as to direct retention and elution of metal traces by covalent binding of typical ligands such as EDTA and DDC. Th e unusual electrical properties of fullerenes can be fully exploited by progressively substituting the conventional carbon forms in building macro- and microelectrodes.

5.3.2. Carbon Nanotubes—A New Material for Chemical Sensors

Carbon nanotubes (CNTs), which have high mechanical and chemical stability, were fi rst fabricated in 1991. Th ere are two types of CNT morphology (Saito et al. 1998; Dresselhaus et al. 1996; Varghese et al. 2001; Terrones et al. 2004). On the one hand, single-walled nanotubes (SWCNTs) consist of a honeycomb network of carbon atoms and can be visualized as a cylinder rolled from a graphitic sheet. On the other hand, multiwalled nanotubes (MWCNTs) are a coaxial assembly of graphitic cylinders generally separated by a plane space of graphite (Dresselhaus et al. 1996) (see Figure 1.7). CNTs seem to be ideal for adsorption and detection of gases due to their hollow center, nanometer size, and large surface area. Gas adsorption on CNTs is today the focus of intense experimental and theoretical studies. Synthesis methods for SWCNTs and MWCNTs include arc discharge, laser ablation, pyrolysis, chemi-cal vapor deposition (CVD), and gas-phase catalytic growth (Terrones et al. 2004). However, these methods do not produce a monodisperse product with controlled physical and chemical properties.


Th e electronic structure of SWCNTs can be either metallic or semiconducting, depending on their diameter and chirality (Dresselhaus et al. 1996). Combining metallic and semiconducting nanotube elements with diff erent electronic properties (Varghese et al. 2001) can create MWCNTs. It is sup-posed (Valentini et al. 2003) that these diverse electronic properties allow the possibility of developing nanoelectronic devices as nanowires, or as metal/semiconductor heterojunctions by combining metallic and semiconducting nanotubes. A possible approach is the modifi cation of diff erent parts of a single nanotube to have diff erent electronic properties using controlled mechanical or chemical processes (e.g., nanotube bending or gas molecule adsorption).

Carbon nanotubes have the same developed surface as fullerenes, and therefore their applications lie in the same general area (Mauter and Elimelech 2008). In particular, the results of recent research have shown that CNTs may fi nd successful applications in design of room-temperature sensors, where their peculiar structural features could be realized. It has in fact been established that such sensors can be extremely sensitive. For example, research conducted by Penza et al. (2004) showed that at room temperature, CNT-based SAW sensors were up to 3–4 orders of magnitude more sensitive than existing organic layer–coated SAW sensors. Th ey had a very low limit of detection, and 1 ppm of ethanol or toluene was easily sensed. It was established that the selectivity to volatile organic compounds (VOCs) can be aff ected by the type of organic solvent used to disperse the carbon nanotubes as sensing materials onto SAW sensors (Penza et al. 2004). Th e interaction between the CNTs’ surface and VOCs plays a main role in the sensing mechanism. Mass spectrometry measurements indicated that the interaction between the CNTs and solvents used becomes stronger with solvents that form hydrogen bonds (e.g., ethanol), suggesting a possible role for the chemisorbed oxygen on CNTs as chemical mediators between the CNTs and the dispersing agents. Th is means that the sensing eff ects are strongly dependent on the chemical affi nity between the analytes to be detected and the solvent used. Also, CNTs form a net sup-porting adsorbed molecules, producing a sensing structure that is stable at room temperature.

CNT-based sensors have shown good electrical response as well (Valentini et al. 2003, 2004). Th ese authors established that NO2 exposure drastically decreases the electrical resistance of CNT-based sensors; NH3, H2O, C6H6, and ethanol exposure increases the electrical resistance; and CO exposure does not

Figure 1.7. Schematic diagrams of (a) a single-wall carbon nanotube (SWNT), (b) a multiwall carbon nanotube (MWNT), (c) a double-wall carbon nanotube (DWNT), and (d) a peapod nanotube consist-ing of a SWNT fi lled with fullerenes (e.g., C60). (Reprinted with permission from Dresselhaus et al. 2003. Copyright 2003 Elsevier.)


aff ect the resistance. Th e threshold of NO2 detection was less than 10 ppb. Further, the sensitivity achieved was pretty good, and also, removing the gas totally restored the initial resistance. Such CNT behavior indicates that charge transfer due to the interaction of carbon nanotubes with adsorbates is an important mechanism in changing conductivity in the CNTs upon adsorption of NO2, water vapor, NH3, C3H6, and ethanol gases. Th e nanotube sensors exhibited fast response and substantially higher sensitivity than existing solid-state sensors at room temperature. Sensor reversibility was characterized by fast recovery at 165°C. Th e experimental fi ndings revealed that p-type semiconductor behavior is present in analyzed CNTs. Th e carbon nanotube thin fi lms studied by Valentini et al. (2003, 2004) were prepared by plasma-enhanced chemical vapor deposition. Valentini et al. (2003, 2004) also found that modifi cation of the sensing material by noble metals could be used to improve the parameters of CNT-based sensors.

It has been established that bulk doping of CNTs also plays an important role in sensing eff ects. For example, Terrones et al. (2004) pointed out that theoretical ab initio calculations have demonstrated that CO and H2O molecules do not react with the surface of pure-carbon SWCNTs. If the surface of the tube is doped with a donor or an acceptor, drastic changes in the electronic properties are observed as a result of the binding of the molecules to the doped locations—because of the presence of holes (B-doped tubes) or donors (N-doped tubes), the surfaces became more reactive.

So, a brief analysis of results obtained indicates that CNTs are really promising materials for sensor applications (Li et al. 2008; Kalcher et al. 2009; Bondavalli et al. 2009). Other interesting results con-nected with CNT preparation and study have also been reported in the literature (Dresselhaus 1996; Harris 1999; Poulin et al. 2002; Valentini et al. 2003; Cantalini et al. 2003; Penza et al. 2004; Mauter and Elimelech 2008).

5.3.3. Metal Oxide One-Dimensional Nanomaterials

As research on carbon fullerenes and nanotubes has shown, the dimensionality is a very important factor in determining the properties of nanomaterials. Th erefore, control of the size and shape of metal oxide crystallites is of great interest with regard to applications of such materials in chemical sensors. Results obtained during study of one-dimensional metal oxide nanomaterials have great importance due to their potential for fundamental studies as well as for application in low-cost, small-sized, and low-power-consumption devices. One-dimensional metal oxide nanomaterials have excellent crystallinity and clear facets. It is expected that these nanomaterials will have less concentration of point defects and specifi c adsorption and catalytic properties, conditioned by a particular combination of crystallographic planes. In addition, one-dimensional metal oxide nanomaterials should be more thermodynamically stable in comparison to nanograins, promoting stable operation of chemical sensors at higher temperatures. Development of nanotechnology gives hope for realizing chemical sensors based on one-dimensional metal oxide nanowires, which optimizes their parameters even more in comparison with devices based on polycrystalline materials.

To date, various kinds of one-dimensional nanomaterials, such as Si, Ge, silica, MgO, CaO, GaN, SiC, In2O3, TiO2, Fe2O3, ZnO, etc., have been synthesized as nanowires, nanotubes, nanospheres, nano rods, and nanobelts (Guha et al. 2004; Li et al. 2002; Kam et al. 2004) (see Figure 1.8). A number of methods, such as physical evaporation, silica-assisted Fe-catalytic growth, thermal reaction between


powders and active carbon, alumina-assisted catalytic growth, carbon-assisted synthesis, etc., have been tried for fabricating these materials. However, there is much more research on synthesis of nanoribbons, nanobelts, and nanowires than on attempts to apply these materials in chemical sensors. Th erefore, we now know more about regularities of nanowires growth (Kam et al. 2004; Jung et al. 2003; Varghese et al. 2003) than about their electrophysical, surface, catalytic, and sensor characteristics. It is necessary to admit, however, that research in this area has recently become much more intensive (Hernandez-Ramırez et al. 2007; Liu 2008; Comini et al. 2009). Th erefore, one may hope that practical application of one-dimensional metal oxide nanomaterials in chemical sensors may not be very long in coming. Th e basis of this optimism is the progress achieved in the synthesis of one-dimensional metal oxide

Figure 1.8. Morphology of as-made ZnO nanowires. (a)–(d) show long (10–15 μm) and thin (30–60 nm) nanowires grown in the higher-temperature region of the furnace. (e)–(h) show short (1–2 μm) and thick (60–100 nm) nanowires grown in the lower-temperature region of the furnace. The scale bar for (a) is 10 μm, for (b), (c), (e), (f), and (g) 1 μm, and for (d) and (h) 200 nm. (Reprinted with permission from Banerjee et al. 2004. Copyright 2004 IOP.)


nanomaterials, which now allows synthesis of high-quality nanomaterials with the length of individual nanowires equaling up to 10–500 m (Li et al. 2002; Zhang et al. 2004).

5.3.4. Nanocomposites

Development of nanocomposites is another promising direction in the design of materials for chemical sensors. Nanocomposite materials have recently attracted increased interest because of the possibilities of synthesizing materials with unique physical-chemical properties (Gas’kov et al. 2001; Zhang et al. 2003). Highly sophisticated surface-related properties, such as optical, electronic, catalytic, mechani-cal, and chemical properties, can be obtained by advanced nanocomposites, making them attractive for chemical sensor applications.

At present, research on elaboration of nanocomposite materials is being carried out in various di-rections, such as composites based on carbon nanotubes and fullerenes, metal oxide nanocomposites, and organic–nonorganic nanocomposites. Materials obtained as a result of this elaboration have their own specifi c advantages. For example, the addition of CNTs to a polymer matrix leads to a very low electrical percolation threshold and allows one to obtain, with only very small amounts of CNTs, an electrical conductivity suffi cient to provide an electrostatic discharge (Flahaut et al. 2000). Transitioning to nanocomposites could improve mechanical properties and promote stabilization of the parameters of the basic material (Konig 1987). For example, in carbon–polymer composites, the presence of carbon nanotubes inside the polymeric matrix can provide mechanical support to the conformational rearrange-ment of the polymeric chains.

Nanocomposites also provide more possibilities for control of the catalytic activity of the sens-ing matrix. For example, the introduction of TiO2 nanoparticles into the polymer matrix of poly(p-phenylenevinylene) (PPV) changes the adsorption properties: Adsorption of oxygen is stronger on the nanocomposite than on PPV (Baraton et al. 1997).

A full description of possible methods of fabricating nanocomposites with particular properties is not possible in this brief review. Interested readers may refer to the literature for details.

It is necessary to remember that design of nanostructured composites for chemical sensors requires consideration of many factors, e.g., the interface volume, crystallite size, interphase interactions, thick-ness, surface and interfacial energy, texture, stress and strain, etc., all of which depend signifi cantly on materials selection, deposition methods, and process parameters (Gas’kov et al. 2001; Zhang et al. 2003). Some aspects of the use of metal oxide nanocomposites in the elaboration of solid-state gas sen-sors were considered in detail by Gaskov and Rumyantseva (2001, 2009).

For example, Gas’kov and Rumyantseva (2001) give the following explanation of the advantages of nanocomposites for gas sensor applications. Th e nanocomposite materials can be represented as M1O/M2O, where M1O is the nanocrystalline matrix and M2O is the doping oxide distributed between the surface and the bulk of the M1O nanocrystalline grain. Th e advantage of nanocomposites over simple nanocrystalline oxides, when used in gas sensors, is associated with the redistribution of M2 between the bulk and the surface of the M1O grains, depending on the redox properties of the gas phase. Low oxida-tion levels of M2 cation, corresponding to larger ionic radius and predominant distribution of M2 over the surface of the M1O grains, occur in a reducing atmosphere. By contrast, cations in higher oxidation


states are formed in an oxidizing atmosphere, with increasing probability that M2 cations will occupy regular cationic positions in M1O. Th e liability of the chemical state of the M2 cation in the nanocrystal-line system may result in a dramatic change in the state of grain boundaries and in modifi cation of the electronic properties of the material in the presence of even trace amounts (0.1–10 ppm) of reducing or oxidizing gas molecules in the gas phase. Table 1.15 lists examples of nanocomposites that are of interest for creating gas-sensitive materials.

5.3.5. Other Materials

Th ere are also other approaches to the elaboration of new sensing materials, based on using ionic liquids (Hardacre et al. 2004), new mesoporous materials (Melde et al. 2008), new wide-band-gap semiconduc-tors (Chaniotakis and Sofi kiti 2008), porous semiconductors (Korotcenkov and Cho 2010), and metal nanoparticles (Luo et al. 2006). Th ese materials were discussed in detail in Volumes 2 and 3 of this series.

For example, as metal particles are reduced in size, the collective oscillation of electrons in the conduction band causes changes in the electrical, optical, and magnetic properties. Such phenomena in nanomaterials may play an important role in sensor technology. By exploiting these nanoscale proper-ties, a highly effi cient chemical sensor can be designed and fabricated (Riu et al. 2006). Nanoparticles, nanoclusters, and nanoarrays of nanoparticles have been used in VOC sensors (Ahn et al. 2004). Th e main problem with metal nanoparticles is related to their low stability. However, metallic nanoparticles can be stabilized in an organic medium using surface functionalization. Th ese materials are known as core–shell nanoparticles. Core–shell metal nanoparticles are being incorporated into various chemiresis-tor sensor applications, particularly in VOC detection. In all of these sensors, nanocrystalline metals are embedded into a polymer matrix. One of the major driving forces for using polymer matrixes is to make the sensor chemically resistant to corrosive gases and VOCs. Th e nanoparticles are chemically susceptible to corrosive analytes and can adversely aff ect any unprotected sensing system. Th e chemical susceptibility of the nanoparticles and the type of polymer matrix used can cause issues for long-term reliability of the sensor (Drake et al. 2007).

Table 1.15. M1O/M2O nanocomposites with promise for gas-sensitive materials

IONIC RADII

r(M2), nm

M1O/M2O r(M1), nm OXIDIZED FORM REDUCED FORM

Cr2O3/SnO2 Cr3+ 0.061 Sn4+ 0.069 Sn2+ 0.093SnO2/CuO Sn4+ 0.069 Cu2+ 0.073 Cu1+ 0.096SnO2/MoO3 Sn4+ 0.069 Mo6+ 0.042 Mo5+ 0.063Ga2O3/Fe2O3 Ga3+ 0.062 Fe3+ 0.064 Fe2+ 0.077In2O3/NiO In3+ 0.079 Ni3+ 0.060 Ni2+ 0.070In2O3/Fe2O3 In3+ 0.079 Fe3+ 0.064 Fe2+ 0.077WO3/TiO2 W6+ 0.058 Ti4+ 0.060 Ti3+ 0.067

Source: Data from Gaskov and Rumyantseva 2001.


Zeolites are another group of compounds that have generated some interest (Rolison 1990; Walcarius 1999). Th is is not surprising considering that they comprise a microporous open framework structure which is accessible to certain guest molecules. Further, their surface and structural properties can be easily modifi ed, which makes them ideal candidates for the selective adsorption of various volatile hydrocarbons and small organic molecules (Pejcic et al. 2007). However, zeolites do have a strong ten-dency to preferentially adsord water relative to analyte, and this appears to be hindering their application in environments with elevated moisture levels. Th ese materials will not be discussed in this chapter, due to their lesser importance in comparison with the materials mentioned above. Zeolites are discussed in detail in Volume 2 of this series.

6. SOME USEFUL DEFINITIONS

In conclusion, in Table 1.16 we provide some defi nitions which will help the reader to understand better the materials discussed in this volume (White 2000; http://en.wikipedia.org).

We would like to note that the development of “smart sensors” is one of the most promising direc-tions in chemical sensor design. A smart sensor is made with the same technology as integrated circuits. Th is means that a smart sensor utilizes the transduction properties of one class of sensing materials and the electronic properties of silicon (GaAs). Integrated sensors provide signifi cant advantages in terms of overall size and the ability to use small signals from the transduction element. Potential advantages of the smart sensor concept include:

Table 1.16. Defi nitions used in chemical sensor technologies

Analyte A substance or chemical constituent that is determined in an analytical procedure.Analytical chemistry Th e study of the chemical composition of natural and artifi cial materials.Chemometrics Th e science of extracting information from chemical systems by data-driven means. It is a highly interfacial discipline, using methods frequently employed in core data-analytic disciplines such as multivariate statistics, applied mathematics, and computer science, used to investigate and address problems in chemistry, biochemistry, and chemical engineering.Signal processing A fi eld of applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time, to perform useful operations on those signals.Environment Th e surroundings of a physical system that may interact with the system by exchanging mass, energy, or other properties.Environment (biophysical) Physical and biophysical factors along with their chemical interactions that aff ect an organism.Environment monitoring Processes and activities that characterize and monitor the quality of the environment.



Pollution Contaminants in an environment that cause instability, disorder, harm, or discomfort to the ecosystem, i.e., physical systems or living organisms. Air (water) pollution is the release of chemicals and particulates into the atmosphere (water).Hazardous materials Chemicals or substances which are physical hazards or health hazards.Target gas Th e specifi c gas that is intended to be monitored.Toxic gas Gas with toxic properties that shuts down the central nervous system.Combustible gas Gas that will be burn when mixed with air (or oxygen) and ignited.Combustion Rapid oxidation of a substance, involving heat and light.Flammable gas Gas that at ambient temperature and pressure forms a fl ammable mixture with air at concentrations of 12% (or less) by volume.Greenhouse gases Gases in an atmosphere that absorb and emit radiation within the thermal infrared range. Th e main greenhouse gases in the Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.Oxidizing agent A substance that oxidizes something, especially chemically. Chemisorption of oxidizing gases is accompanied by electron transfer on adsorbed species.Reducing agent (reductant Th e element or compound in a redox (reduction/reoxidation) reaction thator reducer) reduces another species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox. Chemisorption of reducing gas is accompanied by electron transfer from adsorbed species on the substrate.Concentration Exposure levels are based on concentration in air, which can be measured in percent by volume or in parts per million (ppm) (equivalent to mg/kg) or parts per billion (ppb). To convert percent concentration to ppm, move the decimal point four places to the right. Gas monitoring code requirements usually refer to permissible exposure level (PEL). Permissible exposure limit Concentration deemed safe for 8-h time-weighted average.(PEL)

Immediately dangerous to A concentration of airborne contaminants, normally expressed in parts perlife and health (IDLH) million or milligrams per cubic meter, which represents the maximum level from which one could escape within 30 min without any escape-impairing symptoms or irreversible health eff ects.Lower detectable limit Th e lowest gas concentration that a sensor can reliably and repeatably detect.(LDL)

Lower explosive limit (LEL) Th e concentration of a gas below which the concentration of vapors isor lower fl ammable limit insuffi cient to support an explosive. Th is concentration is always referred to(LFL) as 100% LEL or LFL for that gas.Upper explosive limit (UEL) Th e maximum concentration of gas in air that will combust.or upper fl ammable limit(UFL)

Odor threshold A concentration level that is detectable by the olfactory senses.




Sensing/interaction Fundamental mechanisms and interactions that allow the detection andmechanisms quantifi cation of chemicals (e.g., optical absorption, specifi c affi nity of a material for a chemical, frequency shift). A sensing/interaction mechanism is also known as a transduction mechanism that converts the chemical presence to some form of usable signal.Sensory receptor A structure that recognizes external stimuli.Sensory perception Th e process of acquiring and interpreting sensory information.Poisoning A gas sensor exposed to very high levels of target gas (more than 20 times the designed operating range) becomes saturated and made inoperative. Poisoning also occurs when compounds react and form strong bonds with a sensor’s catalyst surface, thereby tieing up reaction sites for the target gas. Th is action diminishes the sensitivity of the sensor to the target gas.Passive sensor Requires an external AC or DC electrical source.Active sensor Provides its own energy or derives it from the phenomenon being measured.Sensor system Typically, a sensor with associated components (frequently electronics and user input/output devices) that can convert the sensor’s raw signal to usable information on a single target chemical (e.g., chemical concentration). It may be a stand-alone instrument or a sensor module for incorporation in a more comprehensive system.Alarm system A combination of approved compatible devices with the necessary electrical interconnection and energy to produce an alarm signal in the event of system activation.Continuous gas-detection A gas detection system in which the analytical instrument is maintained insystem continuous operation and sampling is performed without interruption.Extractive sampling A gas monitoring method utilizing an air pump to draw air samples through tubing from the sampling location to a remote-mounted gas sensor.Measurement and control Integrated systems developed to meet specifi c client application requirementssystems that can include the following: single or multiple sensors, signal processing and data analysis functions, data monitoring and recording, and control functions.Smart sensor Th e defi nition of “smart” and “intelligent” sensing can be debated. In general, it is diffi cult to identify any features in a smart sensor that parallel intelligence in natural systems; however, the terms have become cemented in the technical jargon. Usually, smart sensors are defi ned as sensors with integrated electronics that can perform one or more of the following functions: (1) logic functions, (2) two-way communication, (3) make decisions (Giachino 1986). Micromachining, microelectromechanical (MEMs), and nanoelectromechanical (NEMs) systems belong to the same class of miniaturized sensor devices. Physically, a smart sensor consists of a transduction element, signal conditioning electronics, and a controller/ processor that support some intelligence in a single package.




• Packaging or assembling of sensing and actuating, or signal processing devices, in the proximity of the analyte

• Lower maintenance • Reduced downtime • Better reliability • Improved resolution and sensitivity, and superior functionality • Fault-tolerant systems • Adaptability for self-calibration and compensation • Lower cost due to batch fabrication • Lower weight • Wireless data transfer • Fewer interconnections between multiple sensors and control systems • Less complex system architecture

Th ese advantages of smart sensors are application-specifi c. Th ere is certainly justifi cation for many applications in distributing the signal processing throughout a large sensor system so that each sensor has its own calibration, fault diagnostics, signal processing, and communication, thereby creating a hierarchical system. Innovations in sensor technology have generally allowed a greater number of sen-sors to be networked, more accurate sensors to be developed, and on-chip calibration to be included. In general, new technology has contributed to better performance by increasing the effi ciency and accuracy of information distribution and by reducing overall costs. However, these performance enhancements have been achieved at the expense of increased complexity of individual sensor systems.

In closing, some defi nitions of sensor characteristics are summarized in Table 1.17. Most of these characteristics are given in manufacturers’ data sheets. However, information on the reliability and ro-bustness of a sensor is rarely given in a quantitative manner.

Microelectromechanical Th e technology of the very small, which merges at the nanoscale intosystem (MEMs) nanoelectromechanical systems and nanotechnology.Nanoelectromechanical Similar to MEMs but smaller.systems (NEMs)

Bulk micromachining A process used to produce micromachinery or MEMs systems. Bulk micromachining creates structures by selectively etching inside a substrate.Surface micromachining Deposition and etching of diff erent structural layers on top of the substrate; i.e., this process creates structure on top of the substrate.Cantilever A beam supported on only one end.Remote sensing Small- or large-scale acquisition of information of an object or phenomenon by the use of either recording or real-time sensing devices that are wireless, or not in physical or intimate contact with the object of control.Sensor network A network consisting of spatially distributed devices using sensors to cooperatively monitor physical or environmental conditions.



PARAMETER DEFINITION

Sensitivity Th e slope of the output characteristic curve (Δy/Δx).Sensitivity error A departure from the ideal slope of the characteristic curve.Range Maximum minus minimum value of the measured stimulus.Dynamic range Total range of the sensor from minimum to maximum.Full-scale output Th e algebraic diff erence between the electrical output with maximum and minimum input stimulus.Resolution Smallest measurable increment in measured stimulus.Detection limit Th e lowest concentration value which can be detected by the sensor in question, under defi nite conditions. Whether or not the analyte can be quantifi ed at the detection limit is not determined.Sensing frequency Maximum frequency of the stimulus which can be detected.Response time Time required for a sensor output to change from its previous state to a fi nal settled value within an error tolerance band of the correct new value.Recovery time Time required for recovery initial state of sensors after interaction with analyte.Accuracy Error of measurement, in percent full-scale defl ection.Hysteresis Capability to follow the changes in the input parameter regardless of the direction.Drift Long-term stability (deviation of measurement over a time period).Zero drift Th e percentage change in the zero point or baseline of a gas sensor or gas detection system over a specifi c period of time.Precision Th e degree of reproducibility of a measurement (repeatability, reproducibility)Linearity Extent to which the actual measured curve of a sensor departs from the ideal curve.Dynamic linearity Ability to follow rapid changes in the input parameter; amplitude and phase distortion characteristics, response time.Monotonicity Th e dependent variable always either increases or decreases as the independent variable increases.Saturation No desirable output with further increase in stimuli (span-end nonlinearity).Off set Output exists when it should be zero.Dead band Insensitivity of a sensor in a specifi c range of input signals.Size Leading dimension of sensors.Weight Weight of sensors.Optimal environment Operating temperature and environment conditions.Reliability Service life in hours or number of cycles of operation.Cost Purchase cost of the sensors.Life cycle Th e length of time over which the sensor will operate. Th e maximum storage time (shelf life) must be distinguished from the maximum operating life. Th e latter can be specifi ed either for continuous operation or for repeated on–off cycles.

Table 1.17. Summary of the main sensor characteristics


7. ACKNOWLEDGMENTS

Th is work was supported by the World Class University (WCU) Program at the Gwangju Institute of Science and Technology (GIST) through a grant (Project No. R31-20008-000-10026-0) provided by the Ministry of Education, Science and Technology (MEST) of Korea, for the project titled “Development of Maritime Environmental Sensor Using Nano and Photonic Technology,” funded by the Ministry of Land, Transport and Maritime Aff airs, Korea, and by the Korean Science and Engineering Foundation (KOSEF) NCRC grant funded by the Korean government (MEST) (No. R15-2008-006-01002-0).

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