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4

Introduction to Chemosensors

H. Nanto, J. R. Stetter

4.1

Introduction

We believe that the 21st century can be the aroma age. The culture of aroma developedwith human civilization and the good smell of foodstuffs gives great comfort to thehuman heart. The sense of smell is, therefore, one of the most interesting of the fivehuman senses and yet is understood the least. The human nose is widely used as ananalytical sensing tool to assess the quality of such as drinks, foodstuffs, perfumes, andmany other household products in our daytime activities, and of many products in thefood, cosmetic, and chemical industries. However, practical use of the human nose isseverely limited by the fact that the human sense of smell is subjective, often affectedby physical and mental conditions, and tires easily. Consequently, there is consider-able need for a device that couldmimic the human sense of smell and could provide anobjective, quantitative estimation of smell or odor.Recently, there has been increasing interest in the development of such a device, the

so-called ‘electronic nose (e-nose)’. This is an electronic instrument that is capable ofdetecting and recognizing many gases and odors, and comprises a sensor array usingseveral chemosensors and a computer. The different types of chemosensors, especiallyodor sensors, that have been employed within an e-nose are described in this chapter.

4.2

Survey and Classification of Chemosensors

A chemosensor is a device that is capable of converting a chemical quantity into anelectrical signal and respondate the concentration of specific particles such as atoms,molecules, or ions in gases or liquids by providing an electrical signal. Chemosensorsare very different from physical sensors. Although approximately 100 physical mea-surands can be detected using physical sensors, in the case of chemosensors this num-ber is higher by several orders of magnitude. The types of chemosensors that can be

Handbook of Machine Olfaction: Electronic Nose Technology.Edited by T.C. Pearce, S.S. Schiffman, H.T. Nagle, J.W. GardnerCopyright ª 2003 WILEY-VCH Verlag GmbH Co. KGaA, WeinheimISBN: 3-527-30358-8

7979

used in an e-nose need to respond to odorous molecules in the gas phase, which aretypically volatile organic molecules with different relative molar masses.

Tab.

4.1

Classification

ofchem

osen

sors

that

have

been

exploited

sofar.Metal

oxidesemicon

ductor,MOS;

MOSfield

effect

tran

sistor,MOSF

ET;qu

artz

crystalmicrobalance,QCM;surfaceacou

stic

wave,

SAW;surfaceplasm

onresonan

ce,SP

R.

Principle

Measurand

Sen

sortype

Fabrication

method

sAvailability/sensitivity

Advantages

Disadvantages

Con

ductom

etricCon

ductan

ceChem

oresistor

MOS

Microfabricated,

Sputtering

Com

mercial,man

y

types,5–500ppm

Inexpen

sive,

microfabricated

Operates

athigh

temperature

Con

ducting

polym

er

Microfabricated,

Electroplating,

Plasm

aCVD,Screen

printing,

Spin

coating

Com

mercial,man

y

types,0.1–100ppm

Operates

atroom

temperature,

microfabricated

Verysensitive

to

humidity

Capacitive

Capacitan

ceChem

ocapacitor

Polym

erMicrofabricated,

Spin

coating

Research

Applicableto

CMOS-based

chem

osen

sor

Verysensitive

to

humidity

Poten

tiom

etric

Voltage/e.m

.f.

Chem

diode

SchottkyDiode

Microfabricated

Research

Integrated,

Applicableto

CMOS-based

chem

osen

sor

NeedsPd,Pt,

Au,Ir

(expen

si-

ve)

I-V/C

-VChem

otransistor

MOSFET

Microfabricated

Com

mercial,

specialorder

only,ppm

Integrated,

Applicableto

CMOS-based

chem

osen

sor

Odoran

t

reaction

product

must

pen

etrate

gate

Calorim

etric

Tem

perature

Them

al

chem

osen

sor

Thermister

(pyroelectric)

Microfabricated,

Ceram

icfab.

Research

Low

cost

Slow

respon

se

Pellistor

Microfabricated

Research

Low

cost

Slow

respon

se

Thermocou

ple

Microfabricated

Research

Low

cost

Slow

respon

se

Gravimetric

Piezoelectricity

Mass-sensitive

chem

osen

sor

QCM

Microfabricated,

Screenprinting,

Dip-coating,

Spin

coating

Com

mercial,

severaltypes,

1.0ngmass

chan

ge

Wellunderstood

technology

MEMsfabrica-

tion

,interface

electron

ics?

SAW

Microfabricated,

Screenprinting,

Dip-

coating,

Spin

coating

Com

mercial,

severaltypes,

1.0ngmasschan

ge

Differential

devices

canbe

quitesensitive

Interface

electron

ics?

Optical

Refractiveindex

Reson

ant-type

chem

osen

sor

SPR

Microfabricated,

Screenprinting,

Dip-coating,

Spin

coating

Research

Highelectrical

noise

immunity

Expen

sive

Intensity/spec-

trum

Fiber-optic

chem

osen

sor

Fluorescence,

chem

oluminescence

Dip-coating

Research

Highelectrical

noise

immunity

Restrictedavai-

lability

ofligh

t

sources

Amperom

etry

curren

tToxic

Gas

Sen

sor

Electrocatalyst

Com

posite

Electrodes

Com

mercial

ppb-ppm

Low

cost

noR

h

interferen

ce

Size

4 Introduction to Chemosensors80

Chemosensors as listed in Table 4.1 have been exploited and some already manu-factured. Principles such as electrical, thermal, optical, and mass can be used to or-ganize these chemosensors according to their device class. The chemosensors usingmetal oxide semiconductors (MOS), organic conducting polymers (CP), chemocapa-citors, MOS field-effect transistors (MOSFET), quartz crystal microbalance (QCM),surface acoustic wave (SAW), surface plasmon resonance (SPR), fluorescence, andothers that can be easily used as the sensor for an e-nose are included in the followingdiscussion. Details about the types of chemosensors discussed here and others can befound in the literature [1–6].

4.3

Chemoresistors

Chemoresistors based on the conductivity change of MOS or organic CPs by chemicalreaction with gaseousmolecules are the simplest of gas sensors, and are widely used tomake arrays for gas and odor measurements.

4.3.1

MOS

Metal oxides such as SnO2, ZnO, Fe2O3, and WO3 are intrisically n-type semiconduc-tors. At temperatures of 200–500 8C, these respond to reducible gases such as H2,CH4, CO, C2H5, or H2S and increase their conductivity. The conductivity r andthe resistivity q is given by

r ¼ 1=q ¼ enl ð1Þ

where e is the charge on the electron (1:6022� 10�19 C), n the carrier (electron or hole)concentration (cm�3) and l the carrier mobility (cm2 V�1s�1). In the atmosphere, someoxygenatomsareadsorbedon thesurfaceofn-type semiconductors to trap freeelectronsfrom the semiconductor, and consequently a highly resistive layer is produced in thevicinity of the semiconductor surface. The adsorption of oxygen atoms on the semi-conductor surface and at grain boundaries of polycrystalline semiconductors creates anelectrical-double layer that acts as the scattering center for conducting electrons. Theconsequent increase in free electrons and decrease in scattering centers results in anincrease in conductivity. The mechanism is similar for p-type semiconductors but is ofopposite sign [101].The mechanism of the increase in carrier concentration by reacting with the redu-

cible gases as described above can be understood from the following reactions:

eþ 1

2O2 ! OðsÞ� ð2Þ

RðgÞ þOðsÞ� ! ROðgÞ þ e ð3Þ

4.3 Chemoresistors 8181

where e is an electron from the conduction band of the oxide semiconductors, R(g) isthe reducible gas, and s and g imply surface and gas, respectively. Equation (2) impliesthat oxygen is physico-chemically adsorbed onto lattice vacancies in the oxide semi-conductor, and consequently the conductivity of the oxide semiconductor becomeslower than that in the case of no adsorbed oxygen. An electron is, however, generatedby the reaction with reducible gases R(g) through Eq. (3). Consequently, the conduc-tivity is increased following Eq. (3) as a result of the increase in carrier concentration.In contrast, p-type semiconductors such as CuO, NiO, and CoO respond to oxidizablegases such as O2, NO2, and Cl2 [101].The schematic diagram in Fig. 4.1 explains the conductivity increase due to the car-

rier mobility for SnO2 gas sensors. In clean air, oxygen atoms that trap free electrons inthe bulk SnO2, is adsorbed onto the SnO2 particle surface, forming a potential barrierin the grain boundaries as shown in Fig. 4.1a. This potential barrier restricts the flow ofelectrons, causing the electrical conductivity to decrease, because the potential barrieracts as the scattering center for electron conduction. When the sensor is exposed to anatmosphere containing reducible gases, e.g. combustible gases, CO, and other similarvapors, the SnO2 surface adsorbs these gas molecules and causes oxidation. This low-ers the potential barrier, allowing electrons to flow more easily, thereby increasing theelectrical conductivity as shown in Fig. 4.1b.The reaction between gases and surface oxygen will vary depending on the operating

temperature of the sensor and the activity of sensor materials. The increasing sensi-tivity and selectivity of the sensors for exposure to gases can be realized by incorpora-tion of a small amount of impurities and catalytic metal additives such as palladium(Pd) or platinum (Pt). The impurities act as extrinsic donors (or acceptors) and, con-sequently, controlling the doped amount of impurities can change the conductivity ofthe sensors. Doping of the catalytic metal to the sensor or coating with thin catalytic

Fig. 4.1 Schematic diagram explaining the conductivity increases

caused by the carrier mobility increase in SnO2 gas sensors. (a) Oxygen

is adsorbed onto the SnO2 particle surface, forming a potential barrier

in the grain boundaries. (b) The potential barrier is lowered bymeans of

reaction of the oxygen atoms with reducing gas, allowing electrons to

flow more easily, thereby increasing the electrical conductivity


Tab. 4.2 Commercially available metal oxide semiconductor chemo-

sensors.

Manufacturer Applications Model Typical detection range and features

FGARO ENG Combustible gas TGS813 For detection of various combustible gases

TGS816 500–10 000 (ppm)

TGS842 Improved sensitivity to CH4

500–10 000 (ppm)

TGS821 High selectivity and sensitivity to H2

500–10 000 (ppm)

Toxic gas TGS203 High selectivity and sensitivity to CO

50–1000 (ppm)

TGS825 High sensitivity to H2S

5–100 (ppm)

TGS826 High sensitivity to NH3 and amine compounds

30–300 (ppm)

Solvent vapor TGS822 High sensitivity to alcohol and organic com-

pounds such as toluene and xylene

TGS823

Halocarbon gas TGS830 High sensitivity to various CFCs, HCFCs

TGS831 100–3000 (ppm)

TGS832

Air quality control TGS800 High sensitivity to gaseous air contaminants

(such as cigarette smoke and gasoline exhaust)

1–10 (ppm)

Cooking control TGS880 Vaporized gases and water vapor form food in

the cooking process

10–1000 (ppm)

TGS882 Alcohol vapor from food in the cooking process

50–5000 (ppm)

TGS883 Water vapor from food in cooking process

1–150 (g m�3)

NEW

COSMOS ELEC.CO,

LTD.

Combustible gas CH-H High sensitivity and selectivity to H2,

50–1000 (ppm)

CH-M High sensitivity to VOCs such as CH4 and i-

C4H10

1000–10 000 (ppm)

CH-CO High sensitivity to CO

100–1000 (ppm)

CH-E2 High sensitivity to alcohol

CH-E3 1–1000 (ppm)

CH-L High sensitivity to LPgas

Toxic gas CH-N High sensitivity and selectivity to NH3

AET-S High sensitivity and selectivity to H2S

Thin film type


metal film of the sensor surface changes the selectivity of the sensor. As describedabove, the crystallographic structure of the semiconductors used as the sensor mate-rial is commonly polycrystalline, and thus includes many grain boundaries. Thesegrain boundaries act as the scattering centers for conducting electrons to producethe change of carrier mobility, and therefore consequently the extent of crystallinityaffects the sensitivity of the sensors.The most widely used semiconducting material as a gas sensor is SnO2 doped with

small amounts of impurities and catalytic metal additives. By changing the choice ofimpurity and catalyst and operating conditions such as temperature, many types of gassensors using SnO2 have been developed. The gas sensors using metal oxide semi-conductors exhibit relatively poor selectivity for gases and remain responsive to amany kinds of combustible gases. Table 4.2 lists some of the commercially availablegas sensors of SnO2 and ZnO that are manufactured by New Cosmos Electric Co., Ltdand Figaro Engineering Inc. (Japan).Figure 4.2 shows schematically the basic construction of the sintering-type and thin-

film-type of gas sensors. The type of sensor materials and operating temperatures oftypical gas sensors using MOSs that have been reported so far are listed in Table 4.3.

4.3.2

Organic CPs

Chemoresistors made from organic CPs also exhibit a change in conductance whenthey are exposed to reducible or oxidizable gases. Organic CPs show reversible

Tab. 4.3 Type of conduction and operating temperatures of typical gas

sensors using metal oxide semiconductors.

Materials (Dopants) n-type or p-type Top ( 8C) Detecting gases Ref.

ZnO(Al) n 200 H2 30

ZnO(Al) n 350 NH3 31

ZnO(Al,In,Ga) n 400 TMA 32

ZnO n 280–470 CO 33

ZnO n 450 CCl2F4, CHClF2, 34

WO3(Pt) n 250–400 N2H4, NH3, H2S, 35

WO3 n 500 CO, CH4, SO2 36

TiO2(Ru) n 560 TMA 37

a-Fe2O3 n 400 H2, CH4 38

c-Fe2O3 n 420 H2, CH4, C3H8C4H10, C2H5OH 39

CdIn2O3 n 300 CO 40

CuTa2O6 n 400 H2, CO 41

CuO/ZnO p/n 250 H2, CO 42

Co3O4 p 200–500 CO, H2, NOX 101

Cr2O3(Ti) n 420 TMA 43

In2O3 (Mg or Zn) n 420 TMA 43

BaSnO3 n 300–500 H2, CO, CH4, H2S, SO2 44

Bi2Sn2O7 p 500 H2, CO, C2H4, NH3 44

Bi6Fe2Nb6O30 n/p 500 C3H8, Cl2, NO2, SO2, H2S 45


changes in conductivity when chemical substances (e.g. methanol, ethanol, and ethylacetate) adsorb and desorb from the polymer. The mechanism by which the conduc-tivity is changed by this adsorption is not clear at present.There are a large number of different electronically conducting polymers. Polypyr-

role was first prepared electrochemically in 1968 [23] and has been most extensivelystudied so far. Electroconducting conjugated polymers (ECP) can exhibit intrinsic elec-tronic conductivity. Their structure contains a one-dimensional organic backbone withalternating single and double bonds, which enables a super-orbital to be formed forelectronic conduction. The most commonly applied polymers for gas-sensing applica-tions have been polypyrrole, polyaniline, polythiophene, and polyacetylene, which are

Fig. 4.2 The basic construction of the sintering-type (a) and thin-film-

type (b) of the gas sensors that are commercially available


based on pyrrole, aniline or thiophenemonomers [24]. Because of their properties theyhave remarkable transductionmatrices that are sensitive to gases and vapors, resultingin a straightforward conductance change via the modulation of their doping level. Theearly studies [25, 26] of the gas-sensing application of organic CPs concentrated on theresponse to reactive gases such as ammonia and hydrogen sulfide. Gustafsson et al.[27] have reported that gas sensors using polypyrrole films exhibit a high sensitivity forammonia gas. Subsequent work [28–30] also showed that gas sensors using organicCPs such as polypyrrole respond to a wide range of organic vapors such as methanol.More recently, studies have been carried out on preparation of thin-film CPs for gas

sensing applications [25, 31]. Thin films of heteroaromatic monomers such as pyr-roles, thiophenes, indoles, and furans were grown electrochemically on interdigitatedelectrodes to produce gas-sensitive chemoresistors [25].Chemoresistors using organic CPs respond to a wide range of polar molecules at

temperatures as low as room temperature (RT) and more recent reports suggest that ahigh sensitivity down to 0.1 ppm is possible. This result indicates that organic CP is apotentially useful material for applications in odor-sensing and e-nose applications.The use of organic CPs as odor sensor materials is very attractive for the following

reasons:

1) a wide range of materials can be simply prepared;2) they are relatively low cost materials;3) they have a high sensitivity to many kinds of organic vapors;4) gas sensors using organic CPs operate at low temperatures.

Comparison between the properties of the organic CP odor sensor and the MOS odorsensor is shown in Table 4.4.

Tab. 4.4 Comparison of the properties of the conducting polymer

odor sensor and the metal oxide odor sensor (thick-film and thin-film

types).

Properties Conducting polymer SnO2 (thick film) SnO2 (thin film)

Key measurand Conductance Conductance Conductance

Fabrication Electrochemical

growth, plasma CVD

paste Sputtering, Sol-gel

Choice of materials Wide Limited Limited

Operating temperature 10–110 8C 250–600 8C 250–600 8CMolecular Receptive range Wide range Combustible vapors Combustible vapors

Detection Range less than 20 ppm 10–1000 ppm 1–100 ppm

Response time 60 s 20 s 20 s

Size Less than 1 mm2 1 � 3 mm Less than 1 mm2

Power Consumption Less than 10 mW 800 mW 80 mW

Integrated array Yes No Yes

Stability Moderate Relatively poor Poor

Interferences Acidic gases, water SO2, Cl2, H2O SO2, Cl2, H2O


Another way to use CPs is to make non-conducting materials, e.g. silicone [32] andpolystyrene [33], conductive by inclusion of carbon-black metal powder. These sensorsare used in e-noses and can exhibit high sensitivity [34].

4.4

Chemocapacitors (CAP)

The principle of chemocapacitors using polymers is schematically shown in Fig. 4.3.There are two steady states for the sensitive layer during operation. In the first state asshown in Fig. 4.3a, no gaseous analyte molecules are present in the sampling envir-onment and consequently only air is incorporated into the polymer. As a result, acertain capacitance C of the sensitive polymer layer is measured and constitutesthe baseline. In the second state, gaseous analyte molecules are present in the sam-pling environment as shown in Fig. 4.3b. When the polymer absorbs the gaseous ana-lyte, the sensitive polymer layer changes its electrical (e.g. dielectric constant e) andphysical properties (e.g. volume V) to produce deviations (De, DV) from the first state(reference state). The changes in electrical and physical properties of polymers are theresult of reversible incorporation of gaseous analyte molecules into the polymer ma-trix.The CMOS-based chemical sensor using chemocapacitive microsensors for detect-

ing volatile organic compounds (VOCs) was built with two interdigitated electrodesspin-coated or spray-coated with polymers such as (poly)etherurethane (PEUT) byKoll et al. [35].

Fig. 4.3 Chemocapacitor based on capacitance measurement of

sensitive layers. There are two steady states for the sensitive layer

during operation; (a) no analyte molecules are present in the sampling

environment, and (b) analyte molecules are present in the sampling

environment


4.5

Potentiometric Odor Sensors

Gas sensors utilizing the electrical characteristics of Schottky diodes and the MOSFEThave also been investigated. Those using the Schottky diode are based on a change inthe work function because of the presence of chemical species on their surface. Ex-amples are catalytic metals (inorganic Schottky diodes) such as Pd and Pt, and organicCPs (organic Schottky diodes) such as polypyrrole. Gas sensors using a MOSFET arebased onmetal-insulator-semiconductor structures in which themetal gate is a catalystfor gas sensing. In this section, mainly potentiometric odor sensors using MOSFETsare included and discussed.

4.5.1

MOSFET

Themicrochemosensor using the structure of a MOSFET in which the gate is made ofa gas-sensitive metal such as Pd was first proposed by Lundstrom in 1975 [36]. Thissensor exhibited a threshold voltage shift depending upon the gas concentration andwas particularly sensitive to hydrogen down to the ppm level with maximum threshold

Fig. 4.4 Basic structures of n-channel MISFET and MISCAP, which

operate on the same basic principle but differ in measurands


voltage shift of about 0.5 V. The use of other metal gate materials such as Pt and Ir andoperating the sensors at different temperatures has led to reasonable selectivity togases such as NH3, H2S, and ethanol [37]. There are two basic structures such asMISFET (metal-insulator-semiconductor FET) and MISCAP (MIS CAPacitor ). Thebasic structures of n-channel MISFET and MISCAP that operate on the same basicprinciple but differ in measurands are shown in Fig. 4.4. In the MISFET, the draincurrent iD flowing through the semiconductor is controlled by the surface potentialdue to the applied gate voltage VG, and in the MISCAP the capacitance of the MISstructure is determined by the surface potential. These devices can respond to expo-sure to any gas that changes the surface potential or the work function of the gatemetal. The materials used in MOSFET-type odor sensors as well as the Schottky-type odor sensors are listed in Table 4.5 in comparison to those of MOS-type andCP-type odor sensors.

4.6

Gravimetric Odor Sensors

Recently, gravimetric odor sensors using acoustic wave devices that operate by detect-ing the effect of sorbed molecules on the propagation of acoustic waves have beeninvestigated for application to an e-nose. Two main types utilizing QCM (or bulkacoustic wave, BAW) and SAW devices have been used as the odor sensors, althoughother types of device have been investigated. In both types, the basic device consists ofa piezoelectric substrate, such as quartz, lithium niobate and ZnO, coated with a sui-table sorbent membrane [38]. Sorption of vapor molecules into the sorbent membranecoated on the substrate can then be detected by their effect on the propagation of the

Tab. 4.5 Materials used in the different odor sensors. MOSFET –

metal oxide semiconductor field effect transistor.

Chemosensor

type

Structure Examples of sensor

materials used

Examples of

detecting gases

MOSFET type Metal-gate MOSFET Pd(Pt)-gate FET

(SiO2, SnO2-Si, SiC)

H2, CO, H2S, NH3

Schottky type Metal/Semiconductor Pd-TiO2 (ZnO) H2, CO, CH3SH

p/n Nb2O3-Bi2O3

p/n ZnO-CuO

Metal/polymer Al/poly(3-octythiophene) NH3, NOx

Chemoresistors n-type semiconductors SnO2, ZnO, a-Fe2O3, TiO2, In2O3,

V2O3, SnO2 þ Pd, ZnO þ Pt,

SnO2þ ThO2 þ Pd,

H2, CO, alcohols,

hydrocarbons,

O2, NO2, Cl2p-type semiconductors CoO, Co3O4, CuO,

Sm0.5Sr0.5CoO3, Co0.3Mg0.7O,

La0.35Sr0.65Co0.7Fe0.3O3-x

H2, O2, CO, alcohols

Conducting polymers Anthracene, phthalocyanine,

polypyrrol, polyacrylonitorile,

polyphenylacetylene

NO, NO2, O2, SO2,

CO, NH3, alcohols

4.6 Gravimetric Odor Sensors 8989

acoustic wave causing changes in the resonant frequency and the wave velocity. Theacoustic waves used are at ultrasonic frequencies ranging typically from 1 to 500 MHz.Both types are discussed in this section.

4.6.1

QCM

QCM or thickness shear mode (TSM) devices using BAWs in piezoelectric materialsare probably the simplest type of odor sensor using a piezoelectric device. Rock crystalsuch as single crystal quartz has an interesting property in that it is distorted by appliedelectric voltage and conversely an electric field is generated by applied pressure. Thisphenomenon is called the piezoelectric effect. Because of this effect, upon excitation byapplication of a suitable a.c. voltage across the quartz crystal, the crystal can bemade tooscillate at a characteristic resonant frequency. A QCM odor sensor comprises of aslice of a single crystal of quartz, typically around 1 cm in diameter, with thin-filmgold electrodes that are evaporated onto both surfaces of the sliced crystal. The quartzcrystal oscillates in such manner that particle displacements on the QCM sensor sur-face are normal to the direction of wave propagation.The thickness of the quartz crystal determines the wavelength of the fundamental

harmonics of oscillation. The resonant frequency of the QCM sensor is related to thechange of the mass of QCM loading by the Sauerbrey equation [39]:

Df ¼ �2f 20 mf=AðqqlqÞ1=2 ð4Þ

where Df is the change in resonant frequency, f0 is the resonant frequency, mf is themass change due to adsorption of gases, A is the electrode area, qq is the density ofquartz and lq is the shear modulus. For typical AT-cut quartz crystal operating at10 MHz, a mass change of the order of 1 ng produces a frequency change of about1 Hz. Thus small changes in mass can be measured using a QCM coated with a mo-lecular recognition membrane on which odorant molecules are adsorbed, as shown inFig. 4.5. The selectivity of the QCM sensor is determined by the coating membranedeposited on the surface of the crystal.The functional design of the polymer-film-coated QCM odor sensor, based on the

solubility parameter of the sensing membrane and detecting gases, was carried out in

Fig. 4.5 Schematic diagram of the structure of a QCM chemosensor.

The sensor consists of polymer membrane that recognizes analyte

molecules and odors, and a QCM as a transducer


order to develop a sensor with excellent selectivity and high sensitivity for harmfulgases such as toluene, xylene, ammonia, and acetaldehyde by Nanto et al [40, 41].The polymer films such as propylene-butyl, polycarbonate, and acrylic resin of whichthe solubility parameters almost coincide with those of toluene, acetaldehyde, andammonia gas, respectively, are chosen as the sensing membrane material coatedon the QCM surface. They found that propylene-butyl-coated sensor exhibited ahigh sensitivity and excellent selectivity for toluene and xylene gases, as expectedfrom the functional design based on solubility parameters. They also found thatthe polycarbonate-coated and acrylic-resin-coated sensors exhibited high sensitivityand excellent selectivity for acetaldehyde and ammonia gases, respectively, also asexpected. The result strongly suggests that the solubility parameter is effective in

Tab. 4.6 Research on e-noses using different types of chemosensors,

including: quartz crystal microbalance, QCM; surface acoustic wave,

SAW; metal oxide semiconductor, MOS; MOS field effect transistor,

MOSFET. Pattern recognition types: multi-layer perception, MLP;

principal component analysis, PCA; fuzzy learning vector quantization,

FLVQ; cluster analysis, CA; Kohonen network, KOH; linear regression,

LR; feature weighting, FW; least square, LS; discriminant function

analysis, DFA; and fuzzy reasoning, FUZ.

Chemosensor type Number of sensors Applications Pattern recognition Ref.

QCM 8 Spirits, perfumes, odors MLP, PCA, FLVQ 46–51

4 Odors 52

8 Odors PCA, CA 53

6 Odors 54, 55

3 Harmful gases PCA 18, 19

SAW 6 Perfumes 56

4 Odors 57

12 58–60

MOSFET 10 Meat MLP, KOH 61

324 Odors 62

MOS 3 Odors 63

3 Odors, tobacco LR, FW 64

12 Odors, coffee LS 65, 66

12 Odors, beverages PCA, CA 67

12 Odors, beers MLP 68, 69

3 Odors CA, LS 70

12 Wines MLP, KOH 71

6 Odors LS 72

8 Odors 73

8 Odors MLP, LS 74

6 Odors LS 75, 76

7–8 Odors LR, PCA, CA 77, 78

6 Spirits, coffee CA, PCA, DFA 79, 80

3 Odors MLP 81

6 Odors KOH 82

3 Fish 83

3 Odors FUZ 84

AGS 4 Grain KNN, NN 106

4.6 Gravimetric Odor Sensors 9191

the functional design of the sensingmembrane of QCM odor sensors. The research one-nose applications using QCM odor sensors as well as those using other type of che-mosensors such as SAW, MOSFET, and MOS are listed in Table 4.6.Recently, studies on QCM odor sensors with plasma-polymerized organic film as

the molecular recognition membrane [42–45] and odor sensors using fundamentaland overtone modes of QCM with high frequency [46, 47] have been reported.

4.6.2

SAW

The SAW device is made of a relatively thick plate of piezoelectric materials (ZnO andlithium niobate) with interdigitated electrodes to excite the oscillation of the surfacewave [87–89]. The SAW is stimulated by applying an a.c. voltage to the fingers of aninterdigitated electrode to lead to a deformation of the piezoelectric crystal surface. TheSAW devices are usually operated in one of two configurations such as a delay line anda resonator. In both cases, the propagation of the SAW is affected by changes in theproperties of the piezoelectric crystal surface. In common gas sensors using a SAWdevice with a dual delay line structure, one arm of the delay line is coated with thesorbent membrane, the other acts as a reference to reduce the change of environmen-tal conditions such as temperature drift and other effects. The change in frequency ofthe SAW with sorption of vapor, Df V, is given by

DfV ¼ DfpcVKp=qp ð5Þ

for a simple mass loading effect, where Dfp is the change in frequency caused by poly-mer membrane itself, cV is the vapor concentration, Kp is the partition coefficient andqp is the density of the polymer membrane used.Considerable work [87] has been reported on the measurement of inorganic gases

such as NO2, H2, H2S, and SO2, and organic gases and vapors such as CH4, C6H6, andC2H5OH. This type of sensor using polymer materials as a sensing membrane can bechemically modified to obtain a higher degree of specificity, because the choice ofchemically sensitive membrane determines the selectivity of the sensor. The SAWodor sensors generally work at much higher frequencies of the order of GHz thanthat of the BAW odor sensor (10 MHz). The main problems with SAW odor sensorare a relatively poor long-term stability and a high sensitivity to humidity. A goodreview of acoustic sensors is available [6].


4.7

Optical Odor Sensors

4.7.1

SPR

SPR is an optical phenomenon in which incident light excites a charge-density wave atthe interface between a highly conductive metal and a dielectric material. The condi-tions for excitation are determined by the permittivities of the metal and the dielectricmaterial. The SPR transduction principle is widely used as an analytical tool for mea-suring small changes in the refractive index of a thin region adjacent to the metalsurface. The optical excitation of surface plasmon on a thin metallic film has, there-fore, been recognized as a promising technique for sensitive detection of chemicalspecies such as odor, vapor and liquid [90]. Several methods have been employedto monitor the excitation of SPR by measuring the light reflected from the sensorinterface. These include analysis of angle modulation [91], wavelength modulation[92], intensity modulation [93], and phase modulation [94].Optical SPR sensors are sensitive to the change in the refractive index of a sample

surface. Recently, it has been reported that toxic gases such as ammonia, toluene,xylene, ethylacetate, 4-methyl-2-pentanone, and propionic acid can be detected bymea-suring the SPR using angle modulation [95]. The SPR was measured using theKretschmann configration, illustrated in Fig. 4.6, with a prism and a thin, highly con-ductive gold metal layer deposited on the prism base. The LED emitting 660 nm lightwas used as the light source to excite the SPR. The SPR reflection spectrum (reflectedlight intensity versus angle of incidence with respect to the normal of the metal/di-electric interface) was measured by coupling transverse magnetically polarized mono-chromatic light into the prism and measuring the reflected light intensity of the rayexiting the prism versus the angle incidence. In order to utilize this system as a gassensor, a very thin film of methyl methacrylate, polyester resin, or propylene ether asthe sensing membrane was deposited on gold metal thin film using a spin-coatingmethod. The reflected light was measured using a CCD camera attached to a personalcomputer. The angle at which the minimum reflection intensity occurs is the reso-nance angle at which coupling of energy occurs between the incident light and the

Fig. 4.6 Kretschmann configuration of SPR apparatus used

in toxic gas detection [29]

4.7 Optical Odor Sensors 9393

surface plasmon waves. Four channel images of reflected light were observed by usingthe CCD camera. The schematic configuration of the SPR sensor is shown in Fig. 4.7.The SPR sensor with synthetic polymer thin film on the gold metal film as a sensingmembrane exhibited high sensitivity for toxic gases such as ammonia, toluene, xylene,ethylacetate, 4-methyl-2-pentanone, and propionic acid.

4.7.2

Fluorescent Odor Sensors

Recently, a new sensing device has been developed that consists of an array of opticallybased chemosensors providing input to a pattern recognition system. This type ofchemosensor consists of optical fibers deposited with fluorescent indicator NileRed dye in polymer matrices of varying polarity, hydrophobicity, pore size, elasti-city, and swelling tendency to create unique sensing regions that interact differentlywith vapor molecules [96].Fiber-optic sensors most often consist of an analyte-sensing element deposited at

the end of an optical fiber. Individual optical fibers with a diameters as small as 2 lmand imaging bundles with a diameter of 500 lm are available, enabling easy minia-turization, and are free from electrical interference. In a fiber-optic chemosensingsystem, the optical sensing element is typically composed of a reagent phase immo-bilized at the fiber tip by either physical entrapment or chemical binding. This reagentphase usually contains a chemical indicator that experiences some change in opticalproperties, such as intensity change, spectrum change, lifetime change, and wave-

Fig. 4.7 Schematic configuration of the SPR

sensor


length shift in fluorescence, upon interaction with analyte gases or vapors. The re-sponses depend upon the nature of the organic vapor and the strength of its interac-tion with the different polymer systems used.The most common configuration of optical fiber chemosensor utilizing fluores-

cence and example of the response are shown in Fig. 4.8. The authors then analyzedthe transient responses of the sensor array to distinguish different organic vapors suchas odor samples a, b, and c.At present, the sensitivity of some types of optical chemosensor is not high (detec-

tion limits of several 1000 ppm) and there is little information about the lifetime,reproducibility or stability of the sensor system. Nevertheless, this is an interestingapproach and one worthy of future work.

4.7.3

Other Optical Approaches

The use of a colorimeter coupled to optical fibers makes an inherently simple sensor[97], can be found in many forms, and was one of the earliest of the optical chemicalsensor approaches. Color changes, or more generally, changes in absorption or emis-sion of radiation, and polymer swelling by changes in refractive index of fiber coatingscan be monitored optically. More recent approaches make e-noses from arrays of mi-crobeads on the end of a fiber [96, 98]. These systems can bemade exquisitely sensitivewith the appropriate chemistry on the fiber tip. The future of optical arrays within thee-nose are very promising.

Fig. 4.8 (a) The most common configuration of an optical fiber chemosensor utilizing fluorescence, and

(b) an example of the response

4.7 Optical Odor Sensors 9595

4.8

Thermal (Calorimetric) Sensors

There are two sensor classes that are based on thermal technology. Those using pyro-electric [38] or thermopile sensors with coatings that absorb the analyte of interest. Theunderlying thermal sensor records the heat of solution of the analyte in the coating.They are quantitative because the more analyte that is absorbed, the more heat isgenerated. The theory and analytical performance of these sensors is similar to thecoated SAW or chemiresistor polymer sensors, except that the underlying transduceris a heat sensor.The second class of thermal sensor is the Pellister, catalytic bead, or combustible gas

sensor [99]. The catalytic sensor is typically a tiny bead of catalyst a millimeter or less indiameter that surrounds a coil of thin, 0.025 mm, Pt wire that acts as a Pt resistancethermometer. When resistively heated to about 500 8C, any contact with a hydrocarboncauses catalytic oxidation of the hydrocarbon with commensurate liberation of the heatof combustion. This heat is at the surface of the catalyst bead and some is lost to thesurroundings while some is transferred to the tiny catalyst sensor bead. The heat trans-ferred to the bead raises the temperature of the sensor, and it is this temperaturechange that is sensed as a change in resistance by the thin Pt wire. The sensor istypically placed in a Wheatstone Bridge circuit to measure the tiny changes in resis-tance of the Pt wire. The larger the resistance change, the higher the concentration ofhydrocarbon. These sensors are typically used for combustible gases and were used invery early e-noses [100]. There are many formulations of the catalyst material and thesesensors are operated at constant temperature or at constant voltage to serve differentapplications.

4.9

Amperometric Sensors

The amperometric gas sensor, or AGS, was one of the first sensors to be used in an e-nose format [100, 101, 103] and has been included in a heterogeneous sensor array-based instrument [132]. Amperometry is an old electroanalytical technique that en-compasses coulometry, voltammetry, and constant potential techniques, and is widelyused to identify and quantify electroactive species in liquid and gas phases. For liquidphase analytes, the electrodes and analytes are immersed in a common electrolyte andthese have resulted in electronic tongues [102]. In contrast, application of amperome-try to gas-phase analytes involves a unique gas-liquid/solid interfacial transport pro-cess. The AGS is a class of electrochemical gas sensors sometimes called voltam-metric, micro-fuel cell, polarographic, amperostatic, or other names [103, 104]. Thecommon characteristic of all AGSs is that measurements are made by recordingthe current in the electrochemical cell between the working and counter electrodesas a function of the analyte concentration. Figure 4.9 illustrates an amperometric sen-sor consisting of working, counter, and reference electrodes dipped in an electrolyte.The analyte is reacted electrochemically, i.e. oxidized or reduced, and this process,


governed by Faraday’s Law, either produces or consumes electrons at the workingelectrode. The amperometric class of electrochemical sensor complements the othertwo classes of electrochemical sensors, i.e. potentiometric sensors that measure theNernst potential at zero current, and conductometric sensors that measure changes inimpedance [130].The AGS, Figure 4.9, is controlled by a potentiostatic circuit and produces its current

or signal when exposed to a gas/vapor containing an electroactive analyte. The analytediffuses into the electrochemical cell and to the working electrode surface and where itparticipates in a redox reaction. The cell current is directly related to the rate of reactiontaking place at the electrode surface and is described by application of Faraday’s Law,relating the mass, W, of a substance of molecular mass M (grams mol�1) as:

W ¼ Q M

F nð6Þ

where Q is the charge per unit electrode area, F is Faraday’s constant in coulombs/equivalent, and n is the number of electron equivalents per mole of the reacting ana-lyte. Assuming there are no other reacting species in the solution, the observed cur-rent, dQ/dt (t ¼ time) or i, is directly proportional to the amount of analyte, W, that issupplied to the working electrode and, this in turn can be related to the gaseous analyteconcentration (see Eq. 7).The potentiostat allows control of the working electrode thermodynamic potential

while the reaction occurs. The AGS is made reactive towards a variety of analytes bychoosing different potentials, working electrode catalysts, electrolytes, porous mem-branes, and different electroanalytical methods. The working electrode reaction thatproduces current in the example of a CO sensor in Fig. 4.9 is usually taken as:

CO½g� þH2O ¼ CO2 þ 2Hþ½aq� þ 2e�:

The CO diffuses or is pumped to the region of the working electrode, dissolves in theelectrolyte, diffuses to the working electrode surface where it undergoes reaction withsubsequent desorption of the CO2 product and conduction of the 2e� away through themetal electrode. The more CO that is present, the larger the current. Typical currentsare in the micro- or pico-ampere level for ppm level reactants. Response times, mea-

Fig. 4.9 An amperometric gas sensor

4.8 Thermal (Calorimetric) Sensors 9797

sured as time to 90% of signal, have ranged from milliseconds for some oxygen sen-sors to several minutes for other analytes.It is usually preferable that a sensor works in the limiting current region in which

themagnitude of the sensor signal is practically independent of the electrode potential.In theory, the limiting current region can be achieved in any case when the rate-limit-ing step is a step prior to electron transfer. The rate of electrode reactionmay be limitedby the rate of diffusion through a membrane or a capillary that is placed somewherebetween the gas stream containing the analyte and the catalyst layer of the electrode. Insuch cases, the limiting current, ilim, can be written:

ilim ¼ k½CO�gas ð7Þ

where the constant k is the proportionality constant relating the gaseous concentrationto the current in some convenient units like lA (ppmv)�1 (parts per million by vo-lume). The amperometric gas sensor is one of the most widely used sensors for toxicgas detection, i.e. CO, NO, NO2, H2S, SO2, O2, and so on. The AGS was used in the e-nose [105] for one of the earliest determinations of bacterial contamination [106] andidentification of discrete analytes [107]. The AGS has been microfabricated [99, 108]but such versions are not yet commercially available. The main advantages of the am-perometric approach are high sensitivity, a good deal of control over selectivity accom-panied by relatively low cost, small size, and long stable lifetimes.

4.10

Summary of Chemical Sensors

Commercially available-nose instruments listed in Table 4.7 are concentrated on twomain types of chemosensors, such as MOS-type and CP-type. More recent work isbeginning to exploit other sensors for application to the food and drink industriesas listed in Table 4.8. There are a number of books and references in other sectionsof this Handbook that point the user towards the myriad of e-noses that have beenconstructed as well as the various classes and types of sensors. New sensors, includingmicro instruments, will also contribute to the growing number of e-noses that willinevitably lead to an improvement in analytical capability.More and more is being demanded of sensors as time goes on. Quantitative and

qualitative analytical results are not enough and we are requested to answer morepertinent and complex questions such as: Where is the contamination? Is this hazar-dous? Is this pure or the same as something else? These questions are often complexchemically. Sensors provide critical data for the e-nose and other analytical instru-ments that can address such complicated analytical tasks. Without good performancewe have no chance for good data or good answers to these types of questions. Sensorsand sensory data must therefore continue to be improved.


Tab.

4.7

Com

merciallyavailablee-noseinstrumen

ts.Abb

reviations:

metal

oxide

semicon

ductor,M

OS;

organiccondu

ctingpolym

er,C

P;quartzcrystalm

icrobalance,

QCM;surfaceacou

stic

wave,

SAW;gaschromatog

raphy,GC;qu

adrupolemass

spectrom

etry,QMS;

infrared,IR;an

dMOSfield

effect

tran

sistor,MOSF

ET.

Pattern

recogn

ition:artificialneuralnetwork,

ANN;distan

ceclassifiers,DC;

principalcompon

entanalysis,PCA;statisticalpattern

recogn

ition,SPR;discrim

inan

t

functionan

alysis,DFA

;clusteran

alysis,CA;an

dprincipal

compon

ents

regression

,

PCR.

Man

ufacturer

Chem

osensortype

Numberof

sensors

Sizeof

Instrument(CostUS$)

Pattern

recogn

ition

Com

ments

Airsensan

alysis

GmbH

(German

y)

MOS

10Laptop(20000–43

000)

ANN,DC,PCA,SPR

Small,fast

&robu

st

AlphaMOS-M

ulti

Organ

olepticSystems(France)CP,MOS,QCM,SAW

6–24

Desktop

(2000

0–10

000

0)ANN,DFA,PCA

Autosampleran

dair

conditioningunitavailable

AromaScanPLC

(UK)

CP

32Desktop

(20000–75

000)

ANN

Autosampleran

dair

conditioningunitavailable

Array

Tech

QCM

8

Blood

hou

ndSen

sors

Ltd.(U

K)

CP

14Laptop

ANN,CA,PCA

Smallcompan

y,instrumen

t

basedon

research

atLees

University

Cyran

oScien

ceInc.

(USA)

CP

32Palmtop(5000)

PCA

EEVLtd.Chem

ical

Sen

sor

System

(UK)

CP,MOS,QCM,SAW

8–28

Desktop

ANN,DFA,PCA

Electronic

Sen

sor

TechnologyInc.

(USA)

GC,SAW

1Desktop

(19500–25

000)

SPR

Hew

lett-PakardCo.

(USA)

QMS

–Desktop

(79900)

Standardchem

ometrix

HKR-Sen

sorsystemeGmbH

(German

y)

QCM

6Desktop

ANN,CA,DFA,PCA

Smallcompan

y.Based

onre-

search

atUniversityof

Munich

Lennartz

Electronic

GmbH

(German

y)

MOS,QCM

16–40

Desktop

(5500

0)ANN,PCA,PCR

MOSESII

MastiffElectronic

SystemsLtd.

CP

16Sniffedpalmsforpersonal

iden

tification

Nordic

Sen

ser

TechnologiesAB(Sweden

)

IR,MOS,MOSFET,QCM

22Laptop(40000–60

000)

ANN,CPA

Iden

tification

ofpurity,

origin.

RSTRostock

Rau

m-fah

rt

undUmweltschatzGmbH

(German

y)

MOS,QCM,SAW

6–10

Desktop

(5000

0)ANN,PCA

Neotron

icsScien

ceLtd.(U

K)CP

12—

—Medium

size

d

compan

y.

Shim

adzu

Co.

(Japan

)MOS

6Desktop

(70000)

PCA

Largecompan

y.

Saw

tekInc.

SAW

2Palmtop(5000)

Proprietary

4.10 Summary of Chemical Sensors 9999

Tab. 4.8 Chemosensors used in recent e-nose studies for application

to food and drink industries.

Food or Drink Test Chemosensor type Number of

sensors

Ref.

Alcohols Identification MOS (SnO2) 12 67

Fish (cod, haddock) Freshness MOS (SnO2) 6 83

Fish (squid) Freshness MOS (MgO-In2O3) 9 85

Coffee Discrimination MOS (SnO2) 12 66

Fish Freshness MOS (Ru-In2O3) 1 43, 86

Soup Quality control MOS (Ru-WO3) 4 87

Sea foods (squid, oyster,

sea bream, sardine)

Freshness MOS (Al-ZnO) 1 88–90

Alcohol Freshness MOS (ZnO-SnO2) 1 91

Ground pork/Beef Discrimination and

effect of ageing

Mixed 15 61

Wine Varieties and vintages

of same wine

MOS (SnO2, WO3) 4 92

Beef Freshness MOS (WO3-ZnO) 1 93

Fish (trout) Freshness MOS 8 94

Wheats Grade quality MOS, AGS 4 � 4 95, [106]

Wheats and cheese Discrimination and

ageing

CP 20 96

Cheeses Maturity of cheddars CP 20 97

Coffees Discrimination

between varieties

CP 12 98

Beers Diacetyl taint in

synthetic beer

CP 12 99

Beers Discrimination

between lager and ales

CP 12 100

Liqors Discrimination

between brandy,

gin and whisky

CP 5 101

Boar Taints in meat MOS 14 102

Sausage meats Discrimination MOS 6 103

Water Taints in drinking

water

MOS 4 104

Colas Discrimination

between diet and

normal colas

MOS 6 103

Coffees Discriminate

C. arabica and C. robustaMOS 6 80, 105

Food flavors

(orange, strawberry,

apple, grape, peach)

Flavor identification QCM 8 46

Tomatoes Effect of irradiation

and stress

Mixed 7 106

Whiskies Discrimination

of Japanese whiskies

QCM 8 51


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