Sensors Report

7/30/2019 Sensors Report

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A

MINOR PROJECT

ON

POLYMER IN CHEMICAL SENSORS ALONG

WITH ITS FABRICATION

Submitted By: Guided By:

BADAL LODHARI & YASH PATEL Prof.NEHA PATNI.

(10BCH011) (10BCH067)

CHEMICAL ENGINEERING DEPARTMENT

INSTITUTE OF TECHNOLOGY

NIRMA UNIVERSITY


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CERTIFICATE

This is to certify that Mr. BADAL VINODKUMAR LODHARI & YASHSUMULBHAI PATEL, students of Chemical Engineering, 7th semester, of NirmaUniversity, has satisfactorily completed the seminar on POLYMER IN

CHEMICAL SENSORS ALONG WITH ITS FABRICATION as a partial

fulfillment towards the degree of B. Tech. in Chemical Engineering.

Date:

Place:

Prof. Neha Patni. Dr. S. S. Patel

Head of Department


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INDEX

Page no.

Acknowledgement I

Abstract II

Content III

List of figures IV

List of tables V


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ACKNOWLEDGEMENTS

First of all, We would like to thank my guide Prof. Neha Patni for his constant support. We

would also like to thank our HOD Dr.S.S.Patel and the whole Chemical Department for their

guidance. We would like to thank my parents and my friends for helping me. We would like tothank the Library of Nirma University for providing me all the essential resources.

I


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ABSTRACT

Chemical sensors have become an important technology in chemical measurement. Theseanalytical devices provide a simple, rapid and reliable means for the detection and quantitation ofspecific chemical species. This technology has enabled the continuous measurement of a markersubstance, and provided a basis for the development of intelligent instrumentation systems.

Chemical Sensors combine a molecular recognition element with a transducer in order toproduce an electronic signal in the presence of a particular analyte. Polymers are used inchemical sensors Because their chemical and physical properties may be tailored over a widerange of characteristics, the use of polymers is finding a permanent place in sophisticatedelectronic measuring devices such as sensors.

During the last 5 years, polymers have gained tremendous recognition in the field of artificialsensor in the goal of mimicking natural sense organs. Better selectivity and rapid measurementshave been achieved by replacing classical sensor materials with polymers involving nanotechnology and exploiting either the intrinsic or extrinsic functions of polymers.

Semiconductors, semiconducting metal oxides, solid electrolytes, ionic membranes, and organicsemiconductors have been the classical materials for sensor devices. The developing role ofpolymers as gas sensors, pH sensors, ion-selective sensors, humidity sensors, biosensor devices,etc., are reviewed and discussed in this paper. Both intrinsically conducting polymers and non-conducting polymers are used in sensor devices.

Polymers used in sensor devices either participate in sensing mechanisms or immobilize thecomponent responsible for sensingthe analyte. Finally, current trends in sensor research and alsochallenges in future sensor research are discussed.

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CONTENT

Chapter

No.

Title Page

No.

1. INTRODUCTION 1.

1.1 THE TERM SENSOR 1.

1.2 CHARACTERISTICS OF A CHEMICAL SENSOR 2.

1.3 ELEMENTS OF CHEMICAL SENSORS 3.

1.3.1 RECEPTORS 4.

1.3.2 TRANSDUCERS 5.

1.4 PARAMETERS OF CHEMICAL SENSORS 5.

2. FUNDAMENTALS 6.

2.1 SENSORS PHSICS 6.

2.1.1 SOLIDS 6.

2.2 SENSOR CHEMISTRY 7.

2.2.1 CHEMICAL EQUILIBRIUM 7.

2.3 SENSOR TECHNOLOGY 9.

3. POLYMERS IN CHEMICAL SENSORS 10.

3.1 GAS SENSORS 10.

3.2 PH SENSORS 19.

3.3 ION SELECTIVE SENSORS 21.

3.4 ALCOHOL SENSORS 24.

4. CONCLUSION 26.

5. REFERENCES 27.

III


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LIST OF FIGURES

Sr. No. Title Page No.

1. 1.2 Two sources in the development of chemical sensors 2.

2. 1.3 Scheme of a typical chemical sensor system 4.

3. 3.1 Basic Representation of a crystaquartz crystal

microbalance(QCM) sensor

18.

4. 3.2 Layout of a Single Acoustic Aperture Wave (SAW) 18.

IV.


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LIST OF TABLES

Sr No. Title Page No.

1. 2 Various sensors and their applications 11.

2. 3 Polymers used in various gas sensors 16.

3. 4. Adsorption of strychnine (a pesticide) and b-ionene (an

odor) at 45 8C by various films immobilized on a QCM

surface

19.

4. 5. Structures of some ionophores used in ion selective sensor

devices

23.

V.


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1. INTRODUCTION.During the last 20 years, global research and development (R&D) on the field of sensors hasexpanded exponentially in terms of financial investment, the published literature, and the numberof active researchers.It is well known that the function of a sensor is to provide information on our physical, chemical

and biological environment. Legislation has fostered a huge demand for the sensors necessary inenvironmental monitoring, e.g. monitoring toxic gases and vapors in the workplace orcontaminants in natural waters by industrial effluents and runoff from agriculture fields. Thus, anear revolution is apparent in sensor research, giving birth to a large number of sensor devicesfor medical and environmental technology.

A chemical sensor furnishes information about its environment and consists of a physicaltransducer and a chemically selective layer[1]. A biosensor contains a biological entity such asenzyme, antibody, bacteria, tissue, etc. as recognition agent, whereas a chemical sensor does notcontain these agents. Sensor devices have been made from classical semiconductors, solidelectrolytes, insulators, metals and catalytic materials.

Since the chemical and physical properties of polymers may be tailored by the chemist forparticular needs, they gained importance in the construction of sensor devices. Although amajority of polymers are unable to conduct electricity, their insulating properties are utilized inthe electronic industry.

A survey of the literature reveals that polymers also acquired a major position as materials invarious sensor devices among other materials. Either an intrinsically conducting polymer isbeing used as a coating or encapsulating material on an electrode surface, or non-conducting apolymer is being used for immobilization of specific receptor agents on the sensor device.

1.1THE TERM SENSORIt would not be sufficient to see sensors merely as some kind of technical sensing organs. Theycan be used in many other fields besides just intelligent machines. A modern definition should becomprehensive.

Actually, there is still no generally accepted definition of the term. On the other hand, it seemsto be rather clear what we mean when we talk about a sensor.We find, however, differencesregarding whether the receptor alone is a sensor or whether the term encompasses the completeunit containing receptor plus transducer.

Regardless of such differences, there is broad agreement about attributes of sensors. Sensorsshould: Be in direct contact with the investigated subject, Transform non-electric information into electric signals, Respond quickly, Operate continuously or at least in repeated cycles, Be small, Be cheap.

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It seems astonishing that sensors are expected to be cheap. Such an expectation can beunderstood as the expression of the self-evident requirement that sensors be available in largequantities, above all as a result of mass production.

1.2. CHARACTERISTICS OF A CHEMICAL SENSOR

The term chemical sensor stems not merely from the demand for artificial sensing organs.Indeed, chemical expertise was necessary to design chemical sensors.Such expertise is the subject of analytical chemistry in its modern, instrumental form. Initially,chemists hesitated to deal with sensors, but later their interest in them grew.

The field of chemical sensors has been adapted and is now largely considered a significantsubdiscipline of analytical chemistry. On the other hand, the field is given little space inanalytical chemistry textbooks. This is true mainly in European textbooks.

Sensors do not fit smoothly into traditional concepts and appear to belong to an unrelated field.Up to now, they have not been a typical constituent of analytical chemistry lectures in Europe.There is no doubt, however, that chemical sensors comprise a branch of analytical chemistry.The latter by definition aims to obtain information about substantial matter, especially about the

occurrence and amount of constituents including information about their spatial distribution andtheir temporal changes (Danzer et al. 1976).

There are two obvious sources for the formationof sensor science as an independent field. One ofthese sources is the above-mentioned development of microtechnologies, which stimulated ademand for sensing organs.

The second source is a consequence of the evolution of analytical chemistry which brought abouta growing need for mobile analyses and their instrumentation. Figure 1.2 attempts to outline theformation of sensor science as a bona fide branch of science.

Figure 1.2. Two sources in the development of chemical sensors

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a definition was given by IUPAC in 1991:[4]A chemical sensor is a device that transforms chemical information, ranging from concentrationof a specific sample component to total composition analysis, into an analytically useful signal.This is rather general. Thus, many pragmatic descriptions exist in the literature.

Consider the following definition by Wolfbe is (1990):Chemical sensors are small-sized devices comprising are cognition element, a transduction

element, and a signal processor capable of continuously andreversibly reporting a chemical concentration.

The attribute of reversibility is considered important by many authors. It means that sensorsignals should not freeze but respond dynamically to changes in sample concentration in the

course ofmeasurement.

The following characteristics of chemical sensors are generally accepted. Chemical sensorsshould: Transform chemical quantities into electrical signals, Respond rapidly,

Maintain their activity over a long time period, Be small, Be cheap, Be specific, i.e. they should respond exclusively to one analyte, or at least be selective to agroup of analytes. The above list could be extended with, e.g., the postulation of a low detectionlimit, or a high sensitivity. Thismeans that low concentration values should be detected.

1.3. ELEMENTS OF CHEMICAL SENSORS

Section 1.1 showed that the functions of a chemical sensor can be considered to be tasks of

different units. This is expressed typically in statements like the following (IUPAC 1999):Chemical sensors usually contain two basic components connected in series: a chemical(molecular) recognition system (receptor) and a physicochemical transducer.

In other documents, additional elements are considered to be necessary, in particular units forsignal amplification and for signal conditioning. A typical arrangement is outlined in Fig. 1.3.

In the majority of chemical sensors, the receptor interacts with analyte molecules. As a result, itsphysical properties are changed in such a way that the appending transducer can gain anelectrical signal.

In some cases, one and the same physical object acts as receptor and as transducer. This is thecase e.g. in metallic oxide semiconductor gas sensors which change their electrical conductivityin contact with some gases Conductivity change itself is a measurable electrical signal. In masssensitive sensors, however, receptor and transducer are represented by different physical objects.A piezoelectric quartz crystal acts as transducer.

The receptor is formed by a sensitive layer at the crystal surface. The latter is capable ofabsorbing gas molecules. The resulting mass change can be measured as a frequency change inan electrical oscillator circuit.

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Figure 1.3. Scheme of a typical chemical sensor system

1.3.1. RECEPTORS

The receptor function is fulfilled in many cases by a thin layer which is able to interact withanalyte molecules, catalyse a reaction selectively, or participate in a chemical equilibriumtogether with the analyte.

Receptor layers can respond selectively toparticular substances or to a group of substances. The

term molecular recognition is used to describe this behaviour. Typical for biosensors is thatmolecules are recognized by their size or their dimension, i.e. by steric recognition.

Among the processes of interaction,most important for chemical sensors are adsorption, ionexchange and liquidliquid extraction (partition equilibrium).

Primarily these phenomena act at the interface between analyte and receptor surface, where bothare in an equilibrium state. Instead of equilibrium, a chemical reaction may also become thesource of information.

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We find this, for example, in receptors where a catalyst accelerates the rate of an analyte reactionsomuch that the released heat from the reaction creates a temperature change that can betransduced into an electrical signal. Processes at the receptor-analyte interface can be classifiedinto interaction equilibria and chemical reaction equilibria.

The differences are not significant for work with sensors. A true chemical equilibrium is formed,for example, in electrochemical sensors where receptor and analyte are partners of the sameredox couple.

1.3.2. TRANSDUCERS

Today, signals are processed nearly exclusively by means of electrical instrumentation.Accordingly, every sensor should include a transducing function, i.e. the actual concentrationvalue, anon-electric quantity,must be transformed into an electric quantity voltage, current orresistance.

The pool of transducers can be classified in different ways. Following the quantity appearing atthe transducer output, we encounter types like current transducer, voltage transducer etc. In

the international literature, there exists no systematic concept for classification. In what follows,an attempt is made to find a classification scheme which reflects the inner function of thetransducers using only a few transducer principles.

It is base on a scheme developed by electronics engineers but has not been applied to sensors tillnow (Malmstadt et al. 1981). Among the examples given are those that develop their sensorfunction only in combination with an additional receptor layer. In other types, receptor operationis an inherent function of the transducer.

1.4 PARAMETERS OF CHEMICAL SENSORS[3]

The following list contains static as well as dynamic parameters which can be used tocharacterize the performance of chemical sensors.

Sensitivity: change in the measurement signal per concentration unit of the analyte, i.e. theslope of a calibration graph.

Detection limit: the lowest concentration value which can be detected by the sensor inquestion, under definite conditions. Whether or not the analyte can be quantified at the detectionlimit is not determined. Procedures for evaluation of the detection limit depend on the kind of

sensor considered.

Dynamic range: the concentration range between the detection limit and the upper limitingconcentration.

Selectivity: an expression ofwhether a sensor responds selectively to a group of analytes oreven specifically to a single analyte. Quantitative expressions of selectivity exist for differenttypes of sensors.

Linearity: the relative deviation of an experimentally determined calibration graph from anideal straight line. Usually values for linearity are specified for a definite concentration range.

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Resolution: the lowest concentration difference which can be distinguished when thecomposition is varied continuously. This parameter is important chiefly for detectors in flowingstreams.

Response time: the time for a sensor to respond from zero concentration to a step change inconcentration. Usually specified as the time to rise to a definite ratio of the final value. Thus, e.g.the value of t99 represents the time necessary to reach 99 percent of the full-scale output. Thetime which has elapsed until 63 percent of the final value is reached is called the time

constant.

Hysteresis: the maximum difference in output when the value is approached with (a) anincreasing and (b) a decreasing analyte concentration range. It is given as a percentage of full-scale output.

Stability: the ability of the sensor to maintain its performance for a certain period of time. As ameasure of stability, drift values are used, e.g. the signal variation for zero concentration.

Life cycle: the length of time over which the sensor will operate. The maximum storage time(shelf life) must be distinguished from the maximum operating life. The latter can be specified

either for continuous operation or for repeated on-off cycles.

2. FUNDAMENTAL

2.1. SENSORS PHSICS

2.1.1 SOLIDS

Many phenomena occurring at the surface of solids are important for sensors. The components ofsolids are typically arranged regularly.These components, i.e. atoms or molecules, are fixed by bonding forces so that they form regionswith a regular lattice structure.Amorphous substances like glasses or polymers are not consideredto be solids, although they have some properties of solid bodies.

The different sorts of solids are characterized preferably by the type of chemical bonding thatpredominates. In metallic solids, free electrons are delocalized in a framework of regularlyarranged cations. Metallic bonding brings about a rigid but ductile structure.

The crystal structure forms as a result of the attempt of spherical atoms to approximate each

other as close as possible and to form a package of maximum density. In an ionic solid, ions ofopposite charge are held together by Coulomb forces.

Every ion tends to be surrounded as uniformly as possible by oppositely charged counterions,thus forming an electrically neutral structure.

The variety of existing structures is a result of the fact that ions often have quite different radiiand often are not shaped like a ball. In solidswith an atomic lattice, atoms are connected bycovalent bonding forces forming a regular network involving the complete crystal.

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The crystal structure is now determined by the tendency of orbitals to overlap rather than tofollow geometric principles.

The diamond structure is a typical example of an atomic lattice. In the diamond crystal, everycarbon atom is interconnected with four equal neighbours in the formof a tetrahedron. Each atomis in an sp3 hybridization state and forms four bonds. Solids with an atomic lattice often arevery hard and chemically inert.

Crystals with molecular lattices are formed by molecules interconnected by intermolecularforces. Such forces are much weaker than chemical bonding forces. The vast majority of organicsolids are molecular crystals. Generally they are soft and have a low melting point. Solidswithmobile electrons are electronic conductors.

They can be classified following the type of temperature dependence of their electricconductivity. The conductivity of metallic conductors decreases with increasing temperature,whereas semiconductors showthe opposite behaviour.

Their temperature dependence is higher than that of metals. Elements like silicon or germaniumas well as compounds like gallium arsenide are typical semiconductors. Substances with a very

low conductivity (e.g. diamond) are isolators. Their conductivity tends to increase withtemperature, like those of semiconductors.

2.2 SENSOR CHEMISTRY

2.2.1 CHEMICAL EQUILIBRIUM.

The most important basis of analytical chemistry is the theory of chemical equilibrium. For ca.350 years chemists have been performing chemical operations with the intention obtaininginformation about chemical composition. This means, first of all, utilizing the laws and therelationships describing chemical equilibria. Chemical equilibrium is a dynamic equilibrium. In asystem which is in equilibrium, reactions do not stop, even if no movement is visible for anexternal observer. When molecules react, they form new products, and the products aredecomposed again, and whereas a certain amount of products is formed, simultaneously an equalamount of the original reactants is generated as a result o the consumption of the products. For atheoretical description of chemical equilibrium and to derive its inherent laws, there exist twofundamentally different models, namely the thermodynamic approach and the kinetic approach.Both approaches result in the same mathematical relationships. For quantitation of mixtures ofsubstances, the following quantities are important:

n the amount of substance, measured in the SI unit mol (mole). One mole means a verylarge number of particles, namely 6.022 1023 pieces. This corresponds to the number ofatoms in 12g carbon or in 197g gold, or to the number of molecules in 2g hydrogen gas.It is advantageous to give a substance amount not by its mass but by the number ofmoles. One mole always indicates the same number of particles, independent of the kindof substance.

c the concentration of solutions, preferably denoted in terms of molarity (the number ofmoles per volume of solution in litres c = n/v). For example, c = 1mol/L = 1M.

the mole fraction (molar fraction). This denotes the number of moles of dissolvedsubstance nB as a proportion of the total number of moles ina solution (nA + nB, wherethe index B denotes the solvent). = nB/nA+Nb

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a the activity a = f c is a concentration where the activity coefficient f is some kind ofcorrecting factor. Activity is measured using the same unitsas concentration c. For lowvalues of c, approximately a c and f 1.

Activity values in electrolyte solutions strongly depend on the concentrations of all the speciespresent in solution and of their charge numbers. In some cases, a can be calculated using theionic strength I.

The chemical reaction rate r = dn/dt, measured inmol s1, depends on the concentrations ofreactants as well as on the concentrations of products. For a simple reaction, if all the partnersare in a gaseous state, e.g. the reaction of iodine vapour with hydrogen gas,H2 + I2 2HI we can write r = k c(H2) c(I2)

whereas for the opposite direction2HI H2 + I2 the rate is r = k c2(HI)

The terms_k and_k are the rate constants of the forward and backward reactions, respectively.They depend only on temperature. When we write a chemical equation for a reaction inequilibrium, it makes sense towrite the double arrow instead of a =, since the dynamic

character of chemical equilibria is symbolized in this way: H2 + I2 2HI.

Under conditions of equilibrium, both reaction rates are equal:r = k c(H2) c(I2) = _k c2(HI) .and finallyK =c2(HI) c(I2) c(H2). (2.12)

Equation (2.12) is well known under the historical name law of mass action. The derivation ofthis law given above is not really a strict one. The result, however, is of strict validity regardlessof the actual shape of reaction-rate equations.Of course, for other reactions with different stoichiometric factors (the numbers which appear inchemical equations left from the chemical formula symbols), the law may look different from the

example given in Eq. (2.12).

Starting with a given stock ofreactant molecules, the rate of forward reaction initially must behigh but decrease more and more in the course of reaction (Fig. 2.22). Just the oppositebehaviour can be expected for the backward reaction.

Alternatively, the law of mass action can be derived on the basis of the assumption that for adefinite initial state (a given set of reactants each with given concentration), a driving forceshould exist.

As in electric engineering, where the current is the result of the driving force voltage, in

chemistry the reaction rate can be considered the result of a chemical driving force.Obviously, this chemical driving force depends on reactant concentrations. Since theseconcentrations decrease in the course of the reaction, the driving force must also decrease, andfinally approach zero.The chemical driving force has a name. It is the so-called change of free Gibbs energy, denotedby the symbol RG. This very important quantity is a function of all reacting species appearingin a chemical reaction scheme:RG = RG

+ R T ln f cii (2.13)In this equation, RG

and R are constants.

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The stoichiometric coefficientsi appear with a positive sign for products generated and with anegative sign for reactants consumed in the course of reaction. The operator means multiplyall the following numbers.

2.3 SENSOR TECHNOLOGY

Chemical sensors are products of quite different fields of science and technology. Consequently,there exist quite differentmanufacturing techniqes.

On the other hand, the field of sensors has formed in the course of the rapid development ofmicrotechnologies, and these technologies have strongly influenced the design of chemicalsensors. In this way, construction details have emerged with sensors which cannot be found inany other field of analytical chemistry.

It is useful to have a closer look at design details characteristic of chemical sensors. Thecommon design of a chemical sensor includes an interface in direct contact with the sample, thereceptor. Very often the receptor represents a thin layer located at the surface of an inert carrier.

The next element cannot be described by a simple rule.

There are electric contacts, devices for signal processing and many other types of units,depending on the kind of sensor. The transducer, i.e. the most important element for samplerecognition, can be manufactured by different means. In thick-film technology, different layersare screen-printed on the surface of a carrier.

Thin-film technology, the predominant technique in microelectronics, also plays an importantrole for sensors.Thin layers are generated by vapour deposition, sputtering or chemical vapourdeposition (CVD).

A combination of thick-film and thin-film technologies can also be found in sensors. Examplesof multilayer sensors are polycrystalline semiconductor gas sensors, where the sensitive tindioxide or titanium dioxide layer is spread by sputtering on the surface of a conductive layerdesigned to be a heater, consisting either of platinum or of ruthenium dioxide. The latter can alsobe produced by sputtering.

Another example is a gas sensor type where a paste of fine oxide particles is deposited on aceramic substrate. The paste is fired and sintered in the next manufacturing step. Layers of equalmaterial and in equal order can be fabricated by thin-film as well as by thick-film techniques.Techniques for generating structures have reached a high degree of perfection and are veryimportant for microelectronics.

The efficiency of such techniques can be illustrated by considering modern integrated electroniccircuits where millions of transistors are located on a single silicon chip. Microelectronicstructures can be two or three dimensional. They strongly vary with respect to resolution.Withthick films, only rough structures can bemade.

With thin films on silicon wafers, extreme resolution is attainable and complex three-dimensional structures can be realized. The latter are fundamental for new technological fieldslike micromechanics and microfluidics.

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Chemical sensors can be integrated into such devices. As a result, complete miniaturelaboratories (lab-on-the chip) can be assembled. Highly complex instruments like liquidchromatographs also are becoming available in miniaturized form.

3. POLYMER IN SENSOR DEVICES

3.1. GAS SENSORS

The emission of gaseous pollutants such as sulphur oxide, nitrogen oxide and toxic gases fromrelated industries has become a serious environmental concern. Sensors are needed to detect andmeasure the concentration of such gaseous pollutants.

In fact analytical gas sensors offer a promising and inexpensive solution to problems related tohazardous gases in the environment. Some applications of gas sensors are included in Table 2.Amperometric sensors consisting of an electrochemical cell in a gas flow, which respond toelectrochemically active gases and vapors, have been used to detect hazardous gases and vapors.

Variation in the electrodes and the electrode potentials can be utilized to identify the gases

present. There have been improvements using a catalytic micro-reactor in the gas flow leading tothe amperometric sensors.

Such a reactor with a heated filament of platinum causes the analyte to undergo oxidation so thatpreviously electrochemically unreactive species can be detected. Conducting polymers showedpromising applications for sensing gases having acidbase or oxidizing characteristics.Conducting polymer composites with other polymers such as PVC, PMMA, etc.

polymers with active functional groups and SPEs are also used to detect such gases. Hydrogenchloride (HCl) is not only the source of dioxin produced in the incineration of plants and acidrain, but it also has been identified as a workplace hazard with a short-term exposure limit of 5

ppm. To detect HCl in sub-ppm levels, composites of alkoxy substituted tetraphenylporphyrinpolymer composite films were developed by Nakagawa.

The sensor response and recovery behavior is improved if the matrix has a glass transitiontemperature below the sensing temperature. The alkoxy group imparts basicity to the material,and hence increases sensitivity to HCl. The changes in the Soret-and Q-bands with HCl gas inppm levels have been examined. It has been found that high selectivity to sub ppm levels of HClgas was achieved using a 5,10,15,20-tetra (40-butoxyphenyl)porphyrin-butylmethacrylate [TP(OC4 H9)PH2-BuMA] composite film Supriyatno et al. showed optochemical detection of HClgas using a mono-substituted tetraphenylporphinpolymer composite films. They achieved ahigher and preferable sensitivity to sub-ppm levels of HCl using polyhexylmethacrylate matrix

in the composite.

Amperometric sensors have been fabricated by Mizutani for the determination of dissolvedoxygen and nitric oxide using a perm selective polydimethylsiloxane (PDMS) (II) membrane.

A hydrophobic polymer layer with a porous structure is useful for the selective permeation ofgases. A very low concentration of nitric oxide (20 nM50 mM) could be measured with thesesensors at 0.85 V versus Ag/AgCl without serious interference from oxidizable species, such asL-ascorbic acid, uric acid and acetaminophen.

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They prepared the electrode by dip coating from an emulsion of PDMS. Being perm selective,the polymer coating is capable of discriminating between gases and hydrophobic species, whichco-exist in the samples to be measured. Gases permeate easily through the pores to reach theelectrode surface, whereas the transport of the hydrophilic compounds is strongly restricted.

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Chou, Ng and Wangprepared a Au-SPE sensor for detecting dissolved oxygen (DO) in water,with Nafion as the SPE. It is a very good sensor for detecting DO in water, with a lower limit of3.8 ppm. The authors also claimed excellent stability for this sensor. Polyacetylene (III) is knownto be the first organic conducting polymer (OCP).

Exposure of this normally resistive polymer to iodine vapor altered the conductivity by up to 11orders of magnitude. Polyacetylene is doped with iodine on exposure to iodine vapor. Then,charge transfer occurs from polyacetylene chain (donor) to the iodine (acceptor) leads to theformation of charge carriers. Above approximately 2% doping, the carriers are free to movealong the polymer chains resulting in metallic behavior.

Later heterocyclic polymers, which retain the p-system of polyacetylene but include heteroatombonded to the chain in a five membered ring were developed. Such heterocyclic OCPs (IV)include polyfuran (X O), polythiophene (X S), and polypyrrole (X NH). Theintrinsically conducting polymers are p-conjugated macromolecules that show electrical andoptical property changes, when they are doped/dedoped by some chemical agent.

These physical property changes can be observed at room temperature, when they are exposed tolowerconcentrations of the chemicals, which make them attractive candidates for gas sensingelements. Nylander investigated the gas sensing properties of polypyrrole by exposingpolypyrroleimpregnated filter paper to ammonia vapor.

The performance of the sensor was linear at room temperature with higher concentrations (0.55%), responding within a matter of minutes. Persaud and Pelosi reported conducting polymersensor arrays for gas and odor sensing based on substituted polymers of pyrrole, thiophene,aniline, indole and others in 1984 at the European Chemoreception Congress (ECRO), Lyon,followed by a detailed paper in 1985.

It was observed that nucleophilic gases (ammonia and methanol, ethanol vapors) cause adecrease in conductivity, with electrophilic gases (NOx, PCl3, SO2) having the opposite effect.Most of the widely studied conducting polymers in gas sensing applications are polythiopheneand its derivatives, polypyrroles, polyaniline and their composites.

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Electrically conducting polyacrylonitrile(PAN) /polypyrrole(PPY) ,polythiophene/polystyrene,polythiophene /polycarbonate, polypyrrole/polystyrene, polypyrrole/polycarbonatecomposites were prepared by electropolymerization of the conducting polymers into the matrixof the insulating polymers PAN, polystyrene and polycarbonates, respectively.

These polymers have characteristics of low powerconsumption, optimum performance at low toambient temperature, low poisoning effects, sensor responseproportional to analyte concentrationand rapid adsorption/desorption kinetics.

Electroactive nanocomposite ultrathin films of polyaniline (PAN) and isopolymolybdic acid(PMA) for detection of NH3 and NO2 gases were fabricated by alternate deposition of PAN andPMA following LangmuirBlodgett (LB) and self-assembly techniques.

The process was based on dopinginduced deposition effect of emeraldine base. The NH3-sensingmechanism was based on dedoping of PAN by basic ammonia, since the conductivity is stronglydependent on the doping level.

In NO2 sensing, NO2 played the role of an oxidative dopant, causing an increase in theconductivity when emeraldine base is exposed to NO2. found that the optical and electrical

properties of p-conjugated polyaniline change due to interaction of the emeraldine salt (ES) (V)with NH3 gas. The interaction of this polymer with gas molecules decreases the polaron densityin the band-gap of the polymer.

It was observed that PANIPMMA composite coatings are sensitive to very low concentrationsof NH3 gas (,10 ppm). Chabukswar synthesized acrylic acid doped polyaniline for use as anammonia vapor sensor over a broad range of concentrations, viz. 1600 ppm. They observed thesensor response in terms of the dc electric resistance on exposure to ammonia.

The change in resistance was found to increase linearly with NH3 concentration up to 58 ppmand saturates thereafter. They explained the decrease in resistance on the basis of removal of a

proton from the acrylic acid dopant by the ammonia molecules, thereby rendering freeconduction sites in the polymer matrix.

A plot of the variation of relative response of the ammonia gas sensor with increase in theconcentration of ammonia gas is shown in Fig. 1. Acrylic acid doped polyaniline showed a sharpincrease in relative response for around 10 ppm ammonia and subsequently remained constantbeyond 500 ppm, whereas the nanocomposite of polyaniline and isopolymolybdic acid (PMA)showed a decrease of relative response with the increase in ammonia concentration.

Yadongreportedthat submicrometer polypyrrole film exhibits a useful sensitivity to NH3. TheNH3 sensitivity was detected by the change in resistance of the polypyrrole film. They

interpreted the resistance change of the film in terms of the formation of a positively chargedelectric barrier of NH4 -ion in the submicrometerfilm.

The electrons of the NH3 gas act as the donor to the p-type semiconductor polypyrrole, with theconsequence of reducing the number of holes in the polypyrrole and increasing the resistivity ofthe submicrometer film. A polypyrrolepoly(vinyl alcohol)(PVA) composite prepared byelectropolymerizing pyrrole in a cross-linked matrix of pyrrole was found to posses significantNH3 sensing capacity.

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The ammonia sensing mechanism of the polypyrrole electrode has been addressed, with evidencethat a mobile counter ion may be required for proper sensor operation. Such evidence supportsthe idea that polypyrrole undergoes a reversible redox reaction when ammonia is detected atsubmillimolar concentrations.

Quartz Crystal Microbalance (QCM) sensors are a kind of piezoelectric quartz crystal with a

selective coating deposited on the surface to serve as an adsorptive surface. The QCM is a verystable device, capable of measuring an extremely small mass change. Fig. 2presents a schematicdiagram for a QCM.

The natural resonant frequency of the QCM is disturbed by a change in mass from the adsorptionof molecules onto the coating. For example, a shift in resonance frequency of 1 Hz can easily bemeasure for an AT-cut quartz plate with a resonance frequency of 5 MHz, which corresponds to achange in mass of just 17 ng/cm2. Table 4 shows how exposure of 19 ppm strychnine (apesticide) or b-ionine (an odor) affects the absorption masses of QCM coated with variouschemically sensitive films.

A number of materials have been investigated as coatings for QCM sensors, includingphthalocyanine , polymerceramic composite, epoxy resin for estimation of ethanol incommercial liquors and cellulose. The general trend observed shows that polymer-coated QCMsare most sensitive towards volatiles possessing a complimentary physicochemical character, e.g.hydrogen bond forming acidic volatiles was best detected by hydrogen bond forming basicpolymers.

Alkanes could be distinguished from alkenes by the use of strongly hydrogen bond formingacidic polymers that could interact with the weak hydrogen bond basicity of the alkenes, thealkanes having no such hydrogen bonding capacity.

If a piezoelectric substance is incorporated in an oscillating electronic circuit a surface acousticwave (SAW) is formed across the substance. Any change in velocity of these waves, due to thechange in mass of the coating on the sensor by an absorbing species, will alter the resonantfrequency of the wave.

The oscillations are applied to the sensor through a set of metallic electrodes formed on thepiezoelectric surface,over which a selective coating is deposited. Fig. 3 shows that the acousticwave is created by an AC voltage signal applied to a set of interdigited electrodes at one end ofthe device.

The electric field distorts the lattice of the piezoelectric material beneath the electrode, causing a

SAW to propagate toward the other end through a region of the crystal known as the acousticaperture. When the wave arrives at the other end, a duplicate set of interdigited electrodesgenerate an AC signal as the acoustic wave passes underneath them.

The signal can be monitored in terms of amplitude, frequency and phase shift. These devicesoperate at ultrahigh frequencies (gigahertz range), giving them the capability to sense as little as1 pg of material.Similar to QCM sensors, the coating on the sensor determines the selectivity ofthe SAW device, for example, LiNbO3 , fluoropolymers for sensing of a pollutantorganophosphorus gas and commercially available gas chromatography phases as coating forsensing toluene in dry air.

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In these sensors the response times can be of the order of 1 s. Although SAW sensors are very

sensitive to physical changes in the sample matrix, this can be overcome by the use of areference cell.

Fig 3.2: Layout of a Single Acoustic Aperture Wave (SAW)

18.

Fig 3.1 : Basic Representation of a crystaquartz crystal microbalance(QCM) sensor


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3.2 PH SENSORS.

The pH indicates the amount of hydrogen ion in a solution. Since the solution pH has asignificant effect on chemical reactions, the measurement and control of pH is very important in

chemistry, biochemistry, clinical chemistry and environmental science.

Munkhol used photochemically polymerized copolymer of acrylamide-methylenebis(acrylamide)containing fluoresceinamine covalently attached to an optical fiber surface (core dia 100 mm) ina pH sensor device. Amongst various organic materials, polyaniline has been found as mostsuitable for pH sensing in aqueous medium.

The use of conducting polymers in the preparation of optical pH sensor has eliminated the needfor organic dyes. Demarcos and Wolfbeisdeveloped an optical pH sensor based on polypyrroleby oxidative polymerization. Since the polymer film has suitable optical properties for optical pHsensor, the immobilization step for an organic dye during preparation of the sensor layer was not

required.

Others have also developed optical pH sensors based on polyaniline for measurement of pH inthe range 212. They reported that the polyaniline films synthesized within a time span of 30min are very stable in water. Jinreported an optical pH sensor based on polyaniline (Table 2).

While they prepared polyaniline films by chemical oxidation at room temperature, theyimproved the stability of the polyaniline film significantly by increasing the reaction time up to12 h. The film showed rapid reversible color change upon pH change.

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The solution pH could be determined by monitoring either absorption at a fixed wavelength orthe maximum absorption wavelength of the film. The effect of pH on the change in electronicspectrum of polyaniline polymers was explained by the different degree of protonation of theimine nitrogen atoms in the polymer chain. The optical pH sensors could be kept exposed in airfor over 1 month without any deterioration in sensor performance.

Ferguson used a poly(hydroxyethyl methacrylate) (IX) hydrogel containing acryloyl fluoresceinas pH indicator. Shakhsher and Seitz exploited the swelling of a small drop of aminated

polystyrene (quaternized) on the tip of a single optical fiber as the working principle of a pHsensor.

Other pH sensor devices using polymers have also been developed. Leiner developed acommercial blood pH sensor in which the pH-sensitive layer was obtained by reactingaminoethylcellulose fibers with 1-hydroxy-pyrene-3,6,8-trisulfochloride, followed by attachmentof the sensitive layer to the surface of a polyester foil, and embedding the composite in an ion-permeable polyurethane (PU) based hydrogel material.

Hydrogen ion selective solid contact electrodes based on N,N0- dialkylbenzylethylenediamine(alkyl butyl, hexyl, octyl, decyl) were prepared. Solid contact electrodes and coated wireelectrodes had been fabricated from polymer cocktail solutions based on N,N0-dialkylbenzylethylenediamine (alkyl butyl, hexyl, octyl, decyl).

They showed that the response range and slopes were influenced by the alkyl chain length. Solidcontact electrodes showed linear selectivity to hydrogen ion in the pH ranges 4.513.0, 4.213.1,3.413.0 and 3.013.2, with Nernstian slopes of 49.7, 50.8, 51.5 and 53.7 mV pH21 at 20 ^ 0.28C, respectively.

Stability was also improved, especially when compared with coated wire electrodes. The 90%response time was ,2 s, and their electrical resistance varied in the range 2.372.76 MV. Solidcontact electrodes with N,N0-didecylbenzylethylenediamine showed the best selectivity andreproducibility of e.m.f.

Pandeydeveloped a solid state poly(3cyclohexyl)thiophene treated electrode as pH sensor, andsubsequently, urea sensor. Later, Pandey and Singhreported the pH sensing function of polymer-modified electrode (a novel pH sensor) in both aqueous and non-aqueous mediums. The sensowas derived from polymer-modified electrode obtained from electrochemical polymerization ofaniline in dry acetonitrile containing 0.5 M tetraphenyl borate at 2.0 V versus Ag/AgCl.

The light yellow color polymer modified electrode was characterized by scanning electronmicroscopy (SEM). They used weak acid (acetic acid) and weak base (ammonium hydroxide) asanalytes. The acetic acid was analyzed in both aqueous and dry acetonitrile, whereas ammoniumhydroxide was analyzed only in aqueous medium.

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3.3. ION SELECTIVE SENSORS

There is a vast literature covering the theory and design of ion selective devices. Generally, ionsensors have been developed taking the polymer as theconductive system/component, or as amatrix for the conducting system.

When such systems come in contact with analytes to be sensed, some ionic exchange/interaction

occurs, which in turn is transmitted as an electronic signal for display. Ion selective electrodes(ISE) are suitable for determination of some specific ions in a solution in the presence of otherions.

The quantitative analysis of ions in solutions by ISEs is a widely used analytical method, withwhich all chemists are familiar. Commercial potentiometric devices of varying selectivity forboth cations and anions are common in most laboratories. Ion sensors find wide application inmedical, environmental and industrial analysis.

They are also used in measuring the hardness of water. Potentiometric ISEs for copper ions havebeen prepared by screen-printing, with the screen-printing paste composed of methyl and butyl

methacrylate copolymer,copper sulphides and graphite (Table 2).Ion-sensitive chemicaltransduction is based on ion selectivity conveyed by ionophoreion-exchange agents, chargedcarriers and neutral carriersdoped in polymeric membranes.

In addition to organic salts, several macrocyclics, such as antibiotics, crown ethers andcalixerenes, are used as neutral carriers, functioning by hostguest interactions. The chemicalstructures of some ionophores are shown in Table 5. The polymeric membrane-based deviceconsists of an internal electrode and reference solution, the selective membrane across which anactivity-dependent potential difference develops, and an external reference electrode to which themembrane potential is compared in the potential measurement.

The response and selectivity of an ion-selective device depend on the composition of themembrane. Polyvinyl chloride (PVC) is the most commonly used as polymeric matrix. A typicalmembrane composition for the usual cations and anions consists of polymer (33 wt%), plasticizer(,65 wt%), ion carrier (,15 wt%), and ionic additives (,02 wt%). In ion-selective sensors,polymers have been utilized to entrap the sensing elements.

Table 6 describes various sensor components, which are entrapped in polymer films for thedetections of different ions, and their sensing characteristics. Silicone rubber and a PU/PVCcopolymer were reportedto be good screen-printable ion selective membranes for sensing arrays.Silicone rubber-based membranecontaining a modified calyx (4) arene was used for detection ofNa in body fluids.

Teixeira studied the potentiometric response of a l-MnO2-based graphite-epoxy electrode fordetermination of lithium ions. The best potentiometric response was obtained for an electrodecomposition of 35% l-MnO2, 15% graphite and 50% epoxy resin. The response time of theproposed electrode was lower than 30 s and its lifetime greater than 6 months.

Further, they discussed the possibility of miniaturization of the electrode by putting thecomposite inside a capillary tube. Such an electrode requires a conditioning time in a Lisolution prior to the measurement of its equilibrium potential.

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Since the epoxy resin absorbs significant amount of water, it is possible that the first layer ofepoxy resin on the electrode surface absorb the Li solution, and thus time is necessary to attainequilibrium. A new Ca2-selective polyaniline (PANI)-based membrane has been developedforall-solid-state sensor applications.

The membrane is made of electrically conducting PANI containing bis [4-(1,1,3,3-tetramethylbutyl) phenyl] phosphoric acid (DTMBP-PO4H), dioctyl phenylphosphonate (DOPP)and cationic (tridodecylmethylammonium chloride, TDMACl) or anionic (potassium tetraki (4-

chlorophenyl) borate, KTpClPB) as lipophilic additives. PANI is used as the membrane matrix,which transforms the ionic response to an electronic signal. Artigas described the fabrication of acalcium ion-sensitive electrochemical sensor.

This sensor device consists of a photocurable polymer membrane based on aliphatic diacrylatedpolyurethane instead of PVC. Moreover, these polymers are compatible with thephotolithographic fabrication techniques in microelectronics, and provide better adhesion tosilanized semiconductor surfaces, such as the gate surfaces of ion selective field effect transistors(ISFETs).

Membranes sensitive to calcium ions were optimized according to the type of plasticizer and the

polymer/plasticizer ratio. Such sensors are stable for more than 8 months, and the resultingsensitivities were quasi-Nernstian (2627 mV/dec) in a range of 5 10268 1022 M. Thesesensors were used to measure calcium activity in water samples extracted from agricultural soils.The authors claimed their results to be well correlated with those obtained by standard methods.

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3.4 ALCOHOL SENSORS.

The determination of alcohol is important in industrial and clinical analyses, as well as inbiochemical applications. Ukeda presented a new approach in the coimmobilization of alcoholdehydrogenase and nicotinamide adenine dinucleotide (NAD) using acetylated cellulosemembrane on glutaraldehyde activated Sepharose and its application to the enzymatic analysis ofethanol.

Since conducting polymers gained popularity as competent sensor material for organic vapors,few reports are available describing the use of polyaniline as a sensor for alcohol vapors, such asmethanol, ethanol and propanol.

Polyaniline doped with camphor sulphonic acid (CSA) also showed a good response for alcoholvapors. These reports discussed the sensing mechanism on the basis of the crystallinity ofpolyaniline. Polyaniline and its substituted derivatives (XI) such as poly(o-toluidine), poly(o-anisidine), poly(N-methyl aniline), poly(N-ethyl aniline), poly(2,3 dimethyl aniline), poly(2,5dimethyl aniline) and poly(diphenyl amine) were found by Athawale and Kulkarni to besensitive to various alcohols such as methanol, ethanol, propanol, butanol and heptanol vapors

(Table 2).

All the polymers respond to the saturated alcohol vapors by undergoing a change in resistance.While the resistance decreased in presence of small chain alcohols, viz. methanol, ethanol andpropanol, an opposite trend in the change of resistance was observed with butanol and heptanolvapors.

The change in resistance of the polymers on exposure to different alcohol vapors was attributedto their chemical structure, chain length and dielectric nature. All the polymers showedmeasurable responses (sensitivity ,60%) for short chain alcohols, at concentrations up to 3000ppm, but none of them are suitable for long chain alcohols.

They explained the results based on the vapor-induced change in the crystallinity of thepolymer. The polypyrrole was also studied as a sensing layer for alcohols. Polypyrroleincorporated with dodecyl benzene sulfonic acid (DBSA) and ammonium persulfate (APS)showed a linear change in resistance when exposed to methanol vapor in the range 875000ppm.

Bartlett also detected methanol vapor by the change in resistance of a polypyrrole film. Theresponse is rapid and reversible at room temperature.

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They investigated the effects of methanol concentration, operating temperature and filmthickness on the response. Mayes reported a liquid phase alcohol sensor based on a reflectionhologram distributed within a poly(hydroxyethyl methacrylate) (IX) film as a means to measurealcohol induced thickness changes.Blum prepared an alcohol sensor in which two lipophilic derivatives of Reichardts

phenolbetaine were dissolved in thin layers of plasticized poly(ethylene vinylacetate) copolymercoated with micro porous white PTFE in order to facilitate reflectance (transflectance)measurements.

The sensor layers respond to aqueous ethanol with a color change from green to blue withincreasing ethanol content. The highest signal changes are observed at a wavelength of 750 nm,with a linear calibration function up to 20% v/v ethanol and a detection limit of 0.1% v/v. Theselayers also exhibit strong sensitivity to acetic acid, which affects effective measurements onbeverages. However, this limitation was overcome by adjusting the pH of the sample solution.

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4.CONCLUSION.

The majority of sensor devices utilize many polymers with definite roles, either in the sensingmechanism or through immobilizing the species responsible for sensing of the analytecomponent.

This has become possible only because polymers may be tailored for particular properties, areeasily processed, and may be selected to be inert in the environment containing the analyte.

While some polymers are intrinsically responsible for a sensor function, other polymers are madeto augment the sensing operation through modification of the polymer by functionalization.

Polymeric thin film deposition technology and the design of more active and sensorspecificpolymers will lead to successful miniature, multiple sensor arrays.

The collaboration of polymer scientists and technologists in sensor research will accelerate theavailability of durable and cheap artificial sensor devices for human consumption .

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5. REFERENCES :

[1] Janata J, Bezegh A. Chemical sensors. Anal Chem 1988;60: 62R74R.[2]Peter Grndler,Textbook of chemical sensors(Introduction for scientist and engineers)

:ISBN978-3-540-45742-8.[3]Danzer K, Than E, Molch D (1976) Analytiksystematischer berblick. Akademische

Verlagsgesellschaft Geest & Portig K.-G. Leipzig[4]IUPAC (1991) Pure Appl Chem 63:12471250.[5]Karl Cammann, Bernd Ross, Andreas Katerkamp, Jorg Reinbold, Bernd Grundig,

Reinhard Renneberg,Chemical and Biochemical Sensors 2012, Wiley-VCH Verlag GmbH &Co. KGaA, Weinhei.

[6] Basudam Adhikari, Sarmishtha Majumdar Polymers in sensor applications,ELSEVIER ,May2004.

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