Chern. Anal. (Warsaw), 42, 199 (1997) REVIEW

Organic Conducting Polymers as Active Materialsin ElectrochemicalChemo-Sensors and··Biosensors

by Marek 'frojanowicz,Tadeusz Krawczyfiski vel Krawczyk and PeterW. Alexander*

Department ofChemistry, University ofWarsali) Pasteura 1,02-093 Warsaw, Poland#Department ofPhysical Sciences, University ofTasmania, Launceston,

P.O.Box 1214, Tasmania 7250, Australia

Key words: conducting polymers, electropolymerisation, electrochemical. detection,

amperometry, potentiometry, biosensors, discrimination of interferences

The electroanalytical applications of organic conducting polymers produced by electro­polymerisation are reviewed based on 101 references. They find applications fordetection of numerous gaseous analytes, for design of amperometric, voltammetric andpotentiometric sensors operating in solutions. There is also increasing number of theirapplications in development ofbiosensors for immobilization of enzymes or antibodies,with simultaneous size-exclusion or ion-exchange discrimination of various interferences.

Opracowany przegl<td, obejmuj<tcy 101 odnosnikow, jest poswiltcony detektorom elektro,..chemicznym opartym na organicznych polimerach przewodZ<l,cych otrzymanych przezelektropolimeryzacjet. Znajduj<t one zastosowanie do detekcji licznych analitow gazowychoraz do konstruowania czujnikow amperometrycznych i potencjometrycznych do ozna­czania analitow w roztworach. S<t one rowniez coraz szerzej wykorzystywane do otrzymy­wania bioczujnikow z unieruchomionymi enzymami lub przeciwcialami, z jednoczesnymwykoizystaniem efektu sitowego oraz jonowymiennego do zmniejszania wpiywu roznychzak16cen.

For many years organic conducting polymers have been the subject of investiga­tion in various fields of science and technology [1-3]. They have gained wide interestas electrode materials in electrochemistry and electroanalysis since development ofelectrolytic procedures of their deposition on the electrode surface at the end of theseventies, initially for polypyrrole and polyaniline, and then for numerous otherheterocyclic and substituted aromatic compounds [4]. Investigations carried out with

200 M. Trojanowicz, T. Krawczynski and P. W Alexander

various physical and chemical methods have shown many of t4eir unique properties.They result not only from the nature of electronic conductivity of these materials,similar to the conductivity of metals, but also from the variety of their chemicalinteractions. The most important feature of conducting polymers in electrochemicalapplications is the possibility of modification of their physical and chemical proper­ties by appropriate polarization and doping with counterions. This allows the modi­fication of their hydrophobic interactions, or hydrogen bonding and ionicinteractions. Their properties utilized in electrochemistry and electroanalysis dependon chemical and physical conditions applied in the electropolymerisation process.The possibility of molecular switching or changes of conductivity by appropriatepolarization is ofgreat interest for electronics. Good adhesion to metallic surfacesand ability to storage the electric charge creates the possibility of their use asanticorrosion coatings and in battery production.

Many different applications of conducting polymers have also been found inelectroanalysis. They have widened the possibility of modification of surface ofconventional electrodes providing new, interesting properties. They were applied inelectrocatalysis, membrane separations and chromatography. They also create newtechnological possibilities in design of chemical and biochemical sensors [5-7].

The literature concerning the electropolymerisation of conducting polymers isalready very extensive. Especially much attention was devoted to polypyrrole and itsderivatives, either to its electropolymerisatione.g. [8-12], as to its properties e.g.

efficient method of producing conducting polymers from solutions of monomer, inwhich the rate and extent of the process can be carefully controlled byelectrochemi­cal conditions. Galvanostatic deposition at constant current provides better controlof the film thickness and is more reproducible than the potentiostatic method,although the latter requiring simpler instrumentation is very often used. The electro­deposition is carried out either in organic or aqueous solutions, however, the choiceof solvent affects the morphology, conductivity and activity of obtained polymer. Inthe most common process, the conductive polymer is formed by mild oxidation ofthe monomer and it involves incorporation of count~rions into the polymer matrixduring electrodeposition. Application of too positive potentials results in irreversibleoxidation of the polymer and a loss of its conductivity and activity. As it is often incase of polymers it is difficult to determine exact structure of the electropolymerizedpolymer. A common opinion is that polymers formed from heterocyclic monomersform a linear chains. It was found, however, that at least under certain polymerizationconditions e.g. polypyrrole with a macrocyclic structure is formed [15]. In modelconsiderations an observation was made about the possibility of a fullerene-likemolecule constructed from pyrrole [16]. Polyaniline evolution can go towards atwo-dimensional polymer with phenazine rings, which can be formed by a cross-link­ing reaction or by insertion of nitrenium cations of aniline [17]. During the electro­polymerisation process also mediators, complexing ligands, enzymes and antibodiescan be incorporated into the conducting polymer. The biocompatibility essential forclinically oriented biosensors can be achieved by incorporating polysaccharides or

Conducting polymers in electrochemical sensors 201

heparin into the polymer during electrodeposition [18]. The combination in the samematerial of the chemical interaction or molecular recognition centers with electronicproperties provides a convenient basis for electronic signal generation for the designof chemical or biochemical sensors both for gaseous species and analytes in solutions.Amperometry, potentiometry and conductivity are most commonly employed asdetection methods with these sensors.

Response to gaseous species

Chemiresistors with conducting polymer layers respond to a variety of gasesincluding many organic vapors [19-21] and inorganic species such as ammonia[20,22,23], hydrazine vapor [22,24], hydrogen cyanide [25], NOz [26], PCl3 and SOz[27]. The adsorbed organic molecules affect the electronic charge-transfer processand have a solvent-type action on the polymer causing physical swelling of thepolymer structure. The aITays of differentconducting polymer sensors can be suc­cessfully employed for olfactory sensing with various types of advanced multivariateprocessing of measured signals [28,29]. Illustration of signals obtained for severaldifferent polymers to beer headspaces is shown in Fig. 1 [28].

Figure 1. Typical responses of conducting polymers to beer headspaces [28]: 8a,b,c - polypyrrole dopedwith p-toluenesulfonate; 9 - polypyrrole doped with tetraethylammonium toluenesulfonate;11a,o - poiyaniiine doped with hydrogensulfate

. The interaction with electrophilic gases attracts electrons out of the polymerphase causing an increase in conductivity, whereas the interaction with nucleophilicgases increase the resistance of the polymer. The irreversible increase in resistanceof the polypyrrole film in the presence of ammonia was attributed to nucleophilicattack by ammonia on the polymer leading to loss of conjugation and ring opening[30]. Reversible response to ammonia gas was observed in piezoelectric measure­ments with polypyrrole sensor interpreted as adsorption by the polymer coating ofthe quartz crystal with simultaneous conductivity changes [31]. D.c. conductivitymeasurements with polypyITole films initially doped with Fe(CN)g- shown that

202 M. Trojanowicz, T. Krawczyfrski and P.l¥. Alexander

ammonia acts as a reversible dedopant, but a poisoning effect of N02 was observed[32]. Sensitivity of polypyrrole to ammonia was utilized in design of air-gap enzymemicroelectrode for urea [33]. Thin films of the electrically conductive polymerpoly(3-hexylthiophene) were found as ultrasensitive chemical sensors for hydrazineand monomethylhydrazine vapor in the presence of NH3, amines, and ambient water[24]. The sensitivity is based on irreversible increase in the electrical resistance basedon annihilation of delocalized charge carriers on the polymer backbone. Compositemembranes from anion-exchange membrane and polypyrrole were employed aspotentiometric humidity sensors [34], whereas polyaniline with metal clusters incor­porated into the bulk of the polymer served as a sensing layer for gaseous hydrogencyanide [25]. In the latter system the interaction of the gas with this layer causes areversible and reproducible change of its work function measured with a Kelvinprobe. The polyaniline film was conditioned in aqueous mercuric chloride solutionprior to exposure to gaseous HeN. When nitrotoluenes are incorporated electro­chemically into the bulk of the polypyrrole it becomes sensitive to some aromaticand hydrogen bonding compounds that interact with the film by hydrogen bondingor through aromatic Jt systems [35]. The copolymerisation of polypyrrole withnitrotoluenes involving the nitrotoluyl radicals produced a substantially differentmaterial with much higher electron affinity.

Amperometric and voltammetric chemical sensors

The voltammetric response of electrodes modified with conducting polymersessentially depends on the kind of counter ion incorporated into the polymer layerduring the electropolymerisation process [36]. The detection with such electrodes isexceptionally advantageous for ions electrochemically inactive which, however,influence the oxidation-reduction equilibrium of the polymer. Determination ofelectroinactive anions is based on the incorporation of anions during the oxidationof the polymer as they neutralize the positive sites generated during oxidation. Theapplication of the reduction potential causes the disappearance of the positive chargesfrom the polymer, which is associated with ejection of anions from the polymermatrix. This mechanism was confirmed by piezoelectric measurements [37]. Theelectrodes modified with polypyrrole and polyaniline were applied for amperometricdetection of inorganic and organic anions [38-42]. Such a detection was utilized alsoin high-performance ion-chromatography of anions [41,42]. A significant improve­ment of detection limit in amperometric determination of anions was observed byusing platinum microelectrodes (10 /-lm diameter) [43]. Then using larger electrodes,it was shown that enhanced sensitivity for detection of electroinactive ions wasobtained when a pulsed potential waveform was used [44]. The same pulsed potentialwaveform was applied in work with polypyrrole coated microelectrodes for flow-in­jection detection of nitrate, chloride, carbonate, phosphate, acetate and dodecylsul­fate with detection limits from 50 nmol 1-1 to 5 /-lmoll-1 [43].

Large surfactant anions such as dodecylsulfate or toluenesulfonate used asdopants in conducting polymers are captured irreversibly by the polymer and during

Conducting polymers in electrochemical sensors 203

reduction process they are not ejected and rather cations are incorporated, which canbe utilized for amperometric detection of electroinactive cations [45]. Paratoluene­sulfonate doped polypyrrole electrode was also used in normal pulse voltammetricdetermination of proteins [46]. For a polyaniline and polypyrrole film formed in thepresence of a cation exchanger Nafion it was observed that immobilized sulfonategroups of Nafion serve as a charge compensator in the course of anodic polymeriza­tion of monomers [47,48]. The immobilization of anionic sites causes that cationsare diffusing species accompanying the redox processes and the polyaniline-Nafioncomposite film electrode can be used for sensitive amperometric detection of alkaliand alkaline earth metal ions in ion-chromatography [49] (Fig. 2).

TO.2nA

1

(8)

o 2 4

t (b)

5nA

1

024 e 8, , , , ,

Elution Time, min

Figure 2. Ion-chromatograms obtained with amperometric detection using polyaniline-Nafion compo­site electrode and 1.5 mmol 1-1 HN03 (a) and 1.0 mmol 1-1 HN03 with 2.0 mmol 1-1

ethylenediamine (b) as eluents. Concentrations in injected sample solutions: Na and K - 0.05mmol 1-1; Mg, Ca, Sr and Ba - 0.2 mmol 1-1 (Reprinted from [49] with kind permission ofElsevier Science - NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)

The interaction ofammonia with some conducting polymers utilized for ammoniadetection in gaseous phase can be also exploited for amperometric detection ofammonia in solution. This can be carried out with polypyrrole [50,51] and polyaniline[52]. Polypyrrole based sensor shown satisfactory dynamic characteristics of detec­tion that permits its application to the detennination of ammonia in a flow injectionsystem up to 100 I.unoll-1 of analyte with detection limit of 0.6 ~moll-l [51]. Theammonia sensitivity of polypyrrole is also maintained after covering the polymerlayer with bilayer lipid membrane [53], which can be used for design of biosensors.More selective and stable amperometric response to ammonia is obtained for poly-

204 M. Trojanowicz, T. Krawczyftski and P. W Alexander

aniline film produced by cyclic voltammetric electropolymerisation from solutionscontaining organic anions [52]. The example of recorded flow-injection response forsuch a sensor is shown in Fig. 3.

2.0 mmoLL-1

ISOnA

1.0

0.5

II I

0.2

0.05°·1 ~~

"-~"""'\I\J10 min

I I

Time

Figure 3. Flow-injection amperometric response recorded for polyani line doped withp-toluenesulfonateto ammonia. Measurements carried out at +1.0 V vs. A,gIAgCI~ IJljected sample volume 20 III

Detection of some organic and biological molecules was carried out by voltam­metry at conducting poly(3-methylthiophene) electrodes [54,55]. The polymer modi­fied surface catalyzes the oxidation of catechol, ascorbic acid, hydroquinone,dopamine, epinephrine, acetaminophen, p-aminophenol and NADH. Differentialpulse and square wave modes were used for the analysis of binary mixtures ofascorbic acid with catechol, NADH, dopamine andp-aminophenol. Differential pulsevoltammetry was also successfully used for a ternary mixtures of ascorbic acid,p-aminophenol and catechol [52]. Poly(3-methylthiophene)/polypyrroie bilayer­coated carbon fiber electrodes were shown to exhibit enhanced electrochemicalreversibility for ascorbic acid oxidation and with differential pulse voltammetrysimultaneous determination of ascorbic acid and dopamine was carried out [55].

Conducting polymers in electrochemical sensors 205

In amperometric measurements it was also found that fine molecular cutoffs canbe obtained by varying the electropolymerisation time and monomer concentration[56]. The exclusion of large electroactive species offers substantial improvements inthe selectivity of amperometric detection in flowing streams and prevention ofelectrode deactivation due to protein adsorption.

Voltammetry at ligand loaded conducting polymer films

Preconcentration on solid sorbents with immobilized complexillg agents is atpresent the. most commonly used method of preconcentration in trace metal analysis.The electrodes modified with conducting polymer with incorporated complexingcenters allow the combination of the chemical preconcentration process with voltam­metric detection. Preconcentration of copper was carried out using a poly(pyrrole-N­-carbodithioate) coated electrode, which was obtained by chemical derivatisation ofpolypyrrole film [57]. The subsequent voltammetry of the complex on the electrodesurface allows copper determination at the ppm level. Similar procedures were usedfor mercury determination [58]. It was observed that the coated electrode removedmore mercury ions from solution and more rapidly than a bare platinum electrode.As a result the stripping signals with the polypyrrole electrode were much larger inmagnitude. Another procedure is based on the incorporation of complexing ligandinto the polymer layer by entrapment during electropolymerisation. This was em­ploved in the detennination ofsilver [59] and copper(I) and copper(II) [60]. however.detection limits at the ppm level obtained in these determinations are substantiallypoorer, than in most conventional electroanalytical methods, especially in strippingmethods. Detennination of oxyanions of Cr(VI) can be based on trapping the analytevia ion exchange in poly(3-methylthiophene) [61] or on preconcentration in overox­idised polypyrrole upon application of an anodic potential [62], followed by linearsweep or differential pulse voltammetry.

Potentiometric chemical sensors

. Conducting polymers have found an increasing number of applications in poten­tiometric chemical sensors. Polypyrrole films doped with chloride, perchlorate,tetrafluoroborate or nitrate produce stable potentiometric responses to a range ofanions [30-32], however, selectivity of obtained sensors is poor. In case of poly­pyrrole based nitrate electrode it was found that while anions more lipophilic thannitrate can be sterically hindered from entering the nitrate-doped polypyrrole filmsif their radii are larger than that of nitrate (e.g. perchlorate or iodide), more lipophilicanions of equal or smaller radii, still cause an interference problem [66]. It was showntheoretically that the potentiometric signal results from the transfer of the electrons(redox sensitivity) and doping ions [68]. The sensitivity of anionic potentiometricresponse depends substantially on kind of doping ion, on polymerization potentialand the pH ofmonomer solution during the electropolymerisation [67]. Apotentiome­tric iodide sensor was produced by the entrapment of iodide/iodine/triiodide intovarious conducting polymers [69]. Superior behavior of iodine-doped poly(3-methyl-

206 M. Trojanowicz, T. Krawczyftski and P. W Alexander

thiophene) electrode towards iodine over that of polyaniline or poly-(N-methyl­-pyrrole) (Fig. 4) was explained in terms of the iodine-induced structural changes inthe poly(3-methylthiophene). A non selective cationic response to several alkali andalkaline earth metal cations was observed also for undopedpoly(3-octylthiophene)[70].

700 (a)

200 ........_ ...........L.__.........-.L._..&............_

J 456 7

-log [I"]

700 (b)

600

500

!>Ea.i

300

100 L..L.._.......--.L.........~......... --L.._.I-

J 456 7-log [Jj

Figure 4. Potentiometric response of iodide electrodes obtained by entrapment of iodide/iodine/triiodidein poly(3-methylthiophene) (a) and polyaniline (b) 1 day (e), 7 days (0),30 days (_), 120days (0) (Reprinted from [69] with kind permission of Elsevier Science - NL, Sara Burger­hartstraat 25, 1055 KV Amsterdam, The Netherlands.)

Another application of conducting polymers in potentiometric sensors is incor­poration of polymers, which are soluble in common organic solvents, into plasticisedPVC based membranes containing an ionophore [71]. This procedure enhances thesignal transduction in the case of ion-selective electrodes with PVC membranes andsolid electronically conducting substrates. It was shown that doped polyaniline in themembrane improves the stability of the standard potential of the electrode. A morestable potentiometric response of membrane ion-selective electrodes can be alsoobtained by contacting the ionophore containing membrane to the solid substrate viaan intermediate conducting polymer layer having mixed ionic and electronic conduc­tivity [72,73].

Conducting polymer based enzyme biosensors

There is now wide interest in development of biochemical methods of chemicalanalysis. The largest numberof electroanalytical applications of conducting polymersis devoted to design of enzymatic biosensors (Tables 1 and 2). Performing theelectropolymerisation process in the presence of soluble enzyme enables its entrap­ment in the polymer layer at the electrode surface without significant loss of its

Conducting polymers in electrochemical sensors 207

biocatalytic activity. Since first works published by Aizawa and Yabuki [74] andFoulds and Lowe [75] these procedures were employed for numerous commonlyemployed enzymes in different polymer layers [5-7]. The alternate way can be acovalent binding of the enzyme to polymer layer via amides or secondary amines[76], or copolymerisation of monomer with a monomer-modified enzyme [77].Recently some examples of coimmobilisation of several enzymes in the conductingpolymer films were also reported. In lactat~ biosensor coimmobilisation of lactatedehydrogenase and lactate oxidase in poly(phenylenediamine) film yields a higWysensitive detection due to amplification by substrate recycling [78]. The creatinineelectrode was made by coimmobilisation of creatininase, creatinase and sarcosineoxidase in a PPy matrix [79]. The enzyme immobilization can be also carried out bythe electropolymerisation of amphiphilic pyrrolyl-alkylammonium ion mixtureswith enzymes. Using tyrosinase a biosensor for the determination of cyanide, chlo­rophenols, atrazine, dithiocarbamate and carbamate pesticides was developed [80].The detection was performed via inhibiting action of analytes on the tyrosinasesensor. The advantages of the procedure of enzyme immobilization in the conductingpolymer were shown by sensor preparation in situ i.e. in a flow injection analysissystem by injecting a plug of a solution containing the monomer and the enzyme [81].

Table 1. Developed glucose biosensors employing conducting polymers

Enzyme Polymer Detection

Glucose oxidase polypyrrole amperometry

potentlometry

poly(N-methylpyrrole) amperometry

polyaniline amperometry

poly(v-phenylenediamine) amperometry

polyindole amperometry

Glucose oxidase and peroxidase polypyrrole amperometry

Glucose oxidase and catalase polypyrrole amperometry

Glucose dehydrogenase polypyrrole amperometry

The structure of the polymer affects the sensitivity and the detection limit of thebiosensor obtained, as well as the ion-exchange and size-exclusion properties, lead­ing to improvement of the detection selectivity. Conducting polymers are highlyeffective in removal of interferences when used for entrapment of redox enzymes[82] and in protection against electrode fouling caused by non-specific adsorption ofhigh-molecular compounds from natural samples [83].

The most important factor affecting the amperometric biosensor is the electrontransfer between the biocatalytic molecule, usually oxidase or dehydrogenase, andthe electrode surface, most often involving a mediato::-. The role of conductingpolymers used for itnmobilization of enzyme in this process is discussed by numerous

208 M. Trojanowicz, T. Krawczynski and P.lv.Alexander

authors. In one of the first approaches the reagentless glucose electrodes have beenprepared by the synthesis ofN-substituted pyrrole containing redox-active ferroceneside chains designed to accept electrons from the reduced form of the enzyme [84].Ferrocene-pyrrole conjugates were efficient oxidants of reduced glucose oxidase,however, sensitivity of such a system to oxygen contrasts with other ferrocene-medi­ated biosensors.

Table 2. Biosensors employing conducting polymers developed for various substrates (except glucose, see, Table 1)

Substrates or speciesEnzyme Polymer Detectionto be determined

D-AJanine D-amino acid oxidase polypyrrole amperometry

Atrazine tyrosinase polypyrrole amperometry

cholesterol oxidase PPD amperometry

Cholesterol cholesterol oxidase and polypyrrole amperometrycholesterol esterase

Choline choline oxidase substituted amperometry

polypyrrole amperometry

Dopamine; tyrosinase polypyrrole

catecholamines

Galactose galactose oxidase polyaniline amperometry

Glutamate glutamate dehydrogenase polypyrrole amperometry

Fructose fructose dehydrogenase polypyrrole amperometry

Hemoglobin pepsin polyaniline conductometry

Hydrogen peroxide peroxidase polypyrrole amperometrypolyaniline

L-Lactate lactate oxidase PPD amperometry

Lipids lipase polyaniline conductometry

NADH NADH dehydrogenase polypyrrole amperometry

Phenols tyrosinase substituted amperometry

polypyrrole

Urea urease polypyrrole amperometry

potentiometry

conductometry

capacitance measurement

admittance measurement

polyaniline conductometry

Uric acid uricase polyaniline amperometry

Triglicerides lipase polyaniline conductometry

PPD - poly(o-phenylenediamine).

Conducting polymers in electrochemical sensors 209

Another observed drawback was complete loss of sensitivity after two days of use.Numerous observations indicating a participation of the polymer in direct electrontransfer were made in the systems without mediators. The same oxidation andreduction peaks observed for flavin adenine dinucleotide (FAD) in poly(N-methyl­pyrrole) film and for glucose oxidase in the sa!lle polymer indicate that electrontransfer occurs between FAD centers in glucose oxidase and the electrode [85].Mediation by the conducting polymer was found to be very effective and no signifi­cant electron transfer to oxygen was observed for biosensors with glucose oxidaseadsorbed irreversibly on conducting microtubulus of polypyrrole [86,87]. The posi­tive response to glucose was observed in oxygen-free solutions at low measuringpotential (100 mV vs. Ag/AgCl), where no response to hydrogen peroxide wasobserved. Biosensors with conducting polymers are considered as third-generationbiosensor, where reoxidation of the reduced enzyme is the rate-limiting process atlow potentials [87]. Similar observations were reported for biosensor with glucoseoxidase immobilized in polyaniline [88]. A similar reduction and oxidation behaviorwas observed for electrodes with polyaniline film loaded with glucose oxidase andisoalloxazine derivative, having the same active redox radical as the active center ofFAD of glucose oxidase. The direct electron transfer between electrode modified withpolypyrrole and peroxidases was also observed [89,90]. It is postulated that anoxidation product(s) of pyrrole, most probably dimeric pyrrole can mediate theenzyme-polypyrrole electron transfer, though a direct electron transfer may also bepossible [90). For fructose biosensor vdth fmctose dehy-drogenase immobilized inpolypyrrole it was found that the enzyme in the conductive thin membrane exhibitsa sharp increase in catalytic activity, which led also to conclusion that polymer matrixserves as an electron-shuttling medium between the enzyme and the electrode [91].

There are also more skeptic reports about the active role of conducting polymermatrix in electron transfer in biosensors. For polypyrrole based electrode withimmobilized glucose oxidase the evidence was presented that electron transportbetween the enzyme and the electrode surface is due to the electrocatalysed oxidationof hydrogen peroxide [92]. For glucose sensor based on a pyrrole-tubule-impregnatedmembrane it was found that the device operates by direct electrochemical oxidationof glucose at the platinum film [93].

Potentiometric biosensors with conducting polymers can be produced using pHsensitivity of polymers [94]. An advantage of replacement of amperometric detectionwith potentiometric measurement has been found to be a greater independence of thesensitivity of detection on aging of the biosensor. Polypyrrole sensitivity to ammoniawas used to produce amperometric biosensors [33,51]. Conductivity biosensorsbased on conducting polymers were developed for penicillin [95], and also forglucose, urea, lipids and hemoglobin [96]. Conducting polypyrrole molecular inter­faces have been also implemented to modulate biological functions of enzymes andliving cells at the electrode surface by adjustment of electrode potential [97]. Veryoften additionally obtained discrimination of electroactive interferences by usingconducting polymers as matrices for enzymes allows the use of such biosensors for

210 M. Trojanowicz, T. Krawczynski and P.lY. Alexander

analysis of natural samples e.g. flow-injection determination oflactate in whole blood(Fig. 5).

2.0 mmoll- f

Ca)Cb)

t5

1100nA

to

0.5

0.20.1

10 minI-----l

8.,.--------------..-,!..'0EE.( 2ii:~

§S

j....

128

Lactate by reference method, mmoll-1

Figure 5. Recorded flow-injection response for standard lactate solutions (a) and correlation plot forlaetate determination ill whole blood (b) with ampelOmehic iacffite biosensOl with lactateoxidase immobilized in poly(o-phenylenediamine) and internal bilayer of electrodepositedpolypyrrole/polyphenol for elimination of electroactive interferences

Besides the advantage of the well defined formation of the polymer layer, thismethodolgy yields also some difficulties, associated with a significant chemical andelectrochemical activity of the polymer matrix due to the sensitivity of these materialstowards ion-exchange processes and redox equilibria. The role of the polymerconductivity in electron transfer in a biocatalytic process requires further studies, aswell as the mechanism of the frequently observed, quite fast degradation of theenzyme activity. One of the reasons can be deterioration of conductivity of conduct­ing polymers by hydrogen peroxide observed, for instance, for polypyrrole [98].

Immunosensors based on conducting polymers

Similarly to enzymes, antibodies can also be incorporated into the conductingpolymer layer during the electropolymerisation process. Polypyrrole immunosensorshave been developed for human serum albumin [99], thaumatin (artificial sweetener)[100] and for p-cresol and other phenolics [101]. Both in determination of humanserum albumin and thaumatin it was shown that sensitive, reversible and rapidresponses can be obtained when pulsed amperometric detection with flow-injectionsample processing is employed. This led to conclusion that if the control of detectionis in the millisecond time domain then the interaction of antigen with antibodyimmobilized in polypyrrole can be reversible (100].

Conducting polymers in electrochemical sensors

Acknowledgments

211

This work waspartly supported by European Community Project COPERNICUS ContractNo. CIPA CT94-0231.

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ReceivedJuly 1996AcceptedJanuary 1997