Conductingpolymers
Conductingpolymers (CP)havea relativelylonghistoryas sensors; Bartlett and
Ling-Chung(1989)describedthe measurement of methanolvapourand so
formedthe basisof severalof the earliergenerationof commercialelectronic
noses(Strikeet al., 1999).Theyoperate, unlikeMOS and MOSFET, at room
temperature.Besidethis a widerangeof materials can be synthesised,whichcan
respond to a broadrangeof organicvolatiles(Bartlett and Ling-Chung,1989).
Thereforea thin polymerfilmis deposited between gold-platedelectrodes.
The detection principle is basedon the changeof the electronflowin the
system if volatilesare in interaction withthe conducting polymer(e.g.pyrroles,
anilines or thiophenes); i.e. the changein voltageacrossthe conductive polymer
is measured. Thischangeis causedby the disturbanceof the conjugated-
electronsystem, whichextends overthe whole backbone. Differentsensitivities
and selectivity are achievedby substitution of side groupsof this backbone, the
selectionof doping ions, the variation of the polymerchainlengthand condition
of the polymerisation.
Although possible applicationssuchas the discriminationof microorganisms
weredescribed by Gibsonet al. (1997)and Cravenet al.(1996), serious
problemssuchas ageingeffectsand poisoning shouldbe solved beforeusing
these devicesin a `non-lab' environment.Furtherdisadvantagesare the poor
batch-to-batch reproducibility duringproduction(Schalleret al., 1998),high
temperaturesensitivity,strong humidityinterferenceand an unpredictabledrift
due to oxidation processes.
19.2.2 Liquiddetection± electronictongue
In the foodindustry thereare a numberof liquidand semi-liquidproducts(drinks,
beverages,dairyproducts) whereliquidsensors maybe applied for quality
control purposes. Quality properties that may be analysedin the liquidphasemay
be relatedto the chemical,microbial or sensory quality of the product.
Theterm `electronic tongue' has been used for liquid sensor systems
containing an arrayof unspecific working electrodesin combination with
multivariatepattern recognition methods(Winquistet al., 2003).If properly
calibrated,the electronictongue is capableof recognisingthe quantitative and
qualitativeoverview of importantingredientsin multicomponentsolutionsof
differentnatures, e.g. beveragesand foodstuffs.
Different techniqueshavebeenusedfor liquidsensing.The mostfrequently
used are basedon potentiometry or voltammetry.Potentiometricdevicesinclude
ion-selectiveelectrodes(ISE), a referenceelectrode and a potentialmeasuring
unit.A typical ISE is the glass-electrodebasedpH meter.Recently,ion-selective
fieldeffecttransistorshavebeendeveloped.In potentiometrya potential is
generatedbetweenthe referenceand working electrodewhenimmersedinto a
liquidelectrolytesolution(Fig.19.2).Thereferenceelectrodeis of constant
potential and the workingelectroderespondsto targetmolecules.Theoutput
signalscorrespond to the potentialgeneratedacrossa surfaceregion on the
292 Handbookof hygiene controlin the foodindustry