Conducting polymer films entrapment

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Fig. 8. Entrapment of mediator-modified enzymes within a conductive polymer film where ( ) represents the mediator ferrocene and (B) the active site...

Enzymes were immobilized using various conducting materials such as sol-gel composites with integrated graphite particles [195] however, there are still problems regarding a low sensitivity and undefined electron transfer. Moreover, in the case of the entrapment of enzymes into conducting-polymer films, possible direct electron transfer [150, 151] is stiU under discussion [196-198]. 

The most popular ways to immobilise enzymes on conducting polymers are either to entrap the enzyme within the growing polymeric films [56, 59, 153, 154] or to use a two-step procedure based on the formation of a functionalised conducting polymer film followed by the covalent binding of the enzyme at the functional groups at the polymer surface [125, 155, 156]. 

Entrapment of Enzymes within Electrochemically-Grown Conducting Polymer Films... 

Figure 9. a) Entrapment of mediator-modified emymes within a conducting-polymer film, b) Calibration graph for glucose obtained with a ferrocene-mo(fi-fied glucose oxidase entrapped within polypyrrole (350 mV exclusion of O2). 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

In recent years the electrochemistry of the enzyme membrane has been a subject of great interest due to its significance in both theories and practical applications to biosensors (i-5). Since the enzyme electrode was first proposed and prepared by Clark et al. (6) and Updike et al. (7), enzyme-based biosensors have become a widely interested research field. Research efforts have been directed toward improved designs of the electrode and the necessary membrane materials required for the proper operation of sensors. Different methods have been developed for immobilizing the enzyme on the electrode surface, such as covalent and adsorptive couplings (8-12) of the enzymes to the electrode surface, entrapment of the enzymes in the carbon paste mixture (13 etc. The entrapment of the enzyme into a conducting polymer has become an attractive method (14-22) because of the conducting nature of the polymer matrix and of the easy preparation procedure of the enzyme electrode. The entrapment of enzymes in the polypyrrole film provides a simple way of enzyme immobilization for the construction of a biosensor. It is known that the PPy-... 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

Polypyrrole was often used as support for platinum particles. Similarly to the case of polyaniline, the activity of such electrodes for the oxidation of methanol depends both on the amount of platinum and on the thickness of the polymer film [43]. In the same study, by using in-situ infrared spectroscopy, it was confirmed that linearly adsorbed CO species are the only detectable species present at the electrode surface. The authors attributed the enhancement of the overall activity observed to the high and uniform dispersion of the metallic particles with, possibly, an effect of the conducting polymer matrix itself. The same conclusions were drawn from another study [44] where the size of the particles obtained by electrodeposition was estimated at 10 nm. In this study, the Pt particles were entrapped into the polymer layer and showed a better activity than particles only deposited on the polymer surface. The authors interpreted their results as a decrease of the poisoning phenomenon in the 3D film in comparison to the only 2D deposit. 

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