Conductivity redox polymers

Habermtiller, K., Reiter, S., Buck, H., Meier, T., Staepels, J., and Schuhmann, W. (2003) Conducting redox polymer-based reagentless biosensors using modified PQQ-dependent glucose dehydrogenase. Microchimica Acta, 143 (2-3), 113-121. 

Zhu, S.S., and T.M. Swager. 1996. Design of conducting redox polymers A polythiophene-Rujbipyja hybrid material. Adv Mater 8 497-500. 

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. 

Because of these important differences between conducting and redox polymers, this chapter is restricted to conducting polymers, which... 

Figure 4 compares cyclic voltammograms for a redox polymer (poly-[Fe(5-amino-1,10-phenanthroline)3]3+/2+)91 and p-doping and undoping of a conducting polymer (polypyrrole).92 The voltammogram for the redox... 

Figure 4. (A) Cyclic voltammograms over a range of scan rates for a redox polymer (poly-[Fe 5-amino-1,10-phenanthrotme)3]3+/>)91 and (B) p-doping and undoping of a conducting polymer (polypyrrole) (B). [(A) Reprinted from X. Ren and P. O. Pickup, Strong dependence of the election hopping rate in poly-tris(5-amino-1,10-phenan-throline)iron(HI/II) on the nature of the counter-anion J. Electroanal. Chem. 365, 289-292,1994, with kind permission from Elsevier Sciences S.A.]...

Although a wide variety of wave shapes have been observed for conducting polymers, most differ from a redox polymer response in the same way as highlighted above for polypyrrole. Since Heinze7 has discussed the origins of these differences in some detail, the discussion here will be brief. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

A Structural characteristic of conducting organic polymers is the conjugation of the chain-linked electroactive monomeric units, i.e. the monomers interact via a 7t-electron system. In this respect they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated. Consequently, and irrespective of their charge state, redox polymers are nonconductors. Their importance for electrochemistry lies mainly in their use as materials for modified el trodes. Redox polymers have been discussed in depth in the literature and will not be included in this review. 

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

Providing an ion exchanger with a sufficient number of redox groups so that conduction can occur by a relay-type redox-change mechanism. Examples are hydroquinone-derived redox polymers and polyvinyl polymers with a tetrathia-fulvalene, ferrocene, or carbazole group, which have been found useful for research and analytical applications. 

The poly(I)-based transistor is the first illustration of a microelectrochemical transistor based on a combination of a conducting and a conventional redox polymer as the active material. The transistor "turns on" at VG corresponding to oxidation of the polythiophene backbone. The resistivity of poly(I) declines by a factor of 105 upon changing VG from 0.4 V to 0.8 V vs. Ag+/Ag. When Vg is moved close to the one-electron reduction potential of V2+/+, the conventional redox conductivity gives a small degree of "turn on". A sharp Iq-Vq characteristic results, with an Ip(peak) at Vq = E° (V2+/+). Though the microelectrochemical devices based on conventional redox conduction have both slow switching speed and a... 

Other popular choices of CMEs are electrodes coated with redox polymers, conducting polymers, or inorganic hlms (see later). 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit-... 

The Menschutkin reaction has been used to prepare poly(viologens) (115) directly (Scheme 31). These polymers were found to be quite useful as polyelectrolyte redox-polymers and for the preparation of conductive polymers when complexed with the 7,7, 8,8 -tetracyanoquinodimethane (TCNQ) radical anion (71MI11100). 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

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