4-Vinylpyridine polymer electrode coating

It is interesting to conclude this section with an example that, in a sense, brings the chapter full circle. The metallization of plastic materials used as metal substitutes is a process with actual and future commercial potential. Usually, plastics are plated after appropriate sensitization by an electroless process which involves reduction of metal ions (e.g. Ni2+, Cu2+) by chemical rather than electrical means.19 There seems no reason why the reducing agent should not be incorporated in the polymer and Murray and his collaborators101 have demonstrated that copper, silver, cobalt and nickel may each be electrodeposited on to films of [poly-Ru(bipy)2(4-vinylpyridine)2]2+ coated on to platinum electrodes. The metal reductions are mediated by the Ru1 and Ru° states of the polymer. 

Binyamin, Chen and Heller reported that wired enzyme electrodes constituted of glassy carbon electrodes coated with poly(4-vinylpyridine) complexed with [Os(bpy)2Cl] and quarternized with 2-bromoethylamine or poly[(iV-vinylimidazole) complexed with [Os(4,4 -dimethyl-2,2 -bypyridine)2Cl] or poly(vinylpyridine) complexed with [Os(4,4 -dimethoxy-2,2 -bypyridine)2Cl] quaternized with methyl groups lost their electrocatalytic activity more rapidly in serum or saline phosphate buffer (pH 7.2) in the presence of urate and transitional metal ions such as Zn and Fe " " than in plain saline phosphate buffer (pH 7.2). It was reported that as much as two-thirds of the current is lost in 2 h in some anodes. However, when a composite membrane of cellulose acetate, Nafion, and the polyaziridine-cross-linked co-polymer of poly(4-vinyl pyridine) quaternized with bromoacetic acid was applied, the glucose sensor stability in serum was improved and maintained for at least 3 days [27,50]. 

The electrolytic oxidation was found to proceed much faster in the presence of Cu-pyridine as a redox mediator in the electrolytic cell divided with a membrane. The electrode coated with Cu/poly(4-vinylpyridine) was also effective for the oxidative polymerization, and what was more, without a partition membrane (Figure 8). Polymer-Cu complex film coated on the electrode prevented formation of the insulating film of the product polymer on the electrode surface and decreased the electrolytic potential. The oxidation using the electrode coated with a macromolecular Cu complex provides a facile method for forming poly(phenylene oxide)s. 

Polymerization of 4-vinylpyridine has also been performed, resulting in a non-electroactive and non-conducting coating . The kinetics of charge transport has been measured when this polymer contains electrostatically trapped Fe(CN)6 ", IrCls " or tris(2,2 -bipyridine)osmium- 111/11) complexes 3 . Incorporating IrCle , the electrode is able to catalyze the oxidation of iron(II) . A composite electrode coated with a mixture of cellulose acetate and poly(vinylpyridine) has been described . This mixture allows the binding of counterionic reactants in acidic media, while maintaining the size exclusion discriminative properties of cellulose acetate. 

One other design developed by Wang s group uses the same base sensor (GCE), which is coated with a layer of poly(4-vinylpyridine) (PVP). This cationic polyelectrolyte was one of the first polymers used to modify electrode surfaces [27]. Much research effort in this context has been directed to the characterization of the transport and electrostatic binding of multi-charged anions at PVP-coated electrodes. The ability of this polymer... 

Polymer films can also be electropolymerized directly onto the electrode surface. For example, Abruna et al. have shown that vinylpyridine and vinyl-bipyridine complexes of various metal ions can be electropolymerized to yield polymer films on the electrode surface that contain the electroactive metal complex (see Table 13.2) [27]. The electronically conductive polymers (Table 13.2) can also be electrosynthesized from the corresponding monomer. Again, a polymer film that coats the electrode surface is obtained [25]. Electropolymerized films have also been obtained from styrenic, phenolic, and vinyl monomers. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

A novel approach to data storage applications involves a glassy carbon electrode with a coating of [Ru(bipy)2(PVP)Cl]Cl, where PVP is poly-4-vinylpyridine. The rotating electrode oxidizes iron(II) at 640 mV [the polymer coating will inhibit iron(II) oxidation until ruthenium(II) centres are oxidized to ruthenium(III)]. If the electrode is irradiated, a photochemically mediated substitution of the chloro hgand by an aquo ligand will occur and, after this radiation, a more positive potential (740 mV) is required to oxidize iron(II). Thus if the electrode potential is held between 640 and 740 mV a yes / no device is possible since irradiation will eliminate the flow of current. ... 

Earlandite structure, 849 Electrical conductivity metal complexes, 133 tetracyanoplatinates anion-deficient salts, 136 Electrical properties metal complexes, 133-154 Electrocatalysis, 28 Electrochemical cells, 1 Electrochemistry, 1-33 hydrogen or oxygen production from water coordination complex catalysts, 532 mineral processing, 831 reduction, 831 Electrodeposi (ion of metals, 1-15 mineral processing difficulty, 831 Electrodes clay modified, 23 ferrocene modified, 20 nation coated, 15 polymers on, 16 polyvinylferrocene coated, 19 poly(4-vinylpyridine) coated, 17 redox centres, 17 Prussian blue modified, 21 surface modified, 15-31 Electrolysis... 

When water is used as a medium for CO2 reduction, it is important to suppress proton reduction to produce H2, which is more favorable in water than CO2 reduction. A hydrophobic polymer environment can provide such conditions by suppressing proton reduction to selectively carry out CO2 reduction. In an electrocatalytic CO2 reduction by CoPc confined in a poly(vinylpyridine) membrane coated on an electrode, selective CO2 reduction takes place, producing CO [119]. It is of further interest that in the electrocatalytic system, a third electron is injected into the Co(I)Pc(-3) - CO2 intermediate to produce CO and Co(I)Pc(-2) complex after the starting Co(II)Pc(-2) complex is reduced by two electrons to Co(I)Pc(-3) to form Co(I)Pc(-3) - CO2 with CO2. Such a mechanistic scheme can be represented by Fig. 20 [119]. Path I is the previously accepted scheme, and path II is the proposal of a new two-electron reduction pathway. Note also that for the CO2 reduction, a proton is also involved in an equilibrium process. It was inferred that in the polyvinylpyridine matrix, protonation and deprotonation takes place easily with the help of the pendant pyridine groups in a concerted fashion, resulting in favorable CO production (see also Fig. 20) [119]. 

Much more effectual and very often applied are polymer-coated electrodes. Especially electrochemical polymerization is an attractive method for the immobilization of redox enzymes at electrode surfaces, and/or accumulation of electroactive reactants. An approximative analytical treatment of the response of an amperometric enzymatic electrode leading to plots of fluxes and concentration profiles has been made in [14]. The electron transport through poly-4-vinylpyridine and polystyrene-sulfonate films (widely used for immobilization of redox centers on electrodes) has been studied in [15]. 

More recently bundles of carbon fibers have been immobilized in copolymers of vinylferrocene or vinylpyridine with crosslinked polystyrene. Alternatively, the fibers were coated by electro-co-polymerization of vinylferrocene and divinylbenzene before immobilization in crosslinked polystyrene. These electrodes are also polishable and present an array of polymer-modified ultramicro disk electrodes to the solution (Creasy and Shaw, submitted). 

---