Redox polymers structure

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. 

FIGURE 12.5 Structure of the osmium redox polymer, OsPVI, formed by coordination of an [Os(2,2 -bipyridine)2Cl]+ complex to polyvinylimidazole in a usually 1 9 ratio. 

Three-dimensional wired enzyme structures, based on crosslinking the redox polymer chains and binding these to glucose oxidase lysine amines, were subsequently designed and syntesized [36, 37]. An example of a polymerization reaction is... 

The inner structure of polyelectrolyte multilayer films has been studied by neutron and X-ray reflectivity experiments by intercalating deuterated PSS into a nondeut-erated PSS/PAH assembly [94, 99]. An important lesson from these experiments is that polyelectrolytes in PEMs do not present well-defined layers but are rather interpenetrated or fussy systems. As a consequence, polyelectrolyte chains deposited in an adsorption step are intertwined with those deposited in the three or four previous adsorption cycles. When polyelectrolyte mobility is increased by immersion in NaCl 0.8 M, the interpenetration increases with time as the system evolves towards a fully mixed state in order to maximize its entropy ]100]. From the point of view of redox PEMs, polyelectrolyte interpenetration is advantageous in the sense that two layers of a redox polyelectrolyte can be in electrochemical contact even if they are separated by one or more layers of an electroinactive poly ion. For example, electrical connectivity between a layer of a redox polymer and the electrode is maintained even when separated by up to 2.5 insulating bUayers [67, 101-103]. 

Structurally well-defined enzyme electrodes wired by redox polymers have been constructed using different interactions, such as antigen-antibody [173-175] or avidin-biotin [176-181]. Hodak et al. described for the first time the layer-by-layer... 

More recently, osmium-based redox polymers of similar structure have been developed as mediators for enzyme-catalyzed reactions relevant to biofuel cells. In this context, the chief development objectives have been tuning the redox potential for both anodes... 

In contrast, a new type of redox polymer-coated electrode has recently been fabricated using the bottom-up method, in which redox-active molecules are connected with molecular wires, and the wires act as the current collector.11-13 In this case, electrons can be transported through the wires, and control of the electron transfer pathway is possible by changing the structure of the molecular wires. If the wire has a linear structure, redox active molecules with the wire connections exhibit a structure similar to that of a beaded curtain (Fig. lb), in which the electron transfers in a straightforward manner along each line. Furthermore, when the wire is composed of redox active molecules, we observe the promising phenomenon that the electron transfers via the redox process in the wire, whose mechanism would... 

In this section, we describe the fabrication of metal complex oligomer and polymer wires composed of bis(terpyridine)metal complexes using the bottom-up method.11 13 This method has an advantage in fabricating organized structures of rigid redox polymer wires with the desired numbers of redox metal complexes. We also present a new electron-transport mechanism applicable to the organized redox polymer wires-coated electrode. 

In this chapter, we presented three different systems of molecular assemblies using molecular wires. The first involved the fabrication of the molecular wire system with metal complex oligomer or polymer wires composed of bis(terpyridine)metal complexes using the bottom-up method. This system showed characteristic electron transfer distinct from conventional redox polymers. The second involved the fabrication of a photoelectric conversion system using ITO electrodes modified with porphyrin-terminated bis(terpyr-idine)metal complex wires by the stepwise coordination method, which demonstrated that the electronic nature of the molecular wire is critical to the photoelectron transfer from the porphyrin to ITO. This system proposed a new, facile fabrication method of molecular assemblies effective for photoelectron transfer. The third involved the fabrication of a bioconjugated photonic system composed of molecular wires and photosystem I. The feasibility of the biophotosensor and the biophotoelectrode has been demonstrated. This system proposed that the bioconjugation and the surface bottom-up fabrication of molecular wires are useful approaches in the development of biomo-lecular devices. These three systems of molecular assemblies will provide unprecedented functional molecular devices with desired structures and electron transfer control. 

In oxidation and dehydrogenation correlations were found with the redox properties of the solid. In the decomposition of nitrous oxide the paramagnetic properties, and with them, the catalytic activity, of organic polymers could be changed at will by modifications in the polymer structure, and in acid catalysis activity could be regulated by changing the type of acidic group and by selective neutralization. 

Figure 1. Structures of redox polymers used as electron relay systems in flavoenzyme-based biosensors. Shown are siloxane (top), ethylene oxide (middle), and branched siloxane-ethylene oxide (bottom) polymers.

It is clear from these results that the ability of the redox polymers to mediate electron transfer from reduced choline oxidase is dependent upon the structure of the polymer backbone. The trend in mediating efficiency is qualitatively the same as that found for the glucose sensors siloxane-ethylene oxide branch polymer > poly(ethylene oxide) > poly(siloxane). 

Structural Assessment of Redox Polymers using Neutron Reflectivity... 

One area where the relationship between the structure of the polymer matrix and the physical processes of the thin layer has been studied in detail is that of electrodes modified with polymer films. The polymer materials investigated in these studies include both conducting and redox polymers. Such investigations have been driven by the many potential applications for these materials. Conducting polymers have been applied in sensors, electrolytic capacitors, batteries, magnetic storage devices, electrostatic loudspeakers and artificial muscles. On the other hand, the development of electrodes coated with redox polymers have been used extensively to develop electrochemical sensors and biosensors. In this discussion,... 

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. 

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