Further Redox Polymer Mediation

Redox species can be incorporated into polymers (then termed redox polymers) either at the monomer stage or after polymerization. These macromolecules have distinct electrochemically active sites at one or more locations on the chain. Examples of redox polymers are shown in Table 9.1. In the previous section, it was described how the redox polymer shown in panel A was used to wire enzymes to electrode surfaces [39]. 

Beginning in 1949, Cassidy and coworkers presented a series of papers demonstrating redox polymers [46 9]. In these early demonstrations, the redox sites were based on hydroquinones. If the redox site is a transition metal complex, however, the redox polymer is also a metallopolymer [50-53]. Ruthenium and osmium metal-lopolymers with polyvinyfimidazole backbones, for example, are shown in panels B and C of Table 9.1. By participating in oxidation-reduction reactions with other species and through self-exchange, a redox polymer in solution can conduct electrons 

A Redox polymer based on osmium bis-bipyridine and polyvinylpyridine used to wire glucose oxidase [39,54,55]. 

An earlier ferrocene redox polymer was uncharged and did not electrostatically bind to the enzyme [33,34], 

B Ruthenium metallopolymer based on ruthenium bis-bipyridine and polyvinylimidazole [52], 

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Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

By the second approach, the enzyme is immobilized in a redox polymer assembly (Figure 39B). Electron-transfer quenching of the photosensitizer by the polymer matrix generates an electron pool for the activation of the enzyme. Photoreduction of nitrate to nitrite was accomplished by the physical encapsulation of NitraR in a redox-functionalized 4,4 -bipyridinium acrylamide copolymer [234]. In this photosystem, Ru(bpy)3 + was used as a photosensitizer and EDTA as a sacrificial electron donor. Oxidation of the excited photosensitizer results in electron transfer to the redox polymer, and the redox sites on the polymer mediate further electron transfer to the enzyme redox center, where the biocatalyzed transformation occurs. The rate constant for the MET from the redox polymer functionalities to the enzyme active site is — (9 + 3) x 10 s. Similarly, the enzyme glutathione reductase was electrically wired by interacting the enzyme with a redox polymer composed of polylysine modified with A-methyl-A -carboxyalkyl-4,4 -bipyridinium. The photosensitized reduction of oxidized glutathione (GSSG) (Eq. 21) ... 

When ferrocene-containing polysiloxane proved to be an efficient electron-transfer relay system, further modification of this type redox pol3uner was investigated to develop optimal enzyme biosensors. Attempts were made to synthesize redox polymers with different mediators and/or different polymer backbones and/or different side chains through which mediators are attached to the polymer backbone. Resulting redox poisoners were tested to construct different types of enzyme sensors. 

Fig. 20.46 Nonequilibrium states of a redox polymer coated electrode (Me/poly) that mediates the oxidation of a solution (S) species (e.g., R— O), The chemical charge transfer reaction takes place at the poly/S interface. ]Lc is the electrochemical potential of electrons in the considered phase, (a) The system at equilibrium at electrode potential 2 [cf. Eq. (19)]. (b) The (nonequilibrium) situation shortly after changing from 2 to 3. (c) Possible further developments, depending on the system parameters and time.

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

The discovery of mediators (small molecules which when oxidised by peroxidases or laccases form highly reactive species), which have the abihty to oxidise high redox potential substrates difficult to be oxidised by the enzyme alone, has further expanded the applications of these enzymes in modifying inert polymers. Examples of widely investigated mediators are 1-hydroxybenzotriazole (HBT), violuric acid (VA), A-hydroxyacetanilide (NHA) and 2,2 -azinobis-(3-ethylbenzothiazoline)-6-sulphonate (ABTS). Alternatively, active research is aimed at developing natural cost-effective lignin-derived mediators [10]. 

In this section we presented a discussion of mediated catalysis using one particular type of electroactive polymer system, that based on polyvinylpyridine containing coordinatively attached bisbip3nidine chloro ruthenium or osmium redox centers. We could of course discuss many more classes of polymer systems. Chapter 2 is intended to be a tutorial, so a comprehensive and exhaustive summary of the experimental literature is unnecessary. For further details the reader is referred to reviews by Hillman and Saveant et alS for a very comprehensive discussion of a wide variety of systems. 

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