Osmium, redox mediator

Figure 19.7—Amperometric detection of glucose. The reaction cycle is shown on the left. A sandwich-type biosensor involving glucose oxidase co-immobilised with an osmium-based redox mediator in a polyvinyl polymer is shown on the right.

Quinone modified polymeric electron transfer systems The redox mechanism of quinone (two electron-proton acceptor/donor) is pH dependent and somewhat more complicated than for ferrocene or osmium (one electron accepter/donor). However, quinones are naturally occurring redox mediators and therefore, many researchers have studied their application to biosensors [107-109]. 

QDHs are independent from classical coenzymes like NAD(P)+. The substrate electrons are preferentially transferred to organic acceptors (quinones) and nonnative redox mediators such as phenazine derivatives, DCPIP, Wursters blue11711, ferrocene11011, ferricyanide1 72- 173], osmium complexes11741, or direct contact to an electrode11751. 

Poly(l-vinylimidazole)i2-[Os-(4,4 -dimethyl-2,2-bipyridyl)2Cl2] and poly(vi-nylpyridine)-[Os-(Ai,Ai -methylated-2,2 -biimidazole)3] were reported for their efficient capability of mediating electrons transfer between bacterial cells to electrodes. With S. oneidensis, the osmium redox polymer modified anode showed a 4-fold increase in current generation and a significant decrease in the start-up time for electrocatalysis. Using an anode modified with electropolymerized polypyrrole, a dramatie improvement in energy output was noticed in the MFCs. MFCs with a polypyrrole/ anthraquinone-2,6-disulfonic disodium salt (PPy/AQDS)-modified anode... 

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. 

Subsequent research on osmium-modified polymers has shown how the redox potential can be controlled by altering bipyridine ligands of the immobilized osmium complexes. The ability to reliably modulate redox potential has significantly broadened the range of enzymes with which osmium redox polymer are compatible and allows for mediation of oxidative enzymatic reactions as well as reductive enzymatic reactions. 

The redox polymer has three functions, it communicates with the active site in the enzyme, transfers the electrons to the electrode surface and it forms a matrix in which the enzyme is immobilized [3], This is the reasons which makes the osmium polymer such a successful mediator. A very sensitive and fast biosensor is achieved with the redox polymer bound enzyme. There are no membrane passages that will delay the reaction and the mediator is immobilized in the same matrix and will not diffuse away. An amperometric glucose sensor with the mediator electrostatically bound to a PVP polymer has been studied in a previous paper [4], where the mediator loss seemed to be a problem. The hydrophilicity of the osmium redox polymer also contributes to the rapid response because of the fast transport of water soluble substrates and products. 

Park T.M., Iwuoha E.I., Smyth M.R., Freaney R., McShane A.J. Sol-gel based amperometric biosensor incorporating an osmium redox polymer as mediator for detection of 1-lactate. Talanta 1997 44(6) 973-978... 

A qiiantitative assay for D-galactose depends on an enzyme sensor based on d-galactose oxidase and a tris(2,2 -bipyridine) complex of osmium as a redox mediator developed on a carbon electrode. ... 

FIGURE 9.2 A mediated glucose-oxygen biological fuel cell. At both anode and cathode, the mediator is an osmium redox couple. Potential difference or overpotential for each electron transfer step is shown. 

In 1955, it was demonstrated that redox polymers could be cross-linked into redox hydrogels [62]. Since the advent of the wired GOx anode described in the previous section, BFC electrodes using an osmium redox polymer hydrogel have appeared extensively in the literature [63-66]. Redox polymer mediators based on species other than osmium have also been demonstrated in recent BFC systems. Redox polymers based on ferrocene, for example, are a continuing area of research [56]. Sato et al. reported a glucose dehydrogenase BFC with a redox mediator based on vitamin K (see panel E of Table 9.1) in 2005 [57]. Six years later, Meredith et al. reported that a polyethylenimine polymer modified with 3-(dimethylferrocenyl)propyl redox centers is a durable and efficient mediator to GOx despite being a neutral molecule (see panel F of Table 9.1) [58]. 

To exploit the 2 term in Equation 9.13, Mao et al. introduced an osmium redox polymer in 2003 in which the redox center was a pendant on the polymer backbone at the end of a 13-atom molecular tether [59]. The increased range of motion for the redox site led to an increase in D pp (see panel G of Table 9.1). A version with a higher redox potential, more appropriate to mediate a cathode enzyme, was also produced [74]. 

What follows is a practical example demonstrating an engineering effort to improve a redox hydrogel-mediated laccase cathode meant to reduce oxygen to water in a BFC [65]. The laccase cathode is produced by combining an osmium redox polymer, laccase from Trametes versicolor, and a diepoxide cross-linker in the proportions 61, 32, and 7% by mass, respectively. After curing in a dry environment, a catalytic film is produced, which will reduce oxygen to water on an electrode surface [73,79]. 

Although the mediator has already been specified as a cross-linked osmium redox polymer, there are many molecular strucmres that fulfill this requirement. The first engineering issue is to select the best available choice for the application. 

For an osmium redox polymer, we select a form based on a polyvinylimidazole backbone (such as in panel C of Table 9.1), in which imidazole rings serve as ligands for attached osmium complexes. The namre of the other ligands can be varied in such a way as to produce a series of mediators with redox potentials spanning a wide range [127]. Such a series is shown in Figure 9.8, in which the highest potential mediator has essenhally the same redox potential as the laccase enzyme. 

XANES can be useful in determining the valence state of a material, provided the appropriate reference spectra have been collected. This is achieved by comparing the edge energy changes of an unknown valence state sample with the edge energy of known standards. An analysis of this type on the osmium mediators, in parallel with experimental data from CVs, should theoretically corroborate that the osmium redox process is a one-electron transfer step. 

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