Redox potentials mediators

However, this potential will not be observed because neither oxidant (glucose) nor reductant (oxygen) interacts with the anode and cathode directly. Rather, this interaction is mediated by the osmium compounds. In practice, the open-circuit potential observed will be shghtly higher than the difference in the mediator redox potentials (Equation 9.5) ... 

This value does not exactly equal the redox potential difference in anode and cathode mediators because of a mixed potential, as there is some DET potential at equilibrium even when mediators are present. When the cell is not at equilibrium and current flows, the electrode potentials are separated from the mediator redox potentials by an electrode polarization overpotential anode or /cathode- For example, at the cathode (Equation 9.6),... 

These potentials are those at the electrode current collector surfaces. Considering this, a disadvantage to MET in comparison to DET becomes apparent. In DET, the overpotential at the anode would be relative to the redox potential of the enzyme active site. In MET, the overpotential is relative to the mediator redox potential. For an anode, we may write an equation similar to Equation 9.6 (Equation 9.17) ... 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

A general theory based on the quantitative treatment of the reaction layer profile exists for pure redox catalysis where the crucial function of the redox mediator is solely electron transfer and where the catalytic activity largely depends only on the redox potential and not on the structure of the catalyst This theory is consistent... 

The application of two successive redox polymer layers at an electrode surface gives rise to rectifying properties because the electron transport between the electrode and the outer layer has to be mediated by the inner redox polymer Among several conbeivable situations, the one where the inner layer possesses two reversible redox potentials (e.g. a Ru"(bipy)j polymer) and the outer layer has one redox transition with a potential between the former ones (e.g. polyvinylferrocene) is most interesting gjj electrode device has two opposite-sign rectifying... 

Table 1 Probes of DNA-mediated CT and their redox potentials (in solution) versus NHE...

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

There are other cases in which triplet-mediated energy transfer might compete with LMCT-mediated energy transfer, particularly where the metal ion is either europium or samarium. In these cases, the redox potentials of the metal ions are such that LMCT is feasible, although the LMCT state will be lower in energy than the emissive state of the metal, precluding luminescence.65... 

In redox mediation, to have an effective electron exchange, the thermodynamic redox potentials of the enzyme and the mediator have to be accurately matched. For biocatalytic electrodes, efficient mediators must have redox potentials downhill from the redox potential of the enzyme a 50 mV difference is proposed to be optimal [1, 18]. The tuning of these potentials is a compromise between the need to have a high cell voltage and a high catalytic current. Furthermore, an obvious requirement is that the mediator must be stable in the reduced and oxidized states. Finally, for operation of a membraneless miniaturized biocatalytic fuel cell, the mediators for both the anode and the cathode must be immobilized to prevent power dissipation by solution redox reactions between them. 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

The thermodynamic redox potential of NAD+/NADH is —0.56 V vs SCE at neutral pH. The NADH cofactor itself is not a useful redox mediator because of the high overpotential and lack of electrochemical reversibility for the NADH/NAD+ redox process, and the interfering adsorption of the cofactor at electrode surfaces. 

While this anode is not useful in the context of implantable fuel cells, it is of interest because methanol is an attractive anodic fuel due to its availability and ease of transport and storage. The oxidation of one equivalent of methanol requires the reduction of three equivalents of NAD+ to NADH. As the NADH cofactor itself is not a useful redox mediator, a benzylviologen/diaphorase redox cycle, with a redox potential of 0.55 V vs SCE at pH 7, was used to regenerate NAD+ for use by the dehydrogenases, as depicted in Fig. 12.10. 

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