Biocatalytic electrodes

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. 

A remaining crucial technological milestone to pass for an implanted device remains the stability of the biocatalytic fuel cell, which should be expressed in months or years rather than days or weeks. Recent reports on the use of BOD biocatalytic electrodes in serum have, for example, highlighted instabilities associated with the presence of 02, urate or metal ions [99, 100], and enzyme deactivation in its oxidized state [101]. Strategies to be considered include the use of new biocatalysts with improved thermal properties, or stability towards interferences and inhibitors, the use of nanostructured electrode surfaces and chemical coupling of films to such surfaces, to improve film stability, and the design of redox mediator libraries tailored towards both mediation and immobilization. 

The practical impact of such considerations is that the reversible potential of a mediated biocatalytic electrode is a mixed potential dominated by the mediator couple. By extension, the open-circuit potential of a biofuel cell comprising two such electrodes is primarily determined by the difference in redox potential of the two mediator couples. The difference in redox potential between the mediator and the consumed reactant represents a driving force for electron transfer and therefore must be nonzero. As... 

A model of such structures has been proposed that captures transport phenomena of both substrates and redox cosubstrate species within a composite biocatalytic electrode.The model is based on macrohomo-geneous and thin-film theories for porous electrodes and accounts for Michaelis—Menton enzyme kinetics and one-dimensional diffusion of multiple species through a porous structure defined as a mesh of tubular fibers. In addition to the solid and aqueous phases, the model also allows for the presence of a gas phase (of uniformly contiguous morphology), as shown in Figure 11, allowing the treatment of high-rate gas-phase reactant transport into the electrode. 

The design of biocatalytic electrodes for activity toward gaseous substrates, such as dioxygen or hydrogen, requires special consideration. An optimal electrode must balance transport in three different phases, namely, the gaseous phase (the source of substrate), the aqueous phase (where the product water is released and ionic transport takes place), and the solid phase (where electronic transport occurs). Whereas the selectivity of biocatalysts facilitates membraneless cells for implementation in biological systems that provide an ambient electrolyte, gas-diffusion biofuel cells require an electro-... 

SCATCHARD PLOT BINDING SITE BINOMIAL THEOREM PASCAL S TRIANGLE BIOAVAILABILITY BIOCATALYTIC ELECTRODE BIOSENSOR... 

Tissue and Bacteria Electrodes The limited stability of isolated enzymes, and the fact that some enzymes are expensive or even not available in the pure state, has prompted the use of cellular materials (plant tissues, bacterial cells, etc.) as a source of enzymatic activity (48). For example, the banana tissue (which is rich with polyphenol oxidase) can be incorporated by mixing within the carbon paste matrix to yield a fast-responding and sensitive dopamine sensor (Fig. 6.14). These biocatalytic electrodes function in a manner similar to that for conventional enzyme electrodes (i.e., enzymes present in the tissue or cell produce or consume a detectable species). 

Current and power densities achieved with electrodes using the direct electron transfer approach will be limited, however, because of the need to have intimate contact between the two-dimensional electrode surface and a coating monolayer of correctly oriented biocatalyst. The use of small redox molecules that can mediate electron transfer between the biocatalyst and the electrode surface offers an opportunity to improve output from biocatalytic electrodes, as three-dimensional films of biocatalysts may now be used. In addition the distance between the active site of the enzyme and the electrode surface is often too great to allow efficient direct electron transfer. In these cases the electron transfer rate is not effective because of the insulation of the redox active site by the surrounding protein. A redox mediator can shuttle electrons between the enzyme and the surface. In the example of redox mediated biocatalytic oxidation of a fuel, depicted in Fig. 12.3, the enzyme catalyzes the oxidation of the mediator... 

In practice, the use of a redox mediator allows for dramatically enhanced current densities and prevents electron leaching by O2. However, use of an artificial redox mediator introduces additional complexity to the design of a biocatalytic electrode, and therefore several factors must be considered in the design of a mediated biocatalytic system. The efficacy of an artificial redox mediator is largely dependent on (l) an electrochemical driving force caused by the difference in potentials of the mediator and enzyme, (2) the ability of the artificial mediator to intercept the electron transport between the enzyme and its natural electron acceptor, and (3) the ability of the artificial mediator to undergo rapid selfexchange to facilitate fast electron transport to the electrode surface. 

The electrochemical potential of a redox mediator (EJJ,) must be such that it provides a thermodynamic driving force to facilitate electron transfer with the enzyme to be used. Because of this requirement, the redox potential of the mediator determines the operational potential of the biocatalytic electrode. Thus for an enzymatic oxidation reaction, must be higher than the redox potential of the enzyme ( en) whereas the reverse is true for an enzymatic reduction reaction. The difference between and is defined as the mediator-induced overpotential (A et) and is the potential required for electron transfer to occur between the enzyme and mediator. In the context of a biosensor, a large overpotential can lead to an artificially inflated signal due the oxidation of biological interferants such as ascorbate. Additionally a large overpotential limits the open circuit potential in the context of a biofuel cell. It is therefore desirable to minimize the electrochemical overpotential however, there exists a limit to the minimum overpotential required to facilitate rapid electron exchange between the enzyme and mediator. 

Biocatalytic electrodes and biofuel cells controlled by biomolecular signals and implantable biofuel cells operating in vivo - towards bioelectronic devices integrating biological and electronic systems... 

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