Electrically contacted enzyme electrodes

Besides the broad applications of electrically contacted enzyme electrodes as amperometric biosensors, substantial recent research efforts are directed to the integration of these functional electrodes as biofuel cell devices. The biofuel cell consists of an electrically contacted enzyme electrode acting as anode, where the oxidation of the fuel occurs, and an electrically wired cathode, where the biocatalyzed reduction of the oxidizer proceeds (Fig. 12.4a). The biocatalytic transformations occurring at the anode and the cathode lead to the oxidation of the fuel substrate and the reduction of the oxidizer, with the concomitant generation of a current through the external circuit. Such biofuel cells can, in principle, transform chemical energy stored in biomass into electrical energy. Also, the use... 

Figure 8. (A) The assembly of an electrically contacted glutathione reductase monolayer. (B) The rate of bioelectrocatalyzed reduction of oxidized glutathione (GSSG) by the electrically contacted enzyme electrode using various connecting chain lengths (a) n = 2, (b) n = 5, (c) = 11. Application of -0.72 V vs. SCE to the enzyme electrode in the presence of GSSG (10 mM). The experiments were performed in 0.1 M phosphate buffer, pH 7.2, under Ar.

A major advance in the construction of electrically contacted enzyme electrodes involves the structural alignment of the enzyme redox center with respect to the electrode interface in conjunction with the site-specific positioning of a redox relay component between the enzyme redox center and the electrode. The design of such electrodes promotes a new level of molecular architecture of biomolecules on surfaces, enabling us to optimize the electrical contact of the resulting enzyme elec-... 

Integrated electrically-contacted enzyme electrodes were prepared by the surface reconstitution of different apo-enzymes on electrode surfaces. The pyrroloquinoline quinone, PQQ, (7), was covalently linked to a cystamine monolayer associated with an Au-electrode, and A -(2-aminoethyl-FAD), (8), was covalently attached to the PQQ units. Fig. 3-4A. The integrated enzyme-electrode was then prepared by the reconstitution of apo-GOx on the FAD units. The surface coverage of the PQQ-FAD units was estimated to be 5.5x10 ° mole cnr. whereas the surface coverage of the reconstituted... 

Since the reconstitution of apo-GOx onto an FAD-monolayer linked to the Au-electrode did not yield an electrically contacted enzyme electrode, and since the electrocatalytic anodic current observed for the integrated electrode was observed at the redox-potential of the PQQ units, the electrical communication between the enzyme redox center and the electrode and its bioelectrocatalytic activation was attributed to the PQQ-mediated electron transfer in the system. Fig. 3-4A. The FAD centers of reconstituted GOx are reduced by glucose, and the reduced cofactors are then oxidized by the PQQ... 

This method was applied to assemble integrated electrically-contacted NAD(P)-dcpcndcnt enzyme electrodes. The direct electrochemical reduction of NAD(l ) cofactors or the electrochemical oxidation of NAD(P)H cofactors are kineticaUy unfavored. Different diffusional redox mediators such as quinones, phenazine, phenoxazine, ferrocene or Os-complexes were employed as electrocatalysts for the oxidation of NAD(P)H cofactors An effective electrocatalyst for the oxidation of the NAD(P)H is pyrroloquinoline quinone, PQQ, (7), and its immobilization on electrode surfaces led to efficient electrocatalytic interfaces (particularly in the presence of Ca ions) for the oxidation of the NAD(P)H cofactors. This observation led to the organization of integrated electrically contacted enzyme-electrodes as depicted in Fig. 3-20 for the organization of a lactate dehydrogenase electrode. 

Zayats, M., Willner, B., Willner, 1. Design of amperometric biosensors and biofuel cells by the reconstitution of electrically contacted enzyme electrodes. Electroanalysis 20(6), 583-601 (2008). doi 10.1002/elan.200704128... 

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