Direct ET Employed for Biosensors and Biofuel Cells

Enzymes that have been much studied in direct ET configuration include peroxidases [217, 218], especially horseradish peroxidase [219, 220], laccase [120,121], 

It is very important to define criteria to unequivocally proof a direct ET pathway between an immobilized redox protein and an electrode surface. The first important prerequisite is the occurrence of the direct electrochemistry of the redox cofactor inside the protein in the absence of the substrate. Hence, a reversible redox wave in a cyclic voltammogram of the protein-integrated cofactor has to be visible with a formal potential which clearly shows that the protein structure is not 

A stability ofthe enzymes and choice thereof poses key challenge 

The most common reaction at the anodic side of biofuel cells is the oxidation of sugars which relies on the catalytic properties of oxidases. This class of enzymes has, however, usually poor potential for direct ET. Direct ET on the anodic site was, however, described for a number of hydrogenases [235, 236] and cellobiose dehydrogenase [225, 237, 238]. Enzymatic catalysis by means of direct ET was also realized on conducting graphite or TiO2 particles [239, 240]. 

In conclusion, biofuel cells have a tremendous potential to be applied in, for example, implantable sensors or similar functional devices. They are a striking example of the continued development and application of the principles of biosensors employing direct ET. 

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