Biocatalyst Electrodes and Their Applications to Sensors and Reactors

Biocatalyst electrodes are the enzyme-modified (i.e., redox enzyme-modified)electrodes in which the electrode substitutes a chemical electron acceptor or donor of the redox reaction of the enzyme that is immobilized on the electrode surface. Enzymemodified electrodes of this type are distinguished by their bioelectrocatalytic nature, that is, the electrochemical control or acceleration of the biological processes that are catalyzed by the enzymes (hence named "enzyme electrode" or, in more general terms, "biocatalyst electrode"), and they appear promising for such novel applications as sensors, reactors, and fuel cells,I4 even in biomolecular electronic devices. FIGURIEA shows a scheme of the bioelectrocatalysis, that is, the electrode process catalyzed by the enzyme immobilized on the electrode surface, E,,/Ercd, in the oxidation of a substrate S to a product P. In such enzyme-modified electrodes, though, the rapid (or reversible)direct electron transfer between the electrode and the redox center of the enzyme is generally difficult to achieve. Then, the presence of an electron transfer mediator, M,,/Mrcd, which acts as an electron shuttle to provide redox coupling between the electrode and the immobilized enzyme (as shown schematically in FIGUREIB ), is useful to accelerate the electron transfer at the enzyme-modified elect rode.