Freely Diffusing Mediator in Solution

In 1964, Yahiro et al. demonstrated enzyme mediation to platinum electrodes [21], They used three enzymes GOx, D-amino acid oxidase, and alcohol dehydrogenase (ADH). If iron was introduced into the system with either GOx or o-amino acid oxidase, an electrical potential was observed. However, this was not the case with ADH, which is dependent on a nicotinamide adenine dinucleotide (NAD) cofactor (see below). 

Electrocatalytic enzyme mediation has been demonstrated using quinones, viol-ogens, 2,2-azinobis(3-ethylbenzothiazohne-6-sulfonate) (ABTS), and complexes of iron, ruthenium, cobalt, osmium, and many other compounds [22-24]. Much early work concerned the GOx anode, intended for a glucose sensor. In 1974, Schlapfer et al. tested 11 different mediators for a GOx electrode with a semipermeable membrane [25]. Ten years later, Cass et al. reported membrane-bound electrodes that operated in whole blood [26]. In 1986, Bourdillon et al. presented an analysis arguing that immobilized enzyme electrodes have higher efficiency than those with free enzyme in solution [27]. These examples demonstrate several possible enzyme/ mediator configurations. Both enzyme and mediator can exist as firee species in the liquid electrolyte, or one or both can be immobilized on the electrode surface. As an alternative to immobilization, enzyme and mediator can also be confined near the electrode by a semipermeable membrane. 

Solution-phase BFCs have been developed by Palmore et al., with anode and cathode compartments separated by a Nafion membrane. Starting in 1998, Palmore et al. used laccase in a mediated cathode and cascades of multiple enzymes designed to fully oxidize methanol to carbon dioxide at a mediated anode [22,29]. The cathode used in Palmore et al. s early work was ABTS, which was chosen for the value of  

FIGURE 93 Mechanism for oxidation of an aldehyde to a carhoxyUc acid hy aldehyde dehydrogenase, in conjunction with an NAD cofactor. The cofactor must hind and he reduced while the aldehyde substrate is bound. Glu is glutamate on the enzyme. 

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