Environmental Engineering Reference
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17.3.1.2 Electrocatalysis of Alcohol Oxidation by Enzymes The most
significant challenge for electrocatalytic applications of Zn alcohol dehydrogenases
is supply of the oxidized NAD(P) þ cofactor. The cofactor is expensive, so a regener-
ation system is necessary. Nonenzymatic oxidation of NAD(P)H is difficult to achieve
electrochemically at mild overpotentials [Blaedel and Jenkins, 1975] and is slow with
O 2 as oxidant. Several approaches have made use of enzymes for regeneration of the
oxidized cofactor. For example, Palmore and co-workers assembled an anode system
for stepwise catalytic oxidation of ethanol to CO 2 by a series of dehydrogenases using
a bacterial NADH oxidase with benzyl viologen as electron transfer mediator to regen-
erate the cofactor at an electrode [Palmore et al., 1998]. Such systems are complicated,
because of the number of steps and components involved. Recently, it has been shown
that a subcomplex comprising two subunits from the mitochrondrial respiratory
Complex I exhibits direct NAD þ /NADH electrocatalysis at a graphite electrode, and
this provides a promising alternative for cofactor regeneration [Barker et al., 2007].
There have been a number of reports of electrocatalysis of alcohol oxidation using
immobilized PQQ-dependent alcohol dehydrogenases or flavin-containing alcohol
dehydrogenases or oxidases with dissolved mediators in solution. Co-immobilizing
the mediator with the enzyme is advantageous, as set out in Section 17.1, and several
such strategies have been employed for electrocatalytic alcohol oxidation.
Heller and co-workers utilized Os-containing redox hydrogels, similar to those
described in Section 17.2, to entrap glucose oxidase or lactate oxidase and facilitate
electron transfer to the flavin [Kenausis et al., 1996]. The redox potential of the
mediator groups can be tuned to a value close to that of the respective enzyme by
modification of the ligands to the Os to minimize the overpotential for alcohol oxi-
dation [Mano et al., 2003]. Supporting these polymers on high surface area multiscale
carbon supports comprising multiwalled carbon nanotubes grown on carbon fiber
paper results in high electrocatalytic currents for glucose oxidation: .20 mA cm 22
geometric area (Fig. 17.10) [Barton et al., 2007].
Figure 17.10 Electrocatalytic current ( per geometric area) versus potential for glucose oxi-
dation by glucose oxidase in an Os-containing redox polymer supported on carbon nanotubes
grown for various periods (times indicated) on carbon paper. Reproduced by permission of
ECS—The Electrochemical Society, from Barton et al., 2007.
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