Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 606

Figure 17.11 Schematic representation of an approach for achieving efficient electrocatalysis
of glucose oxidation by glucose dehydrogenase on Au nanoparticles tethered on an Au elec-
trode. The nanoparticles are modified with a PQQ-capped linker that interacts with the unoccu-
pied PQQ site of cofactor-deficient glucose dehydrogenase [Zayats et al., 2005].
Willner and co-workers have developed an immobilization and mediation strat-
egy that involves “wiring” a flavin cofactor to an electrode surface via a conducting
linker that may incorporate a PQQ or ferrocene derivative. The immobilized flavin
is then used to reconstitute cofactor-free glucose oxidase so that electrons can be
channeled directly to or from the active site. Using variations on this method, glu-
cose oxidase and glucose dehydrogenase (Fig. 17.11) have been attached to a
range of conducting surfaces, including Au nanoparticles and carbon nanotubes,
and very high rates of electron transfer through the wire (up to 5000 s 21 at high
overpotential) have been reported [Xiao et al., 2003; Zayats et al., 2005; Willner
et al., 2007]. Enzyme electrodes assembled in this way have been used in a fuel
cell [Katz et al., 1999].
There have been several recent reports of direct electrocatalytic oxidation of sugars
by dehydrogenases that possess electron relay centers in addition to the PQQ or flavin
active sites. Ikeda and co-workers studied a PQQ-dependent alcohol dehydrogenase
from Gluconobacter suboxydans that has several heme-containing subunits and
allows direct electrocatalysis of ethanol oxidation on a range of metal and carbon elec-
trodes [Ikeda et al., 1993]. Related enzymes were used more recently by Minteer and
co-workers for glycerol oxidation in a fuel cell [Arechederra et al., 2007]. Sode and co-
workers have reported direct electrocatalysis of sugar oxidation at a carbon paste elec-
trode modified with a thermostable flavin glucose dehydrogenase from Burkholderia
cepacia that naturally incorporates a heme-containing subunit [Kakehi et al., 2007] or
PQQ - glucose dehydrogenase genetically fused to a heme domain [Okuda et al.,
2007]. In both of these reports, the catalytic currents are very low, indicating that
few enzyme molecules are engaged in sugar oxidation. Impressive electrocatalytic cur-
rent densities were achieved by Kano and co-workers using a bacterial D -fructose
dehydrogenase that possesses a flavin catalytic site and a heme electron relay center
at a carbon powder electrode in the presence of 200 mM D -fructose (Fig. 17.12)
[Kamitaka et al., 2007].
Heme-containing dehydrogenase enzymes are promising for technological appli-
cations, and as more enzymes are isolated and studied, it is likely that systems with
further attractive electrocatalytic properties will be uncovered.