Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 401

Glucose dehydrogenase (GDH) is a class of enzyme that is gaining increased attention
as a catalyst for the oxidation of glucose in biosensing and biocatalytic fuel cell applica-
tions. GDH is either a NAD - or PQQ-dependent enzyme. The pyrrolo-quinoline quinine
(PQQ) cofactor (prosthetic group) has a thermodynamic redox potential of 0.125 V vs

SCE at pH 7 [20], which is less negative than that for the NAD /NADH couple. In addi-
tion, the application of the PQQ-dependent GDH to biocatalytic fuel cells may be limited
because of its relative instability with respect to GOx [84]. Nevertheless, the application
of electrodes modifi ed with the PQQ-dependent GDH as glucose sensors and biocata-
lytic anodes has been explored owing to its oxygen insensitivity, the fact that the cofac-
tor is bound to the enzyme, unlike the NAD -dependent GDH, and the high catalytic
effi ciency of this PQQ-dependent GDH [84-87].
For example, Ye et al. [85] have used carbon electrodes coated with cross-linked
fi lms of soluble PQQ-dependent GDH, from Acinetobacter calcoaceticus , and an
osmium redox polymer to achieve glucose oxidation current densities of 1.8 mAcm 2
at

0.4 V vs SCE in solutions containing glucose concentrations above 40 mM.
Tsujimura et al. [87] have recently utilized this approach to devise a biocatalytic fuel
cell anode for glucose. Zayats et al. [74], in an approach similar to that devised for the
apo-GOx system [15, 20, 21], have reconstituted PQQ-dependent apo-GDH on PQQ-
functionalized gold nanoparticles assembled onto a gold surface. Unfortunately, while
the onset of bioelectrocatalytic oxidation of glucose is close to the redox potential of
the PQQ, 0.125 V vs SCE at pH 7, appreciable glucose oxidation currents are only

observed at an overpotential of approximately 0.35 V vs the PQQ redox potential, at

0.2 V, precluding its use as a biocatalytic anode for glucose oxidation. This overpo-
tential is similar to that observed for reconstituted apo-GOx on gold nanoparticles (see
above) and is again attributed to a tunneling barrier introduced by the dithiol spacer.
Yuhashi et al. [84] have described a biocatalytic fuel cell anode for glucose based
on PQQ-dependent GDH carbon paste electrodes. The enzyme mixed with carbon
paste is lyophilized and then packed into the end of a carbon electrode and treated with
glutaraldehyde for immobilization. The research was extended to investigate improv-
ing the stability of the GDH using a Ser415Cys mutant GDH, previously developed
for biosensor applications [88]. For the wild type GDH, after 24 hours of operation
only 40% of the initial catalytic response remained. Meanwhile, the Ser415Cys mutant
GDH showed improved stability, with 80% of the initial response remaining after the
same period [84].
The use of the NAD -dependent GDH as a glucose biocatalytic anode has been
investigated recently by Sato et al. [89]. This research focused on evaluating the use
of glassy carbon electrodes coated with diaphorase/GDH and a polymeric vitamin K 3 -
based mediator with a redox potential of 0.25 V vs Ag/AgCl at pH 7, as a glucose

oxidizing anode. Vitamin K 3 had previously been identifi ed as a promising mediator of
the diaphorase oxidation of NADH to NAD [90]. The anode operation is thus based
on vitamin K 3 mediation of diaphorase oxidation of NADH to NAD and GDH oxidation
of glucose coupled to NAD reduction.
Palmore et al. [91] have reported on a graphite plate biocatalytic anode that uses
solution-phase dehydrogenases to catalyse the successive oxidation of methanol to CO 2 .