Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 405

diaphorase layer may be used with other NADH dependent enzymes such as alcohol
dehydrogenase. The platinum cathode was coated with polydimethylsiloxane to yield
an O 2 -selective electrode. The assembled biocatalytic fuel cell was studied at 37ºC in
air saturated phosphate buffer solution at pH 7 containing glucose (10 mM) and NADH
(0.5 mM). The maximum power density was 14.5 Wcm 2 (40.3 Acm 2 current den-

µ µ

sity) at a potential of 0.36 V with an external load of 200 k . The cell power dropped

steadily for fi ve days to 30% of its initial value and was then stable at about 4

Wcm 2
for more than 2 weeks. The use of a non-biocatalytic cathode is interesting in terms of
electrode stability, while obvious bio-compatibility of vitamin K 3 is attractive for the
prospect of an implantable device. However, the reduction product of oxygen at pH 7
is inevitably hydrogen peroxide.
Despite the known loss of activity of fungal laccase enzymes at neutral pH, and
its reported inhibition by chloride, we were curious to investigate what cell perform-
ance could be reached with a Trametes versicolor laccase in a glucose-O 2 biocatalytic
fuel cell working in pseudo-physiological conditions [18]. Our pseudo-physiological
media was a pH 7.4 phosphate buffer containing 0.1 M sodium chloride and 10 mM
glucose. The solution was thermostated under air at body temperature (37ºC).
Graphite electrodes were modifi ed by a mixture of enzyme, redox polymer, and die-
poxide cross-linker. The GOx-based anode redox polymer was [Os(4,4

µ

-diamino-

bipyridine) 2 Cl] conjugated to polyvinylimidazole, of redox potential

2,2 0.11 V vs

Ag/AgCl. An [Os(1,10-phenanthroline) 2 ] 2 complex conjugated to two units of imida-
zole in a polyvinylimidazole polymer, of redox potential 0.49 V vs Ag/AgCl, was used
at the cathode. At pH 7.4 the maximum power density for the cell was 16 Wcm 2 at

a cell voltage of 0.25 V. The fact that the cathode was limiting the device in this case
is exemplifi ed by the higher maximum power density of 40

µ

Wcm 2 at pH 5.5, where
laccase is much more active. Liu et al. [73] have exploited the pH dependence of lac-
case to build a biocatalytic fuel cell with a tunable power ouput. To increase power
density porous carbon was chosen as the electrode material and GOx and laccase
enzymes were entrapped in suspensions of carbon nanotube/chitosan and then cast on
the porous carbon matrix. Solution redox mediators ferrocene monocarboxylic acid
and ABTS were dissolved, respectively, in the anolyte and catholyte which were sepa-
rated by a Nafi on membrane. The maximum power density dropped from 100

µ

Wcm 2

µ

W/cm 2 at pH 7.
The biocatalytic fuel cell prototypes presented have not as yet been refi ned in
a rational manner. The situation is different for the cells designed by the Heller
group, where an extensive literature is available. The publication of the fi rst oxygen-
glucose biocatalytic fuel cell from this group appeared in 2001 [38], after several years
of refi ning each individual electrode [3]. One interesting aspect of their study is the
miniaturization of the cell by the use of small carbon fi bers (

at pH 4 to 2 µ

m diam-
eter and 0.44 mm 2 active area) as the electrode material. The fi bers were fi rst rendered
hydrophilic by plasma oxidation, and modifi ed by a mixture of enzyme, an osmium-
based redox polymer and a diepoxide cross-linker. The GOx anode was modifi ed with
[Os(4,4

3 cm long, 7 µ

-bipyridine) 2 Cl] complex conjugated to a polyvinylimidazole
co -acrylamide polymer, redox potential of

-dimethyl-2,2

0.10 V vs Ag/AgCl, while the laccase