Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 391

of the microperoxidase, to the terminal amine group of a self-assembled monolayer
of cystamine on a gold electrode, yielding electrodes with redox potentials for micro-
peroxidase-11 heme of 0.4 V vs SCE [20]. There is some doubt about whether

a single monolayer of microperoxidase-11 forms using this coupling approach, or if
multi-layers are formed, based upon examination of the rate of heterogeneous electron
transfer to the microperoxidase-11 using this and other immobilization approaches
[22]. The microperoxidase-11 modifi ed electrodes were capable of biocatalytic reduc-
tion of peroxide at potentials as positive as 0.3 V vs SCE, with the reason for this

potential shift postulated to be a result of the formation of an Fe(IV) intermediate spe-
cies in the presence of peroxide [20, 23].
Recently, direct electron transfer to microperoxidases adsorbed on carbon
nanotube-modifi ed platinum electrodes has been observed [24]. The redox potential
for this direct electron transfer is 0.4 V vs SCE, the same as that for the microper-

oxidase-11 on the cystamine-modifi ed gold. However, curiously, biocatalytic reduction
of peroxide proceeds at this redox potential,

0.4 V vs SCE, at the carbon nanotube-
modifi ed electrodes, and not shifted positively, as was reported for the cystamine-
modifi ed gold [20].
Conversion of a peroxide-reducing cathode into a cathode that reduces dissolved
oxygen is also possible, as recently demonstrated by Ramanavicius et al. [25]. In this
study, microperoxidase-8 was co-immobilized with glucose oxidase to provide a cath-
ode that couples glucose oxidation, producing peroxide from the oxygen co-substrate,
with peroxide reduction by the microperoxidase, and subsequent direct electron trans-
fer from the electrode to the microperoxidase. While this is an elegant demonstration
of a novel combination of biocatalyst to provide high potential ( 0.15 V vs SCE)

reduction of oxygen, the fact that glucose is depleted, as it effectively acts as a co-
substrate, would mitigate against adoption of this approach for an implantable biocata-
lytic fuel cell using glucose as a fuel.

12.4.3 Oxygenases
The theoretical thermodynamic reduction potential for oxygen is

1.23 V vs NHE at

pH 0, or 0.82 V vs NHE at pH 7, and it is thus, like peroxide, a strong oxidant [1-3].

The reduction of oxygen at electrodes is, again like peroxide, hampered by large over-
potentials, with direct electrochemical reduction occurring only at 0.1 V vs NHE

at gold and carbon electrodes at neutral pH. Catalysts, such as platinum, are therefore
used to decrease this overpotential in fuel cell cathodes. As stated previously, how-
ever, the use of expensive, and non-selective, platinum catalysts is not compatible with
operation of a putative miniaturized membraneless fuel cell in vivo . An additional dis-
advantage of oxygen reduction at platinum catalysts is that it occurs, at neutral pH, via
a two-electron reduction, to produce peroxide, a toxic reactive oxygen species in vivo .
Catalytic reduction of oxygen directly to water, while not as yet possible with tra-
ditional catalyst technology at neutral pH, is achieved with some biocatalysts, particu-
larly by enzymes with multi-copper active sites such as the laccases, ceruloplasmins,
ascorbate oxidase and bilirubin oxidases. The fi rst report on the use of a biocatalyst