Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 399

In the case of these redox-polymer mediators a key parameter to take into account is
usually the polymer-enzyme ratio [30, 77, 78]. An excessive enzyme weight fraction
can decrease the current density because the enzyme is an electronic insulator. When
the weight fraction of redox polymer is excessive, the fl ux of electrons is reduced
because of the smaller number of enzyme molecules. The initial studies on these redox
polymer GOx fi lms for mediated oxidation of glucose focused on co-immobilization
with the OsPVI polymer shown in Fig. 12.5 on carbon electrodes [42, 54]. Refi nement
of the redox potential of the polymer was achieved by replacing the 2,2 -bipyridine

ligands of osmium with, initially, 4,4-dimethyl-2, -bipyridine, resulting in biocata-

lytic oxidation of glucose at 0.1 V vs Ag/AgCl compared to 0.25 V vs Ag/AgCl

[79]. Replacement of this ligand with 4,4 -bipyridine, subsequent

to a report by Zakeerudin et al. [71] on increased electron transfer rates from GOx to
osmium complexed to such ligands, yielded a redox polymer with a redox potential of

-dimethoxy-2,2

0.07 V vs Ag/AgCl [54]. Finally, further refi nement of the redox polymer is possi-
ble by replacement of the 2,2

-bipyridine ligands of osmium with 4,4 -diamino-2,2 -

bipyridine, to yield a redox potential of 0.15 V vs Ag/AgCl [77]. A current density

A/cm 2 was observed for oxidation of 10 mM of glucose in room temperature
pH 7.4 phosphate buffered saline solution at fi lms of GOx co-immobilized with the lat-
ter redox polymer on graphite electrodes. This output may be improved upon by the
use of electrode surfaces of higher micro- and nanoscopic areas. For example, a current
density of

of 65 µ

A/cm 2 was observed for similar fi lms immobilized on a 7

m diam-
eter carbon fi ber electrode, in solutions containing 15 mM glucose, pH 7.4 in phosphate
buffered saline at 37ºC [77, 80].
A signifi cant limiting factor to current density output in these redox polymer fi lms
is the ease of physical displacement of the redox center in the polymer fi lm to allow
intimate contact, and electron transfer, between the redox complex and the biocata-
lyst [81]. To address the mobility of the redox complex sites in the redox polymer,
represented by an apparent diffusion coeffi cient D , Mao et al. [82] have grafted a
13-atom-long fl exible spacer between a poly(vinylpyridine) polymer backbone and
an alkyl functional group of an [Os(N,N

150 µ µ

-biimidazole) 3 ] 2 /3 redox
center. This strategy allows both improved electron transfer between GOx-FADH 2
centers and the redox polymer, and charge transport within the redox polymer. Carbon
fi bers coated with cross-linked fi lms of GOx and this polymer have a redox potential
of

-dialkylated-2,2

0.19 V vs Ag/AgCl, and provide a current density of 1.15 mAcm 2 in solutions
containing 15 mM glucose, pH 7.4 in phosphate buffered saline at 37ºC. A further
improvement in current density, to yield 1.5 mAcm 2 at a potential of 0.1 V vs Ag/

AgCl, is obtained when the ratio of osmium complex to polymer monomeric unit is
optimized [83].
It should be noted that biocatalytic fuel cell anodes based on the GOx enzyme face
a signifi cant problem: O 2 is the natural electron acceptor of GOx. GOx therefore cata-
lyzes the oxidation of glucose to gluconolactone, producing also hydrogen peroxide
when dioxygen is the electron acceptor. The mediators described previously attempt
to compete with the oxygen reduction in order to avoid oxygen depletion, because it
is required for the cathode, and production of peroxide, a highly toxic product. It may,