Biomedical Engineering Reference
In-Depth Information
FIGURE 12.8
Structure of the FAD/FADH
2
active site of glucose oxidase.
transfer occurs between the FAD/FADH
2
active site of the GOx and the electrode
surface, at redox potentials close to the
0.41 V vs SCE reduction potential of FAD
[62] at neutral pH. Examples of direct electron transfer to GOx have been reported for
GOx adsorbed or attached to carbon paste [63], carbon nanotube [64-66] and colloidal
gold nanoparticle [67, 68] modifi ed electrodes. However, in these cases the electrode
replaces the glucose substrate, thus resulting in the biocatalytic reduction of oxygen to
hydrogen peroxide, in the presence of oxygen.
2e
2H
7
GOD(FADH
2
)
GOD(FAD)
(1)
GOD(FADH
2
)
O
2
→
GOD(FAD)
H
2
O
2
(2)
In the presence of glucose and oxygen the bioelectrocatalytic reduction of oxygen
is diminished due to competition for the FAD active site between glucose and the elec-
trode surface. A method for the determination of glucose levels, based on this decrease
in reduction current in the presence of glucose, has been proposed [67]. In the presence
of anaerobic glucose solutions, only the redox process for FAD/FADH
2
is observed,
with no biocatalytic oxidation of glucose. In conclusion, the direct electron transfer
to GOx at nanostructured electrode surfaces yields biocatalytic oxygen reduction cur-
rents. GOx direct electron transfer processes cannot therefore be usefully used as yet
to develop an anode for biocatalytic fuel cells.
Since the fi rst report on the ferrocene mediated oxidation of glucose by GOx
[69], extensive solution-phase studies have been undertaken in an attempt to eluci-
date the factors controlling the mediator-enzyme interaction. Although the use of
solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell,
such studies can help elucidate the relationship between enzyme structure, media-
tor size, structure and mobility, and mediation thermodynamics and kinetics. For
example, comprehensive studies on ferrocene and its derivatives [70] and polypy-
ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been
undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many
others, because of factors such as their mediation effi ciency, stability in the reduced
form, pH independent redox potentials, ease of synthesis, and substitutional versatility.
Ferrocenes are also of suffi ciently small size to diffuse easily to the active site of GOx.
However, solution phase mediation can only be used if the future biocatalytic fuel cell
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