Biomedical Engineering Reference
In-Depth Information
including Cu, Zn-SOD, Fe-SOD, and Mn-SOD onto SAM-modifi ed Au electrodes [138].
The concept of the biosensing technique for the determination of O
2
•
can be illus-
trated in Scheme 3 together with the mechanism of O
2
•
dismutation catalyzed by the
SODs in biological systems [150], by using Cu, Zn-SOD as an example. In biological
systems, SODs effi ciently catalyzes the dismutation of O
2
•
into O
2
and H
2
O
2
via a
redox cycle of the metal complex moiety [
M
Cu, Fe or Mn] in the SODs. During the
dismutation reactions, two O
2
•
ions are stoichiometrically converted to one O
2
molecule
and one H
2
O
2
molecule with consumption of two H
ions. That is, one O
2
•
reduces
the SOD [
M
(oxidized form)
] to produce O
2
and the SOD [
M
(reduced form)
], while another
O
2
•
oxidizes the SOD [
M
(reduced form)
] to produce H
2
O
2
and the SOD [
M
(oxidized form)
].
The biological homogeneous catalytic reactions can be split into two independent het-
erogeneous electrode reactions, in which the SODs are immobilized on the electrode
surface, as shown in Scheme 3. In the cathodic process, the redox reaction between
O
2
•
and SOD [
M
(reduced form)
] takes place to produce H
2
O
2
and SOD [
M
(oxidized
form)
]. The generated SOD [
M
(oxidized form)
] can be reduced at the electrode. On the
other hand, in the anodic process, O
2
•
reduces SOD [
M
(oxidized form)
] to produce SOD
[M
(reduced form)
], which can be reoxidized at the electrode. Thus, by measuring the oxi-
dation or reduction current at the SOD-modifi ed electrode in the presence of O
2
•
, one
may detect O
2
•
. This kind of third-generation SOD-based O
2
•
biosensor is essen-
tially based on the direct electron transfer properties and the catalytic activities of
the SODs.
The thing to be noted here is that the
E
0
values of the O
2
/ O
2
•
and O
2
•
H
2
O
2
redox couples are
0.35 and 0.68 V vs Ag/AgCl at pH 7.4 and thus the SODs, for
example, Cu, Zn-SOD (Cu (I/II)) with
E
0
65 mV can mediate both the oxidation of
O
2
•
to O
2
and the reduction of O
2
•
to H
2
O
2
. Such a bi-directional electromediation
(electrocatalysis) by the SOD/SAM electrode is essentially based on the inherent spe-
cifi city of the SOD enzyme which catalyzes the dismutation of O
2
•
to O
2
and H
2
O
2
via a redox cycle of their metal complex moiety (Scheme 3).
Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based
O
2
•
biosensors with Cu, Zn-SOD confi ned onto cystein-modifi ed Au electrode as
an example. The presence of O
2
•
in solution essentially increases both the cathodic
and anodic peak currents of the SOD compared with its absence [150]. Such a redox
response was not observed at the bare Au or cysteine-modifi ed Au electrodes in the
presence of O
2
•
. The observed increase in the anodic and cathodic current response
of the Cu, Zn-SOD/cysteine-modifi ed Au electrode in the presence of O
2
•
can be
considered to result from the oxidation and reduction of O
2
•
, respectively, which are
effectively mediated by the SOD confi ned on the electrode as shown in Scheme 3.
Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modifi ed
Au electrode is essentially based on the inherent specifi city of SOD for the dismuta-
tion of O
2
•
, i.e. SOD catalyzes both the reduction of O
2
•
to H
2
O
2
and the oxidation
to O
2
via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron
transfer of SOD realized at the cysteine-modifi ed Au electrode. Thus, this coupling
between the electrode and enzyme reactions of SOD could facilitate the development
of the third-generation biosensor for O
2
•
.
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