Biomedical Engineering Reference
In-Depth Information
carbon electrode (PACE) for the determination of O
2
•
[76]. The SOD-based O
2
•
bio-
sensor was polarized at
0.32 V (vs Ag/AgCl) to obtain the oxidation current of H
2
O
2
produced by the enzymatic disproportionation of O
2
•
. In order to distinguish H
2
O
2
pro-
duced from the SOD-catalyzed dismutation reaction from that produced from the natural
disproportionation reaction through the other routes, a second bovine serum albumin-
coated PACE electrode was employed in that work in conjunction with the bipotentiostat
poised at
0.32 V vs Ag/AgCl. Song and his coworkers showed that the selectivity of
the SOD-based O
2
•
biosensor could be greatly improved by using a Tefl on membrane
[82]. They demonstrated that the outer Tefl on membrane could discriminate O
2
•
from
endogenous H
2
O
2
and other interferents, but the sensitivity and the response time of the
biosensor were decreased somewhat. In a different way, Lvovich and Sheeline have devel-
oped a two-channel biosensor for the simultaneous detection of H
2
O
2
and O
2
•
based on
the immobilization of both horseradish peroxidase and SOD into a polypyrrole layer at
a glassy carbon electrode [77]. O
2
•
generated by the interaction of xanthine and xan-
thine oxidase, or by injection of KO
2
at basic pHs, was disproportionated into H
2
O
2
and
O
2
under the catalysis of immobilized SOD. The produced H
2
O
2
both from the dispro-
portionation reaction and from XOD-based O
2
•
-generating system was reduced under
the catalysis of HRP. This biosensor could have a high sensitivity and good stability for
ten days.
The second-generation O
2
•
biosensors are mainly based on the electron transfer of
SOD shuttled by surface-confi ned or solution-phase mediators, as shown in Scheme
2(b). In 1995, Ohsaka
et al.
found that methyl viologen could effi ciently shuttle the
electron transfer between SOD and the glassy carbon electrode and proposed that
such a protocol could be useful for developing O
2
•
biosensors [125]. Recently, Endo
et al.
reported an O
2
•
biosensor based on mediated electrochemistry of SOD [148].
In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process
of SOD. The as-developed O
2
•
biosensor showed a high sensitivity, reproducibility,
and durability. A good linearity was obtained in the range of 0
100
µ
M. In the fl ow
cell system, tissue-derived O
2
•
was measured.
Besides the mediator-based second-generation SOD biosensors, much attention is
being devoted to the development of SODs-based third-generation O
2
•
biosensors on
the basis of the direct electron transfer properties of the SODs as shown in Scheme 2(c).
This is because the third-generation biosensors are advantageous over the other two types
of biosensors in, for example, the simple procedure required for the biosensor design
and for mechanistic understanding. Di
et al.
developed a third-generation O
2
•
biosensor
by immobilizing SOD into sol-gel thin fi lm confi ned on an Au electrode [149]. The
uniform porous structure of the silica-PVA sol-gel matrix resulted in a very low
mass transport barrier and a rapid and direct electron transfer of SOD. Based on bio-
molecular recognition for specifi c reactivity of SOD toward O
2
•
, the SOD-immobilized
electrode enabled a sensitive and selective detection of O
2
•
with a low potential
of
0.15 vs SCE.
On the basis of the direct electron transfer properties of SODs at the SAM-
modifi ed Au electrodes as described in the previous section, Ohsaka
et al.
have
developed SOD-based third-generation O
2
•
biosensors by immobilizing SODs
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