Biomedical Engineering Reference
In-Depth Information
SOD
H
2
O
2
O
2
•
e
(a)
O
2
O
2
H
2
O
2
Mediator (Ox)
e
O
2
(b)
O
2
•
Mediator (Red)
H
2
O
2
O
2
SOD (Ox)
e
(c)
SOD (Red)
O
2
•
SCHEME 2
Schematic illustration of analytical mechanism of (a) fi rst-, (b) second-, and (c) third-generation
O
2
•
biosensors. Note that the reactions shown in (b) and (c) are bi-directional since SODs are enzymes spe-
cifi cally catalyzing the O
2
•
dismutation, i.e. oxidation into O
2
and reduction into H
2
O
2
.
fabricated based on the direct electron transfer of enzymes, that is, by creating an electrode
surface that can interact with the active center of the redox enzymes.
Similarly, the SOD-based enzymatic biosensors for the O
2
•
determination can
also fall into the three categories mentioned above, i.e. the fi rst-, second-, and third-
generation biosensors with the analytical mechanism as schematically illustrated in
Scheme 2. Most of the SOD-based fi rst-generation biosensors reported thus far have been
based on the detection of H
2
O
2
produced from the SOD dismutation of O
2
•
as shown
in Scheme 2(a). Mesaros and coworkers have designed an amperometic O
2
•
biosensor
at the platinum electrode with electropolymerized pyrrole fi lm containing SOD [79, 80].
The O
2
•
penetrates from the bulk solution into the polypyrrole fi lm, where it is dismu-
tated into H
2
O
2
and O
2
under the catalysis of SOD. The generated H
2
O
2
is then oxidized
at platinum electrode at a potential of
0.7 V (vs SCE). The biosensor was demonstrated
to have a low detection limit (15 nM), good temperature stability, and short response
time (
5 s). However, the high potential required for the oxidation of H
2
O
2
may cause
the simultaneous oxidation of some coexisting electroactive species in biological sam-
ples, e.g. ascorbate and uric acid. Moreover, H
2
O
2
is a metabolite of the dismutation of
O
2
•
and a product from the enzymatic reaction of endogenous oxidases, such as mono-
amine oxidase and L-amino acid oxidase. Thus, the fi rst-generation biosensors based on
the detection of H
2
O
2
seem to be limited in distinguishing H
2
O
2
produced by the SOD-
catalyzed dismutation of O
2
•
from that endogenously produced in the biological sys-
tems. McNeil and his coworkers have fabricated the SOD-coated platinized activated
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