Biomedical Engineering Reference
In-Depth Information
TABLE 6.6
Analytical properties of three kinds of O
2
•
biosensors
Cu, Zn-SOD
Fe-SOD
Mn-SOD
Applied potential (mV)
300
100
300
10
300
100
Surface coverage (
10
1
mol cm
2
)
1.1
1.1
1.9
1.9
1.2
1.2
Sensitivity (nA cm
2
/nM min
1
)
19
25
25
31
17
30
Detection limit (nA/nM min
1
)
0.38
0.49
0.49
0.63
0.34
0.61
Linear range (nM min
1
)
13-130
13-130
13-130
13-130
13-130
13-130
(Reprinted from [138], with permission from the American Chemical Society.)
those of the species co-produced with O
2
•
in the xanthine-XOD generating system,
e.g. uric acid and H
2
O
2
, was evident by adding Cu, Zn-SOD, a selective scavenger of
O
2
•
, into the solution containing O
2
•
. As expected, the presence of Cu, Zn-SOD in
solution greatly decreases the anodic current by
96% (a).
On the other hand, an obvious cathodic current was clearly recorded with the addi-
tion of xanthine into PBS containing XOD when the Fe-SOD/MPA/Au electrode was
polarized at
100 mV (b). The introduction of catalase, an enzyme specifi cally cat-
alyzing the dismutation of H
2
O
2
, resulted in no change in the current response, pre-
cluding the originality of the recorded current response from H
2
O
2
co-produced in the
xanthine-XOD system. In contrast, the addition of Cu, Zn-SOD yielded a large
decrease in the cathodic current almost to the background level. These observations
may allow us to ascribe cathodic response to the reduction of O
2
•
at the Fe-SOD/
MPA-modifi ed Au electrode [138].
The sensitivity of the SOD-based biosensors for O
2
•
determination was found to
be dependent on the operation potential and the surface coverage of each kind of SOD.
The analytical properties of three kinds of O
2
•
biosensors under optimized conditions
are summarized in Table 6.6.
The main purpose of the development of O
2
•
biosensors lies in their applications
for monitoring O
2
•
in biological systems and thus the developed biosensors should be
studied with respect to their relevance for
in-vivo
biological measurements. It is known
that there are a variety of interferents coexisting in biological samples, suggesting that
the biosensors used for the practical measurements should have signifi cant specifi city
against potential interferents. In biological systems, H
2
O
2
is a metabolite in the degra-
dation of O
2
•
and a product of enzyme reactions of endogenous oxidases, such as
monoamine oxidase and L-amino acid oxidase. In addition, H
2
O
2
is one of the main
byproducts in the XOD-xanthine system to generate O
2
•
. Therefore, the specifi city
of the SOD-based biosensors against H
2
O
2
is of great importance in their applications
for the determination of O
2
•
in biological systems as well as in their calibration with
the XOD-xanthine O
2
•
-generating system. Thus, the interference from H
2
O
2
was fi rst
examined. In addition, the current responses of the SOD-based biosensors against other
potential interferents, such as the principal metabolites (DOPAC, HVA, and 5-HIAA)
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