Biomedical Engineering Reference
In-Depth Information
obtained at the apo-SOD/cysteine-modifi ed Au electrode compared with that at the Cu,
Zn-SOD/cysteine-modifi ed Au electrode. This demonstrates that the oxidation of O
2
•
at
the Cu, Zn-SOD/cysteine-modifi ed Au electrode was based on the Cu, Zn-SOD enzyme
amplifi cation. The minor current response observed at apo-SOD/Cys/Au may be consid-
ered to be due to the direct oxidation of O
2
•
, independent of the Cu, Zn-SOD catalytic
dismutation (b), probably because of the small size of O
2
•
and its permeation through
the apo-SOD/cysteine layer and the pinhole of cysteine monolayer. As described in the
previous section, the formal potential of O
2
/O
2
•
redox couple is
0.31 V vs Ag/AgCl
and thus this direct oxidation of O
2
•
may occur at ordinary electrodes at not so positive
potentials and thus is common to most kinds of amperometric O
2
•
biosensors, e.g. Cyt.
c
-based O
2
•
biosensor. It should be mentioned that the ratio of the oxidation current of
O
2
•
obtained via SOD enzyme amplifi cation to its direct oxidation current should be
higher than that at the Cyt.
c
-based O
2
•
biosensor because the catalytic activity of SOD
is several hundreds times higher than that of Cyt.
c
and the specifi city of SOD toward
O
2
•
is also signifi cant [151].
Superior to previous enzyme biosensors for the O
2
•
determination, O
2
•
could also
be detected with the SOD/cysteine-modifi ed Au electrode by utilizing the catalytic
activity of SOD toward the reduction of O
2
•
to H
2
O
2
, as mentioned in Scheme 3.
The typical current-time response obtained at the Cu, Zn-SOD/cysteine-modifi ed Au
electrode at
200 mV is shown in Fig. 6.9. The addition of xanthine to the solution
resulted in an obvious increase in the cathodic current, but such a cathodic current was
not observed at the bare or cysteine-modifi ed Au electrodes. As can be seen from Fig.
6.9, the introduction of catalase, which has a high catalytic activity toward the dismu-
tation of H
2
O
2
, to the solution did not result in any change in the current response,
suggesting that the observed cathodic current is not due to the reduction of H
2
O
2
coproduced in the XOD/xanthine-based O
2
•
generating reaction. On the contrary, the
1.7 nA
xanthine
4 s
catalase
Cu, Zn-SOD
FIGURE 6.9
Typical current-time responses obtained at the Cu, Zn-SOD/cysteine-modifi ed Au electrode
in phosphate buffer (O
2
-saturated) containing 0.002 unit of XOD upon the addition of 30 nM xanthine and the
subsequent addition of 590 units of catalase and 6 µM Cu, Zn-SOD. The electrode was polarized at
200 mV,
and other conditions are the same as those in Fig. 6.8.
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