Biomedical Engineering Reference
In-Depth Information
addition of Cu, Zn-SOD caused the cathodic current to decrease to the background
level. Thus, the observed cathodic response is reasonably ascribable to the reduction of
O 2 mediated by the Cu, Zn-SOD confi ned on the electrode [151].
The steady-state amperometric responses of the Cu, Zn-SOD/cysteine-modifi ed Au
electrode to successive addition of O 2 in solution examined at
200 mV
were found to be proportional to O 2 concentration in a range of ca. 13 to 130 nM
min 1 . The sensitivity of the SOD/cysteine-modifi ed Au electrode was 24 and 22 nA
cm 2 /(nM min 1 ) at
300 mV and
200 mV, respectively. The detection limit was evalu-
ated based on a signal-to-noise ratio of 3 to 1 and calculated to be 5 nM at
300 and
300 mV and
6 nM at
200 mV, respectively. The response time of the biosensor was measured as the
time to reach 95% of the maximum change in response to a step injection of xanthine
and found to be less than 6 s. For the stability test, the anodic and cathodic responses for
O 2 generated by the XOD-xanthine system were recorded four times each day and the
current responses were reported to be constant for at least one week [151].
The protocol demonstrated for the development of Cu, Zn-SOD-based third-
generation O 2 biosensors is also sutiable for other kinds of SODs, such as Fe-SOD and
Mn-SOD in the SOD family [138]. In those cases, MPA was used as promoter for the
SODs instead of cysteine because cysteine was unable to promote electron transfer of
Fe-SOD and Mn-SOD. Figure 6.10 compares the CVs obtained at the Cu, Zn-SOD- (a),
Fe-SOD- (b), and Mn-SOD- (c) based electrodes in the absence and presence of O 2 .
As shown, the presence of O 2 in solution obviously increases both anodic and cathodic
peak currents of the SODs confi ned on the electrodes, suggesting the good bifunctional
catalytic activity for the reduction and oxidation of O 2 at the SODs, which is similar to
the results obtained with the Cu,Zn-SOD/cysteine-modifi ed Au electrode. It should be
mentioned that the same response was observed neither at the MPA-modifi ed Au elec-
trode nor at the bare Au electrode under the same conditions. Such a bidirectional elec-
tromediation of the SOD-based biosensors is essentially based on the inherent specifi city
0.5
b
b
0.5
a
a
0
0
0
a
b
0.5
1
1
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(a)
E / V vs Ag/AgCl
(b)
E / V vs Ag/AgCl
(c)
E / V vs Ag/AgCl
FIGURE 6.10 CVs obtained at (a) Cu, Zn-SOD/MPA-modifi ed, (b) Fe-SOD/MPA-modifi ed, and (c)
Mn-SOD/MPA-modifi ed Au electrodes in 25 mM phosphate buffer (pH 7.5) in the absence (dotted lines)
and presence (solid lines) of 1.8 µM min 1 O 2 . Potential scan rate: 100 mV s 1 . (Reprinted from [138],
with permission from the American Chemical Society.)
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