Biomedical Engineering Reference
In-Depth Information
TABLE 6.5
Electrochemical Parameters of SODs at MPA-modifi ed electrode at various pH values
Cu, Zn-SOD
Fe-SOD
Mn-SOD
E
0
E
0
E
0
∆
E
p
k
s
∆
E
p
k
s
∆
E
p
k
s
(s
1
)
(s
1
)
(s
1
)
pH
(mV)
(mV)
α
c
(mV)
(mV)
α
c
(mV)
(mV)
α
c
5.8
282
121
0.98
0.63
210
120
1.5
0.59
275
104
1.2
0.61
7.0
212
100
1.1
0.61
135
40
3.9
0.5
225
85
1.9
0.58
8.0
153
115
0.94
0.63
70
76
2.4
0.55
222
112
1.6
0.59
9.0
93
146
0.46
0.74
38
132
0.74
0.65
155
133
0.35
0.76
(Reprinted from [138], with permission from the American Chemical Society.)
the Fe-SOD (curve a) indicates that the redox process of Fe-SOD involves one electron
and one proton probably at pH
8.5. Unlike
those of Cu, Zn-SOD or Fe-SOD, the formal potential of Mn-SOD showed more com-
plicated pH dependence as shown in Fig. 6.6 (curve c). The formal potential decreases
linearly with pH with a slope of ca.
8.5 and is independent of pH at pH
40 mV/pH between pH 5.8 and pH 7.0, retains a
constant between 7.0 and 8.5, and then decreases sharply between pH 8.5 and 9.5 (the
slope is ca.
140 mV/pH). This
E
0
-pH profi le likely suggests that the Mn-SOD has
two p
K
a
s; one around 7.0 and the other about 8.5. This almost coincides with earlier
results obtained with optical titrations, in which two p
K
a
s of 6.7
0.1 and 8.5
0.3
were suggested for Mn-SOD [146, 147].
The rate constant of electron transfer (
k
s
) and anodic and cathodic electron transfer
coeffi cients (
α
c
) of the SODs at various pH values were estimated with Laviron's
equation and summarized in Table 6.5. Interestingly, the fastest electron transfer of
the SODs was essentially achieved in a neutral solution, probably in agreement with the
biological conditions for the inherent catalytic mechanisms of the SODs for
O
2
•
dismutation, although the electrode processes of the SODs follow a different
mechanism.
α
a
and
6.4.4 SOD-based electrochemical biosensors for O
2
Generally, the enzyme-based biosensors can be divided into three categories, i.e.
fi rst-, second-, and third-generation biosensors. The fi rst-generation enzyme-based biosen-
sors are preliminarily based on the measurement of compounds involved in the enzymatic
reactions, such as hydrogen peroxide and dissolved oxygen. These kinds of biosensors
unfortunately have their essential limits in sensitivity, selectivity, response time, and so on.
The development of electrochemical studies on electron transfer mediation provides the
possibility to construct the second-generation biosensors, of which the electron transfer
between the enzymes and electrode is shuttled by artifi cial electron transfer mediators. The
mediators can be retained at the electrode surface by a discrete membrane or mixed with
the enzyme in a carbon paste or entrapped in a fi lm. The third-generation biosensors are
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