Superoxide electrochemical sensors and biosensors: principles, development and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 156

TABLE 6.3
p K a values of the compounds capable of facilitating electron transfer of Cu, Zn-SOD

Compounds p K a Compounds p K a

Mercaptoacetic acid 3.73 0.20 3-Mercaptopropionic acid 4.34 0.20

Cysteine 2.07 0.10, 11.05 0.16 Dimercaptosuccinic acid 2.74 0.25

L-Cystine 1.70 0.10, 8.72 0.16 Mercaptosuccinic acid 3.49 0.23

Thiolactic acid 3.74 0.20 N-Acetyle-L-cysteine 3.25 0.10

D,L-Homocysteine 2.21 0.10, 9.33 0.16

While the interactions between the ˆ COOH groups of the SAMs (as listed in
Table 6.1) and the Cu, Zn-SOD are critical to the direct electron transfer of the SOD as
addressed above, the constituents of the central moieties linking X and Y are found to
be also of great importance for the electron transfer because of their remarkable infl u-
ences on the kinetics of electron transfer and the interaction of the SAM with the SOD.
Generally speaking, the alkylene chains linking X and Y act as a potential energy bar-
rier for the electron transfer between the SOD and the Au electrode, and consequently
the electron transfer rate decreases with increasing the chain length. The increase in
the length of the alkylene chain linking X and Y, ( ˆ (CH 2 ) ˆ n ), was found to obvi-
ously tune the reversibility of electrode reactions of the SOD from reversible ( n

1)

to quasi-reversible ( n 2, 3), and they are actually suppressed when the chain length

is longer than ˆ (CH 2 ) ˆ 3 , as listed in Table 6.4, although the interaction between the
ˆ COOH groups of the SAMs and the SOD is expected to occur in these cases [137].
Besides the dependency on the length of the alkylene chain linking X and Y, the
electron transfer of the Cu, Zn-SOD was also found to be greatly dependent on the
constitution (aliphatic or aromatic) of such a section. As shown in Table 6.4, the SAMs
with an aromatic group linking X and Y could not promote the electron transfer of the
Cu, Zn-SOD although some of them could facilitate the electron transfer of Cyt. c .
Perhaps this is due to the fact that the aromatic ˆ COOH-terminated SAMs are not
favorable for interactions between the SAMs and the Cu, Zn-SOD, probably because
of the spatial barrier of these aromatic SAMs.
The uses of SAMs of alkanethiols for facilitating electron transfer of the Cu, Zn-
SOD can be extended for other kinds of the SODs, such as Fe-SOD and Mn-SOD in
the SOD family [138]. Figure 6.5 shows typical CVs obtained at 3-mercaptopropionic
acid (MPA)-modifi ed Au electrodes in 5 mM phosphate buffer (pH 7.0) containing
Fe-SOD (1), Mn-SOD (2) or Cu, Zn-SOD (3). The concentrations of the SODs used
here represent those of the Cu 2 site of Cu, Zn-SOD, Fe 3 site of Fe-SOD, or Mn 3
site of Mn-SOD, respectively.
A pair of redox peaks, which could be ascribed to the electron transfer of the SODs,
was observed for the three kinds of SODs at the MPA-modifi ed Au electrode, while it was
not obtained at a bare Au electrode (not shown in Fig. 6.5). This result demonstrates that
the electron transfer between the SODs and the Au electrode can be well promoted by the