Biomedical Engineering Reference
In-Depth Information
The electron transfer property of Cyt.
c
can also be obtained at Au electrodes modifi ed
with self-assembled monolayers of, for example, N-acetyl cysteine [91], or 4,4-dithio-
bipyridine [92] or 3,3-dithiobis (sulfosuccinimidylpropionate) [93-95]. These modifi -
ers were employed as promoters for facilitating the electron transfer of Cyt.
c
. From a
practical application point of view, a free orientation of the Cyt.
c
without denaturation
is generally required for achieving an effective electron transfer between Cyt.
c
and the
electrode and for the subsequent biosensing of O
2
•
. In this case, self-assembled mono-
layers of alkanethiols formed onto Au electrodes are remarkable because they can not
only facilitate direct electron transfer of Cyt.
c
but also prevent the electrode from foul-
ing by the potential interferents in the solution. As a consequence, the self-assembled
monolayer (SAM) of HS(CH
2
)
10
COOH confi ned on the Au electrode has been used for
constructing a Cyt.
c
-based amperometric O
2
•
biosensor [72, 76]. In addition, mixed
SAMs, e.g. those consisting of short alkanethiol SAM, such as 3-mercaptopropinic acid
and 3-mercaptopropanol, and long alkanethiol SAM, such as 11-mercaptoundecanoic
acid and 11-mercaptoundecanol [96], have been also used for facilitating the electron
transfer of Cyt.
c
and this direct electron transfer property has been further developed
for O
2
•
biosensing. In a different way, Campanella
et al.
have developed a Cyt.
c
-based
O
2
•
biosensor by using hemin as an electron transfer mediator between Cyt.
c
and a
carbon paste electrode [87]. The oxidation current of hemin (Fe(II)) constituted the ana-
lytical current for continuous O
2
•
determination at
0.8 V vs SCE. The response time
and the detection limit of the O
2
•
biosensor was 2 min and 0.2 mM, respectively. The
lifetime of the biosensor was 3 days. More recently, a multilayer Cyt.
c
-modifi ed elec-
trode was used for biosensing of O
2
•
, of which the electrode assembled with six layers
of Cyt.
c
exhibits the highest sensitivity toward O
2
•
[97].
Although the Cyt.
c
-based electrochemical biosensors have been demonstrated to
be useful for the determination of O
2
•
in biological samples, it is known that Cyt.
c
is not an enzyme specifi c for O
2
•
. For example, Cyt.
c
also shows an inherent cata-
lytic activity like the peroxidase to reduce oxidants including H
2
O
2
and ONOO
. This
non-specifi c catalytic activity of Cyt.
c
somewhat limits the application of Cyt.
c
-based
electrochemical biosensors for selective determination of O
2
•
in biological systems
even though the peroxidase activity of Cyt.
c
has been reported to be controlled by
electrode design [75]. In this aspect, the utilization of SODs would be a good alter-
native because SODs are the enzymes for catalyzing the dismutation of O
2
•
into O
2
and H
2
O
2
with a strong activity and great specifi city. As such, the SOD-based electro-
chemical biosensors have been recently studied and developed for selective and sensi-
tive determination of O
2
•
.
6.4.2 Brief introduction to SODs
SOD comprises a family of metalloproteins primarily classifi ed into four groups: cop-
per, zinc-containing SOD (Cu, Zn-SOD), manganese-containing SOD (Mn-SOD),
iron-containing SOD (Fe-SOD) and nickel-containing SOD (Ni-SOD). In the following
studies, we will only focus on the uses of the former three kinds of SODs to construct
SOD-based O
2
•
biosensors since the last one, Ni-SOD, is not commercially available.
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