Biomedical Engineering Reference
In-Depth Information
with hydrophobic surfactants, such as quinoline and isoquinoline, Ohsaka
et al.
have
successfully achieved the redox reaction of the O
2
•
/O
2
redox couple and thus could
determine the biomolecular reactivity of O
2
•
in aqueous media [84, 85]. The O
2
•
/
O
2
couple exhibits a reversible electrochemical behavior with a formal potential of
135 mV vs Ag/AgCl in 0.10 M NaOH and 0.5 M KCl solution. Such an electrochem-
ical property of O
2
•
has been used for the development of electrochemical sensors for
O
2
•
determination. For example, on the basis of the direct oxidation of O
2
•
at carbon
fi ber microelectrode, Tanaka
et al.
have successfully determined periodic fl uctuations
in O
2
•
production by a single phagocytic cell [68]. In that case, the carbon fi ber was
sealed into a glass capillary by heating the glass to melt it around the fi ber. Also, by
virtue of the direct oxidation of O
2
•
on carbon fi ber microelectrodes at
0.12 V vs
SCE (saturated calomel electrode), Privat and coworkers simultaneously determined
O
2
•
and nitric oxide [86]. This method has been demonstrated to be useful for real-
time monitoring of extracellular O
2
•
production in stimulated human vascular cells.
Although the determination of O
2
•
based on the direct oxidation of O
2
•
has been
proved to be mechanistically simple and to possess a low detection limit and quick
response, its application has been limited by its poor selectivity. Such a problem could
be expected to be solved by a combination of suitable electrochemical methods with
specifi c biomolecular recognition of enzymes toward substrate, such as SODs toward
O
2
•
, as will be illustrated in the following sections.
6.4 ELECTROCHEMICAL SENSORS FOR O
2
•
6.4.1 Biosensors with enzymes other than SODs
In addition to a family of SODs, several other kinds of enzymes and proteins, includ-
ing tyrosinase [87], galactose oxidase [87], hemin, and cytochrome
c
(Cyt.
c
), have
been employed to construct enzyme-based biosensors for the O
2
•
determination.
Here, we will use Cyt.
c
as an example to illustrate the analytical mechanism of such a
kind of O
2
•
biosensors. For constructing a Cyt.
c-
based biosensor, Cyt.
c
is normally
immobilized on the electrode surface and acts as an electron transfer mediator between
the electrode and O
2
•
. The O
2
•
radical reduces the immobilized Cyt.
c
(Fe(III)) to
Cyt.
c
(Fe(II)) and the Cyt.
c
(Fe(II)) is reoxidized on the electrode at a potential of
0.15-0.25 V (vs Ag/AgCl). In this regard, the electron transfer between Cyt.
c
and the
electrode becomes essential. However, such an electron transfer could not be readily
obtained at electrodes frequently used in electrochemistry, such as glassy carbon, gold,
and platinum. The electron transfer could be achieved at the platinized activated car-
bon electrode (PACE) and such a property has been further exploited for O
2
•
deter-
mination [88-90]. The sensitivity of the Cyt.
c
-modifi ed PACE electrode toward O
2
•
was evaluated to be 34.0 (
M
1
), which was greater than that obtained at
a planar Au electrode. The increase in sensitivity was ascribed to the larger effective
surface area of the PACE compared with that of the planar Au electrode because the
PACE is an extremely porous material capable of binding a large amount of Cyt.
c.
A cm
2
)/(
µ
µ
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