Biomedical Engineering Reference
In-Depth Information
as to suppress the dismutation reaction and thereby to obtain detectable levels of O
2
•
.
Chemical methods for the detection of O
2
•
are integrative and offer the advantages
of sensitivity and simplicity. In those methods, O
2
•
can be trapped with an indicat-
ing scavenger. The reaction between O
2
•
and the trapping agents can be followed by
a suitable optical, manometric, or polarographic method to constitute analytical pro-
tocols for the O
2
•
measurements. The scavenger can be used at concentrations that
compete effectively with the dismutation reactions such that the produced O
2
•
will
be trapped and thus detected. Thus far, some chemical methods have been used for
the detection of O
2
•
. For example, the O
2
•
radical can reduce ferricytochrome
c
[60]
(reaction shown below), tetranitromethane, and nitroblue tetrazolium [61] and these
reductions can be followed spectrophotometrically in terms of the accumulations of
ferrocytochrome
c
, the nitroformate anion, and blue formazan, respectively, to estab-
lish spectrophotometric methods for the O
2
•
determination. There are, of course,
other agents that can be involved in these reductions and thus interfere with the O
2
•
determination. In this case, the net response for O
2
•
can be differentiated by using
SOD since SOD can specifi cally catalyze the dismutation of O
2
•
.
Cytochrome
c
(Fe(III))
O
2
•
O
2
The O
2
•
radical can act as an oxidant as well as a reductant and chemical esti-
mates of its production can also be based on its ability to oxidize epinephrine to adren-
ochrome [62]. These chemical methods have the additional advantage of not requiring
highly specialized equipments. Also based on its redox property, the O
2
•
radical can
be determined by chemiluminescence methods through the measurement of the inten-
sity of the fl uorescence radiation emitted after chemical oxidation of O
2
•
by, e.g.,
lucigenin [63-67]. These methods, however, are limited by the poor selectivity and
lack of capability for
in-vivo
performance.
Considerable interest has been devoted to the application of electrochemical method
for O
2
•
determination because of its high selectivity, sensitivity, and capability for
in-vivo
use [68-83]. In this chapter, we will mainly focus our attention on electrochemi-
cal methodologies for O
2
•
determination, highlighting recent attempts on this aspect by
using SODs. As will be described, a combination of the promoted direct electron trans-
fer of the SODs with the biomolecular recognition by virtue of specifi c and signifi cant
enzyme-substrate reactivity of the SODs toward O
2
•
essentially results in a sensitive
measurement of O
2
•
without a virtual interference from physiological levels of H
2
O
2
,
ascorbic acid (AA), uric acid (UA), and metabolites of neurotransmitters. Furthermore,
these strategies could be further accomplished with carbon fi ber microelectrodes, which
can be readily employed for
in-vivo
determination of O
2
•
in biological systems.
→
Cytochrome
c
(Fe(II))
6.3 O
2
•
ELECTROCHEMISTRY AND
O
2
•
ELECTROCHEMICAL SENSORS
O
2
•
is not stable in aqueous media, especially in acidic solutions. This poor stability
has essentially made it diffi cult to study O
2
•
electrochemistry in aqueous media. By
carefully designing the electrode-solution interface of a hanging mercury drop electrode
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