Biomedical Engineering Reference
In-Depth Information
the same rate of O
2
•
generation at various pH values as shown in Fig. 6.12. As shown,
slight pH dependence was observed for O
2
•
responses at the SOD-based biosensors
within a pH range from pH 5.8 to 9.5 [138].
The interference from O
2
was also investigated with the SOD-based biosensors, in
which O
2
•
was generated from KO
2
rather than from the xanthine-XOD system since
the enzymatic system requires O
2
for the O
2
•
generation. The removal of O
2
from the
phosphate buffer by bubbling N
2
into the solution was found to produce no observable
change in the current response of the SOD-based biosensors toward KO
2
, suggest-
ing that O
2
does not interfere with O
2
•
determination under the present experimental
conditions.
6.4.5 SOD-based micro-sized biosensors for O
2
•
Even though the demonstrated good analytical properties of the SOD-based biosensors,
such as low detection limit and high selectivity, substantially make them very poten-
tial for
in-vivo
determination of O
2
•
, the miniaturization of those SOD-based bio-
sensors remains essential for such a purpose. As detailed in the above sections, other
groups have successfully fabricated the third-generation biosensors for O
2
•
based on
the direct electron transfer of SODs on SAM-modifi ed Au electrodes. However, this
concept can not be readily realized on carbon fi ber microelectrodes because the pro-
moters could hardly be stably anchored at the carbon-based electrode directly. From a
practical point of view, the utilization of carbon fi ber microelectrodes (CFMEs) is one
of the most powerful analytical protocols, because CFMEs possess a number of unique
features. Their small dimensions result in an increased mass transport of an electro-
active species to the electrode surface, a low double-layer capacitance and ohmic loss.
Moreover, carbon fi bers have better mechanical characteristics than noble-metal elec-
trodes of a micrometer size. In addition to the capability for insertion in a single cell
and for implantation into biological tissues with a relatively good biocompatibility of
the material itself [156, 157], they are rather easily prepared and handled.
To fulfi ll both the requirement of CFME for the practical applications and the neces-
sity of Au substrate to assemble so-called promoters to construct the third-generation
biosensor, Tian
et al
. have combined the electrochemical deposition of Au nanoparticles
(Au-NPs) onto carbon fi ber microelectrodes with the self-assembly of a monolayer on
these Au-NPs to facilitate the direct electron transfer of SOD at the carbon fi ber micro-
electrode. The strategy enabled a third-generation amperometric O
2
•
biosensor to be
readily fabricated on the carbon fi ber microelectrode. This CFME-based biosensor is
envisaged to have great potential for the detection of O
2
•
in biological systems [158].
As shown in Scheme 4, all electrodes were fabricated from a single carbon fi ber
(10
m in diameter) connected to the copper wire using a silver paste. After cutting the
tip of the prepulled glass to obtain a smooth blunt opening of 20
µ
m diameter,
each carbon fi ber was inserted into a pulled glass capillary. Diluted epoxy solution was
used to make the seal between the carbon fi ber and glass by capillary action. After the
epoxy had cured, the extruded tip portion of carbon fi ber was cut to the desired length
30
µ
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