Biomedical Engineering Reference
In-Depth Information
2.0 10
6
1.0 1 0
6
A
0.0
1.0 10
6
2.0 10
6
B
3.0 10
6
4.0 10
6
5.0 10
6
6.0 10
6
7.0 10
6
C,D,E,F
10
6
8.0
10
6
9.0
0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs Ag/AgCl)
FIGURE 12.6
Cyclic voltammograms at a laccase biosensor in the absence (a) and presence (b) of O
2.
Sparging of O
2
through the electrolyte between scans (2 min) yields reproducible and increased catalytic
reduction currents (C-F) compared to ambient O
2
levels. Scan rate 10 mVs
1
in 0.05 M acetate buffer of pH
4.5. (From [55], with permission from Wiley.)
verrucaria
(
Mv
) in phosphate buffer, pH 7.0 [56]. Subsequent optimization of the
BOD cathode focused on immobilization of both mediator and enzyme, and match-
ing of mediator redox potential to that of the enzyme. The Ikeda group have achieved
current densities of 17 mAcm
2
at
0.25 V vs Ag/AgCl for mediated oxygen reduc-
tion by electrostatically entrapping
Mv
BOD with [W(CN)
8
]
3
/4
in poly(L-lysine) at
carbon felt electrodes rotated at 4000 rpm [57]. The Heller group have co-immobilized
Mv
BOD and a redox polymer prepared by substitution of one of the chloride ligands
of an Os(4,4
-bipyridine)
2
Cl
2
complex with imidazole units of a copoly-
mer of poly(vinylimidazole) and polyacrylamide on carbon cloth fi bers to yield bio-
catalytic oxygen cathodes that provide current densities of 0.7 mAcm
2
at a potential
of
-dichloro-2,2
0.3 V vs Ag/AgCl in non-stirred phosphate buffered saline at 37ºC [58]. Further
improvements in the BOD cathode were obtained by replacement of the
Mv
BOD by
a BOD from
Trachyderma tsunodae
, which is claimed to have a T1 redox potential
of
0.44 V vs Ag/AgCl. Current densities of 6.25 mAcm
2
at potentials of
0.3 V vs
Ag/AgCl were obtained at carbon cloth electrodes rotated at 4000 rpm in oxygenated
phosphate buffered saline at 37ºC [59]. Purifi cation of the BOD yielded biocatalytic
fi lms that provided higher current densities of 9.5 mAcm
2
under these conditions
[60]. More recently, direct electron transfer “type” reactions of the
Mv
BOD enzyme
immobilized in poly(L-lysine) layers at carbon electrodes containing a high density of
crystal edges have been reported [61]. Using this approach, from the cyclic voltammo-
grams (CVs) in Fig. 12.7, steady-state current densities of 0.85 mAcm
2
for oxygen
reduction at potentials of
0.2 V vs Ag/AgCl in oxygen saturated phosphate buffer
at pH 7.0 with rotation at 1400 rpm can be estimated.
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