Biomedical Engineering Reference
In-Depth Information
the preservation of enzyme activity, and its fi ne catalytic ability. The Hb-kieselgubr-
PG electrode can catalytically reduce H
2
O
2
and make an H
2
O
2
biosensor [237]. With
the addition of H
2
O
2
to the electrochemical cell with an Hb-kieselgubr-modifi ed PG
working electrode, an obivious increase in the cathodic peak and a decrease in the
anodic peak are observed, which is characteristic of an electrochemically catalytic
reaction. The catalytic peak is located at about
250 mV. On the other hand, no elec-
trochemical signal corresponding to H
2
O
2
can be observed in the potential range at a
bare PG or an electrode modifi ed with kieselgubr alone. Therefore, the catalytic peak
should come from the interaction between Hb and H
2
O
2
. This result shows that Hb
incorporated in the kieselgubr fi lm can act as an effective catalyst to the reduction of
H
2
O
2
. Meanwhile, the height of the reduction peak current apparently increases with
the concentration of H
2
O
2
and reaches a plateau at a higher concentration of 500
M
in a stirring solution, suggesting a typical electrocatalysis process that coincides with
a Michaelis-Menten kinetics model. The decrease of the anodic peak current with the
addition of H
2
O
2
implies that the oxidized form of Hb can be quickly reduced to its
reduced form by acquiring an electron from the PG electrode. There is a linear relation
between the catalytic peak current and H
2
O
2
concentration in the range of 5.0
µ
10
6
10
4
mol L
1
, while it reaches a plateau at a concentration of 6.0
10
4
mol L
1
to 3.0
10
6
mol L
1
(from three times S.D.).
The apparent Michaelis-Menten constant for the Hb-kieselgubr-modifi ed H
2
O
2
sensor
is found to be 975
and the detection limit is estimated to be 2.1
M.
The porous inorganic sol-gel matrix is particularly attractive for the development
of electrochemical biosensors because the matrix possesses physical rigidity, chemical
inertness, high photochemical, biodegradational, and thermal stabilities, and experi-
ences negligible swelling in aqueous solutions. When hemoglobin was immobilized
by a thin silica sol-gel fi lm which derived from tetraethylorthosilicate (TEOS) on a
carbon paste electrode (CPE), the modifi ed electrode can electrocatalytically reduce
O
2
, NO
2
, and H
2
O
2
[197]. Electrochemical catalytic reduction of oxygen by Hb is
observed through CV. When a volume of air is passed through a pH 7.0 phosphate
buffer, a signifi cant increase in reduction peak at about
µ
0.3 V is observed for the
Hb/sol-gel fi lm-modifi ed CPE, as compared to the reduction peak of Hb Fe(III) of the
Hb/sol-gel fi lm-modifi ed CPE without oxygen. This increase in the reduction peak is
accompanied by the decrease of the oxidation peak of HbFe(II), which suggests that
HbFe(II) partially reacted with oxygen. For sol-gel-modifi ed CPE without any Hb, the
reduction peak of oxygen is observed at about 0.83 V, far more negative than the cata-
lytic peak potential. Electrochemical catalytic reduction of NO
2
was also tested with
Hb immobilized on CPE by sol-gel fi lm. The addition of NaNO
2
in a pH 5.5 buffer
results in a new reduction peak at about
0.6 V. Further addition of NaNO
2
caused
an increase of the peak. It has been demonstrated that this new reduction peak does
not come from NO
2
but from the [Hb Fe(II) (NO)
]
nitrosyl adduct [101]. The Hb/
sol-gel fi lm-modifi ed CPE was also used to reduce H
2
O
2
. When H
2
O
2
was added
to a pH 5.5 buffer solution, an increase in the reduction peak at about
0.3 V was
observed. The reduction peak current increases with the concentration of H
2
O
2
in the
solution. The concentration between 5
10
6
and 7
10
4
M, the electrocatalytic
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