Biomedical Engineering Reference
In-Depth Information
Recently, many inorganic porous materials such as clay [62], montmorillonite [63-64],
porous alumina [65], sol-gel matrix [66], etc. have been shown to be promising as
immobilization matrices. They have many signifi cant advantages of high mechanical,
thermal, and chemical stability and good adsorption and penetrability due to their reg-
ular structures and appreciable surface areas.
Hexagonal mesoporous silica (HMS) is a mesoporous silica material which proc-
esses a porous size of nanoscale dimension to make it more suitable for enzyme inter-
calation and loading. The Mb-HMS/GCE electrode exhibits a good electrocatalytic
behavior with respect to the pseudo-peroxidase activity to H
2
O
2
reduction and a fast
amperometric response to NaNO
2
and the prepared sensors could be used for the deter-
minations of H
2
O
2
and NO
2
[235]. From the amperometric response of Mb-HMS/
GCE with successive additions of H
2
O
2
to 0.1 M, pH 7.0, PBS at an applied potential
of
400 mV, we can see that the reduction current increased steeply to reach a stable
value upon addition of an aliquot of H
2
O
2
to the buffer solution. Under optimal condi-
tions the linear response range of the sensor to H
2
O
2
concentration was from 4.0 to
124
10
8
M at a signal-to-noise
ratio of 3. The
K
app
value for the electrocatalytic activity of Mb-HMS/GCE to H
2
O
2
was determined to be 0.065
µ
M and the detection limit was estimated to be 6.2
0.005 mM, indicating that the Mb entrapped in the HMS
matrix was of a high affi nity to H
2
O
2
. The cyclic voltammograms of Mb-HMS/GCE
in 0.1 M, pH 6.0, PBS upon addition of NaNO
2
showed that there was a reduction peak
of NO at about
0.96 V. NO is produced from the nitrite disproportionation reaction:
NO
2
2H
e
→
NO
H
2
O
(18)
1.0 V was applied to amperometric detection of NO
2
. The linear response
range of the sensor to NO
2
concentration is from 8.0 to 216
Thus
µ
M and the detection
limit of NO
2
concentration was 8.0
10
7
M at a signal-to-noise ratio of 3. The
K
m
app
value of the Mb-HMS/GCE to NaNO
2
was estimated to be 0.72 mM, indicating
a high affi nity of the Mb-HMS/GCE to NO
2
.
Clays are usually cation-exchangeable aluminosilicates, and exfoliated clay parti-
cles have a platelet shape with nanoscopic size. Cast protein-clay fi lms on electrodes
have been used to immobilize proteins. The {Clay/Mb}
n
electrode has good electro-
catalytic properties for the reduction of oxygen and hydrogen peroxide [236] and the
biosensors can also be made based on these properties.
17.3.1.4 Biosensors based on direct electron transfer of hemoglobin
Hemoglobin (Hb) is a heme protein that can store and transport oxygen in blood in
vertebrates. It has a molar mass of approximately 67 000 g mol
1
and comprises four
polypeptide subunits (two
subunits). The structure of each subunit is simi-
lar to that of Mb. The materials that are used to immobilize Mb for biosensors are also
suitable for Hb. The substrates that can be catalyzed by Mb also can be catalyzed by
Hb. Kieselgubr is a kind of inorganic material with a porous structure, which is prom-
ising as an immobilization matrix because of its good mechanical, thermal, and chemi-
cal stability. Moreover, kieselgubr is of special interest because of its effectiveness in
α
and two
β
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