Biomedical Engineering Reference
In-Depth Information
sensitivity. These authors report that ammonia gas sensors have been used in chemical plants,
food technology, fertilizers, in environmental pollution monitoring, and in food technology.
Matsuguchi et al. (2002)
have used thin films of polyaniline-insulating matrix polymer blend
to detect ammonia near room temperature.
Roy et al. (2005)
point out that polymer-based
materials may degrade, and often suffer a limited cycle of operation. They further explain
that inorganic materials are better for gas sensing.
Sen et al. (2003)
have used an elemental
thin film of tellurium that was prepared by the thermal evaporation technique for the detec-
tion of ammonia at concentrations lower than 100 ppm.
Roy et al. (2005)
have recently used the ammonia-sensing behavior of barium strontium tita-
nate (BST) films to detect ammonia. The BST films were deposited by the sol-gel spin coat-
ing technique, and these thin films showed an increase in resistance when exposed to
ammonia gas. They further state that the sensitivity variation is from 20% to 60%. The lowest
detection limit was around 160 ppm.
Figure 10.3a
shows the binding and dissociation of NH
3
in air to the sol-gel derived thin film
sensor where the presintering was performed at 873
K(
Roy et al., 2005
). A single-fractal
analysis is adequate to describe the binding and the dissociation kinetics. The values of the
rate coefficient and the fractal dimension for the binding and the dissociation phases are
given in
Table 10.3
.
Figure 10.3b
shows the binding and dissociation of NH
3
in air to the sol-gel derived thin film
sensor where presintering was performed at 773
K. A single-fractal analysis is adequate to
describe the binding and the dissociation kinetics. The values of the rate coefficient and
the fractal dimension for the binding and the dissociation phases are given in
Table 10.3
.
In this case, the affinity,
K
(
¼
k/k
d
), value is 4.57.
Figure 10.3c
shows the binding and dissociation of NH
3
in air to the sol-gel derived thin film
sensor where presintering was performed at 673
K. A single-fractal analysis is adequate to
describe the binding and the dissociation kinetics. The values of the rate coefficient and
the fractal dimension for the binding and dissociation phases are given in
Table 10.3
. In this
case, the affinity,
K
(
k
/
k
d
), value is 0.420. A decrease in temperature leads to a decrease in
the affinity,
K
, value in the range 673-873
K temperature.
¼
Figure 10.3d
and
Table 10.3
show the increase in the binding rate coefficient,
k
, with an
increase in the fractal dimension,
D
f
. For the data shown in
Figure 10.3d
, the binding rate
coefficient,
k
, is given by:
D
5
:
008
0
:
3361
k
¼ð
0
:
1931
0
:
0250
Þ
ð
10
:
5a
Þ
f
The fit is good. Only three data points are available. The availability of more data points
would lead to a more reliable fit. The binding rate coefficient,
k
, for a single-fractal analysis
is very sensitive to the fractal dimension,
D
f
, or the degree of heterogeneity that exists on the
biosensor surface as noted by the fifth (equal to 5.008) order of dependence exhibited.
