Biomedical Engineering Reference
In-Depth Information
In a classical situation, to demonstrate fractality, one should make a log-log plot, and one
should definitely have a large amount of data. It may be useful to compare the fit to some
other forms, such as exponential, or one involving saturation, etc. At present, no independent
proof or physical evidence of fractals in the examples is presented. It is a convenient means
(since it is a lumped parameter) to make the degree of heterogeneity that exists on the surface
more quantitative. Thus, there is some arbitrariness in the fractal model to be presented. The
fractal approach provides additional information about interactions that may not be obtained
by conventional analysis of biosensor data.
There is no nonselective adsorption of the analyte. The present system (environmental
pollutants in the aqueous or the gas phase) being analyzed may be typically very dilute. Non-
selective adsorption would skew the results obtained very significantly. In these types of sys-
tems, it is imperative to minimize this nonselective adsorption. It is also recognized that, in
some cases, this nonselective adsorption may not be a significant component of the adsorbed
material and that this rate of association, which is of a temporal nature, would depend on sur-
face availability. If the nonselective adsorption were to be accommodated into the model,
there would be an increase in the heterogeneity on the surface, as, by its very nature, non-
specific adsorption is more homogeneous than specific adsorption. This would lead to higher
fractal dimension values since the fractal dimension is a direct measure of the degree of
heterogeneity that exists on the surface.
Shinde et al. (2007)
recently investigated the use of ZnO thin films prepared by the spray
pyrolysis method for the sensing of LPG. These authors deposited nanocrystalline ZnO films
onto glass substrates by the spray pyrolysis of zinc nitrate solution, and used this as a LPG
gas sensor. These authors further explain that ZnO has replaced the more toxic and expensive
materials such as CdS, TIO
2
, GaN, and SiO
2
for applications in gas sensors (
Rao and Rao,
1999
).
Ismail et al. (2001)
have shown that owing to its resistivity control in the range
10
3
to 10
5
ohm cm ZnO is particularly suitable for gas sensors. Besides, it exhibits high
electrochemical stability, absence of toxicity, and is readily available in nature.
Shinde
et al. (2007)
point out that thin films are particularly suited for gas sensors, as the gas sensing
properties of metal oxides (a) may be related to the material surface, and (b) gases are readily
adsorbed and react with the thin film biosensor surface (
Liu et al., 1997
). Furthermore, Zhu
et al. (1993) and
Chai et al. (1995)
report that thin film gas sensing materials have good gas
sensitivity and selectivity.
Patil (1999)
has demonstrated the versatility of using the spray
pyrolysis method for the deposition of metal oxides.
Shinde et al. (2007)
report that the gas sensing properties of oxide materials may be related to
surface morphology and are grain size dependent. This surface morphology is what we will
attempt to characterize and make quantitative using the fractal analysis method. As indicated
in the different chapters throughout the topic, the fractal dimension (a) provides a quantitative
measure of the degree of heterogeneity on the sensing surface, and (b) an increase in the
