Environmental Engineering Reference
In-Depth Information
of characteristic features in the absorption spectrum of clusters. Fluorescence quenching
was more highly selective for Cys than for the other 19 amino acids. Fluorescence of clus-
ters was not quenched in the presence of other thiols such as GSH and proteins, e.g., BSA.
Owing to the large size of these molecules, they cannot penetrate to reach the silver core
through the ligand shell. In the presence of other thiols such as mercaptopropionic acid
and cysteamine, luorescence was quenched where the steric hindrance was overcome.
26.3 Devices and Market
It is important to work toward making use of materials as devices for use at community
levels. Gold and silver NPs exhibit unusual properties such as optical absorption, lumi-
nescence, stability to various chemical environments, and catalytic activity. The absorp-
tion or luminescence is highly sensitive to ions/molecular interactions. Small quantities of
materials are enough to cause considerably large changes in optical properties at ultra-low
concentrations of analyte. This fact makes the fabrication of silver and gold nanosystems-
based devices more feasible. It is also important to note that coating of nanomaterials on
oxide surfaces such as alumina, silica, titania, and magnesia can be done for eficient dis-
persion of nanomaterials and their reuse, if the process of contaminant interaction is just
physisorption. If the material involves certain chemical reactions, the reactions should
be green and the products must be nontoxic. Red-emitting Au
25
clusters, encapsulated in
BSA protein, were incorporated into a nitrocellulose strip for detection of Hg
2+
. This strip
was red luminescent under a UV lamp. After dipping a piece of such a strip into various
solutions of metal ions, luminescence of clusters was completely quenched selectively in
the presence of Hg
2+
ions. Similarly, β-CD-encapsulated GSH-protected Au
15
clusters were
incubated into chitosan, which was made as a ilm. This ilm was red-luminescent due to
the presence of Au
15
clusters. Those ilms were used to selectively detect Cu
2+
ions down
to 1 ppm, which is below the maximum contamination level (1.3 ppm), on the basis of
quenching of luminescence that was seen by the naked eye.
Silver NPs, supported on neutral alumina, were used in cartridges of a water ilter for
the removal of pesticides, CP, and malathion. The same material was also used to decon-
taminate water from chlorocarbons such as CCl
4
and C
6
H
5
CH
2
Cl. There are several nano-
systems yet to be commercialized for water puriication applications. The most important
factors that are limiting commercialization of these materials are cost, reuse, and lack of
detailed understanding of the effect of leachates from the materials, on human health. It
is important to consider the disasters in communities where the drinking water problem
is unexplainable. In such cases, materials that are cheap, highly portable, nontoxic, easy
to use and distribute, and require low energy input, need to be addressed. A set-up like
blotting paper containing Ag NPs for simple inactivation of bacteria is one such example.
26.4 Perspectives
Water pollution has become a major problem of the world. Water resources are contami-
nated severely and the complexity of the problem increases with time as the population
