Environmental Engineering Reference
In-Depth Information
due to intraband (sp→sp) and interband (d→sp) transitions, respectively, which are
entirely different from the SPR peak of NPs at 520 nm. Its structure consists of an Au
13
icosahedral core in which one Au atom is at the center. The Au
13
core is protected by six
RS-Au-SR-Au-SR units, as shown in the inset of Figure 26.1.
7
The Au
25
cluster protected
with glutathione (GSH) ligands exhibits red luminescence
8
around 700 nm (Figure 26.1b).
Progress is slow in obtaining the crystal structure of silver clusters because of their poor
stability, unlike in gold cluster systems. Only a few reports of crystal structure of silver
clusters are available in the literature thus far. The molecular formula of one such cluster
is Ag
14
(SC
6
H
3
F
2
)
12
(PPh
3
)
8
and the structure contains an
Ag
4+
octahedral core that is very
different from analogs of gold clusters.
9
It has absorption bands at 368 and 530 nm, and
it shows yellow emission both in the solid and solution states. Two other clusters with
molecular formulae Ag
16
(DPPE)
4
(SC
6
H
3
F
2
)
14
and {Ag
32
(DPPE)
5
(SC
6
H
4
CF
3
)
24
}
2−
(where DPPE
is 1,2-bis(diphenylphosphino)ethane, SC
6
H
3
F
2
is 3,4-diluorothiophenol-H, and SC
6
H
4
CF
3
is 4-(triluoromethyl)thiophenol-H) were crystallized by Yang et al.
10
The composition of
these clusters reveals the presence of mixed ligands. Both the clusters exhibit molecule-
like absorption spectra that contain a main peak at 485 nm. In addition to the 485-nm peak,
the Ag
32
cluster showed a shoulder at 720 nm. Both the clusters exhibit weak blue emis-
sion when excited with ultraviolet (UV) light. Photoluminescence (PL) of both the clusters
was strong in CH
2
Cl
2
at 440 nm when excited at 360 nm. These clusters have core-shell
structures.
Ag
6+
and
Ag
22
12+
are the cores in Ag
16
and Ag
32
, respectively. In Ag
16
, the
Ag
6+
unit was encapsulated in a complex shell of {Ag
8
(DPPE)
4
(SC
6
H
3
F
2
)
14
}
6−
, whereas in Ag
32
,
the
Ag
22
12+
core was
coprotected by one {Ag
6
(DPPE)
3
(SC
6
H
4
CF
3
)
12
}
6−
, two {Ag
2
(DPPE)(SC
6
H
4
CF
3
)
4
}
2−
, and four
(SC
6
H
4
CF
3
)
−
units.
In this chapter, we discuss the applications of NMNs as sensors for toxins in water. It
focuses mainly on the chemistry of various nanomaterials and mechanisms of interactions
responsible for sensitivity. We present the sensors that are able to detect ultra-low levels
of contaminants even in the presence of common interfering molecules/ions.
We note that
some of the applications of relevance to drinking water using NMNs have already been
commercialized. Developments in this area of the past several years have been reviewed
previously
2
and have also been the subject of a topic chapter.
11
In the present work, we
focus on the most recent literature of the past 4 years.
12+
core was encapsulated in an Ag
10
(DPPE)
5
(SC
6
H
4
CF
3
)
24
shell. The
Ag
22
26.2 Sensing/Removal of Pollutants of Water Using NMNs
26.2.1 Inorganic Metal Ions
26.2.1.1 Mercury
Use of gold and silver NPs has emerged as an option for capturing various forms of mer-
cury, at low concentrations, of signiicance to drinking water as these NPs possess high
adsorption capacities. Adsorption capacity was found to be low when Au@citrate NPs
were treated with Hg
2+
. This is due to poor electrostatic interactions between NPs and
Hg
2+
ions. Lisha and coworkers
12
have used metal-alloying chemistry for sorption of toxic
Hg
0
. They found that alumina-supported Au@citrate NPs (10-20 nm in diameter) show an
adsorption capacity of 4.065 g of mercury per gram of gold NPs. In this study, Hg
2+
was
reduced to Hg
0
by using dilute aqueous NaBH
4
. Formation of an amalgam (Au
3
Hg) was
