Environmental Engineering Reference
In-Depth Information
peak at m/z 220 in the reaction mixture was due to the TCP after addition of one sodium
ion per molecule, conirming the degradation of CP to TCP. It was proposed that the degra-
dation of CP proceeds through the formation of an AgNP-S surface complex, the presence
of which was conirmed by Raman spectroscopy. In this complex, the P-O bond cleaved
to give a stable aromatic species, TCP. The rate of degradation of CP increased with an
increase in temperature and pH. There is a large need for the development of new methods
that are simple and cost-effective for the detection of pesticides in water and other parts
of the environment such as food, soil, etc. Nanomaterials-based techniques are promis-
ing for sensing pesticide molecules, which explore the changes in unusual properties of
NPs, such as optical absorption, PL, and surface enhancement in Raman signals. One of
the important and commonly explored properties is SPR. Metal NPs (especially Ag and
Au) are well known to enhance the Raman signals of analyte molecules at their hotspots
due to electromagnetic ield and chemical enhancement mechanisms. The shell-thickness-
dependent Raman enhancement of thiocarbamate and organophosphorous compounds
by Au@Ag core-shell nanostructures has been demonstrated.
43
The core-shell NPs are
better candidates compared with monometallic (Ag/Au) NPs for SERS. With the increase
of the silver shell thickness to 7 nm, there was a wide range and strong SPR in the 320-560 nm
range. This results in the strong enhancement in Raman signals of pesticide molecules
enabling the detection of pesticides to nanogram levels.
26.2.3.2 Halocarbons
Unusual reactions of Ag and Au NPs with halocarbons were found by Nair and Pradeep.
44
Here, halocarbons such as CCl
4
, CHCl
3
, C
6
H
5
CH
2
Cl, CHBr
3
, and CH
2
Cl
2
react with noble
metal (Ag and Au) NPs to form corresponding metal halides and amorphous carbon.
Mineralization of halocarbons by bulk silver or gold was impossible, whereas, at the
nanoscale, this was possible under suitable experimental conditions. In the reaction, 2.5 mL
of 1:1 (v/v) Ag@citrate NPs (in water) and isopropyl alcohol (IPA) were treated with 50 μL
of CCl
4
. Here, IPA was used to achieve good mixing of reactants. Absorption spectra of
the reaction mixture were recorded at different intervals after mixing. The intensity of the
SPR peak decreased with time accompanied by a slow disappearance of color, indicating
the participation of NPs in the reaction. After 12 h of reaction, the light yellow color of the
Ag NPs solution turned colorless; a gray precipitate was formed at the bottom of the reac-
tion vessel. There was no SPR peak at this stage, indicating the completion of reaction. The
precipitate was found to be AgCl by XRD. The same precipitate was analyzed with Raman
spectroscopy before and after washing with ammonia. The presence of characteristic
Raman features of amorphous carbon conirms the carbonaceous material. Washing the
precipitate was performed to remove AgCl as [Ag(NH
3
)
2
]
+
Cl
−
soluble complex. A similar
reaction was also noticed with other halocarbons but at different reaction rates. In the case
of Au@citrate NPs, the SPR peak was red-shifted after the introduction of benzyl chloride,
indicating the aggregation of NPs along with a change in color. Reaction rate was slow (48 h)
compared with that of Ag NPs. The reaction product was AuCl
3
. The rate of reaction of
halocarbons varies with the size of the particles.
Monolayer-protected noble metal QCs (especially clusters of silver), which are smaller
in size (in-between molecules and NPs), show eficient reactivity with halocarbons com-
pared with NPs.
45
An absorption spectrum and PL spectra of an Ag
9
(MSA)
7
cluster are
shown in Figure 26.6a. Reaction of CCl
4
with the Ag
9
(MSA)
7
cluster was complete in 1.5 h.
The precipitate formed in the reaction was conirmed to be AgCl (Figure 26.6b) using
XRD. The mechanism of mineralization of halocarbons by noble metal NPs/clusters has
