Environmental Engineering Reference
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The PL of GSH-protected silver nanoclusters was selectively quenched by Hg 2+ . The sensi-
tivity of these clusters for Hg 2+ was found to be appreciable down to 0.1 nM (0.02 ppb). PL
quenching was due to aggregation of clusters due to the strong interaction between Hg 2+
and the carboxylic groups of clusters. 16
26.2.1.2 Lead
Colorimetric, luorescence, and electrochemical methods have been used to detect lead
species in water samples. Tetra- n -octylammonium bromide (TOAB)-protected gold NPs
were treated with different concentrations of Pb 2+ solutions in THF. 17 It was found that
the absorbance of gold particles decreased along with a red shift of the SPR peak, as Pb 2+
concentration increased. At higher concentrations, the color of the solution turned from
red to blue and inally to colorless. The red shift of the SPR peak and the blue color were
attributed to aggregation of NPs and change in the dielectric constant of the particles. Pb 2+
ions interact with Br ions on the surface of particles to form PbBr 2 . During this interac-
tion, some of the TOAB was removed, resulting in the aggregation and color change. Red
emission was seen after illumination of lead ion-treated gold particle solution with UV
light (300 nm), and the solution exhibits an absorption band at 300 nm due to the formation
of PbBr 2 . Wang et al . 18 reported the detection of lead in the nanomolar range using gold
NPs. They propose the formation of AuPb alloy on the surface of NPs at pH 9. This alloy
increases the catalytic activity of NPs in H 2 O 2 -mediated oxidation of Amplex UltraRed
(AUR). Here, AUR is nonluminescent, whereas its oxidation product is highly luminescent
(showing emission at 584 nm when excited at 540 nm). The intensity of the emission peak
at 584 nm is proportional to the concentration of Pb 2+ solution in the 0.05-5 μM range. The
detection limit was 4 nM. This method was used to test the presence of Pb 2+ in lake water
and in blood samples.
Yuan et al . 19 have synthesized nonluminescent Au nanodots (Au NDs) protected with
tetrakis(hydroxymethyl)phosphonium chloride (THPC) and GSH. After exchange of THPC
with 11-mercaptoundecanoic acid (MUA), Au NDs exhibited a green emission due to MUA
ligand-to-Au(I) metal charge transfer. These dual ligand (MUA and GSH)-stabilized Au
NDs were used for selective detection of Pb 2+ ions. After the addition of Pb 2+ ions, green
luorescence became quenched because of the aggregation of Au NDs, which was attrib-
uted to the interaction of GSH with Pb 2+ ions. In this method, the detection limit was 5 nM.
Yoosaf et al . 20 have synthesized gallic acid-protected gold and silver NPs that were
highly stable in the pH range 4.5-5.0. The stability was attributed to strong electrostatic
interactions between the NP surface and the carboxylate anion, and a high negative zeta
(ζ) potential (−45 mV). Pb 2+ ions interact with carboxylate groups of Ag/Au NPs, leading
to aggregation and color change. Owing to aggregation, the SPR peak became red-shifted.
The SPRs of individual particles couple in aggregates (Figure 26.3). The extent of the red
shift was dependent on the concentration of lead ions. Aggregation of particles was evi-
dent from transmission electron microscopy (TEM) analyses (Figure 26.3). Interaction and
shift of the SPR peak was highly speciic to lead ions. Unlike Pb 2+ , other metal ions such
as Hg 2+ , Cd 2+ , Cu 2+ , Zn 2+ , and Ni 2+ did not lead to a shift of the SPR. GSH-protected red-
emitting silver NPs were found to selectively sense Pb 2+ ions down to parts-per-quadrillion
levels. 21 The reason for luorescence quenching is believed to be the strong complexation
of Pb 2+ with GSH molecules. This led to the release of ligands into the solution resulting in
the dissolution of NPs. At higher Pb 2+ concentrations, TEM analysis showed the absence of
NPs. These luorescent Ag NPs were also tested for their ability to detect Pb species in real
samples such as river water, battery, paint, and toys. The concentrations of Pb 2+ obtained
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