Environmental Engineering Reference
In-Depth Information
attributed to a transformation of the interconnected and irregularly shaped NPs to the
single, separated, and nearly spherical ones, as conirmed by TEM and UV/visible (Vis)
absorption spectroscopy.
26.2.2.3 Thiocyanate (SCN
−
)
Citrate-capped Au NPs were synthesized according to the literature, and Tween 20 (poly-
sorbate surfactant) was subsequently added as a capping agent. After the addition of SCN
−
,
some of the citrate ions on the surface of the Au NPs were replaced owing to the higher
afinity of gold for SCN
−
ions.
34
As a result, Tween 20 molecules, which are adsorbed on the
surfaces of Au NPs, were separated and an aggregation of Au NPs occurred. Aggregation
of particles was accompanied by a visible color change from red to blue within 5 min. The
sensing of SCN
−
could easily be conirmed by a UV/Vis spectrophotometer. The effects
of relevant experimental parameters including concentration of Tween 20, pH of solution,
incubation temperature, and time, were evaluated to optimize the method. Under the opti-
mized conditions, this method yields excellent sensitivity, down to 11.6 ppb and selectivity
toward SCN
−
. This method was also applied successfully to test the presence of SCN
−
in
saliva of smokers and real water samples.
26.2.2.4 Sulide (S
2−
)
Au@Ag core-shell nanocubes were used for sensing of S
2−
ions based on the change in
optical properties. When S
2−
ions (from Na
2
S) were added to Au@Ag core-shell nano-
cubes, there was a change in the position of the SPR peak and the color of the solution.
35
Detection limits of S
2−
using this strategy was 10 ppb by UV/Vis spectroscopy and 200 ppb
by the naked eye. The SPR was tuned in the 500-750 nm window. A possible reason for the
SPR tuning with concentration of S
2−
could be the increase in thickness of the Ag
2
S layer
formed on the Ag shell. There were slight distortions of nanocubes after reaction with S
2−
,
which may also account
for this tuning. Owing to the change in refractive index of the new
composition (Ag
2
S), color would be changed. The reaction can be represented as
4Ag + 2S
2−
+ O
2
+ 2H
2
O → 2Ag
2
S + 4OH
−
26.2.3 Organic Contaminants
26.2.3.1 Pesticides
Endosulfan, CP, and malathion are important pesticides being used in developing coun-
tries. Use of noble metal NPs for the removal of pesticides from water has been demon-
strated for the irst time by Nair et al.
36
Gold and silver NPs, protected with trisodium
citrate, have been utilized for this purpose. Au@citrate NPs interact with endosulfan mol-
ecules leading to a color change (from wine red to blue) and a red shift of the SPR peak.
SPR was sensitive down to the 2 ppm range. It was found that Au particles showed faster
interaction compared with Ag particles. Au and Ag@citrate NPs were also used for the
removal of CP and malathion.
37
After the introduction of CP and malathion to Ag and
Au@citrate NPs, there were color changes and red shifts of SPR peaks due to the adsorp-
tion of pesticide molecules on the surface of NPs. In this method, the limit of detection
was only moderate: a few ppm, typically the limit of solubilities. Ag and Au@citrate NPs
