Environmental Engineering Reference
In-Depth Information
A linear increase in current was observed up to 15 ppb. GNEEs were prepared by immo-
bilizing colloidal gold NPs on a thiol functionalized sol-gel derived 3D silicate network
preformed on a polycrystalline gold electrode.
GSH-functionalized Ag NPs based on a surface enhanced Raman spectroscopy (SERS)
platform, for highly selective and sensitive detection of trace levels of As
3+
, was developed
(Figure 26.4b) by Li et al
.
30
Initially, Ag NPs were functionalized with GSH and 4-mercap-
topyridine (4-MPY). Here, 4-MPY was used as a Raman analyte. As
3+
solutions of differ-
ent concentrations were added into the modiied Ag NPs. The color of NPs changed from
yellow to brown. This color change was due to aggregation of NPs and was conirmed by
microscopic analyses. When NPs undergo aggregation, the number of hotspots increases
and hence Raman signals of analyte molecules become enhanced. Aggregation of Ag NPs
was selective to As
3+
; other heavy metal ions did not lead to aggregation. With this method,
the detection limit for As
3+
was 0.76 ppb. Real water samples were also analyzed using the
same technique.
26.2.2 Inorganic Anions
26.2.2.1 Cyanide (CN
−
)
Au@citrate NPs have been shown to be better SERS candidates after treating with CN
−
ions. Senapati et al
.
31
used the SERS technique for selective and highly sensitive detection
of CN
−
. After the addition of CN
−
ions to the Au@citrate NPs, they aggregate leading to the
red shift of the SPR peak. SERS of aggregates show Raman shifts at ~300, ~370, and ~2154
cm
−1
originating from Au-CN bending, Au-C, and C≡N stretching frequencies, respec-
tively. This indicates that the aggregation was due to the formation of Au-C bonds. The
intensity of the C≡N peak was increased with increase of CN
−
ions concentration, indicat-
ing the possibility of quantiication of CN
−
ions. Using the SERS method, CN
−
ions were
detected down to 110 ppt levels. They showed the detection of CN
−
ions in real samples
as well. Time-dependent SERS suggested a decrease in intensity of C≡N stretching. This
was attributed to the reaction of CN
−
ions with gold particles leading to the formation of
an
Au CN
)
2
−
complex.
(
26.2.2.2 Iodide (I
−
)
Au
25
(SG)
18
clusters were shown to selectively detect I
−
ions down to the 400 nM level. After
the introduction of I
−
solutions of different concentrations into the Au
25
clusters, a red shift
of the emission peak was seen. Increase in the intensity of the red-shifted emission peak
was noticed with an increase in I
−
ion concentration. Wang et al
.
32
called this as afinity-
induced ratiometric and enhanced luorescence, which is rare, whereas, in most of the
cases, aggregation-induced luorescence quenching occurs. The reason for this luores-
cence enhancement was obtained by XPS analysis of I
−
-treated Au
25
clusters. Quantiication
of elements suggests the composition to be Au
25
(SG)
17. 6 8
I
1.09
, indicating the partial replace-
ment of GSH ligands by I
−
. Formation of a new composition is the likely reason for the
luorescence enhancement. Another possibility is the binding of small amounts of I
−
with
the core of the cluster without disturbing the ligands.
Zhang et al
.
33
used citrate-capped core-shell Cu@Au NPs for selective colorimetric
detection of I
−
ions. After the addition of I
−
ions into the purple core-shell Cu@Au NPs,
there was a color change from purple to red. Using the color change, they could detect
0.76 ppm of I
−
within 20 min by observation with the naked eye. The color change was
