Environmental Engineering Reference
In-Depth Information
Au NPs. This was due to the less/no interaction of added species with G/T-rich ssDNA. Now,
G/T-rich ssDNA was able to protect the Au NPs. Hence, after the addition of NaCl, there was
no color change. However, there was sensitivity to Hg
2+
due to the T-Hg-T complex, which
was overcome by using EDTA. With this strategy, arsenite was detected down to 2 ppb.
Selective detection of arsenic in groundwater was done by Kalluri et al
.
28
using modiied
gold NPs. Modiication of the surface of NPs was done using thiol group (-SH)-containing
molecules such as GSH, DTT, and Cys, separately. After the addition of As
3 +/5 +
ions into
the functionalized Au NPs, there was a color change from red to blue, which was seen
in the absorption spectrum as a red shift of SPR of Au NPs. The blue color was attrib-
uted to the presence of aggregated NPs and the presence was conirmed by TEM analysis.
Aggregation of NPs was brought about by the attractive interaction between carboxy-
late groups of ligands and As
3 +/5 +
. Other heavy metal ions did not show the color change.
Selectivity toward As
3+
was attributed to the high binding constants of As
3+
with all the
three ligands. DLS, which accurately measures the size of the particles, was used to detect
low levels of arsenic, whereas the colorimetric method is applicable at high concentrations.
Lower detection limits using DLS and colorimetric methods were 3 ppt and 1 ppb, respec-
tively. Functionalized Au NPs were used to test the presence of As in Bangladeshi wells
and Mississippi tap and drinking water. Bangladeshi wells were found to have >1 ppm of
As, as seen by the color change of NPs.
Square-wave anodic stripping voltammetry (SWASV) is one of the highly sensitive tech-
niques in the electrochemical analysis of trace levels of metals in different samples. Using
this method, Jena et al
.
29
have detected As
3+
selectively (in presence of Cu
2+
) with a limit of
0.02 ppb under optimized conditions. This method involves mainly two steps: (i) deposi-
tion of As
0
at an optimized potential for a particular time (−0.35 V for 100 s) and (ii) anodic
stripping of deposited As
0
. The anodic stripping signal was used to monitor the concen-
tration of As
3+
in solution. The SWASV response of the gold nanoelectrode ensembles
(GNEEs) electrode at different As
3+
concentrations (in 1 M HCl) is shown Figure 26.4a.
(a)
(b)
15
3000
3 ppb
10
2000
0
5
1000
0
-0.3
0.0
0.3
0.6
E
(V) (Ag/AgCl)
FIGURE 26.4
(a) SWASV response of the GNEE electrode toward As
3+
at different concentrations in 1 M HCl. Each addi-
tion increased the concentration of As
3+
by 0.1 ppb. (Adapted from Jena, B.K., and Raj, C.R.,
Anal. Chem
., 80,
4836, 2008. Copyright with permission from American Chemical Society.) (b) Metal ion-induced SERS intensity
changes of a proposed SERS-based As
3+
sensor. The concentration of each of As
3+
, Cd
2+
, Cu
2+
, Cr
3+
, Zn
2+
, Ni
2+
, Fe
3+
,
and As
5+
was 1.34 μM. The concentration of other metal ions (Ca
2+
, Hg
2+
, K
+
, Mg
2+
, Mn
2+
, and Pb
2+
) was 13.4 μM.
The incubation time was 2 min. (Adapted from Li, J. et al.,
ACS Appl. Mater. Interfaces
, 3, 3936, 2011. Copyright
with permission from American Chemical Society.)
