Environmental Engineering Reference
In-Depth Information
Cu
2+
ions down to 87 nM.
24
Detection of Cu
2+
was on the basis of luorescence quenching. With
increase in concentration of Cu
2+
in the range of 10
−8
-10
−6
M, there was a linear decrease in
the luorescence intensity. Fluorescence was recovered after the addition of a stoichiometric
amount of ethylenediaminetetraacetic acid (EDTA). This indicates the blocking of the charge
transfer (leading to luorescence quenching) from the complex of S-Au to gold NPs due to
the interaction of Cu
2+
with carboxylic groups of 11-MUA. Cu
2+
(hard acceptor) binds prefer-
entially with carboxylate groups (hard donor). The possibility of luorescence quenching by
aggregation was ruled out as TEM analysis revealed that the particles were well dispersed.
The other reason might be the paramagnetic nature of Cu
2+
ions.
Liu et al
.
25
have synthesized Ag clusters in azobenzene-modiied poly(acrylic acid)
(MPAA) templates. Clusters exhibit red and yellow emissions depending on the excita-
tion wavelength, indicating the formation of clusters of different size. Fluorescence was
sensitive speciically to Cu
2+
ions and was not sensitive to a large number of other cations
and anions. Fluorescence quenching was due to the binding of Cu
2+
on the surface of clus-
ters leading to energy transfer. Fluorescence was recovered after the addition of chelating
agents such as EDTA. The energy transfer mechanism for quenching of luorescence was
conirmed by absorption spectroscopy: absorption spectra of MPAA and Cu
2+
+ MPAA
were recorded in which the Cu
2+
+ MPAA complex exhibits a strong and broad absorption
in the 540-800 nm range. An overlap of the emission peak of the cluster and the absorption
peak of Cu
2+
+ MPAA complex indicates that energy transfer is responsible for luorescence
quenching. These clusters were integrated onto a cellulose ilter paper, just by dipping.
After drying, the paper strips were exposed to different concentrations of Cu
2+
solutions
and were kept under a UV lamp. There was a clear absence of emission from the high con-
centration Cu
2+
solutions. These paper strips were used to detect copper ions in real water
samples as well. Homocysteine and dithiothreitol (DTT)-modiied Ag NPs were found to
show color change from yellow to orange and then to green-brown after the addition of
Cu
2+
. This change is due to the aggregation of NPs.
26
The change of color was observed
only in the case of copper, indicating high selectivity toward copper. These Ag NPs were
coated on normal paper for testing copper ions. The limit of naked-eye detection of copper
was 7.8 nM. It was tested in real waters such as tap and pond water.
26.2.1.4 Arsenic
Arsenic exists in groundwater in +3 and +5 oxidation states, among which As
3+
is highly
toxic. Arsenic exists mainly as anions (arsenite and arsenate). Liang et al
.
27
developed a
label-free colorimetric method for the detection of arsenite using G/T-rich oligonucleotides
and unmodiied Au NPs. Sensing was on the basis of a color change of gold NPs due to
aggregation. At pH 8, arsenic exists in water as H
3
AsO
3
having three hydroxyl groups that
can form strong hydrogen bonds with amine groups (R
2
NH, RNH
2
) or carbonyl groups (C =
O) of bases in G/T-rich single-stranded DNA (ssDNA). On the other hand, arsenate exists as
HAsO
2−
, which has one As=O bond that may disturb the formation of hydrogen bonds. It
is well known that by the addition of common salt, Au@citrate NPs can be made to undergo
aggregation. This aggregation is due to the decrease of electrostatic repulsion between nega-
tively charged NPs. G/T-rich ssDNA has the lexible property to bind with gold NPs through
N atoms of DNA. After the addition of arsenic species, arsenite forms hydrogen bonds, lead-
ing to the folding of G/T-rich ssDNA. Gold NPs were introduced into the folded DNA in
which NPs were not protected. Addition of NaCl leads to aggregation of NPs resulting in a
color change and appearance of a new peak around 700 nm. Aggregation of NPs was con-
irmed by TEM analysis. Unlike arsenic, other metal ions did not show any color change of
