A Bis-Quinoline Appended Azobenzene Based Naked Eye Sensor for Selective Detection of Cd2+ Ion

Article abstract of DOI:10.1007/s10953-016-0473-y

A new azobenzene chromophore (AZQU), appended with a bis-quinoline unit as a turn-on chemosensor for selective detection of Cd²⁺ ions, has been designed and synthesized. The fluorescent receptor AZQU has an excellent selectivity and sensitivity towards Cd²⁺ ions over other metal ions in CH₃CN:H₂O (80:20, v/v) solution. AZQU is red colored and non-fluorescent in the absence of Cd²⁺ ions, but upon addition of Cd²⁺ ions it turns colorless with appreciable fluorescence. This “off–on” type signaling behavior is attributed to the CHEF (chelation-enhanced fluorescence) effect of the quinoline unit and the change in color from red to yellow occurs by an ICT process with Cd²⁺. The association constant for AZQU?+?Cd²⁺ was calculated to be 1.68?×?10⁵ L·mol^?1 with binding in the 1:1 stoichiometric ratio.

Full text of DOI:10.1007/s10953-016-0473-y

J Solution Chem (2016) 45:907–919
DOI 10.1007/s10953-016-0473-y
A Bis-Quinoline Appended Azobenzene Based Naked Eye
Sensor for Selective Detection of Cd2+ Ion
Elango Hrishikesan1 Rangasamy Manjunath1
•
•
Palaninathan Kannan1
Received: 14 September 2015 / Accepted: 22 April 2016 / Published online: 23 May 2016
Ó Springer Science+Business Media New York 2016
Abstract A new azobenzene chromophore (AZQU), appended with a bis-quinoline unit as
a turn-on chemosensor for selective detection of Cd2? ions, has been designed and syn-
thesized. The fluorescent receptor AZQU has an excellent selectivity and sensitivity
towards Cd2? ions over other metal ions in CH3CN:H2O (80:20, v/v) solution. AZQU is
red colored and non-fluorescent in the absence of Cd2? ions, but upon addition of Cd2?
ions it turns colorless with appreciable fluorescence. This ‘‘off–on’’ type signaling behavior
is attributed to the CHEF (chelation-enhanced fluorescence) effect of the quinoline unit and
the change in color from red to yellow occurs by an ICT process with Cd2?. The asso-
ciation constant for AZQU ? Cd2? was calculated to be 1.68 9 105 LÁmol-1 with binding
in the 1:1 stoichiometric ratio.
Keywords Azobenzene Á Chemosensor Á 8-Hydroxyquinoline Á Naked eye sensor
1 Introduction
Metal cations play a major role in many biological and environmental processes. The
qualitative and quantitative determinations of metal ion present in biological and envi-
ronmental systems are of current interest in the field of supramolecular chemistry [1–3].
Recently, numerous chemosensors have been developed by various research groups to
selectively detect and determine the cation with a variety of sensing mechanisms [4–6].
Transition metals such as Cu2?, Zn2?, Hg2?, Pb2?, Ag? and Cd2? are essential in various
Electronic supplementary material The online version of this article (doi:10.1007/s10953-016-0473-y)
contains supplementary material, which is available to authorized users.
&
Palaninathan Kannan
pakannan@annauniv.edu
1
Department of Chemistry, Anna University, Chennai 600 025, India
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biological systems at the allowed levels of concentration suggested by WHO. However,
excessive concentration of such metals may leads to acute and chronic toxicity to all living
organism [7]. Of these transition metals, cadmium is extremely toxic and carcinogenic in
nature. Cadmium effluents are mainly discharged from steel processing and paint industries
and prolonged exposure to cadmium leads to liver, kidney and cardiovascular diseases and,
most importantly, cancer risk to humans [8]. Hence, recognition and detection of Cd2? is
of great interest since it is highly toxic to all living organisms. Very few colorimetric as
well as fluorometric chemosensors for Cd2? have been reported so far [9, 10]. Therefore,
detection of Cd2? in biological and environmental concern has attracted a lot of attention
of the research community. There are numerous methods known for cation detection in
biological and environmental samples. But, recently the most familiar technique is by
analyzing modulation in fluorescence and absorption signals which ensure high sensitivity
and ease of use that can be well read by the naked eye without acquiring any sophisticated
analytical instrumentation [11, 12].
There is great demand for the design and synthesis of specific colorimetric and fluo-
rometric sensors for selective detection of toxic metal ion in the environment. The design
of such optical probes typically involves selective construction of a binding unit in a
chromophore that allows the binding event, while in turn the photo-physical behavior of
the chromophore results in easily measurable colorimetric or fluorimetric responses [13].
Moreover, there are only a few reports on the combination of an azobenzene sensing unit
with another fluorophore. Cheng et al. [14] reported synthesis of azobenzene coupled with
a pyrene molecule that selectively detects Hg2? both colorimetrically and fluorometrically.
Similarly, Das et al. [15] constructed a molecule with two phthalimide moieties connected
to the tertiary amine part of the azobenzene chromophore that selectively detects Nd3? ions
both colorimetrically as well as fluorometrically.
Based on the reported literature, the present sensor was designed and synthesized with
8-hydroxy quinoline appended azobenzene chemosensor for selective sensing of Cd2? ions
in aqueous medium. 8-Hydroxyquinoline is a well-known fluorophore that chelates metal
ions like Hg2?, Zn2? and Al3? because of its electron rich N atom in the 8-position on the
pyridine ring and O atom of phenol group, which tend to coordinate with these metal ions
[16–19]. In our previous works, coupling of the azobenzene chemosensor with electron
rich bis-triazole and bis-pyridine units to identify a chemosensing property with Cu2? ion
was investigated by the naked eye detection method. However, it lacks a detection limit
even at lower concentrations [20, 21]. Hence, the fluoro-ionophore in the receptor molecule
has been constructed. Coupling of the 8-hydroxyquinoline fluorophore with non-fluores-
cent azobenzene has hitherto not been reported for selective chemosensors. The azoben-
zene-based chemosensor has attracted considerable attention from fundamental and
applied points of view, attributed to its ease of synthesis and ability to tune its spectral
properties by substituting a desired functionality.
2 Experimental
2.1 Materials and Techniques
All the chemicals for synthesis were purchased from commercial suppliers and used
without further purification. Perchlorate salts of Na?, K?, Fe2?, Ni2?, Cu2?, Zn2?, Cd2?
,
Hg2? and Al3? were purchased from Alfa Aesar, 8-hydroxyquinoline was purchased from
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Sigma Aldrich, India. Aniline, 2-chloroethanol, potassium carbonate, potassium iodide,
phosphorus oxychloride and sodium hydroxide were purchased from Merck, India and
used as received. All the reagent-grade solvents were purified by reported procedures and
freshly distilled prior to use [22]. Thin layer chromatography (TLC) was conducted on
silica gel 60 F254 plates (Merck KGaA).
Column chromatography was performed on silica gel (60–120 mesh). FT-IR spectra
were recorded with an IR spectrometer (Perkin–Elmer) using KBr pellets. Elemental
analysis was performed on a Perkin–Elmer 240 instrument. 1H and 13C NMR spectra were
recorded on Bruker 300 and 500 MHz spectrometers using tetramethylsilane (TMS) as an
internal reference. Electron spray mass spectra (ESI–MS) were carried out on a Bruker
MaXis 10138 HRMS instrument. UV–Vis spectra were recorded at room temperature
using a Shimadzu UV-1650PC spectrophotometer. Fluorescence spectra were recorded at
room temperature using a Perkin Elmer LS-45 Fluorescence spectrometer.
2.2 General Procedures of Spectral Detection
All experiments were carried out in acetonitrile/water (80/20, v/v) solution. Stock solutions
(0.1 mmolÁL-1) of the receptor AZQU were prepared in acetonitrile: water (80:20, v/v)
mixture. The absorption and fluorescence spectra were measured using 5 lmolÁL-1 AZQU
in aqueous solution with varying concentrations of metal cations.
2.3 General Procedure for the Synthesis of Compounds
2.3.1 Synthesis of 2,20-(Phenylazanediyl)diethanol (1)
In a three-necked round-bottomed flask, equipped with a stirrer, dropping funnel, and a
reflux condenser topped by a calcium chloride tube, was placed 15.83 g of potassium
carbonate (128.4 mmol) in dry acetone (100 mL). To a stirred mixture 2 g of aniline
(21.4 mmol), 6.91 g of 2-chloroethanol (85.9 mmol) was added dropwise with stirring and
then the solution was refluxed for 3 days with a catalytic amount of potassium iodide.
Then, the reaction mixture was filtered. The solvent was removed using a rotary evapo-
rator; the crude product was subjected to column chromatography using ethyl acetate:
hexane (2:8) as solvent mixture to obtain 4 g (63 %) of a colorless white solid as product.
FT-IR (KBr, cm-1) t: 3392 (O–H), 3052, 3024 (C–H), 2942, 2878 (CH2), and 1605 (C=C).
1H NMR (300 MHz, CDCl3, Me4Si) d (ppm): 7.24 (m, 2H, Ar–H), 6.73 (d, J = 6.9 Hz,
2H, Ar–H), 6.68 (dd, J = 8.4 Hz, 1H, Ar–H), 4.12 (s, 2H, –OH), 3.78 (t, J = 4.8 Hz, 4H, –
O–Me), and 3.52 (t, J = 4.8 Hz, 4H, –CH2–); 13C NMR (75 MHz, CDCl3: Me4Si) d
(ppm): 147.86, 129.31, 116.83, 112.55, 60.72, and 55.40.
Analysis Calculated for C10H15NO2:C, 66.27; H, 8.34; N, 7.73; found C, 66.25; H, 8.30;
N, 7.71.
2.3.2 Synthesis of N,N-Bis(2-chloroethyl)aniline (2)
2,20-(Phenylazanediyl)diethanol (2.0 g, 11 mmol) in phosphorus oxychloride (25 mL) was
refluxed for 6 h and the hot reaction mixture was poured with stirring into crushed ice. The
reaction mixture was extracted with ethyl acetate and washed with 10 % sodium bicar-
bonate solution. The combined organic layer was dried, filtered and the solvent was
removed by a rotary evaporator; the crude product was subjected to column
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chromatography using hexane as eluent to obtain 2.1 g (92 %) of an off white solid as
product. FT-IR (KBr, cm-1) t: 2973 (C–H), 1602 (C=C), 1169 (C–N), and 748 (C–Cl). 1H
NMR (400 MHz, CDCl3, Me4Si) d (ppm): 7.12 (m, 2H, Ar–H), 6.68 (d, J = 6.9 Hz, 2H,
Ar–H), 6.67 (dd, J = 8.4 Hz, 1H, Ar–H), 3.90 (t, J = 3.3 Hz, 4H, –CH2–), and 3.76 (t,
J = 3.3 Hz, 4H, –CH2–); 13C NMR (100 MHz, CDCl3: Me4Si) d (ppm): 149.14, 128.49,
121.80, 111.41, 77.77, 77.35, 76.92, 56.09, 56.07, 42.94, and 42.87.
Analysis Calculated for C10H13Cl2N:C, 55.06; H, 6.01; N, 6.42; found C, 54.98; H, 5.98;
N, 6.39.
2.3.3 Synthesis of N,N-Bis(2-chloroethyl)-4-((4-nitrophenyl)diazenyl)aniline (3)
The diazonium salt of 4-nitroaniline was prepared by adding an aqueous solution of
sodium nitrite (0.414 g, 6 mmol) drop wise into a solution of 4-nitroaniline (0.7 g,
5 mmol) dissolved in a HCl:H2O mixture. The mixture was stirred at 0 °C for 30 min. The
diazonium salt solution was added drop wise into a solution containing 1.1 g of N,N-bis(2-
chloroethyl) aniline, 240 mg (6 mmol) of sodium hydroxide in a small amount of water
and 15 mL of THF; immediately afterwards the color of the solution turned to dark red and
a precipitate of azo compound formed. Then, the mixture was stirred for few hours and it
was poured into an excess amount of ice cold water. The precipitate was collected by a
filtering funnel and repeatedly washed with water. The crude product was subjected to
column chromatography using a solvent mixture of (1:10) ethyl acetate and hexane to
obtain as dark red needles with yield of 85 % (1.1 g). FT-IR (KBr, cm-1) t: 2992 (C–H),
1
1605 (–C=C–), 1510, 1350 (–NO2), 1178 (C–N), and 743 (C–Cl). H NMR (400 MHz,
CDCl3, Me4Si) d (ppm): 8.21 (d, J = 8 Hz, 2H, Ar–H), 8.14 (d, J = 8 Hz, 2H, Ar–H),
7.96 (d, 2H, J = 8 Hz, Ar–H), 6.98 (d, J = 8 Hz, 2H, Ar–H), 4.04 (t, J = 8 Hz, 4H,
–CH2–), and 3.82 (t, J = 8 Hz, 4H, –CH2–); 13C NMR (100 MHz, CDCl3: Me4Si)
d (ppm): 155.35, 149.14, 148.52, 145.19, 125.89, 123.24, 121.80, 112.66, 56.09, and 40.88.
Analysis Calculated for C16H16Cl2N4O2:C, 52.33; H, 4.39; N, 15.26; found C, 52.31; H,
4.36; N, 15.19.
2.3.4 Synthesis of 4–9(4-Nitrophenyl)diazenyl)-N,N-bis(2-(quinolin-8-
yloxy)ethyl)aniline (AZQU)
In a 250 mL three-neck round-bottom flask was placed 100 mL of dry acetone. Then,
8-hydroxyquinoline (2.69 mmol, 2 g), K2CO3 (10.8 mmol, 1.48 g) and a small amount of
KI were added under a nitrogen atmosphere. N,N-Bis(2-chloroethyl)-4-((4-nitrophenyl)-
diazenyl)aniline (3) was added drop wise over an hour in the addition funnel containing
20 mL of acetone and the mixture was stirred for 24 h at 80 °C under a nitrogen atmo-
sphere. After completion of the reaction (which was monitored by TLC), the reaction
mixture was filtered through a filtering funnel and concentrated under reduced pressure,
The resulting crude product was purified by silica gel column chromatography using
hexane/EtOAc to obtain a dark red solid (1.47 g, 67 %). FT-IR (KBr, cm-1) t: 3004, 2942
1
(C–H), 1640 (–C=N–), 1448 (–N=N–), and 1376 (NO2). H NMR (500 MHz, CDCl3,
Me4Si) d (ppm): 8.88 (d, J = 4.2 Hz, 2H, Ar–H), 8.36 (d, J = 9.0 Hz, 2H, Ar–H), 8.14 (d,
J = 8.3 Hz, 2H, Ar–H), 7.94 (d, J = 8.9 Hz, 2H, Ar–H), 7.45–7.35 (m, 6H, Ar–H),
7.12–6.99 (m, 6H, Ar–H), 4.62 (t, J = 6.3 Hz, 4H, –CH2–), and 4.31 (t, J = 6.3 Hz, 4H,
–OMe–).13C NMR (125 MHz, CDCl3: Me4Si) d (ppm): 156.16, 151.87, 150.15, 149.00,
144.45, 140.71, 134.82, 129.65, 129.20, 127.92, 127.00, 126.63, 123.85, 121.01, 118.98,
105.97, 66.94, and 59.80.
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Analysis Calculated for C34H28N6O4:C, 69.85; H, 4.83; N, 14.38; found C, 69.79; H,
4.80; N, 14.35. ESI–MS (m/z): calculated for C34H28N6O4 [M ? H]?: 584.22, found:
584.15.
3 Results and Discussion
The general synthetic routes to prepare the bis-quinoline appended azobenzene receptor
AZQU are summarized in Scheme 1. Briefly, the synthesis of 2,20-(phenylazanediyl)di-
ethanol (1) was achieved by reaction of aniline with 2-chloroethanol in refluxing acetone,
catalyzed by basic K2CO3 and a small pinch of KI as reactivity enhancer with a 63 % yield.
N,N-bis(2-chloroethyl) aniline (2) was prepared by refluxing with excess of POCl3 for 24 h
with 92 % yield. Then it was condensed with diazotized nitroaniline in the presence of
sodium acetate to get N,N-bis(2-chloroethyl)-4-((4-nitrophenyl)diazenyl)aniline (3) with
85 % yield. Finally, the receptor AZQU was prepared with good yield by using precursor 3
with 8-hydroxyquinoline in the presence of K2CO3/KI as catalyst. The synthesized AZQU
1
was characterized by FT-IR, H NMR, 13C NMR and mass spectra.
3.1 Colorimetric Investigation
The free receptor AZQU formed a red color solution in acetonitrile/water medium and with
the addition of cadmium ion the red color suddenly turned pale yellow. No significant color
change was observed on the addition of other studied metal ions (Fig. 1). This feature
shows that the receptor AZQU acts as a naked eye sensor for the cadmium ions.
3.2 Photophysical Studies of Receptor AZQU
The photophysical behavior of receptor AZQU were investigated with several metal
cations: Na?, Al3?, K?, Mn2?, Fe2?, Ni2?, Cu2?, Zn2?, Cd2?, Hg2? and Pb2? as their
perchlorate salts in CH3CN/H2O (80:20, v/v) by using UV–Vis measurement and fluo-
rescence measurement at pH 7.2 (HEPES buffer). The absorption spectrum of AZQU alone
has absorption maxima at 287 and 466 nm. The absorption at 466 nm is assigned to
n - p* transition of the azobenzene chromophore by their strong push–pull effect from
donor nitrogen to the acceptor –NO2 group [23, 24]. Absorption at 287 nm is assigned to a
p - p* transition of the alkylated 8-hydroxy quinoline. Figure 2 shows the absorption
spectral changes of receptor AZQU in CH3CN/H2O (80:20, v/v) upon addition of various
competitive metal ions. The UV–Vis spectra of AZQU clearly shows the formation of a
new absorption band centered at 322 nm in the presence of five equivalents of Cd2? ion,
with a distinct color change from red to pale yellow. However, no appreciable spectral
changes were observed upon addition of competitive metal cations, which indicates that
the selectivity of the receptor AZQU for Cd2? ion over other competitive cations is high
enough.
To investigate the binding behavior, a UV–visible titration experiment was performed,
During titration, the concentration of AZQU was kept constant at 5 lmolÁL-1 and the
moles of Cd2? were varied from 0.25 to 5 lmolÁL-1. Titration of the receptor AZQU with
increasing concentrations of Cd2? revealed that the intensity of the absorption band at
287 nm decreased and a new red-shifted absorption band at 322 nm gradually appeared,
accompanied by three clear isobestic points at 256, 295 and 383 nm, respectively,
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Scheme 1 Synthetic route for receptor AZQU
indicating that AZQU complexes with Cd2? (Fig. 3). The absorption maximum attains
saturation at 322 nm with addition of 5 lmolÁL-1 of Cd2? and also a gradual decrease in
the absorption band at 466 nm reveals that azobenzene takes part in coordination.
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Fig. 1 Pictures of receptor I (100 lmolÁL-1) upon addition of various metal ions (five equivalents) in
CH3CN/H2O (80/20, v/v)
Fig. 2 UV–Vis absorption spectra of receptor AZQU [1 9 10-5 molÁL-1 in CH3CN/H2O (80:20, v/v)] in
the presence of different cations (five equivalents)
3.3 Fluorescence Spectroscopic Studies
Emission spectroscopic studies were carried out to prove the effectiveness of receptor
AZQU as a fluorescent probe for the detection of Cd2?. The fluorescence ability of AZQU
with various metal ions was investigated to examine the selectivity of the receptor by
exciting at 322 nm and its emission was monitored from 350 to 600 nm. The fluorescence
spectra of AZQU with different metal cations are shown in Fig. 4. Free AZQU is ascribed
to a weak fluorescence at 400 nm upon excitation at 322 nm in CH3CN/H2O (80:20, v/v)
because the electron rich oxygen atom of the quinoline unit quenches the fluorescence
behavior of 8-hydroxyquinoline by an effective PET process. Upon the addition of Cd2?
ions to the solution of AZQU, the fluorescence is appreciably enhanced at 400 nm
(/ = 0.18). The fluorescence enhancement of AZQU with Cd2? ion may be attributed by
the chelation-enhanced fluorescence (CHEF) mechanism of 8-hydroxyquinoline in CH3-
CN/H2O (80:20, v/v). Titration of AZQU with increasing concentration of Cd2? ion
revealed that the fluorescent intensity increases linearly with the concentration of Cd2? as
shown in Fig. 5. Addition of competing metal ions in large excess does not produce any
substantial changes in fluorescence intensity. Hence, the receptor is specific for Cd2? only.
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Fig. 3 UV–Vis absorption spectra of receptor AZQU with increasing concentration of Cd2?
(0–100 lmolÁL-1) in CH3CN/H2O (80:20, v/v)
Fig. 4 Fluorescence spectra of receptor AZQU (100 lmolÁL-1) upon addition of various metal ions (five
equivalents) in CH3CN/H2O (80/20, v/v)
We further conducted competition experiments using solutions containing Cd2? and all the
other anions. As can be seen, these competitive species did not lead to any significant
absorption or fluorescence changes of AZQU, because the azo chromophore does not
involve a deprotonation process.
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Fig. 5 Fluorescence spectra of receptor AZQU with increasing concentration of Cd2? ion
(0–100 lmolÁL-1) in CH3CN/H2O (80:20, v/v)
Fig. 6 Job’s plot of a 1:1 complex for receptor AZQU and Cd2? in CH3CN/H2O
Stoichiometric analysis of the complex between receptor AZQU and Cd2? was per-
formed by means of continuous variation by using Job’s method [25] where the absorption
of complex at 322 nm was plotted against mole fraction of AZQU under the condition of
an invariant total concentration (Fig. 6). Job’s plot depicts a maximum absorption center at
0.5 mol fraction of Cd2?, further supporting 1:1 complex formation between AZQU and
Cd2?. From UV–Vis absorption measurements, the association constant (Ka) of Cd2?
complex with AZQU was calculated using the Benesi–Hildebrand (B–H) equation [26] and
it was found to be Ka = 1.68 9 105 LÁmol-1 (Fig. 7).
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Fig. 7 Benesi-Hildebrand plot for complexation of receptor AZQU with Cd2?
Fig. 8 FT-IR spectra of receptor AZQU with Cd2? ion
1
3.4 FT-IR and H-NMR Spectroscopic Studies for Receptor AZQU
The detailed complexation behavior of receptor AZQU with Cd2?, studied by FT-IR and
1H-NMR, was performed by adding different equivalent of Cd2? to AZQU. Coordination
sites of receptor AZQU with Cd2? were identified by IR spectra in the region around
1000–3000 cm-1. In the IR spectra (Fig. 8), the stretching frequency of C=N in the
quinoline ring appears at 1642 cm-1, but coordination with Cd2? results in a shift from
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Fig. 9 Partial 1H-NMR in CH3CN-d3: (i) receptor AZQU only, (ii) receptor AZQU ? 0.5 equivalents of
Cd2? and (iii) receptor AZQU ? 1 equivalent of Cd2?
Scheme 2 Proposed binding mode of receptor AZQU with Cd2?
1642 to 1636 cm-1 ascribed to electron transfer from the electron rich quinoline nitrogen
to the Cd2? ion, which clearly gives information about coordination of the Cd2? ion with
AZQU.
To further elucidate the precise coordination site of receptor AZQU for Cd2?, 1H-NMR
1
spectra were measured. Figure 9 shows the stacked plots of H-NMR spectra of AZQU
(10 mmolÁL-1) in the absence and also in the presence of various equivalents of Cd2?
.
Before addition of Cd2?, all individual proton signals of AZQU were assigned. Upon
complexation with 0.5 equivalents of Cd2?, there is a significant upfield shift in the
aliphatic methyl proton of receptor, from 4.31 ppm (a) and 4.62 ppm (b) to 3.17 ppm (a*)
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and 3.59 ppm (b*), respectively, indicating the oxygen atom of methoxy group is strongly
bonded with Cd2? ions. Similarly, the proton adjacent to the quinoline nitrogen undergoes
a downfield shift from 8.88 ppm (e) to 9.30 ppm (e*) due to change in the electron density
of quinoline by coordination with Cd2? (Scheme 2).
The tertiary nitrogen of the azo chromophore also participated in coordination with
Cd2?. This can be identified because the ortho proton (c) of aniline shifts from 7.04 ppm
(c) to 8.06 ppm (c*). This gives prominent results about electron density of the tertiary
amine being transferred to the lower electron density of Cd2? ions [27, 28].
4 Conclusions
In conclusion, a new bis-quinoline appended azobenzene receptor AZQU has been suc-
cessfully synthesized and its dual mode in sensing with Cd2? was studied by colorimetric
as well as fluorimetric techniques. Upon additions of Cd2? there is a color change from red
to pale yellow due to hindrance of ICT mechanisms in the azo chromophore, which can be
identified by the naked eye and by an increase in fluorescence due to the CHEF effect by
the two quinoline moieties. Similarly, fluorescence sensors could be further developed and
modified for detection of cations in living cells.
Acknowledgments The author E.H thanks thank the Department of Science and Technology (DST), India
for financial support (DST-SR/S1/PC-07/2010). The authors P.K and R.M are thankful to the Anna Cen-
tenary Research Fellowship (ACRF), Anna University, Chennai (CR/ACRF/2013/38). The instrumentation
facility was provided to our department through DST-FIST and UGC-DRS from New Delhi.
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