Novel chemosensor for multiple target anions: The detection of F- and CN- ion via different approach

Article abstract of DOI:10.1016/j.jfluchem.2015.04.014

A novel chemosensor based on azo linked Schiff base (receptor 1) was synthesized by simple condensation method. The sensor studies of receptor 1 disclose the selectivity toward fluoride and cyanide ion with a color change among the other common anions in

Full text of DOI:10.1016/j.jfluchem.2015.04.014

Journal of Fluorine Chemistry 175 (2015) 180–184
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Novel chemosensor for multiple target anions: The detection of FÀ and
CNÀ ion via different approach
b
Duraisamy Udhayakumari a, Sivan Velmathi a, , Maria Susai Boobalan
*
a Department of Chemistry, National Institute of Technology, Organic and Polymer Synthesis Laboratory, Tiruchirappalli 620 015, India
b Department of Chemistry, St. Joseph’s College (Autonomous), Tiruchirappalli 620002, India
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel chemosensor based on azo linked Schiff base (receptor 1) was synthesized by simple
condensation method. The sensor studies of receptor 1 disclose the selectivity toward fluoride and
cyanide ion with a color change among the other common anions in aqueous medium. The binding
affinity of receptor 1 was also enlightened by 1H NMR titration and DFT calculation.
ß 2015 Elsevier B.V. All rights reserved.
Received 10 February 2015
Received in revised form 30 March 2015
Accepted 18 April 2015
Available online 12 May 2015
Keywords:
Azo dye
Nucleophilic addition
Cyanide detection
Aqueous solution
Colorimetric
1. Introduction
sensor that can sense differentially FÀ and CNÀ ion has rarely been
reported [21–23]. Recently we reported azo Schiff base [24,25] to
Selective sensing of anions has received considerable attention
over recent years due to their important role in the biological
process, medicinal chemistry, environmental chemistry and
catalysis. In addition, colorimetric chemosensors have many
advantages like low cost, suitability for biological concern, easy
detection and high selectivity [1–5]. During the last few years, a
number of colorimetric anion sensors have been developed using
urea, thiourea, amide and pyrrole based binding units. Most of the
receptors sense different anions such as fluoride, acetate, nitrate,
dihydrogen phosphate and hydroxyl group [6–12]. Cyanide is one
of the most toxic anion, harmful to the environment. It plays an
important role in various industrial processes, for example gold
mining, synthetic fibers and resins [13–17]. On the other hand,
fluoride is a biologically important anion due to its wide role in
dental care and treatment of osteoporosis [18–20]. Novel
colorimetric cyanide ion sensors, which allow naked eye detection
of the color change without resorting to the use of expensive
instruments is most attractive approach. However, finding a
simple receptor selectively sensing cyanide anion in aqueous
medium is still a challenge. To the best of our knowledge, a simple
act as a colorimetric, fluorescent chemosensors for FÀ/AcOÀ ions in
organic and semi-aqueous medium. In continuation of our
research interest in the chemosensors, we herein report the
application of receptor 1 toward sensing of CNÀ in pure aqueous
medium.
In this paper, we report a simple receptor which can selectively
detect cyanide in aqueous medium via colorimetric and UV–vis
spectroscopic method. With appropriate analysis, it is possible to
detect fluoride and cyanide ions separately in the same sample by
tuning the solvent medium. The absorption spectra indicated the
formation of complex between receptor and anions in 1:1
stoichiometric ratio with micromolar detection limit. The interac-
tion mechanism between the receptor 1 with fluoride and cyanide
was investigated using 1H NMR titration experiment.
2. Materials and methods
All reagents used were obtained commercially and used
without further purification. In the titration experiments, all the
anions were added in the form of tetrabutylammonium (TBA) salts
and analytical grade solvents such as acetonitrile (CH3CN) and
ethanol (EtOH) were purchased commercially. Shimadzu UV-2600
UV-vis spectrophotometer was used to record UV–visible spectra
using quartz cell with 1 cm path length. 5 Â 10À5 M solution of
receptor 1 in CH3CN and 1.5 Â 10À3 M solutions of the anions in
*
Corresponding author. Tel.: +91 431 2503640/9486067404;
fax: +91 431 2500133.
E-mail address: velmathis@nitt.edu (S. Velmathi).
http://dx.doi.org/10.1016/j.jfluchem.2015.04.014
0022-1139/ß 2015 Elsevier B.V. All rights reserved.

D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
181
CH3CN, H2O were prepared. 0.2 equiv. (20
guest solution was added to 3 mL of R1 taken in the UV cuvette.
m
L)–2 equiv. (200
m
L) of
(0%, 20%, 40%, 60%, 80% and 90% water, respectively) with receptor
1 (CH3CN). 20% aq solution of FÀ resulted in a marginal color
change, but did not show any optical changes (Fig. S5). So the
outcome of the second titration indicates, receptor can sense
fluoride, only in organic medium and not in aqueous medium. In
the third titration, in order to gain systematic knowledge on the
effect of water on the binding affinity and selectivity for cyanide
(100% H2O), we took receptor 1 in water–acetonitrile mixtures (0%,
20%, 40%, 60%, 80% and 90% water) and CNÀ ion in 100% H2O. When
receptor 1 was taken in 20:80 water–acetonitrile mixtures as
solvent, CNÀ showed obvious interaction with the receptor 1. The
color changed to green upon addition of CNÀ, correspondingly in
optical spectrum the band at 350 nm shifted (red shift) to 390 nm.
When increase the percentage of water from 40% to 90%, same
result was observed in color change and UV–vis change (Fig. 2a and
b). From these results, it is confirmed that cyanide can be
selectively sensed in 90% aqueous medium.
3. Results and discussion
The synthesis of chemosensor (receptor 1) is outlined in
Scheme 1. 1 mmol of 4-amino azobenzene (0.200 g) and 1 mmol of
2-hydroxy-5-(phenyldiazenyl) benzaldehyde (0.229 g) in ethanol
(10 mL) was reflux for 2–3 h at 85 8C. After cooling, the precipitate
formed was filtered, washed, recrystallized from EtOH and dried in
vacuum oven. Yield: 85%, mp: 160 8C. 1H NMR (300 MHz, DMSO-d6
d
ppm): 9.25 (s, 1H, OH), 8.37 (s, 1H, CH55N), 8.02–8.07 (d, 3H, Ar,
J = 15 Hz), 7.87–7.94 (m, 4H, Ar, J = 5.7 Hz), 7.55–7.71 (m, 9H, Ar,
J = 8.4 Hz), 7.18–7.21(d, 1H, Ar, J = 9 Hz). HRMS: Calcd: 406.1600,
Found: 406.1660 (Figs. S1 and S2).
The sensing behavior of receptor 1 with anions was observed by
‘‘naked-eye’’ and detection was further confirmed by UV–vis
spectrophotometer. In the presence of different counter anions,
receptor 1 showed selective sensing of CNÀ in aqueous environ-
ment. To investigate the anion recognition property of receptor 1,
four different types of colorimetric and UV–vis titrations were
carried out. The binding affinity of receptor 1 (5 Â 10À5 M in
CH3CN) in the presence of various anions such as FÀ, ClÀ, BrÀ, AcOÀ,
H2PO4À, HSO4À, HOÀ and CNÀ (1.5 Â 10À3 M in CH3CN) was carried
out. Fig. 1a and b shows the color changes and the absorption
spectrum of receptor 1 with different anions in CH3CN. The solution
of receptor 1 resultedinan immediatecolor change fromcolorless to
orangeforFÀ, yellowcolor for AcOÀ and green color forCNÀ ions. The
other ions like ClÀ, BrÀ, H2PO4À, HSO4À and HOÀ did not induce any
color changes and absorption changes even in large excess. The
incremental addition of FÀ and CNÀ shows a band at 450 nm
increased, at the same time the band at 350 nm decreased with the
clear and single isosbestic point at 385 nm (Figs. S3 and S4).
In second titration, the effect of water on the binding affinity for
fluoride was carried out in different water–acetonitrile mixtures
In general, hydration will terminate the anion sensing behavior,
water the strong competitive solvent in the hydrogen bonding
and lead to the leaching of anions and regenerate the receptor
molecule. We chose receptor 1 in 90% H2O (5 Â 10À5 M) and
anions in 100% H2O (1.5 Â 10À3 M), only CNÀ ion showed obvious
interaction with the receptor 1 (Fig. 3a). The color changed to
green and the band shifted to 350–390 nm (40 nm) only for CNÀ
ion (Fig. 3b). Increasing the amount of water content is avoiding
completely the interfÀerence of FÀ and ACOÀ ions. The anions such
as FÀ, AcOÀ, H2PO4 and HSO4 should interact with water
À
through hydrogen-bonding leading to a large decrease in their
nucleophilicity. While cyanide has weaker hydrogen-bonding
ability and stronger nucleophilicity toward the carbonyl group.
Cyanide has much weaker hydrogen bonding ability in
comparison with other anions like FÀ and AcOÀ and stronger
nucleophilicity toward the imine group [26–28], which results
in the addition of CNÀ ions to the carbon atom of an electron
deficient imine group and, subsequently, fast proton transfer of
Scheme 1. Synthetic route of receptor 1.
Fig. 1. (a) The corresponding color changes and (b) absorption spectra of receptor 1 when treated with 2 equiv. of various anions (FÀ, ClÀ, BrÀ, AcOÀ, H2PO4À, HSO4À, HOÀ and
CNÀ) in CH3CN.

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D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
Fig. 2. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water–acetonitrile mixtures (0%, 20%, 40%, 60%, 80% and 90% water, respectively) when
treated with 2 equiv. of CNÀ ion in 100% H2O.
Fig. 3. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water (90%)–acetonitrile (10%) mixtures when treated with 2 equiv. of various anions
(FÀ, ClÀ, BrÀ, AcOÀ, H2PO4À, HSO4À, HOÀ and CNÀ) in 100% H2O.
the phenol hydrogen to the neighboring nitrogen anion through
an intramolecular hydrogen bond.
The detection of anions by receptor 1 is further supported by 1H
NMR titration. 1H NMR titrations were carried out in DMSO-d6.
Tetra butyl ammonium salt of fluoride and cyanide were used as
anions sources. Without the addition of anions, receptor 1 showed
Binding constant (Ka) for all anion complexes of the receptor 1
were calculated using Benesi–Hildebrand plot and stoichiometry
of the complex was calculated using Job’s plot. The detection limit
(LOD) of FÀ and CNÀ ions by receptor 1 was calculated using the
formula, LOD = 3s/m. Where ‘‘s’’ the standard deviation and ‘‘m’’ is
the slope obtained in the plot of absorbance vs. concentration of
anions. The detection limit value implies that the receptor 1 can
easily detect FÀ and CNÀ at micro molar level in both organic and
aqueous medium. Binding constants and detection limit are
calculated with repeated titrations and the values are given in
Table 1. The receptor 1 was compared with some reported
chemosensors for CNÀ (Table 2). While each chemosensor showed
some advantages such as easy synthesis, high sensitivity, no
interference and naked eye detection. Our receptor 1 exhibited
quite appealing analytical features such as easy synthesis, high
selectivity and naked-eye sensing (R155CNÀ) [29–34].
Density Functional Theory (DFT) calculations have been used to
understand the behavior of receptor 1 with the FÀ and CNÀ ions.
Highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) for receptor 1 and their complexes
have been generated from the optimized structures of the receptor
signals at
d 8.37 ppm due to HC55N proton and 9.25 ppm
corresponds to OH proton. After the addition of 2 equiv. of FÀ
ions, the resonance signal of –OH proton becomes disappeared
(Fig. 4). This might be due to the formation of hydrogen bonds
between the anions and the –OH group of receptor 1. This indicates
that deprotonation of the receptor 1 can possibly occur. In addition,
the signals correspond to imine and aromatic protons were shifted
to upfield with the addition of FÀ ions. The 1H NMR titration studies
sustain the proposed sensing mechanism of receptor 1 with FÀ ions
(Scheme 2). But in the case of CNÀ ions to receptor 1, the –OH signal
of receptor 1 disappeared and a new peak at 6.36 ppm started
appearing. All the aromatic protons were shifted to upfield. This
result strongly supports the nucleophilic attack of CNÀ ions toward
the carbon atom of an electron deficient imine group of receptor 1
(Scheme 3).
Fig. 4. 1H NMR spectrum of receptor 1 recorded in DMSO-d6 in the absence and presence of FÀ and CNÀ ions.

D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
183
Scheme 2. The proposed sensing mechanism for receptor 1 with fluoride.
Scheme 3. The proposed sensing mechanism for receptor 1 with cyanide.
1 and their respective anion complexes (Fig. 5). The structural
optimization and the computational calculations were carried
out using Gaussian09 quantum chemistry package and results
were viewed with GaussView5 GUI. The appropriate choice of
model chemistry for the computation of electronic structure
property is indispensable. In the present scenario, Density
Functional Theory (DFT) supported B3LYP/6-31G (d, p) model
chemistry was implemented particularly for Schiff base ligands
under gas phase condition. DFT calculations were carried out to
investigate the change of the absorption spectra upon the
addition of FÀ and CNÀ. From the energy level diagram (Fig. 6), it
can be seen that the addition of the anions lead to the
stabilization of the HOMO and LUMO of the sensor molecule.
From the calculated results, the energy levels of HOMO and
LUMO of receptor 1 increased after the addition of FÀ and CNÀ
ions when compared to that of the free receptor 1 (Table 3). For
FÀ and CNÀ complexes of receptor 1, the absorption bands
predicted by the theoretical methods is red shifted compared
with those of the independent probes, which is in good
agreement with the experimental observations.
Table 1
Binding constant, stoichiometry, detection limit value of the complex formed for receptor 1 with CNÀ and FÀ ion.
S. no
Receptor + ions
Solvent system
Binding constant (MÀ1
)
Stoichiometry
LOD (MÀ1
)
1
2
3
R1 + FÀ
R1 + CNÀ
R1 + CNÀ
100% CH3CN
4.6 Â 104
3.3 Â 105
5.5 Â 104
1:1
1:1
1:1
1.5 Â 10À6
1.9 Â 10À6
5.2 Â 10À6
100% CH3CN
90% H2O (R1) 100% H2O (CNÀ
)
Table 2
Comparison of some of the reported chemosensors for CNÀ with the present work.
Sensors
Solvent system
Sensing ions
LOD
Binding constant
(MÀ1
Linear range
Reference
[29]
)
(R2)
Thiazole based sensor
DMSO/bis-tris buffer (8:2)
CNÀ
AcOÀ
CNÀ
CNÀ
CNÀ
CNÀ
60
m
M
M
3.2 Â 102
1.0 Â 102
–
0.994
140
m
Azo based sensor
H2O
0.01–0.07
m
M
–
[30]
[31]
[32]
[33]
Azo coupled tripodal based sensor
Azo coupled NBD unit based sensor
Bis-pyrene derivative based sensor
CH3CN:DMSO:HEPES (93:1:6)
EtOH:H2O (3:7)
0–1.9
1.5
m
m
m
m
m
m
M
M
2.3 Â 103
0.990
0.990
0.990
–
CH3CN:DMSO (99:1)
1.79
M (R1)
M (R2)
M
5.89 Â 104 (R1)
3.98 Â 108 (R2)
6.80–8.96
6.31–8.50
4.6 Â 104
5.5 Â 104
2.43
3.31
4.97
Imidazo-anthraquinones based sensor
Azo linked Schiff base sensor
CH3CN:H2O (97:3)
CNÀ
FÀ
–
[34]
M
CH3CN
FÀ
1.5
mM
0.993
This work
CH3CN:H2O (1:9, v/v)
CNÀ
1.9–5.2 mM
Table 3
Energies of HOMO and LUMO of the R1 and complexes.
R1 and complexes
HOMO (eV)
LUMO (eV)
DE (eV)
Theoretical wavelength (nm)
Observed wavelength (nm)
R1
À5.774
À3.434
À3.282
À2.361
À0.807
À0.633
3.413
2.627
2.649
363
471
350
354
450
385
R1 + FÀ
R1 + CNÀ

184
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
Fig. 5. Optimized structures of (a) receptor 1, (b) receptor 1 + FÀ and (c) receptor 1 + CNÀ
.
Fig. 6. Energy levels of various HOMO and LUMO’s of receptor 1 in the absence and presence of FÀ and CNÀ
.
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distinct color changes, where FÀ ion shows colorless to orange and
CNÀ ion shows colorless to green. Further, receptor 1 showed
selective detection of cyanide ion in 90% aqueous medium based
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Acknowledgment
Authors express their thanks to DRDO (ERIP/ER/1006004/M/01/
1333 dated 23-05-2011) for financial assistance in the form of a
major sponsored project.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.jfluchem.2015.04.014.
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