Ratiometric fluorescent chemosensor for fluoride ion based on inhibition of excited state intramolecular proton transfer

Article abstract of DOI:10.1016/j.saa.2014.11.026

ESIPT based benzimidazole derivative has been synthesized and investigated their photophysical behavior towards various anions. The probe 2 has been used for selective estimation of F^- ions as compared to other anions and signaled the binding event through formation of new absorption band at 360 nm and emission band at 420 nm. The probe 2 showed fluorescence behavior towards fluoride ions through hydrogen bonding interactions and restricted the ESIPT emission at 540 nm from OH to nitrogen of benzimidazole moiety to release its enol emission at 420 nm.

Full text of DOI:10.1016/j.saa.2014.11.026

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ratiometric fluorescent chemosensor for fluoride ion based on inhibition
of excited state intramolecular proton transfer
⇑
Akul Sen Gupta, Kamaldeep Paul, Vijay Luxami
School of Chemistry and Biochemistry, Thapar University, Patiala 147004, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis of hydroxyaryl-
benzimidazole based ESIPT sensor.
Ratiometric chromofluorescent
sensor for fluoride ions.
Broad range of fluoride ion detection
limit.
Practical usability of probe 2 by dip
coating method.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2014
Received in revised form 21 October 2014
Accepted 7 November 2014
Available online 15 November 2014
ESIPT based benzimidazole derivative has been synthesized and investigated their photophysical behav-
ior towards various anions. The probe 2 has been used for selective estimation of Fꢁ ions as compared to
other anions and signaled the binding event through formation of new absorption band at 360 nm and
emission band at 420 nm. The probe 2 showed fluorescence behavior towards fluoride ions through
hydrogen bonding interactions and restricted the ESIPT emission at 540 nm from OH to nitrogen of benz-
imidazole moiety to release its enol emission at 420 nm.
Keywords:
ESIPT
Ó 2014 Elsevier B.V. All rights reserved.
Chemosensor
TD-DFT
Fluorescence
Benzimidazole
Introduction
health, thus intake of acute amount of it is beneficial to the treat-
ment of osteoporosis and dental health [1–3]. But an overdose of
Fluoride ion, being the smallest and most electronegative anion
has received much attention due to its profound effects on human
fluoride ion can cause many problems such as bone disorder, colla-
gen breakdown, thyroid activity and depression [4]. Thus, to deter-
mine the level of fluoride ions, various detection methods have
been reported including highly accurate but expensive analytical
instruments that use ion selective electrodes and chromogenic
and fluorescent probes [5–7]. Fluoride ions can contaminate the
human body and other materials in the surroundings during the
manufacture of rocket fuel and fireworks [8]. Detection of these
traces is a major concern in the field of analytical and forensic
Abbreviations: ESIPT, excited state intramolecular proton transfer; TMS, tetra
methyl silane; ASC II, American Standard Code for Information Interchange; B3LYP,
Becke, 3-Parameter, Lee–Yang–Parr; TD-DFT, Time Dependent-Density Functional
Theory; Mpt., melting point; Excel TM, Microsoft Excel Trade Mark; 6-31G(d,p),
basis set method-double diffuse function.
⇑
Corresponding author.
E-mail addresses: vluxami@thapar.edu, vj_luxami@yahoo.co.in (V. Luxami).
http://dx.doi.org/10.1016/j.saa.2014.11.026
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

68
A.S. Gupta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
sciences. Colorimetric sensors do not need any spectroscopic
instrument since colorimetric changes can be detected by the
naked eye. But use of such sensors is limited because of their low
absorption. Fluorescent sensors based on excited-state intramolec-
ular proton transfer (ESIPT), as seen from 2-(2-hydroxyphenyl)
benzimidazole/thiazole/oxazole have been attracted more atten-
tion [9]. ESIPT sensors exhibit dual emissions from both the excited
enol and keto tautomers [10]. Fluorescent sensing of fluoride ion
could realize by prohibiting ESIPT through coordination or depro-
tonation induced by fluoride with ESIPT centers, resulting in
detectable spectral change. Compared with widely used photoin-
duced electron transfer (PET) mechanism for sensing [11], the fluo-
rescent sensors based on ESIPT can afford many advantages
including dual fluorescence intensity changes and large stokes
shift, thus the excited state intramolecular proton transfer (ESIPT)
has attained a special importance due to vast applications as laser
dyes, UV-photostabilizer, scintillators, membrane and protein
probes and as a potential component for photo switches and
organic LEDs. ESIPT usually occur in the molecules bearing an
H-bond donor group usually a phenolic/amino moiety associated
with basic site (O, N, S) in the ground state through H-bond
interactions. This proton which is covalently attached in the elec-
tronically excited state migrates to a neighboring hydrogen-
bonded atom which is less than 2 Å. The significant amount of
excited state energy is dissipated in this process. The formed
phototautomer emits light at a lower energy with an unusually
large Stokes shift (range from 100 to 500 nm) and thermally
equilibrates back to the ground state with the proton bound to
its original atom. This large stokes shift is an important property
for fluorophores because the self-absorption, or the inner filter
effect, can be avoided and the fluorescent analysis can also be
improved. They can detect metal ions with ratiometric fluores-
cence response, providing self calibration function and avoiding
photodamage, scattering light and strong interference derived
from short wavelength emission in biological media.
1 and 2 was prepared at 10ꢁ3 M in distilled CH3CN. Tetrabutylam-
monium salts were used for the anions. All absorption and fluores-
cence scans were saved as ASC II files and further processed in
Excel™ to produce all graphs shown. Solutions of 1 and 2 were typ-
ically 20
lM for UV–Vis studies. Stability constants were deter-
mined using Benesi–Hildebrand Equation [15] (Eq. (1))
1
1
1
¼
þ
ð1Þ
n
K½Fꢁꢂ ðAmax ꢁ A0Þ
ðA ꢁ A0Þ ðAmax ꢁ A0Þ
where A0, A, Amax are the absorption considered in the absence of
ion, at an intermediate, and at a concentration of saturation. K is
binding constant, [Fꢁ] is concentration of ion. n is the stoichiometric
ratio.
Computational methods
The ground state (So) geometry of probe 2 was optimized using
Density Functional Theory (DFT) [16]. The functional used was
B3LYP and basis set used for all atoms were 6-31G(d,p). The verti-
cal excitation energy and oscillator strengths at the ground state
equilibrium geometries were calculated using with the same
hybrid functional and basis set. The low-lying first singlet excited
state (S1) of the probe 2 was relaxed using to obtain its minimum
energy geometry. Frequency computations were also carried out
on the optimized geometry of the low-lying vibronically relaxed
first excited state of the probe 2 and probe 2.Fꢁ.
Synthesis of probes 1 and 2
Probe 1 – 3,4 Diaminobenzophenone (500 mg, 2.3 mmol) and
benzaldehyde (374 mg, 3.5 mmol) were dissolved in 10 mL
nitrobenzene. The reaction mixture was refluxed for about 24 h,
and then cooled to room temperature. The solid separated was fil-
tered by using sintered crucible with the help of vacuum filtration
method and washed with diethyl ether to obtain colorless solid of
probe 1 (225 mg, 89% yield); Mpt. 220–221 °C; 1H NMR (CDCl3 + -
DMSO-d6, 400 MHz): d 8.26–8.25 (m, 2H, Ar), 8.17 (s, 1H, Ar), 7.85–
7.76 (m, 4H, Ar), 7.66–7.48 (m, 7H, Ar), 4.97 (bs, NH); 13C NMR
(CDCl3 + DMSO-d6, 100 MHz): d 195.52 (C@O), 137.50, 130.94,
128.72, 127.90, 127,17, 123.08, 120.82, 117.33, 113.28, 110.96
(Ar H); (Figs. S1 and S2). MS (ESI) m/z 299 (M++H) (Fig. S3).
Probe 2 – 3,4 Diaminobenzophenone (1 g, 4.7 mmol) and sali-
cylaldehyde (630 mg, 5.1 mmol) were dissolved in 30 mL nitroben-
zene. The reaction mixture was refluxed for about 24 h, and then
cooled to room temperature. The solid separated was filtered and
washed with diethyl ether to obtain yellow colored solid which
was further purified by column chromatography to get pure probe
2 (528 mg, 92% yield); Mpt. 240–241 °C; 1H NMR (CDCl3 + DMSO-
d6, 400 MHz): d 12.57 (bs, 1H, NH), 9.63 (bs, 1H, OH), 8.35 (s, 1H,
ArH), 7.96 (s, 1H, ArH), 7.88 (d, 1H, J = 8.28 Hz, ArH), 7.61–7.57
(m, 4H, ArH), 7.29–7.24 (t, 1H, J = 9.16 Hz, ArH), 7.14–7.10 (t, 1H,
J = 11.0 Hz, ArH), 7.00–6.96 (t, 1H, J = 7.36 Hz, ArH), 6.86–6.79 (m,
1H, ArH), 5.77 (s, 1H, ArH); 13C NMR (CDCl3 + DMSO-
d6,100 MHz): d 195.56 (C@O), 158.29, 137.86, 131.68, 129.31,
127.90, 126.06, 124.82, 124.29, 120.72, 118.71, 117.09, 113.75,
112.06, 110.97 (ArH); (Figs. S4 and S5). MS (ESI) m/z 314.11
(M++H) (Fig. S6) (see Scheme 1).
We have recently reported some ESIPT based cation and anion
sensors based on hydroxyarylbenzimidazole moiety [12]. In the
present manuscript ESIPT fluorescent probe 2 has been synthesized
with different strategy other than reported scheme [13] which
showed sensitivity changes towards fluoride ions. The presence
of fluoride ions caused decrease in absorption intensity at
335 nm and immergence of new absorption band at 360 nm. How-
ever, in case of emission presence of fluoride ion caused quenching
of keto tautomer at 540 nm and enol tautomer band is released at
420 nm.
Experimental section
Materials and equipments
All chemicals were purchased from Loba and Sigma Aldrich
Chemical Co and used without further purification. Column chro-
matography was performed using silica gel (60–120 mesh). All
reactions were monitored by thin layer chromatography. Chloro-
form: Methanol was the adopted solvent system. Melting points
were carried out by the open capillary tube method [14] and
uncorrected. 1H NMR and 13C NMR spectra were carried out using
a JEOL ECS-400 MHz spectrometer in SAI Labs, Thapar University,
Patiala with TMS as an internal reference. Mass Spectra of the syn-
thesized compounds were recorded at MAT 120 in SAIF, Punjab
University. All chemical shifts are reported in ppm relative to the
TMS as an internal reference. UV–Vis studies were carried out on
a Specord PC machines using slit width of 1.0 nm and matched
quartz cells. Fluorescence spectra were determined on a Varian
Cary Eclipse fluorescence spectrometer. Stock solution of probes
Results and discussion
Photophysical properties of probes 1 and 2
The UV–Vis spectroscopic properties of probe 1–2 were investi-
gated in CH3CN at 20
lM. Probe 1 and 2 displayed an absorption
band centered at 320 nm and 335 nm respectively. Addition of

A.S. Gupta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
69
O
intensity at 335 nm and increase at 360 nm provided the opportu-
nity to determine fluoride ions ratiometrically. The absorption
ratio at these two wavelengths varied from 0.1356 to 1.383, indi-
cating 10.19 fold absorption ratio change. Thus, probe 2 can be
used to estimate a wide range of fluoride ions between 2 and
H
O
HN
O
N
R
120 o
Nitrobenzene
C
R
+
2400
acetate ions new band at 272 and 350 nm appeared with addition
of higher equivalents of acetate ions i.e. 1000 M (Fig. S8).
lM through ratiometric approach (Fig. 2(b)). But in case of
H2N
NH2
1. R = H
l
2. R = OH
UV-Vis absorption changes of probe 1 in the presence of fluo-
ride ions were similar as that of probe 2, which clearly indicated
that both molecules in the ground state caused NH deprotonation
which was also supported through theoretical calculations.
Scheme 1. Synthesis of probes 1–2.
different tetrabutylammonium anions viz. CNꢁ, Fꢁ, Clꢁ, Brꢁ, AcOꢁ,
Iꢁ, HSOꢁ4 , OHꢁ, H2POꢁ4 , etc. caused no significant change except
presence of fluoride ions in case of probe 1 (Fig. S7). Incremental
addition of fluoride ions to probe 1 caused the decrease in absorp-
tion intensity at 320 nm and formation of new absorption band at
360 nm with two isobestic points at 275 and 335 nm on addition of
The probe 2 (20 lM, CH3CN) on excitation at 340 nm displayed
keto emission band at 540 nm. On addition of various anions like
CNꢁ, Fꢁ, Clꢁ, Brꢁ, AcOꢁ, Iꢁ, HSO4ꢁ, OHꢁ, H2POꢁ4 , etc., no significant
change was observed except fluoride and acetate ions (Fig. 1(b)).
Upon incremental addition of fluoride ions keto emission band at
540 nm was quenched and new emission band at 420 nm was
formed. The new emission band at 420 nm was typically due to
enol emission because presence of fluoride caused deprotonation
at –OH group which restrict the ESIPT phenomenon and thus
released the enol emission at 420 nm (Fig. 3(a)). The binding con-
stant was calculated to be 1.148 ꢃ 104 Mꢁ1 with lowest detection
1000 lM solution of fluoride ions (Fig. S10). Addition of different
tetrabutylammonium anions viz. CNꢁ, Fꢁ, Clꢁ, Brꢁ, AcOꢁ, Iꢁ, HSO4ꢁ,
OHꢁ, H2POꢁ4 , etc. caused no significant change except presence of
fluoride and acetate ions in case of probe 2 (Fig. 1(a)). Upon incre-
mental addition of fluoride ions, absorption intensity at kmax
335 nm decreased gradually with simultaneous increase in new
absorption band at 360 nm with well defined isobestic point at
340 nm and 270 nm (Fig. 2(a)). The presence of isobestic points
showed that different species are in equilibrium and there is no
stepwise formation of complexed species. The binding constant
limit of 2
appeared with addition of higher equivalents of acetate ions i.e.
1000 M (Fig. S9).
lM. But in case of acetate ions new band at 420 nm
l
The gradual addition of fluoride ions caused decrease in emis-
sion intensity of keto tautomer at 540 nm and increase in emission
intensity of enol tautomer at 420 nm and thus provided the oppor-
tunity to determine fluoride ions ratiometrically. The emission
was calculated to be 6.6 ꢃ 103 Mꢁ1 with detection limit of 2
lM.
The gradual addition of fluoride ions caused decrease in absorption
Fig. 1. Effect of various anions on (a) absorption spectra and (b) emission spectra of probe 2 (20 lM, CH3CN).
Fig. 2. (a) Effect of addition of fluoride ions on absorption spectrum of probe 2; (b) plot of absorption intensity ratio between 360 and 335 nm (A360/A335) vs [Fꢁ] ions of probe
2 at 20 M in CH3CN.
l

70
A.S. Gupta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
Fig. 3. (a) Fluorescence spectra of probe 2 (20
l
M, kex = 340 nm) CH3CN upon complexation with increasing concentration of Fꢁ ions. (b) Plot of emission intensity ratio
M in CH3CN.
between 420 and 540 nm (I420/I540) vs [Fꢁ] ions of probe 2 at 20
l
mixed with 50 equiv. of each of the various metal ions. The com-
plex formed in the presence of acetate ions is very week as com-
pared to fluoride complex. Whereas, no significant variation in
the fluorescence intensity was found when comparing the results
with or without the other anions (Fig. 4).
Practical application of probe 2 by dip coating technique
In order to check the practical applicability of probe 2, TLC
strips were prepared by dip-coating a solution of probe 2, followed
by drying them in a vacuum in order to check for residual contam-
ination in contact mode. We prepared several samples of solution-
coated TLC strips and studied the response of their fluorescence
towards fluoride ions in contact mode and solution phase. Solution
of fluoride ions were placed over coated TLC strips for 5 s. Upon
illumination with a UV lamp, blue spots were observed in the con-
tact area (Fig. 5). The minimum amount of fluoride ions detectable
by the naked eye was as low as, thereby registering a detection
limit at the 10ꢁ6 M.
Fig. 4. Red bars represent selectivity of probe
2 (20 lM, kex = 340 nm) upon
addition of different metal ions in CH3CN and blue bars shows the competitive
selectivity of probe 2 in the presence of interfering metal ions. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
Theoretical studies
ratio at these two wavelengths varied from 0.376 to 4.058, indicat-
ing 10.86 fold emission ratio change. Thus, probe 2 can be used to
To explore the origin of each induced absorption band, theoret-
ical calculation were employed to optimize the structure through
DFT and TD calculation with B3LYP-631G methods. The probe 2
was optimized using DFT calculations. It was found that O-H bond
acquires partial double bond character and hydrogen of O-H orient
itself to the closer vicinity to imine Nitrogen of benzimidazole moi-
ety (Fig. 6). The presence of fluoride ion caused deprotonation at
hydroxyl group and acquired keto form.
estimate a wide range of fluoride ions between 2 and 75
through ratiometric approach (Fig. 3(b)).
lM
In case of probe 1, absence of hydroxyl group led to exhibition
of very weak enol emission at 420 nm with lesser stokes shift of
ꢄ90 nm than probe 2 of ꢄ200 nm at higher concentration. These
studies clearly predicted that –OH participates in ESIPT
phenomenon.
The Theoretical calculation of probe 2, showed that band at
330 nm was due to HOMO to LUMO+1, which has oscillating
strength 0.2424 and excitation energy 4.0026 eV. The HOMO of
probe 2 locates mainly on benzimidazole ring and LUMO mainly
on benzophenone moiety but for LUMO+1 it exhibit an expanding
Interference studies of probe 2
Addition of other anions (100 equiv.) such as Clꢁ, Brꢁ, AcOꢁ, Iꢁ,
HSOꢁ4 , OHꢁ, H2POꢁ4 , no significant change in fluorescence intensity
was observed by comparison with and without other anions. To
determine the practical applicability of probe 2, we carried out
competitive experiments in the presence of 10 equiv. of Fꢁ ions
p-nature orbital covering both benzimidazole and benzophenone
moiety. The other bands at 260 and 280 nm were due to HOMOꢁ1
Fig. 5. Photographs of probe 2 coated TLC strips under different experimental conditions.

A.S. Gupta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
71
Probe 2
Probe 2 + F-
Fig. 6. Optimized structures and HOMO and LUMO of probe 2.
Table 1
Excitations for probe 2 that contribute to the transitions in the 250–700 nm and their expansion coefficients in gaseous phase.
kmax (nm) Experimental
kmax (nm) Theoretical
Eexc (eV)
Oscillator strength
Excited state
Expansion coefficient
Probe 2
335
262
330
280
4.0026
4.3661
0.2424
0.3049
HOMO to LUMO+1
HOMOꢁ3 to LUMO
HOMO to LUMO+2
HOMOꢁ2 to LUMO+2
0.63898
0.52184
Probe 2+Fꢁ
360
280
360
280
3.1007
4.5082
0.2974
0.1449
0.65909
0.60937
to LUMO+1 and HOMOꢁ3 to LUMO respectively (Table 1). In case
of probe 2.Fꢁ band at 360 nm was due to HOMO to LUMO+2 tran-
sitions, which has oscillating strength 0.2974 and excitation energy
3.1007 eV. The HOMO of probe 2+Fꢁ locates mainly on benzimid-
azole ring and LUMO mainly on benzophenone moiety but for
enol emission. The probe 2 showed practical applicability to sense
fluoride ions at micromolar level through dip coating method.
Acknowledgement
LUMO+2, it exhibit an expanding
p-nature orbital covering both
benzimidazole and benzophenone moiety. The other bands at
280 and 305 nm were due to HOMOꢁ2 to LUMO+2 and HOMOꢁ3
to LUMO respectively (Table 1).
We thank DST-INSPIRE (IFA12-CH-59), New Delhi for fellow-
ship and financial assistance. We also thank to SAIF, Panjab Univer-
sity, Chandigarh for mass analysis and SAI Labs, Thapar University,
Patiala for NMR facility.
Conclusion
Hydroxyaryl benzimidazole derivative has been synthesized for
selective sensing of fluoride ions. Probe 2 showed ratiometric
absorption and emission behavior towards fluoride ions to sense
Appendix A. Supplementary material
fluoride ion between a broad ranges of 2–2400
of fluoride ion caused inhibition of keto tautomer and release of
l
M. The presence
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.11.026.

72
A.S. Gupta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 67–72
[9] J. Seo, S. Kim, S.Y. Park, Strong solvatochromic fluorescence from the
References
intramolecular charge-transfer state created by excited-state intramolecular
proton transfer, J. Am. Chem. Soc. 126 (2004) 11154–11155.
[1] K.L. Kirk, Biochemistry of Halogenated Organic Compounds, vol. 9, Plenum Pub
Corp, New York, 1991.
[2] J. Wergedal, D. Baylink, Fluoride directly stimulates proliferation and alkaline
phosphatase activity of bone-forming cells, Science 222 (1983) 330–332.
[3] M. Kleerekoper, The role of fluoride in the prevention of osteoporosis,
Endocrinol. Metab. Clin. North Am. 27 (1998) 441–452.
[4] B.L. Riggs, Bone and Mineral Research, Annual 2, Elsevier, Amsterdam, 1984.
[5] (a) B. Zhu, F. Yuan, R. Li, Y. Li, Q. Wei, Z. Ma, B. Du, X. Zhang, A highly selective
colorimetric and ratiometric fluorescent chemodosimeter for imaging fluoride
ions in living cells, Chem. Commun. 47 (2011) 7098–7100;
[10] J. Zhao, S. Ji, Y. Chen, H. Guo, P. Yang, Excited state intramolecular proton
transfer (ESIPT): from principal photophysics to the development of new
chromophores and applications in fluorescent molecular probes and
luminescent materials, Phys. Chem. Chem. Phys. 14 (2012) 8803–8817.
[11] (a) V. Thiagarajan, P. Ramamurthy, D. Thirumalai, V.T. Ramakrishnan, A novel
colorimetric and fluorescent chemosensor for anions involving PET and ICT
pathways, Org. Lett. 7 (2005) 657–660;
(b) T. Gunnlaugsson, H.D.P. Ali, M. Glynn, P.E. Kruger, G.M. Hussey, F.M.
Pfeffer, C.M. dos Santos, J. Tierney, Fluorescent photoinduced electron transfer
(PET) sensors for anions; from design to potential application, J. Fluoresc. 15
(2005) 287–299.
(b) R.M. Duke, E.B. Veale, F.M. Pfeffer, P.E. Kruger, T. Gunnlaugsson,
Colorimetric and fluorescent anion sensors: an overview of recent
developments in the use of 1,8-naphthalimide-based chemosensors, Chem.
Soc. Rev. 39 (2010) 3936–3953;
[12] (a) V. Luxami, S. Kumar, Molecular half-subtractor based on 3,30-bis(1H-
benzimidazolyl-2-yl)[1,10]-binaphthalenyl-2,20-diol, New J. Chem. 32 (2008)
2074–2079;
(c) S.H. Mashraqui, S.S. Ghorpade, S. Tripathi, S. Britto,
A new indole
(b) V. Luxami, S. Kumar, ESIPT based dual fluorescent sensor and
incorporated chemosensor exhibiting selective colorimetric and fluorescence
ratiometric signaling of fluoride, Tetrahedron Lett. 53 (2012) 765–768.
[6] A.-F. Li, J.-H. Wang, F. Wang, Y.-B. Jiang, Anion complexation and sensing using
modified urea and thiourea-based receptors, Chem. Soc. Rev. 39 (2010) 3729–
3745.
[7] (a) Y. Zhou, J.F. Zhang, J. Yoon, Fluorescence and colorimetric chemosensors for
fluoride ion detection, Chem. Rev. 114 (2014) 5511–5571;
(b) J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, New sensing mechanisms for design
of fluorescent chemosensors emerging in recent years, Chem. Soc. Rev. 40
(2011) 3483–3495.
concentration dependent reconfigurable Boolean operators, RSC Adv.
(2012) 8734–8740.
2
[13] V.A. Sontakke, S. Ghosh, P.P. Lawande, B.A. Chopade, V.S. Shinde, A simple,
efficient synthesis of 2-aryl benzimidazoles using silica supported periodic
acid catalyst and evaluation of anticancer activity, ISRN Org. Chem. (2013).
[14] V. Dorofeev, A. Arzamastsev, O. Veselova, Melting point determination for the
analysis of drugs of the fluoroquinolone group, Pharm. Chem. J. 38 (6) (2004)
333–335.
[15] I. Kuntz Jr, F. Gasparro, M. Johnston Jr., R. Taylor, Molecular interactions and
the Benesi–Hildebrand equation, J. Am. Chem. Soc. 90 (18) (1968) 4778–4781.
[16] M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, J.
Montgomery Jr., T. Vreven, K. Kudin, J. Burant, Gaussian 03, Revision c. 02;
Gaussian. Inc., Wallingford, CT 4, 2004
[8] P. Singh, M. Barjatiya, S. Dhing, R. Bhatnagar, S. Kothari, V. Dhar, Evidence
suggesting that high intake of fluoride provokes nephrolithiasis in tribal
populations, Urol. Res. 29 (2001) 238–244.