“Switch on” fluorescent sensor for the detection of fluoride ions in solution and commercial tooth paste

Article abstract of DOI:10.1016/j.saa.2019.117361

A simple and novel uracil based chemosensor (1) has been developed by one step reaction, which selectively detected F^? ions via “switch on” fluorescence mode. Upon the addition of F^? ion to CH₃CN solution of 1, the non-flu

Full text of DOI:10.1016/j.saa.2019.117361

Accepted Manuscript
“Switch on” fluorescent sensor for the detection of fluoride ions
in solution and commercial tooth paste
Navneet Kaur, Gitanjali Jindal
PII:
S1386-1425(19)30751-6
DOI:
https://doi.org/10.1016/j.saa.2019.117361
Article
Number:
Reference:
117361
SAA 117361
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
23 May 2019
2 July 2019
7 July 2019
Please cite this article as: N. Kaur and G. Jindal, “Switch on” fluorescent sensor for
the detection of fluoride ions in solution and commercial tooth paste, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/
j.saa.2019.117361
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ACCEPTED MANUSCRIPT
“Switch on” fluorescent sensor for the detection of fluoride ions in solution
and commercial tooth paste
Navneet Kaur* and Gitanjali Jindal
Department of Chemistry, Panjab University, Chandigarh 160014, India
 Corresponding author. Tel.: +91 172 2534430; fax: +91 172 2545074;
e-mail: neet_chem@yahoo.co.in; neet_chem@pu.ac.in
Abstract: A simple and novel uracil based chemosensor (1) has been developed by one step
reaction, which selectively detected F- ions via “switch on” fluorescence mode. Upon the addition
of F- ion to CH3CN solution of 1, the non-fluorescent probe became highly fluorescent, showing
a color change from colorless to fluorescent blue, when irradiated with 280 nm light. 1H NMR
studies revealed the binding sites of chemosensor 1, where C-5 hydrogen and amine hydrogens
formed hydrogen bonding with F- ion. This binding mode was further confirmed using DFT
calculations. Significantly, the detection limit of chemosensor 1 towards F- has been evaluated to
be 47.6 nM, which is lower than the maximum values of F- (1.5 mg/L) ions permitted by WHO.
The in-situ generated 1-F- complex has been used for secondary sensing of Ca(NO3)2, one of the
component of the fertilizer. Moreover, the sensor has been successfully applied for detection of
fluoride ion in commercial tooth paste.
Keywords: Uracil; F- sensor; fluorescence on; Secondary sensor
1. Introduction
Recently, the field of anion sensing and recognition is in continuous growth as this area of
supramolecular chemistry have a great relevance and applications, such as in organocatalysis
where metal-free molecules can catalyze reactions via hydrogen-bonding interactions, and in
separation of mixtures including anions at industrial scale [1]. Anions play a vital role in a number
of biological and environmental processes and they can be either fundamental to sustain growth or
work as harmful pollutants [2]. So, the development of sensors for anions that can be used under
real-world to sense trace quantities of anions is the main concern of today’s world. It is therefore
not of any surprise that their synthesis have become a target of research in the last decade [3-4].
Amongst all the biologically essential anions, the fluoride (F-) ion is attracting scientist’s
interest due to its small size, high charge density and hard Lewis basic nature. The F- ion is known
to play important roles in various biological, medical and environmental processes and is very

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important for dental health and treatment of osteoporosis[5,6]. However, its overexposure may
have a severe effect on human health and can cause gastric and bone diseases as fluorosis,
urolithiasis, and even cancer [7,8]. The variance in its utility, equally valuable and otherwise,
makes its recognition considerable. Further, calcium nitrate (a calcium salt), perform an essential
role in agricultural industry as it is a very useful part of fertilizer and also employed for disease
control and wastewater treatment [9]. It is the only water soluble source of calcium available for
plants. Diseases like blossom end rot are easily controlled with calcium nitrate. Consequently,
effective and investigative methods have to be applied for development of a sensing system for
recognition of F- ion and calcium nitrate (Ca(NO3)2).
Now-a-days, in the midst of all the constantly used techniques, the research on constructing
sensors with good sensitivity and selectivity has been of great concern [10]. Among them, the
fluorescent sensors have attracted a considerable interest which has led to the development of
highly specific probes with a wide range of applications in environmental chemistry, biochemistry,
and cell biology [11]. The fluorescent methods present the advantages of simplicity, rapid
response, sensitivity, low cost, non-destructive methodology, real-time monitoring and high spatial
resolution via microscopic imaging [12-14]. Also, the analyses based on fluorescent sensors are
easy to handle and more economical.
In the present study, we have designed and synthesized simple uracil appended chemosensor
1, possessing uracil backbone, which is known to display a wide range of pharmacological
activities and free-NH2 group that can be used for detection of anions. This chemosensor 1 has
been used for the selective recognition of F- in CH3CN using fluorogenic “switch-on” modes. This
can be attributed to the perturbation of excimer band of uracil due to F- binding with 1, resulting
in released fluorescence of individual molecule of 1. Furthermore, the complexed 1-F- species
generated in situ has been used as a platform for recognition of Ca(NO3)2 via fluorescence
quenching. Thus, chemosensor 1 behaves like a fluorescence sensor for both anionic and cationic
species, based on a different approach to that of a chemosensing ensemble. The analytical
applications of 1 were also tested for the detection of F- in commercial tooth paste powder.
2. Experimental
2.1 Materials and methods

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6-Amino-1, 3-dimethyluracil, 2-hydroxybenzaldehyde, trifluoroacetic acid, calcium nitrate,
tetrabutylammonium salts of various anions and perchlorate salts of various metal ions were
purchased from Aldrich. All other chemicals were used as received without further purification.
Acetonitrile (CH3CN) was of HPLC grade. Melting points were determined in capillary and are
uncorrected. 1H and 13C NMR spectra were recorded on a BRUKER AVANCE 400 and 100 MHz
instrument using tetramethylsilane as an internal standard. Various anions such as F-, Cl-, Br-, I-,
-
-
-
OH-, AcO-, HSO4 , H2PO4 , SO4 were added as their tetrabutylammonium salts, whereas all the
metal ions such as Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ and Hg2+ were added
as their perchlorate salts for UV-vis and fluorescence experiments. Aliquots of analytes (anions or
metal ions) under investigation were then injected into the sample solution and the solutions were
kept for some time to get stabilized after each addition and then they were scanned.
2.2 General procedure for UV-vis and fluorescence experiments
The UV-vis and fluorescence titrations were performed using 20 µM and 2 µM solution of 1,
respectively, in acetonitrile solution. All the UV-vis experiments were conducted on Shimadzu
UV-240 spectrometer; while fluorescence spectra were carried out using HITACHI-7000
spectrophotometer equipped with 220-240 V Xe lamp with quartz cell of 1 cm width and 3.5 cm
height. The excitation wavelength was taken as 350 nm for sensor 1 with 5 nm excitation as well
as emission slit widths for fluorescence studies. Stock solutions of the sensor 1 (0.01 M) were
prepared in DMSO and were diluted with CH3CN solution for further different spectroscopic
analyses. All absorption scans were saved as ACS II files and further processed in Excel(tm) to
produce all graphs shown.
2.3 General procedure for 1H NMR experiments
Two stock solutions were prepared in CDCl3, one of them containing host (1 of 3.59 x10-2 M conc.)
only and other containing an appropriate concentration of guest (F-). Aliquots of these two
solutions were mixed directly in NMR tube, which then was diluted to 0.5 mL with CDCl3 if
required.
2.4 Synthesis of 10-(6-Amino-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydro-pyrimidin-5-yl)-1,3-
dimethyl-1,10-dihydro-9-oxa-1,3-diaza-anthracene-2,4-dione (1)
2-Hydroxybenzaldehyde (0.343 ml, 3.22 mmol) was dissolved in 40 ml absolute ethanol in a round
bottom flask. To this mixture, 6-amino-1, 3-dimethyluracil (0.5 g, 3.22 mmol) and few drops of
trifluoroacetic acid were added. The reaction mixture was heated at 60oC for 5 h (as indicated by

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TLC). The reaction mixture was then cooled to room temperature. The solid formed was filtered
and recrystallized from chloroform/methanol mixture to get the desired product (1). Off-white
soild; 86% yield; m.p. (oC) 245-248; FT-IR (neat, ν cm-1): 3381 (υO-Hstr.), 1694 (υC=Ostr.), 1581 (υAr-
C=Cstr.); 1H NMR (400MHz, CDCl3, ppm): δ 7.22 (s, 1H, Ar-H), 7.13 (s, 1H, Ar-H), 7.11-7.08 (m,
2H, Ar-H), 5.81 (s, 2H, NH2, exchanges with D2O), 4.84 (s, IH, C-H), 3.53 (s, 3H, CH3), 3.48 (s,
3H, CH3), 3.33 (s, 3H, CH3), 3.10 (s, 3H, CH3); 13C NMR (100MHz, CDCl3, ppm): δ 164.1, 161.3,
154.3, 151.5, 128.3, 127.9, 125.2, 123.4, 115.6, 93.2, 87.8, 30.3, 29.3, 28.2, 27.7; HRMS : m/z
1
(relative abundance (%), assignment) = 817.23 [100, (2xM + 23)+]. H NMR (in CDCl3), 1H NMR
(in CDCl3+D2O), 13C NMR and HRMS spectra of 1 have been shown in Figs. S1-S4.
3. Results and discussion
3.1 Synthesis of chemosensor 1
Chemosensor 1 has been synthesized by refluxing 6-amino-1,3-dimethyluracil and salicyladehyde
in ethanol containing few drops of TF. Here, initially open di(uracilyl) derivative, 1a, was formed,
which finally underwent cyclocondensation to form chemosensor 1. Earlier, Dabiri et al. also
reported that 6-amino-1,3-dimethyluracil upon reaction with aromatic aldehydes catalysed by
HOAc in ionic liquid as solvent, resulted to the formation of such closed ring structure [15].
O
O
H3C
CH3
O
H3C
CH3
O
N
H
N
N
H
N
O
H
O
H2N
O
H3C
O
H2N
O
N
OH
TFA, 60oC
Abs. EtOH
+
H3C
O
H3C
O
-NH3
N
NH2
N
N
CH3
HO
N
O
N
NH2
CH3
CH3
(1a)
(1)
Scheme 1. Synthesis of the target chemosensor 1.
3.2 Absorption spectral studies towards anions
-
-
The recognition behaviour towards various anions such as CN-, F-, Cl-, Br-, I-, AcO-, HSO4 , H2PO4
2-
, SO4 was examined by observing the changes in the UV-vis absorption spectra of the
chemosensor 1 (20 µM) in CH3CN (Fig. S5). The UV-vis spectrum of sensor 1 displayed a peak
at 280 nm, which decreased in intensity along with appearance of new band around 370 nm with
the addition of F- ions. The addition of other ions did not show any changes in absorption spectra.
To quantitatively sense F- ions, UV-vis titrations were carried out by adding different

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concentrations of F- ions to the CH3CN solution of 1 (20 M) (Fig 1). Upon subsequent addition
of F- ions, a new absorption band starts forming at 372 nm, whose intensity increased with increase
in concentration of F- ions and attained plateau after addition of 120 µM of F- ion. The appearance
of new band at longer wavelength has been attributed to increased ICT within whole molecule
arising due to F- ion binding [16]. The binding constant of 1 with F- was evaluated to be 1.1 x 104
M-1 based on UV-vis titration using Benesi-Hildebrand equation given as below [17];
Here, Ao, A, and Amax is the absorbance of free 1, measured with F- and measured with
1
1
1
-
=
+
excess amount of F at A3-7A2 nm, respectively. K is the association constant and [C] is the
Amax-Ao
[Amax-Ao] K [C]
o
concentration of F- ions added. A linear plot of 1/(A-A0) versus 1/[F-] indicated 1:1 stoichiometry
of 1 with F- ions . The limits of detection (LOD) value has been reasonably estimated to be 75 μM
for F- using the relation [18]:
LOD = 3s–1
where  = standard deviation of response and s = slope of the calibration curve.
Fig. 1. UV-vis titration spectrum of 1 (20 M) in CH3CN in the presence of various concentrations of F-
ion; Inset: Plot of absorption intensity at 372 nm versus the concentration of added F- ions.
3.3 Fluorescence emission spectral studies towards anions
The sensor 1 exhibits a non-fluorescent nature when excited at 280 nm. However, upon addition
-
-
2-
of various anions such as CN-, F-, Cl-, Br-, I-, AcO-, HSO4 , H2PO4 , SO4 , only addition of F- ions
to solution of 1 (2 µM; CH3CN) enhanced its fluorescence intensity (Fig. S6). All other tested ions
displayed negligible fluorescence response under the same spectroscopic environment. This

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fluorescence enhancement was also accompanied by color change from colorless to dark
fluorescent blue, when irradiated with light of wavelength 280 nm (Fig. 2 inset).
To study the interaction between 1 and F- ion quantitatively, fluorescence titration
experiment was performed by the addition of different concentrations of F- ions to solution of
sensor molecule 1 (2 µM). The chemosensor 1 (2 µM) exhibited a weak excimer emission band at
419 nm with a stoke’s shift of 139 nm, when excited at 280 nm (Fig. 2). The dimer peak
corresponding to [2x M+23]+ in mass spectra (Fig. S3) also supported to the excimer formation in
chemosensor 1.With the gradual addition of F-, the fluorescence intensity increased progressively
with negligible spectral shift. This has been attributed to the perturbation of excimer band and
fluorescence emission of individual molecules of chemosensor 1 due to formation of 1-F- structure
[19]. The Job’s plot, 1H NMR titrations and DFT calculations further supported this assumption.
Fig. 2. Fluorescence titration spectrum of 1 (2 M; λex 280 nm) in CH3CN in the presence of various
concentrations of F- ions; Inset: Plot of fluorescence intensity at 419 nm versus the concentration of added
F- ions.
The job’s plot revealed the 1:1 binding stoichiometric ratio of chemosensor 1 with F- ion (Fig. S7).
The binding constant between sensor 1 and F- and the limit of detection was calculated to be 3.15
x 104 M-1 and 47.6 nM, respectively. Importantly, the calculated LOD values is lower than WHO
limits of detection for F- (1.5 mg/l) [20].
3.3 1H NMR titration

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To examine the binding sites responsible for the optical (absorption and fluorescence) changes
observed in the sensor 1 upon the addition of F- ion, 1H NMR titration experiment was performed
(Fig. 3). With the addition of 0.05 equiv. of F- ions, the signal due to –NH2 of free sensor 1 at δ
5.7 ppm was initially broadened. However, clear downfield shifting along-with further broadening
of –NH2 signal with the addition of 1.5 equiv. of F- ion pointed to formation of hydrogen bonding
between –NH2 of sensor 1 and F- ion. Also the peak due to C-5H broadened upon the addition of
1.5 equiv. of F- ion, indicating its involvement in F- binding.
O
H3C
H
CH3
O
N
N
1+1.5eq F-
1+0.5eq F-
1+0.2eq F-
1+0.05eq F-
N
H
//
//
//
F-
H3C
H
O
5
N
O
N
O
CH3
1-F- complex
//
//
NH2
C-H
5.0
Free 1
6.5
6.2
5.9
5.6
5.3
4.7
4.4
4
3
2
1
ppm (δ)
Fig. 3. 1H–NMR titration of 1 (3.59 x10-2 M) with incremental addition of F- in CDCl3.
3.4 DFT Computational studies
To evaluate the structural properties and binding nature of the chemosensor 1, computational
calculations were carried out with and without F- ion. The optimization of the structures of
chemosensor 1 and the 1-F- complex (1:1) were performed by Density Functional Theory (DFT)
method. The DFT calculations were done using the Gaussian 09 package using the exchange
correlation function B3LYP and the basis set 6–31G (d, p) for C, H, N, O and Fatoms. The energy
evaluation diagram (Fig. 4) showed that on complexation of 1 with one F- ion, the energy gap of
HOMO and LUMO decreased with respect to free chemosensor indicating the formation of a stable
complex and validating the 1:1 binding stoichiometry. Also, the bond length of one N-H got
increased from 1.007 Å (in 1) to 1.503 Å (in the 1-F- complex); whereas the bond distance of the

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other N-H remained the same, i.e., 1.009 Å. Moreover, there was a slight increase in the C-5H
bond distance from 1.0953 Å (in 1) to 1.1455 Å (in 1-F- complex). This clearly indicated the
interaction of F- ion with only one N-H of free –NH2 group and the hydrogen of C-5H in 1- F-
complex. Furthermore, the electron density of the LUMO of 1 lied on the electron donating part
of the respective ligands and for the HOMO, the electron density was spread over both the electron
donating and electron withdrawing parts. The contour plots of the LUMO after the addition of F-
ions illustrated that the electron density was over the electron donating part of chemosensor 1,
however the electron density of the HOMO was only on the electron withdrawing part (Fig. 4).
Fig. 4. DFT computed LUMO and HOMO diagrams of sensor 1 and 1 + F- complex.
3.5 Selectivity in the presence of competitive anions

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The selectivity of chemosensor 1 was endorsed towards F- ions in the presence of various
-
2-
interfering ions (Cl-, Br-, I-, AcO-, H2PO4 , HSO4 and SO4 ) by using fluorescence spectroscopy.
100 equiv. of aforementioned ions were added to 1-F- (2 µM of 1 with 10 added equiv. of F-)
acetonitrile solution. The bar graphs (Fig. 5) clearly represent the non-interference of various
anions in the recognition process.
Fig. 5. Anion selectivity graph of 1 (2 μM) via changes observed in fluorescence emission spectra with
addition of 10 equiv. of F- and 100 equiv. of other interfering cations in CH3CN.
3.6 Fluorescence switch off sensing of Ca2+
As sensor 1 possessed various heteroatoms, so, its interaction with different metal ions such as
Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ and Hg2+ (perchlorate was used as a counter
ion) along with Ca(NO3)2 was investigated through UV–vis and Fluorescence spectra by adding
their 100 equivalents to CH3CN solution of 1 (20 M for UV–vis spectra / 2 M for fluorescence
spectra). The absorption band of 1 lying in range of 275–290 nm was negligibly perturbed with
addition of 100 equiv. of mentioned metal ions (Fig. S8a). The increased absorbance due to Hg2+
and Cu2+ ions at 270 nm was due to their own absorbance below 300 nm and for the same reason,
any actual changes observed with Hg2+ and Cu2+ ions cannot be identified. Also, the chemosensor
1 remained non-fluorescence in the absence or presence of any of the said metal ions (Fig. S8b).
Recently, various F- ion possessing moieties have been utilized for the recognition of calcium
compounds, which are one of the components of fertilizers as calcium salts are known to have
excellent affinity for fluoride [21-23]. For checking the viability of this approach, 1-F- complex
was utilized as a platform for detection of above mentioned metal ions by fluorescence

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spectroscopy in CH3CN (Fig. 6a). Thus, the probe 1-F- was prepared in situ by addition of 30
equiv. of Ca2+ to 1 solution (2 µM). As shown in fig. 6b, 1-F- has strong emission peak at 419 nm
(λex 280 nm). The addition of Ca(NO3)2 ions to the solution of in situ generated 1-F- complex i.e.
“on state” resulted in the quenching of fluorescence emission i.e. “off state”. It is important to
mention that addition of calcium perchlorate to solution of 1-F- complex also quenched
fluorescence, but, even then Ca(NO3)2 is selected to study the sensing properties as Ca(NO3)2 is a
very useful part of fertilizer and is the only water soluble source of calcium available for plants.
Fig. 6. Fluorescence spectra of L’ (1-F- complex) in CH3CN in the presence of 50 equiv. of different
metal ions.
The fluorescence spectra of 1-F- complex in acetonitrile solution in the presence of various
amounts of Ca2+ is shown in figure 8. Upon addition of incremental amounts of Ca2+ ions (0-30
equiv.) to solution of 1-F- complex, a significant fluorescence quenching has been observed at 419
nm along with change in the color of the fluorescence back to colorless (Fig. 7). The binding
constant between 1-F- complex and Ca2+ has been determined as 4.13 x 104 M-1 with LOD to be
0.80 μM.

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Fig. 7. Fluorescence titration spectrum of 1-F- complex in the presence of various concentrations of Ca2+
excited at 280 nm in CH3CN; Inset A: Fluorescence color change in 1-F- complex before and after addition
of Ca2+ ions; Inset B: Plot of fluorescence intensity at 419 nm versus the concentration of added Ca2+.
To check the reversible nature of the chemosensor 1, the F- ion and Ca(NO3)2 were
alternatively added to the solution of 1 and the fluorescence spectrum was recorded (Fig. S9). It
was observed that the F- ion enhanced the emission intensity remarkably, hence, showing “switch
on” behavior. Furthermore, the addition of Ca(NO3)2 to the 1-F- complex provoked a noticeable
fluorescence change by showing “switch off” behaviour. Therefore, the repeated exhibition of
“switch on-off” behaviour of the complex in fluorescence clearly implies that 1 is a reversible
sensor.
3.7 Plausible sensing model for chemosensor 1
The plausible sensing mechanism of chemosensor 1 towards F- and then Ca(NO3)2 has been
proposed on the basis of fluorescence titration studies, job’s plot, DFT computational calculations
and 1H NMR titration studies. Here, the free amino group of uracil along with C-5H takes part in
hydrogen bonding with F- ion in 1:1 stoichiometric manner (Scheme 2). The known strong binding
of calcium salts towards F- has been utilized to reverse the fluorescence as well as color changes.

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CH3
N
O
O
O
N
CH3
H
N
O
O
O
O
NH2
CH3
F-
H3C
CH3
O
N
N
H
N
H3C
H2N
O
ON
H3C
O
N
H3C
N
CH3
N
N
H
O
N
CH3
H
F-
O
H
(1)
O
CH3
N
H3C
O
O
N
N
CH3
O
N
O
H
O
O
O
CH3
(1-F-)
NH2
H3C
CH3
N
N
N
H
N
CH3
H3C
OFF
H2N
O
O
O
Emission
at 419nm
H3C
N
Ca(NO3)2
Emission
O
N
O
quenched
CH3
(1)
Scheme 2: Proposed sensing mechanism for chemosensor 1 with F- and Ca(NO3)2 in solution.
3.9 Real-life applications
To check the applicability of the proposed chemosensor 1 in real-life, it was tested for the
determination of F- ions in the toothpaste samples. For this analysis, 20 mg of toothpaste was taken
and it was dispersed in 2 mL of acetonitrile. The obtained mixture was first sonicated, centrifuged
and then filtered to separate the precipitates. The filtrate was used for the qualitative investigation
of F- ion. The addition of 100 μL of toothpaste solution into the 20 μM solution of the sensor 1 in
acetonitrile solution resulted in fluorescent color changes from colorless to light fluorescent blue
(Fig. 8), which clearly indicates the presence of F- ion in the sample. Also, fluorescence emission
spectrum was recorded for this sample which also showed the presence of F- ion in the mixture.
Hereafter, it can be said that the chemosensor 1 can be effectively applied for detection of F- in
real-life samples.

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500
(a)
400
300
200
100
0
(b)
F ion in
solution
Toothpaste
Toothpaste
F- ion
1
in solution
Fig. 8. (a) Fluorescence color changes and (b) fluorescence spectra in the presence of F- ion and
toothpaste.
3.10. Comparison with literature sensors for F-
The comparison of the present chemosensor 1 with some previously reported receptors for F- has
been given in Table 1. From this table, it is clearly observed that the most of the reported sensors
involve F- induced deprotonation of acidic OH or NH. In contrast, in the present work the free
amino group of uracil along with C-5H in chemosensor 1 takes part in hydrogen bonding with F-
ion and detected F- ions with lowest LOD value as compared to other reported sensors. Moreover,
chemosensor 1 has the real life application in terms of detection of F- ion in toothpaste. The 1.F-
complex has also been used for secondary sensing of Ca(NO3)2. Thus, the simple, one-step
methodology of synthesis of chemosensor 1 makes it superior in terms of its easy synthesis, low
LOD values and real life applications.
S.
No.
1
Chemosensor
Binding sites
Fluorescence
LOD
Ref.
changes with F- ions (in μM)
O
-NH2 and
-CH
Fluorescence
enhancement
0.0476 Present
work
H3C
H2N
O
CH3
N
N
H
O
H3C
O
N
N
O
CH3
CH3
2
amidic NH
Fluorescence
quenching
24
26
O
S
O
O
HN
O
H3C
O
O

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3
-OH
Fluorescence
quenching
1.37,
1.24
25
O
N
O
OH
O
N
O
= H, O-C(CH3)2
4
5
-OH
Fluorescence
enhancement
0.72,
0.46
26
27
N
N
O
OH
S
O
= phenyl, naphthyl
C4H9
=N-NH
Fluorescence
quenching
0.41
N
O
O
HN
N
N
S
Conclusions: To conclude, a simple and novel uracil based chemosensor (1) has been synthesized
for fluorescent detection of F- ion in acetonitrile. The addition of F- ion to solution of 1 made it
highly fluorescent along with color change from colorless to fluorescent blue. The sensing
mechanism and the binding stoichiometry were confirmed by Job’s plot, DFT computational
calculations and 1H NMR studies. The in-situ generated 1-F- complex has been used for secondary
sensing of Ca(NO3)2, one of the component of the fertilizer. Moreover, the sensor can be applied
in daily-life for the detection of fluoride ions. So, the work described in this paper strongly supports
the excellent prospects of design envision of a simple multichannel chemosensor 1 and afforded a
propitious outcome which is paramount for the advancement of research on chemosensing formats.
Acknowledgements

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The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the NMR
spectra and are grateful to DST PURSE-II (Grant no. 48/RPC) for the financial assistance.
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ACCEPTED MANUSCRIPT
CH3
N
O
O
O
N
CH3
H
N
O
O
O
NH2
CH3
F-
H3C
CH3
N
N
N
H3C
H2N
O
O
H
ON
H3C
O
N
H3C
N
CH3
N
N
H
O
N
O
CH3
H
F-
O
H
(1)
O
CH3
N
H3C
O
O
N
N
CH3
O
N
O
H
O
O
O
CH3
(1-F-)
NH2
H3C
CH3
N
N
N
H
N
CH3
H3C
OFF
H2N
O
O
O
Emission
at 419nm
H3C
N
Ca(NO3)2
Emission
O
N
O
quenched
CH3
(1)
Graphical abstract

ACCEPTED MANUSCRIPT
Research Highlights



A simple, novel, tailor-made chemosensor synthesized by one-step reaction.
Selective detection of F- ions via “switch on” fluorescence mode.
1
Binding mode was confirmed using fluorescence studies, H NMR studies and DFT
calculations



LOD value for F- detection was 47.6 nM, which is lower than the maximum values of F-.
Secondary sensing of Ca(NO3)2, one of the component of the fertilizer.
Detection of fluoride ion in commercial tooth paste