An easy prepared dual-channel chemosensor for selective and instant detection of fluoride based on double Schiff-base

Article abstract of DOI:10.1016/j.saa.2016.05.040

A colorimetric and fluorescent dual-channel fluoride chemosensor N,N′-bis (4-diethylaminosalicylidene) hydrazine (sensor S) bearing two imine groups has been designed and synthesized. This structurally simple probe displays rapid response and high selecti

Full text of DOI:10.1016/j.saa.2016.05.040

An easy prepared dual-channel chemosensor for selective and instant detection
of fluoride based on double Schiff-base
Yan-Li Leng, Jian-Hui Zhang, Qiao Li, You-Ming Zhang, Qi Lin, Hong
Yao, Tai-Bao Wei
PII:
DOI:
Reference:
S1386-1425(16)30298-0
doi: 10.1016/j.saa.2016.05.040
SAA 14461
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Revised date:
Accepted date:
23 July 2015
19 May 2016
25 May 2016
Please cite this article as: Yan-Li Leng, Jian-Hui Zhang, Qiao Li, You-Ming Zhang,
Qi Lin, Hong Yao, Tai-Bao Wei, An easy prepared dual-channel chemosensor for se-
lective and instant detection of fluoride based on double Schiff-base, (2016), doi:
10.1016/j.saa.2016.05.040
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ACCEPTED MANUSCRIPT
An easy prepared dual-channel chemosensor for selective and
instant detection of fluoride based on double Schiff-base
Yan-Li Leng a,b , Jian-Hui Zhang b , Qiao Li a, You-Ming Zhang a, Qi Lin a,
Hong Yao a, Tai-Bao Wei a, 
------------------------------------------------------------------
E-mail: weitaibao@126.com *
------------------------------------------------------------------------
a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China,
College of Chemistry and Chemical Engineering,Northwest Normal University, Lanzhou, Gansu
730070, PR China
b Department of Metallurgical Engineering, Lanzhou Resources & Environment Voc-Tech College,
Lanzhou, Gansu 730021, PR China
Abstract: A colorimetric and fluorescent dual-channel fluoride chemosensor N,N’-bis
(4-diethylaminosalicylidene) hydrazine (sensor S) bearing two imine groups has been
designed and synthesized. This structurally simple probe displays rapid response and
−
high selectivity for fluoride over other common anions (Cl−, Br−, I−, AcO−, H2PO4 ,
−
−
HSO4 , ClO4 , CN− and SCN−) in a highly polar aqueous DMSO solution. Mechanism
studies suggested that the sensor firstly combined with F− through hydrogen bonds
and then experienced the deprotonation process at higher concentrations of F− anion
to the two Ar-OH groups. The detection limits was 5.78×10−7 M of F−, which pointing
to the high detection sensitivity. Test strips based on sensor S were fabricated, which
could act as a convenient and efficient F− test kit to detect F− for “in-the-field”
measurement.
Corresponding author
Tel: +8609317973120. E-mail address: weitaibao@126.com

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Keywords: Fluoride (F−); Chemosensor; Colorimetric; Fluorescent; Imine group;
DFT calculations.
1. Introduction
In the past few decades, design and synthesis of organic molecules based on
supramolecular concepts for the colorimetric detection in the presence of various
anions from industrial effluents, environmental contaminants and the potential for
applications in biology and medicine has received current attention [1-5]. Fluoride is
considered as a serious health hazard in the environment because it may lead to bone
disease in the event of excessive intake of it [6,7]. Therefore, considering the wide
application of fluoride, there is a great demand for the development of methods that
can rapidly, sensitively and selectively detect the fluoride anion [8-11]. Up to now,
many efforts have been made to design simple and low cost chemosensors, several
anion receptors such as imidazoles [12,13], pyrroles [14,15], calixpyrroles [16,17],
amides [18-20], carbamides [21,22], urea [23,24], and thiourea [25] have been
developed.
In many cases, the anion binding unit is primarily composed of H-bonding donors.
A variety of neutral receptors have been reported for selective anion recognition based
on H-bonding owing to the strength and selectivity of this interaction [26,27].
Meanwhile, H bonding interactions between an anion and the H-bond donors in a
sensing system can induce internal charge transfer (ICT), which cause changes in the
absorption and emission of the chromogenic probe, thus allowing the naked eye
chemosensing [28,29]. Among them, taking the Ar-OH group as the sensing site
possesses several merits. For example, it can be used for the hydrogen bond donor site
and act as anion binding site. Furthermore, owing to the fact that the acidity of

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“Ar-OH” is stronger than that of “H-C=N-”, the Ar-OH group can sense F− not only
by forming hydrogen bond (F−…HO-Ar) but also by a deprotonation process
(deprotonation of Ar-OH). As a result, it can afford two kinds of sense approaches
and enhance the sense selectivity for F−.
With this background, as a part of our research interest in molecular recognition
[30-34], we reported a simple-to-synthesize, yet sensitive and selective fluoride anion
sensor S. The strategy employed in the design of this sensor is as follows: firstly, we
introduced hydroxyl and imine groups to the sensor molecule as binding sites to
detect F− by hydrogen-bonding interactions. Secondly, in order to achieve high
sensitivity for F−, the fluorescent signal report mode has been adopted, because the
fluorescent sensors often provide higher sensitivity than other optic sensors. Therefore,
we introduced 4-diethylaminosalicylaldehyde as the fluorescence signal group.
Finally, the sensor molecular was designed easily to synthesize with low cost. As a
result, sensor S could detect F− with specific selectivity and high sensitivity.
Moreover, we also demonstrate that the F- anion recognition mechanism is both
hydrogen bonding and deprotonation with the Ar-OH in sensor S, which was
investigated by various methods (such as 1H NMR spectroscopic titration, ESI-MS and
Density functional theory (DFT) calculations), both of which give rise to changes in
the UV-vis and fluorescent spectra.
2. Experimental section
2.1. Materials and physical methods
All reagents and solvents were commercially available at analytical grade and were
used without further purification. 1H NMR spectra were recorded with a
1
Mercury-400BB spectrometer at 400 MHz. H chemical shifts are reported in ppm
downfield from tetramethylsilane (TMS, δ scale with the solvent resonances as internal

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standards). UV-visible spectra were recorded on a Shimadzu UV-2550 spectrometer.
Photoluminescence spectra were performed on a Shimadzu RF-5301 fluorescence
spectrophotometer. Melting points were measured on an X-4 digital melting-point
apparatus (uncorrected). The infrared spectra were performed on a Digilab FTS-3000
FT-IR spectrophotometer. Elemental analyses were performed by Thermo Scientific
Flash 2000 organic elemental analyzer.
2.2 Synthesis of sensor 1
4-diethylaminosalicylaldehyde (0.424 g, 2.2 mmol), hydrazine monohydrate (0.100
g, 2 mmol), and a catalytic amount of acetic acid (five drops of AcOH) were
combined in absolute ethanol (10 mL). The solution was stirred under reflux for 4 h.
The reaction mixture was refluxed for 3-4 h and the progress of reaction was
monitored by thin layer chromatography (TLC). After the completion of the reaction,
the reaction flask was allowed to cool to room temperature. The solid product that
appeared in the flask was filtered and recrystallized with ethanol to get orange
crystalline product S (0.592 g) in 77% yield (mp: 244-246℃); IR: (KBr, cm-1) ν: 2978,
1627, 1590, 1516, 1409, 1355, 1229, 1128, 967, 782; 1H NMR (DMSO-d6, 400 MHz):
δ 1.09-1.13(t, 12H, -CH3), 3.35-3.39 (t, 8H, N-CH2), 6.12 (s, 2H, ArH), 6.30-6.32 (d,
2H, ArH), 7.27-7.29 (d, 2H, ArH), 8.61 (s, 2H, N=CH), 11.51 (s, 2H, Ar-OH); Anal.
Calcd for C22H30N4O2: C 69.08, H 7.91, N 14.65. Found: C 69.20, H 7.89, N 14.62.
ESI-MS: m/z calcd for C22H30N4O2, [M-H]+ = 382.24, found [M-H]+ = 383.29.
2.3. General procedure for UV-vis experiments
The solution of sensor S (2.0×10−4 M) in DMSO was prepared and stored in dry
atmosphere. The solution was used for all spectroscopic studies after appropriate
dilution. The DMSO solutions of each anions (1.0×10−2 M) were prepared,
−
−
respectively, via tetrabutylammonium salts for F−, Cl−, Br−, I−, AcO−, H2PO4 , HSO4 ,

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−
ClO4 and the sodium salts for CN−, SCN−. Any changes in the UV-vis spectra of
sensor S were recorded upon the addition of salts while keeping the concentration of
sensor S (2.0×10−5 M) in all experiments.
2.4. General procedure for Fluorescence spectra experiments
The solution of sensor S (2.0×10−4 M) in DMSO was prepared and stored in dry
atmosphere. The solution was used for all spectroscopic studies after appropriate
dilution. The DMSO solutions of each anions (1.0×10−2 M) were prepared,
−
−
respectively, via tetrabutylammonium salts for F−, Cl−, Br−, I−, AcO−, H2PO4 , HSO4 ,
−
ClO4 and the sodium salts for CN−, SCN−. The fluorescence spectra were obtained by
excitation at 422 nm. The excitation slit widths were 5 nm and emission slit widths
were 5 nm, respectively. Any changes in the fluorescence spectra of sensor S were
recorded upon the addition of salts while keeping the concentration of sensor S
(2.0×10−5 M) in all experiments.
2.5. General procedure for 1H NMR experiments
1
For H NMR titrations, the appropriate concentrated solution of sensor S and the
guest were both prepared in DMSO-d6. Aliquots of the two solutions were mixed
directly in NMR tubes.
3. Results and discussion
In this work, F− chemosensor S was synthesized by the condensation of
4-diethylaminosalicylaldehyde and hydrazine monohydrate (Scheme 1) which has
1
been characterized by H NMR, ESI-MS and elemental analyses. The structure of S
was further confirmed by X-ray crystallography, as shown in Figure 1 (Summary of
Data CCDC 1060898). A summary of the crystallographic data and structural
refinements for S are listed in Figure 1, Table S1 (see supporting information).
(Insert Scheme 1)

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(Insert Figure 1)
The absorption spectrum of sensor S in DMSO solution exhibits the absorption
band at 422 nm. When 50 equiv. of F- was added to the DMSO solution of sensor S,
the band at 422 nm is red-shifted to 456 nm (Figure 2). This absorption was
responsible for the color change (from yellow to dark-yellow) and perceptible to the
naked eyes (Figure 2, insert). To validate the selectivity of sensor S, the same tests
−
−
−
were applied using Cl−, Br−, I−, AcO−, H2PO4 , HSO4 , ClO4 , CN− and SCN− anions,
and none of these anions induced any significant changes in the UV-vis spectrum. The
selective fluorescence response of S to F− anion was also investigated (Figure 3). The
emission of S appeared at the maximum emission wavelength was 488 nm in DMSO
solution when excited at λex = 422 nm. When 50 equiv. of F− was added to the
solution of sensor S, the strong fluorescence emission band at 488 nm is quenched
and the fluorescence color changed from blue-green to pale-yellow (Figure 3, inset),
which could be easily distinguished by the naked eye under the aid of a normal UV
lamp (λex =365 nm). To validate the selectivity of sensor S, the same tests were also
−
−
−
applied using Cl−, Br−, I−, AcO−, H2PO4 , HSO4 , ClO4 , CN− and SCN− anions, and
none of these anions induced any significant changes in the fluorescent spectrum of
the sensor.
(Insert Figure 2)
(Insert Figure 3)
The binding affinities of S and F− have been examined through the titration
experiments. As shown in Figure 4, with the increasing concentrations of F−, the
strong absorption band of S centered at 422 nm gradually decreased along with the
appearance of a new absorption band centered at 456 nm. The formation of new band
may be due to the hydrogen bond formation of the F– with Ar-OH protons or

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deprotonation of Ar-OH protons [32]. Similarly, in the florescence titration spectrum
(Figure 5), while the concentration of F− was increased, the emission maximum at 488
nm gradually decreased. The sensor gives an immediate response to the fluoride anion
(see supporting information, Figure S5) and the detection limit of the S for the
determination of F− was estimated to be 5.78×10−7 M, which pointing to the high
detection sensitivity (see supporting information, Figure S6). The Job plot between S
and F− was implemented, demonstrating a 1:4 stoichiometry for the S and F−, as
shown in Figure 6 [35].
(Insert Figure 4)
(Insert Figure 5)
(Insert Figure 6)
1
To explore the sensing mechanism of sensor S to F−, the H NMR titration were
investigated, which illustrated the characteristic structural changes occurring upon
interaction with F−. As shown in Figure 7, before the addition of fluoride anion, the 1H
NMR chemical shifts of the Ar-OH and -HC=N- protons on S were at δ 11.51, 8.61
ppm, respectively. Upon the addition of 0.5 and 1.0 equiv. of fluoride anion, the
resonance for Ar-OH were gradually decreased and then completely disappeared.
Further addition of F− to 2.0 equiv. resulted in the formation a new peak at 16 ppm
which pertained to [HF2]− [36]. These observations obviously indicate the
deportonation process of the fluoride anion with the two Ar-OH groups in sensor S. A
plausible mechanism for the formation of S-F− has been proposed in Scheme 2.
(Insert Figure 7)
(Insert Scheme 2)
Further to confirm the proposed mechanism of sensor S with F−, we performed
DFT calculations done at the B3LYP/6-311+g (2d, p) level of theory [37]. The

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optimized stationary structures (minima, saddle points) on the potential surfaces of the
reactions are depicted schematically in Figure 8. Calculations results show: 1) the
sensor S firstly combined with F− through hydrogen bond and then experienced the
deprotonation process at higher concentrations of F− anion to the two Ar-OH groups;
2) the reaction process of IM1 to IM2 is endothermic by 19.5 kJ·mol−1 on the ground
potential energy surface, while the whole reaction is exothermic by 277.7 kJ·mol−1,
which showed that the reaction of probe S with F− is easy to carry out. These
calculations results are consistent with the experimental results.
(Insert Figure 8)
To investigate the practical application of sensor S, test trips were prepared by
immersing filter papers into DMSO solution of S (0.1 M) and then drying in air. The
test strips containing S were utilized to sense F−. As shown in Figure 9, when F− anion
was added on the test kits, the obvious color change was observed both in visible light
and under the 365 nm UV-lamp.
(Insert Figure 9)
4. Conclusion
In summary, we have proposed a facile, low-cost, and efficient double Schiff-base
example of a highly selective chemosensor for F− through hydrogen bonding and
deprotonation between S and the anion. The sensor gives an immediate response to the
fluoride anion both by visible color changes as well as fluorescence turn-off response.
Fluoride anion are detectable by hydrogen bonding and deprotonation to the Ar-OH
groups and the mechanism of the reaction was investigated by various means (such as
1H NMR spectroscopic titration，ESI-MS and DFT calculation). Moreover, test strips
based on sensor S was fabricated, which could serve as practical colorimetric to detect
F– for “in-the-field” measurement and did not require any additional equipment but

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just by virtue of “dip-sticks” approach. We believe the test strips could act as a
convenient and efficient F− test kit. More applications of this efficient strategy might
be found in extended occasions of this concept, such as with combinations of
anion-neutral molecules.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No.
21064006, 21262032, and 21161018), the Program for Changjiang Scholars and
Innovative Research Team in University of Ministry of Education of China (No.
IRT1177), the Natural Science Foundation of Gansu Province (No. 1010RJZA018),
the Youth Foundation of Gansu Province (No. 2011GS04735), and
NWNU-LKQN-11-32.
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O
HO
NEt2
OH
.
H NNH H O,
AcOH
2
2
2
N
2
N
EtOH, reflux
Et2N
OH
NEt2
Scheme 1. Synthetic procedures for sensor S.
Et2N
O
Et2N
Et2N
F-
F-
O
F-
O
N
N
H
N
N
-
H
-HF2
H
N
H
N
F-
O
O
O
NEt2
NEt2
NEt2
Scheme 2 A possible mechanism of the sensor S to F−.

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Figure 1 The single crystal X-ray structure of S.

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Figure 2 Absorbance emission data for probe S in the DMSO solution.
Inset: color changes observed for S upon the addition of F−, Cl−, Br−, I−,
−
−
−
AcO−, H2PO4 , HSO4 , ClO4 , CN−, SCN− (50 equiv., respectively) in
DMSO.

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Figure 3 Fluorescence emission data for probe S in the DMSO solution
(excitation wavelength=422 nm). Inset: Visual fluorescence emissions of
−
−
−
S upon the addition of F−, Cl−, Br−, I−, AcO−, H2PO4 , HSO4 , ClO4 , CN−,
SCN− (50 equiv., respectively) in DMSO on excitation at 365 nm using
UV lamp at rt.

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Figure 4 Absorbance spectra of probe S in the presence of different
concentration of F− in DMSO. Inset: A plot of absorbance depending on
the concentration of F− in the range from 0 to 120.56 equiv. The detection
wavelength was 480 nm.

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Figure 5 Fluorescence spectra of probe S in the presence of different
concentration of F− in DMSO. Inset: A plot of fluorescence intensity
depending on the concentration of F− in the range from 0 to 111.64 equiv.
The detection wavelength was 488 nm.

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Figure 6 Job’s plot of S with F− in DMSO, indicating the 1:4
stoichiometry for S and F− clearly.

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1
Figure 7 Partial H NMR spectra of S (0.05M, DMSO-d6) and in the
presence of varying amounts of F−.

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50.0
)
1
-
S
l
o
0.0
m
·
J
-50.0
k
(
y
-100.0
g
r
e
IM2
n
e
-150.0
-200.0
-140.6
IM1
e
v
i
-160.1
t
a
IM3
l
e
-209.5
R
-250.0
-300.0
P
-277.7
Figure 8 The optimized stationary structures on the potential surfaces of the reactions.
(R1 is sensor S, IM1 is intermediate 1, IM2 is intermediate 2, IM3 is intermediate 3, P
is S-F−, pale-blue ball is fluoride, grey ball is carbon, blue ball is nitrogen, red ball is
oxygen, white ball is hydrogen).

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Figure 9 Photographs of S on test papers (a) left: only S, right: after
immersion into DMSO solutions with F−; (b) left: only S, right after
immersion into DMSO solutions with F− under irradiation at 365 nm.

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Graphical Abstract
A colorimetric and fluorescent dual-channel fluoride chemosensor N,N’-bis
(4-diethylaminosalicylidene) hydrazine (sensor S) bearing two imine groups has been
designed and synthesized. This structurally simple probe displays rapid response and
−
high selectivity for fluoride over other common anions (Cl−, Br−, I−, AcO−, H2PO4 ,
−
−
HSO4 , ClO4 , CN− and SCN−) in a highly polar aqueous DMSO solution. Mechanism
studies suggested that the sensor firstly combined with F− through hydrogen bonds
and then experienced the deprotonation process at higher concentrations of F− anion
to the two Ar-OH groups. The detection limits was 5.78×10−7 M of F−, which pointing
to the high detection sensitivity. Test strips based on sensor S were fabricated, which
could act as a convenient and efficient F− test kit to detect F− for “in-the-field”
measurement.

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Highlights
1. Synthesize an effective F− chemosensor.
2. Study the mechanism by different way.
3. Test trips can be used to detect F− easily.