A novel fluoride ion colorimetric chemosensor based on coumarin

Article abstract of DOI:10.1016/j.saa.2011.04.068

A novel visible colorimetric sensor (L₁) with high selectivity for fluoride ion based on coumarin has been synthesized by a simple modification of our earlier report. The chemosensor L₁ shows an obvious color change from yellow to bl

Full text of DOI:10.1016/j.saa.2011.04.068

Spectrochimica Acta Part A 79 (2011) 1352–1355
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A novel fluoride ion colorimetric chemosensor based on coumarin
Xiaoqing Zhuang, Weimin Liu∗, Jiasheng Wu, Hongyan Zhang, Pengfei Wang
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel visible colorimetric sensor (L1) with high selectivity for fluoride ion based on coumarin has been
synthesized by a simple modification of our earlier report. The chemosensor L1 shows an obvious color
change from yellow to blue upon addition of fluoride ion with a large red shift of 145 nm in acetonitrile,
and without interference of other anions such as Cl−, Br−, I−, NO3−, H2PO4−, HSO4−, and AcO−. The
investigation of 1H NMR spectrum titration indicates the proposed mechanism is that F− first establishes
a hydrogen bonding interaction with L1, and then the formation of [F–H–F]− induces deprotonation.
© 2011 Elsevier B.V. All rights reserved.
Received 22 March 2011
Received in revised form 15 April 2011
Accepted 27 April 2011
Keywords:
Colorimetric chemosensor
Anion recognition
Fluoride sensing
Coumarin
Schiff base
1. Introduction
fore, there is a great challenge to construct the colorimetric sensor
with specific receptor for the fluoride ion and suitable chromophore
Anion recognition is an important issue in supramolecular
chemistry as anions play a significant role in chemistry industry,
biochemistry, and environmental science [1,2]. As the first anion
in the periodic table of elements, fluoride ion has unique biology
and chemistry properties. It is an essential trace element in human
and shortage of fluoride results in dental caries and osteoporosis.
In many countries, fluoride has been added to water widespread
to prevent tooth decay. However, high dose or chronic ingestion of
lower doses of fluoride may result in fluorosis, and lead to nephro-
toxic changes and urolithiasis in humans [3–8]. In addition, with the
development of industry and the increase in the number and the
scale of enterprises on fluorine products, the contamination of fluo-
rine becomes more serious. For these reasons, selective recognition
of fluorine ion is of realistic significance.
In recent years, many analytical methods have been devel-
oped for fluoride ion detection. Among all the analysis methods,
colorimetry, known as naked eye detection, has attracted much
attention due to its rather straightforward and fast measurement
protocols and no need of valuable instruments [9,10,2,11–13].
These methods are mostly based on N–H proton transfer from the
donor unit to fluoride ion, which induced ꢀ-electron delocalization,
or N–H deprotonation [14]. The hydrogen donator includes amine
[15], amide [16], pyrrole [17,18], and urea [19–21]. However, highly
basic anions such as CH3CO2− or H2PO4− can also act as an acceptor,
and cause similar spectral changes with fluoride ion [22,23]. There-
for obvious color change. Up to now, the colorimetric sensors based
on N–H proton transfer that satisfy the above conditions are rarely
reported.
Present study is a part of our current research interest towards
chemosensors based on coumarin with Schiff base (–C N–) as
bridge [24–26]. The present chemoreceptor in this work is a simple
structural modification of one of our earlier report [27]. This modi-
fication was worthwhile to modulate the acidity of N–H proton by
decreasing the numbers of nitro-group, so that the new receptor
can be able to discriminate between anions of high and compa-
rable basicity like fluoride and dihydrogen phosphate. Compound
L1 was designed and synthesized by the condensation of 7-N,N-
diethyl-aminocoumarin and 4-nitrophenylhydrazine (Scheme 1).
The result shows that the fluoride ion could be detected by naked
eyes without interference from other anions. The details of anion
binding characteristics of the compound L1 have been investigated
by UV–vis and 1H NMR titrations. The maximum of absorption of L1
shifts 145 nm in the presence of the fluoride ion in CH3CN solution
due to the N–H deprotonation and formation of the particularly
stable [F–H–F]− dimer.
2. Experimental
2.1. Reagents and instruments
7-(Diethylamino)-2-oxo-2H-chromene-3-carbaldehyde
was synthesized according to reported methods [24]. 4-
nitrophenylhydrazine hydrochloride and all anions were
purchased from Sigma Aldrich and used without any further
∗
Corresponding author. Tel.: +86 10 82543475; fax: +86 10 82543475.
E-mail address: wmliu@mail.ipc.ac.cn (W. Liu).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.04.068

X. Zhuang et al. / Spectrochimica Acta Part A 79 (2011) 1352–1355
1353
H
N
CHO
N
+
O2N
NHNH2
N
O
O
N
O
O
NO2
L1
Scheme 1. Preparation of L1.
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
purification. N,N-dimethyl formamide was dried with calcium
hydride and distilled at reduced pressure prior to use. Acetonitrile
was chromatographical grade, and other chemicals were analytical
grade.
All UV–vis spectra in this work were recorded using Hitachi
U3010 spectrometer. NMR spectra were recorded on Varian Gemin-
400 NMR instrument (400 MHz), and MS spectra were performed
on Finnigan 4021C and APEXII FT-ICR.
Blank, Cl -, Br-, I-, AcO-, HSO4-, NO3-
F -
H2PO4-
2.2. Synthesis
7-(Diethylamino)-2-oxo-2H-chromene-3-carbaldehyde
(245 mg, 1 mmol) and 4-nitrophenylhydrazine hydrochloride
(189 mg, 1 mmol) was added in ethanol (40 mL) (Scheme 1). The
mixed solution was refluxed for 6 h, cooled to room temperature.
Then the solvent was concentrated to half and garnet crystals were
precipitated, washed with hot ethanol for several times to yield L1
198 mg (yield 52.1%). 1H NMR (DMSO-d6) ı 1.13 (6H, t, J = 7.0 Hz),
3.45 (4H, q, J = 7.1 Hz), 6.57 (1H, d), 6.74 (1H, dd), 7.15 (1H, d),
7.58 (1H, d), 8.07 (1H, s), 8.11 (2H, m), 8.37 (1H, s), 11.39 (1H, s).
TOF-HRMS found [M]+: m/z, 380.1254, Calcd. 380.15.
300 350 400 450 500 550 600 650 700
Wavelength(nm)
Fig. 1. Absorption spectra changes of L1 (5 ␮M in MeCN) upon addition of different
anions (20 equiv).
2.3. General spectroscopic procedures
Stock solutions of L1 was prepared by dissolving L1 in acetoni-
trile to the concentration of 1.0 × 10−3 M and stored at −25 ◦C in
the dark. Then the stock solution was diluted with acetonitrile to
a final concentration of 5 ␮M for spectra analysis. All anion stock
solutions were prepared by dissolving corresponding tetra-n-butyl
ammonium salts into acetonitrile. Each time 2 mL of L1 solution
was filled in a quartz cell of 1 cm optical path length, and different
stock solutions of anions were added into the quartz cell gradually
using a pipette. The volume of anion stock solution added was less
than 100 mL to keep the total volume of testing solution without
obvious change. The temperature of analysis was 25 ◦C.
Fig. 2. Color changes upon addition of 20 equiv different anions in 10 ␮M MeCN
solution of L1.
than other anions. This is mainly because the ion radius of F− is
smaller and the charge density is higher compared to other anions.
To evaluate the effect of the concentration of F− on UV–vis
spectrum, the UV–vis titration experiment of L1 was carried out
in acetonitrile. Upon the addition of F−, the absorption peak at
3. Results and discussion
5
4
3
2
1
The interactions between L1 and anions were investigated by
UV–vis spectroscopy. The maximum absorption of L1 is 475 nm and
the molar absorption coefficient is 5.0 × 105 in acetonitrile. Fig. 1
shows the absorption spectra of L1 in the presence of several anions.
On the addition of 20 equiv of F−, the color of L1 turned from yellow
to blue (Fig. 2). (For interpretation of the references to color in this
text, the reader is referred to the web version of the article.) A new
peak appears at 620 nm with a shoulder peak at 650 nm, indicat-
ing a red shift of 145 nm. However, L1 has no color change in the
presence of 20 equiv Cl−, Br−, I−, NO3−, H2PO4−, HSO4−, and AcO−.
This result shows that L1 exhibits highly selective recognition of
F−.
0
F -
Cl -
+
Br -
+
I - AcO- H2PO4- HSO4- NO3
-
It deserves to be specially noted that the presence of other
anions do not interfere the color reaction of L1 with F− except for
HSO4−, as shown in Fig. 3. On the addition of 20 equiv of F−, red
shift occurs in the absorption spectra in the presence of 20 equiv
Cl−, Br−, I−, NO3−, H2PO4−, HSO4−, and AcO−, which indicates that
the hydrogen bonding interaction between L1 and F− is stronger
+
+
+
+
+
F -
F -
F -
F -
F -
F -
F -
Fig. 3. Variation of absorption spectra of L1 (5 ␮M in MeCN) in the presence of
20 equiv F− with coexisting competitive anions (20 equiv).

1354
X. Zhuang et al. / Spectrochimica Acta Part A 79 (2011) 1352–1355
F -
H
N
H
F -
N
N
N
N
O
O
NO2
N
O
O
NO2
L1
F -
N
N
N
N
O
N
O
O
N
O
N
O
O
NO2
Scheme 2. Proposed mechanism for L1 with F−
.
475 nm was decreasing, whereas new absorption peaks at 620 nm
and 650 nm were observed and gradually increased (Fig. 4a). As
shown in titration curves at 475 nm and 620 nm (Fig. 4b), when the
addition of F− is less than 2 equiv, there are no significant changes
in spectra. When more than 2 equiv F− is added, the spectra signif-
icantly change. These changes indicate the reaction takes place in
steps and the first step shows little influence on absorption spec-
tra [28]. The appearance of new absorption peaks at 620 nm and
650 nm may indicate the existence of similar structures or similar
conjugate structures in the reaction system.
In order to clarify the function mechanism of the interaction
of L1 with F−, 1H NMR spectroscopy was investigated. In par-
ticular, a DMSO-d6 solution in L1 was titrated with F−, which
was added stepwise up to 5 equiv. Fig. 5 shows different 1H NMR
spectra obtained in the course of the titration. After the addi-
tion of 5 equiv of F−, the –NH proton (11.39 ppm) disappeared,
while a typical triplet influenced by F− was appeared at 16.14 ppm
which indicated the formation of [F–H–F]−. The mechanism is
that –NH interacts with F− through hydrogen bonding at first,
and then more F− participate in the interaction inducing the pro-
ton transfer reaction [29]. As shown in Scheme 2, when –NH is
deprotonated, the electron density is increased. This leads to the
intramolecular charge transfer in the whole system and the red
shift of absorption spectrum. Meanwhile, the electron density of
nitroaromatic ring is greatly increased and the hydrogens on aro-
matic ring shift upfield significantly. This explains the changes in
the UV–vis titration of L1 with F−. The absorption spectra shift
little when the hydrogen bond complex is formed in the first
step. But when the deprotonation reaction undergoes further, the
absorption spectra shift dramatically. Besides, we also performed
UV–vis titration of L1 with 20 equiv F− in proton polarity sol-
vent methanol. The absorption peaks at 620 nm and 650 nm were
decreasing and showed a blue shift, while the absorption peak
at 475 nm was increasing. At last, the titration spectrum over-
lapped with the absorption spectrum of L1. This indicates that
0.25
a
[F-] 10-5mol/L
0.20
0
0.15
0.10
0.05
0.00
10
300
400
500
600
700
Wavelength(nm)
b
0.25
0.20
0.15
0.10
0.05
0.00
620nm
475nm
0
2
4
6
8
10
[F-] / [L1]
Fig. 4. (a) Absorption spectra changes of L1 (5 ␮M in MeCN) in the course of the
titration of F− (0–50 ␮M). (b) The absorbance was measured at 475 nm and 620 nm,
respectively with titration of F− (0–50 ␮M).
Fig. 5. 1H NMR spectra of (a) L1 in DMSO-d6 and (b) L1 in DMSO-d6 upon addition
of 5 equiv F−
.

X. Zhuang et al. / Spectrochimica Acta Part A 79 (2011) 1352–1355
1355
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4. Conclusion
In summary, a new colorimetric chemosensor L1 based on
coumarin with –NH as hydrogen donator and –C N as bridge has
been synthesized by a simple modification of our earlier report.
When treated with F−, a dramatic color change from yellow to blue
is observed. The study of UV–vis absorption spectroscopy indicates
that L1 exhibits highly selective recognition of F− without interfer-
ence from other anions, which results a 145 nm red shift of the
maximum absorption. The study of 1H NMR spectroscopy indi-
cates the mechanism is that the proton transfer reaction between
L1 and F− leads to the delocalization of electron in the system,
which increases the intermolecular charge transfer and makes the
absorption spectra red shift significantly. Thus, L1 can be applied as
a highly selective naked-eye colorimetric chemosensor for F−.
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This project was supported by the Main Direction Program of
Knowledge Innovation of Chinese Academy of Sciences, the NNSF
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