A novel colorimetric and fluorescence anion sensor with a urea group as binding site and a coumarin group as signal unit

Article abstract of DOI:10.1016/j.dyepig.2010.04.007

A novel colorimetric and fluorescent anion sensor based on urea substituted with a coumarin moiety and a pendant phenylazo unit was synthesized and evaluated according to the binding site-signalling subunit approach. The azo group was exploited as the chromogene, the urea moiety providing the anion binding site and the coumarin system was responsible for fluorescence. As such, the dye enabled visual detection of acetate ion in DMSO (color change light yellow to red) without resorting to spectroscopic instrumentation. More importantly, the sensor displayed significant fluorescence enhancement response to anions such as acetate, because of complex formation.

Full text of DOI:10.1016/j.dyepig.2010.04.007

Dyes and Pigments 87 (2010) 272e276
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel colorimetric and fluorescence anion sensor with a urea group
as binding site and a coumarin group as signal unit
*
Jie Shao
Department of Chemistry and Materials Science, Nanjing Forestry University, Nanjing 210037, 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 colorimetric and fluorescent anion sensor based on urea substituted with a coumarin moiety and
a pendant phenylazo unit was synthesized and evaluated according to the binding site-signalling subunit
approach. The azo group was exploited as the chromogene, the urea moiety providing the anion binding
site and the coumarin system was responsible for fluorescence. As such, the dye enabled visual detection
of acetate ion in DMSO (color change light yellow to red) without resorting to spectroscopic instru-
mentation. More importantly, the sensor displayed significant fluorescence enhancement response to
anions such as acetate, because of complex formation.
Received 10 November 2009
Received in revised form
15 April 2010
Accepted 16 April 2010
Available online 27 April 2010
Keywords:
Anion sensor
Urea
Ó 2010 Elsevier Ltd. All rights reserved.
Coumarin
Azo
Fluorescence
Turn on
1. Introduction
Hitherto, numerous colorimetric and/or fluorescent chemo-
sensors for anions, cations or neutral molecules have been reported;
In recent years, considerable attention has attended the devel-
opment of colorimetric and fluorescent chemosensors for sensing
anionic species [1e5]. Commonly, such sensors offer many advan-
tages such as high sensitivity and simplicity, especially for real-time
and on-line analysis of analytes [6,7]. Anions are widely found in
the environmental and biological systems, as exemplified by fluo-
ride, which is commonly used in dental care applications and
exhibits inhibition of certain enzyme functions [8]; however,
overexposure to fluoride can also result in the most widespread
side effect of fluoride (fluorosis) [9] which induces nephrotoxic
changes in both humans and animals and leads to urolithiasis [10].
Acetate ion has been found to be a possible tracer for malignancies
and has been extensively investigated in prostate cancer and its
metastases [11]. Accordingly, the determination of anionic species
and their concentration is of interest in many areas, including food
processing, environmental evaluation and clinic analysis [12e14],
in which context, the development a novel colorimetric and fluo-
rescent chemosensor for the rapid and convenient detection of
biologically important anions such as fluoride and acetate [15e17]
is required.
however, almost all of these are designed to rely upon the same
principles namely, that the functional moiety is covalently (binding
site-signalling subunit approach) [18e21] or non-covalently
(displacement approach) [22,23] linked to the signal moiety
(eg chromogene, electrochemical group, fluorescent group). The
functional moiety is responsible for binding guests and the signalling
moiety acts as a signal transducer, that converts binding events into
an optical signal expressed as a change in either color, electrochem-
istry or fluorescence [24]. Mechanisms that involve the photophysics
of fluorogenic anion sensors, include photoinduced electron transfer
(PET) [25], photoinduced charge transfer (PCT) [26] and excimer/
exciplex formation [27]. In general, a PET fluorescent sensor displays
an ‘ONeOFF’ response towards anionic guests [28] and a fluorescent
sensor based on PCT or excimer/exciplex formation shows a ratio-
metric response towards anions, which can quantify the analyte
concentration by using the ratio of intensities [29,30]. However,
despite many advantages, excimer/exciplex formation or PCT-based
fluorogenic sensors for anions are not common in the literature.
This paper concerns a novel colorimetric and fluorescent anion
sensor 1 that comprises a urea group (anion binding site) coupled
to with a 6-(phenylazo)-coumarin group (chromogenic and fluo-
rescence unit). The behaviour of this new compound towards
anions eg acetate, was investigated by UVevis and fluorescence
spectroscopy in DMSO.
* Tel.: þ86 25 85427621; fax: þ86 25 85427080.
E-mail address: njshao@live.cn
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.04.007

J. Shao / Dyes and Pigments 87 (2010) 272e276
273
2. Experimental section
(5 ml; 0.05 ml) was dissolved in a solution comprising 18 g NaCO3
and 150 ml H2O and the resulting solution of salicylaldehyde was
added dropwise to the bright yellow coloured solution over 1 h.
After stirring for 4 h, the reaction mixture was neutralized with HCl,
the brown crude solid was filtered and recrystallized from ethanol
to afford a pure yellow product. Mp: 120 ꢃC (Fig. 2).
2.1. Apparatus
1H NMR spectra were obtained on a Varian UNITY Plus-400 MHz
Spectrometer. ESI-MS was performed with a MARINER apparatus.
C, H, N elemental analyses were made on an elementar vario EL.
UVevis spectra and fluorescence spectra were recorded on a Shi-
madzu UV2450 Spectrophotometer and a Shimadzu RF-5301PC
fluorimeter, respectively, at 298.2 ꢀ 0.1 K.
2.4.2. 6-Phenylazo-coumarin-3-formylethylester (3)
3 was synthesized according to the literature [32]. In a 250 ml
round-bottomed flask was added 3 (4.52 g, 20.0 mmol), 5 (3.2 g,
20.0 mmol), a catalytic amount of piperdine and ethanol (140 mL).
The resulting mixture was heated to reflux under magnetic stirring
for 6 h after which the solution was cooled to room temperature
over 10 h and the brown precipitate which appeared was collected
by filtration, washed with ethanol and recrystallized from ethanol
to give the pure brown coloured product (3.24 g). Yield ¼ 50%.
2.2. Materials
All reagents for synthesis obtained commercially were used
without further purification. In the titration experiments, all the
anions were added in the form of tetra-n-butylammonium (TBA)
salts, which were purchased from Chemical Company Ltd., stored in
a vacuum desiccator containing self-indicating silica and dried fully
before use. DMSO was dried with CaH2 and then distilled under
reduced pressure.
2.4.3. 6-Phenylazo-coumarin-3- formylhydrazine (4)
4 (0.644 g, 2 mmol) was dissolved in ethanol (40 ml), to which
was added hydrazine in ethanol (5 ml) dropwise and the ensuing
mixture was stirred at reflux for 4 h. The reaction solution was
cooled to room temperature and the brown precipitate was filtered,
washed with ethanol and used in the next step without further
purification.
2.3. General method
All titration experiments were carried out at 298.2 K, unless
otherwise mentioned. UVevis spectra and fluorescent spectra were
measured using a Shimadzu UV2450 Spectrophotometer and
2.4.4. N-(6-Phenylazo-coumarin-3-formyl)-N0-4-
(nitrophenyl)urea (1)
a Shimadzu RF-5301PC fluorimeter, respectively. A 5.0 ꢁ 10ꢂ5
M
1 was prepared according to the literature procedure [14,33]. 2
(0.31 g, 1.0 mmol) was dissolved in hot CH3CN (25 mL) to which was
then added, dropwise, 4-nitrophenylisocyanate (0.16 g, 1.0 mmol)
and the ensuing mixture was stirred under an inert atmosphere at
reflux for 5 h. The reaction mixture was cooled to room tempera-
ture and the brown precipitate formed was filtered and washed
with CH3CN, yield ¼ 0.36 g, 76%. 1H NMR (400 MHz; DMSO-d6;
Me4Si): dH 11.07 (s, eNH, 1H), 10.98 (s, eNH, 1H), 9.71 (s, eNH, 1H),
8.65 (s, AreH, 1H), 8.41 (s, AreH, 1H), 8.24 (d, J ¼ 9.2 Hz, AreH, 2H),
8.01 (d, J ¼ 9.2 Hz, AreH, 2H), 7.88 (d, J ¼ 7.6 Hz, AreH, 2H), 7.83 (m,
AreH, 1H), 7.62 (m, AreH, 2H), 7.56 (d, J ¼ 7.2 Hz, AreH, 1H), 7.10 (d,
J ¼ 8 Hz, AreH, 1H); ESI-mass: m/z calcd. for C23H16N6O6 [M]
472.41, found: 473.13; Elemental analysis calcd for C23H16N6O6: C
58.48%, H 3.41%, N 17.79%, found: C 58.21%, H 3.32%, N 18.01%.
solution of the compound 1 in dried DMSO and solutions of 0.10 M
tetrabutylammonium (TBA) salts of the respective anions were
prepared in dried DMSO and were stored under a dry atmosphere.
These solutions were used for all spectroscopic studies after
appropriate dilution. Then, a given amount of the solution of 1 was
added to the quartz cuvette and the increased amount of anions
tested (0.1 M in DMSO-d6) was added to the solution. The absor-
bance/emission spectra were then recorded immediately. The slit
width used for fluorescence measurements was 5 mm.
2.4. Synthesis
2.4.1. 5-Phenylazo-salicylaldehyde (2)
The route to 1 is shown in Figs. 1 and 2 was prepared according
to the reported method [21,31]. To a solution of aniline (5 ml;
0.05 mol) in a small quantity of water was slowly added 6 ml of 37%
aq HCl solution at 0e5 ꢃC with stirring. 20 ml of 20% aq NaNO2
solution was added to the mixture and the resulting solution was
stirred for 1 h, affording a bright yellow solution. salicylaldehyde
3. Results and discussion
Coumarin derivatives have been extensively investigated for
electronic and photonic applications, such as fluorescence probes,
charge-transfer agents, solar energy collectors, and nonlinear
optical properties due to their inherent photochemical character-
istics, reasonable stability, good solubility and their relative ease of
synthesis [4,34e36]. A great number of fluorescent probes based
on coumarin derivatives for various transition-metal ions have
been developed in recent years [37,38], however, fluorescent
anion sensors derived from coumarins have been scarcely repor-
ted [39]. In addition, the urea group has been proven to be an
excellent hydrogen bonding donor exploited in design and
synthesis of anion sensors [40e42]. Bearing these in mind, we
OH
CHO
Ethanol, Piperdine
NH2
COOC2H5
COOC2H5
OH
HCl, Na2CO3
+
+
reflux, 50%
N
NaNO2, H2O 75%
CHO
N
2
CONHNH2
O
O
O
O
CO2Et
NO2
Ethanol, reflux
45%
CH3CN, N2
1
N2H4
+
ref lux, 76%
Chromophore Unit
O
O
O
NO2
O
N
N
N
N
NCO
H
N
Fluorescence Unit
Anion Binding Site
N
N
N
H
N
H
3
4
Fig. 1. Synthesis of N-(6-phenylazo-coumarin-3- formyl)-N0-4-nitrophenylurea (1).
Fig. 2. Structure of the sensor 1.

274
J. Shao / Dyes and Pigments 87 (2010) 272e276
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.8
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1 only
1+3 equiv AcO-
1+3 equiv AcO- + other anions
1+3 equiv F-
0.6
0.4
1+3 equiv H2PO-4
1+excess Cl-
1+excess Br-
1+excess I-
0.0
8.0x10
1.6x10
-
2.4x10
3.2x10
0.2
0.0
300
400
500
600
700
wavelength/nm
300
400
500
600
700
Fig. 5. Variation in the absorption spectra of 1 (2 ꢁ 10ꢂ5 M) induced by different
Wavelengh/nm
anions in DMSO.
Fig. 3. Variation in the absorption spectra of 1 (2 ꢁ 10ꢂ5 M) in DMSO with increasing
concentrations of AcOꢂ
stoichiometry.
.
Inset: The titration curves for AcOꢂ show the 1:1
complex having a certain stoichiometric ratio between 1 and AcOꢂ
formed in solution. The titration profile of 1 with AcOꢂ shown in
inset of Fig. 3 demonstrated 1 equiv of AcOꢂ reacting with 1 equiv of
1 could quickly reached an equilibrium, indicating the formation of
a 1/AcOꢂ complex of 1:1 stoichiometry.
introduced an azo moiety (a chromophore unit) to the coumarin
derivative (a fluorescence unit) and thus the intermediate (3-
substituted-6-(phenylazo)-coumarin) was obtained. Then, the
colorimetric and fluorescent anion sensor 1 was successfully
prepared through coupling the urea group with the intermediate.
Firstly, the anion binding and sensing ability of 1 was investi-
gated in detail in DMSO. Fig. 3 demonstrates the changes in the
absorption spectrum of 1 (2 ꢁ 10ꢂ5 M) in DMSO in the absence and
the presence of AcOꢂ. The free compound 1 exhibited one main
band at 348 nm with a shoulder band at 458 nm, which could be
The sensor 1 gave a similar spectral response towards Fꢂ ion.
The presence of the other anions (Clꢂ, Brꢂ and Iꢂ) with weak
basicity resulted in negligible changes in UVevis spectra of 1, which
could be seen from Fig. 5. Surprisingly spectral changes were
induced by addition of H2POꢂ4 ions, which possibly resulted from an
uncertain stoichiometric ratio between 1 and H2POꢂ4 (see Fig. 6).
Fig. 4 displayed color changes of 1 (2 ꢁ 10ꢂ5 M, DMSO) in presence
of the different anions tested. Obviously, addition of AcOꢂ, Fꢂ and
H2POꢂ4 ions induced similar color changes from light yellow to red
and no significant color changes were seen when Clꢂ, Brꢂ and Iꢂ
ions were added, which was consistent with results observed from
spectral titrations. In particular, the selectivity for acetate anion
over a complex background of potentially competing species was of
interest and was thus evaluated (see Fig. 5). Obviously, the spectral
changes of 1 resulting from AcOꢂ were influenced slightly by the
subsequent addition of miscellaneous anions.
attributed to
pep* transition of the azo moiety. As soon as the
acetate ion was added at room temperature, the absorption
maximum of 1 at 348 nm decreased gradually whereas the
absorption at 259 nm and a new absorption maximum at 458 nm
increased. The color changes of sensor 1 were also observed by eye
simultaneously from light yellow to red in presence of AcOꢂ (see
Fig. 4). Additionally there were two well-defined isosbestic points
at 301 nm and 385 nm, respectively, indicating that a stable
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength/nm
Fig. 4. Color changes of the sensor 1 (2 ꢁ 10ꢂ5 M) in absence and presence of different
anions (A: the free 1; B: 1 þ 5 equiv AcOꢂ, Fꢂ or H2PO4ꢂ; C: 1 þ excessive equiv Clꢂ
,
Fig. 6. Variation in the absorption spectra of 1 (2 ꢁ 10ꢂ5 M) in DMSO with increasing
Brꢂ or Iꢂ).
concentrations of H2PO4ꢂ
.

J. Shao / Dyes and Pigments 87 (2010) 272e276
275
Table 1
28
Equilibrium constants (Ks molꢂ1 L) of the sensor
298.2 ꢀ 0.1 K.
1 with anions in DMSO at
26
24
22
20
18
16
14
12
10
8
Anionsa
Ks (Mꢂ1
AcOꢂ
Fꢂ
H2POꢂ4
Clꢂ
Brꢂ
Iꢂ
b
c
)
9.87 ꢁ 105
8.29 ꢁ 103
_
_
_
_
_
_
R^2
0.996
0.994
_
_
a
All the anions were added in the form of tetrabutylammonium salts.
b
The titration profile was not suitable for determination of the equilibrium
constant.
c
The equilibrium constants could not be determined due to slight spectral
changes.
6
4
The equilibrium constants (Ks) of 1 with different anions were
determined by nonlinear fitting analyses of the titration curves
according to the Eq. (1), 1:1 hosteguest complexation [43].
2
0
350
400
450
500
Wavelength/nm
A ¼ A0
ꢀ
ꢁ
h
i
Fig. 8. Variation in the emission spectra of 1 (2 ꢁ 10ꢂ5 M) in DMSO with increasing
1=2
ðAlim ꢂAÞ cH þcG þ1=Ks ꢂ ðcH þcG þ1=KsÞ2ꢂ4cHcG
concentrations of AcOꢂ
(
lexc ¼ 382 nm).
þ
2cH
(1)
state less probable; consequently, the emission intensity increased
(see Fig. 8).
Where, cG and cH are the concentration of guest and host,
respectively and A is the intensity of absorbance at a certain
concentration of host and guest. A0 is the intensity of absorbance of
host only and Alim is the maximum intensity of absorbance of host
when guest is added. Ks is the equilibrium constant. The equilib-
rium constant (Ks) could be determined from the plot of A against
[Anion] and the high value (R^2) of the plots (Table 1) confirmed
a 1:1 complex formation between 1 and anions. This further
corroborates 1:1 complex formation that we have concluded based
on the spectral titration profile.
The selectivity of the host for a specific anion of interest could be
rationalized on the basis of not only the guest basicity but also
complementary shape between the host and the anionic guests
[44]. In particular, multiple hydrogen-bonding interactions were
necessary in high-affinity anion binding sites. Consequently, it was
reasonable that the sensor 1 showed a high selectivity for acetate
ion among the other anions tested such as Fꢂ and H2PO4ꢂ (see Fig. 7).
The proposed binding mode of the host 1 with AcOꢂ ion in solution
was described in Fig. 7, which corresponds with observations from
previous studies [45].
4. Conclusion
In summary, a colorimetric and turn-on fluorescence anion
sensor was designed and synthesized according to the binding site-
signalling subunit approach. The urea group was exploited as an
anion binding site in 1 and 6-(phenylazo)-coumarin moiety acted
as both a fluorescence unit and a chromophore unit. In addition, the
fluorescence enhancement in the emission spectra of 1 were
observed due to anion-induced conformational restriction.
Acknowledgment
The author thank Prof. Z. Y. Luo for his help in preparation of the
paper and also thank anonymous reviewers for their valuable
suggestions on improvement of the present paper.
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