The synthesis and study of the fluorescent probe for sensing Cu 2+ based on a novel coumarin Schiff-base

Article abstract of DOI:10.1016/j.cclet.2014.05.001

A novel, fluorescent probe was synthesized from 2,4-dihydroxybenzaldehyde and 8-hydroxyquinoline for sensing Cu²⁺ by the naked eye. The structure was confirmed by IR, MS, ¹H NMR, ¹³C NMR and the spectral properties of the probe were investigated. It exhibited strong fluorescence responses toward Cu²⁺ and high selectivity over other metal ions. The binding constant between the probe and Cu²⁺ was calculated using Benesi-Hildebrand equation.

Full text of DOI:10.1016/j.cclet.2014.05.001

G Model
CCLET 2978 1–5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
3
4
The synthesis and study of the fluorescent probe for sensing Cu2+
based on a novel coumarin Schiff-base
a
a
Q1 Yu-Wei Duan a, Hao-Yang Tang b, Yuan Guo a, , Zhan-Ke Song , Meng-Jiao Peng ,
*
5
6
Yong Yan a
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9
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science,
Northwest University, Xi’an 710069, China
b School of Automation, Xi’an University of Posts and Telecommunications, Xi’an 710121, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel, fluorescent probe was synthesized from 2,4-dihydroxybenzaldehyde and 8-hydroxyquinoline
for sensing Cu2+ by the naked-eye. The structure was confirmed by IR, MS, 1H NMR, 13C NMR and the
spectral properties of the probe were investigated. It exhibited strong fluorescence responses toward
Cu2+ and high selectivity over other metal ions. The binding constant between the probe and Cu2+ was
calculated using Benesi–Hildebrand equation.
Received 27 February 2014
Received in revised form 6 April 2014
Accepted 16 April 2014
Available online xxx
ß 2014 Yu-Wei Duan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Keywords:
Fluorescent probe
Schiff-base
Copper ion
Coumarin
10
11
1. Introduction
detection of anions, cations and neutral molecules have been 33
reported [17,18]. Bai reported the S2ꢀ fluorescent sensors based 34
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Besides zinc and iron, copper ranks as third among the essential
heavy metals in abundance in human bodies [1]. Copper plays an
important role in the areas of biological, environmental, and
chemical systems [2]. It is an essential trace element for both
plants and animals, including humans, but alterations in its cellular
homeostasis are connected to serious neurodegenerative diseases,
including Menkes and Wilson diseases [3–5], familial amyotropic
lateral sclerosis [6,7], Alzheimer’s disease [8], and prion diseases
[9]. Hence, it is very essential to detect copper.
Fluorescence sensing has wide use in the environment,
chemistry, biology, and medicine due to its high sensitivity, high
selectivity, fast analysis with spatial resolution, and its sample/cell
non-destructive nature [10–12]. Thus, the design and synthesis of
fluorescent copper probes have attracted intense attention, most of
which, however, were synthesized by very difficult methods and
expensive raw materials [13–15]. Since coumarin derivatives
exhibited several advantages, such as ease of synthesis, not very
expensive, large Stokes shifts, good photostability and high
fluorescence quantum yields, they could be used as efficient
fluorophores [16]. Consequently, in recent years, the design and
synthesis of many coumarin-based fluorescent probes for the
on an 8-hydroxyquinoline-appended fluorescein derivative, which 35
had high selectivity to Cu2+ and exhibited a visible color change 36
[19]. Inspired by the idea, we synthesized a novel fluorescent probe 37
1
(CQP, Scheme 1) by ethyl-7-hydroxy-8-formylcoumarin-3- 38
carboxylate and 2-(quinolin-8-yloxy) acetohydrazide. 39
The fluorescent metal ion probes which were synthesized by 40
the PET (photo-induced electron transfer) recognition mechanism 41
always bear the selection of suitable energy donors, energy 42
acceptor and linkers [20–23]. Now, we report one novel fluorescent 43
probe, CQP, which was rationally designed on the basis of a 44
coumarin-quinoline scaffold as the PET mechanism platform. It 45
exhibited strong fluorescence responses and high selectivity 46
toward Cu2+ over other metal ions. With the addition of Cu2+
,
47
compound CQP could give rise to a visible colorless-to-yellow 48
color change and clear fluorescence quenching. The binding 49
constant between the compound CQP and Cu2+ was calculated 50
using Benesi–Hildebrand equation [24].
51
2. Experimental
52
The NMR spectra were measured by Varian Unity INOVA-400 53
spectrometer with tetramethylsilane (TMS) as internal standard. 54
Infrared (IR) spectra in cmꢀ1 were recorded on a Bruker EQUIOX-55 55
spectrometer (Germany). Mass spectra were acquired on a Bruker 56
*
Corresponding author.
E-mail address: guoyuan@nwu.edu.cn (Y. Guo).
http://dx.doi.org/10.1016/j.cclet.2014.05.001
1001-8417/ß 2014 Yu-Wei Duan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: Y.-W. Duan, et al., The synthesis and study of the fluorescent probe for sensing Cu2+ based on a novel
coumarin Schiff-base, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.001

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Y.-W. Duan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 1. The synthesis of compound 1 (CQP).
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micrOTOF-Q II Mass Spectrophotometer (Germany). All fluores-
cence measurements were made on a Hitachi F-4500 Fluorescence
Spectra (Tokyo, Japan) with excitation slit set at 5.0 nm and
emission at 5.0 nm in 1 cm ꢁ 1 cm quartz cell. Absorption spectra
were measured on Lambda 35 UV/vis spectrophotometer.
The solution of CQP was then diluted to 1 ꢁ 10ꢀ4 mol/L with
methanol water. A standard test solution was prepared by dilution
of the solution of CQP (1 ꢁ 10ꢀ4 mol/L). A stock solution of cupric
sulfate (1 ꢁ 10ꢀ2 mol/L) was prepared by dissolving 14.37 mg of
cupric sulfate in 50.0 mL water. A standard test solution was
prepared by dilution of the solution of cupric sulfate
(1 ꢁ 10ꢀ2 mol/L). A 0.2 mol/L Na2HPO4–citric acid buffer solutions
(pH 7.4) were employed in all test.
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All reagents of analytical grade were obtained from
a
commercial source and used without further purification. Deio-
nized water was used throughout all experiments. The Na2HPO4–
citric acid buffer solutions were prepared by using proper amount
of Na2HPO4 and citric acid under adjustment by a pH meter. All
inorganic salts used were of analytical grade. Co(NO3)2ꢂ6H2O,
Mn(NO3)2, Al2(SO4)3, Zn(NO3)2ꢂ6H2O, Ca(NO3)2ꢂ4H2O, HgCl2,
3. Results and discussion
117
Free CQP shows absorption at 275 nm and 385 nm in ethanol.
Upon the gradual addition of Cu2+, the intensity of the absorption
at 275 nm and 385 nm increased while at the same time the
hypochromatic shift appeared 50 nm and 20 nm, respectively, in
Fig. 1, resulting in a color change from colorless to yellow. Other
metal cations did not show the same phenomenon.
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Cu(NO3)2ꢂ3H2O,
SnCl2,
Ni(NO3)2ꢂ6H2O,
Cd(NO3)2ꢂ2H2O,
Cr(NO3)3ꢂ9H2O, AgNO3, Fe(NO3)3ꢂ9H2O, Mg(NO3)2ꢂ6H2O and
Pb(NO3)2 were dissolved in deionized water, but Al2(SO4)3 and
Fe(NO3)3ꢂ9H2O were dissolved in acidic deionized water.
Synthesis of ethyl-7-hydroxy-8-formylcoumarin-3-carboxylate
(compound 2): Ethyl-7-hydroxycoumarin-3-carboxylate (1.64 g,
7 mmol) and hexamine (1.47 g, 10.5 mmol) were added to a stirred
TFA (15 mL). This solution was refluxed for 24 h. Then, 30 mL water
was added, and the mixture was warmed to 60 8C while stirring for
The effect of pH on the fluorescent response of CQP to Cu2+ was
studied in
a concentration of Na2HPO4–citric acid fixed at
10 mmol/L, and the result shown in Fig. 2. It can be seen that
the fluorescence emission of the free CQP was stable from pH 2.2 to
7.0, however, the fluorescence emission at pH > 7 slightly increase,
at the same time the slight hypochromatic shift can be seen in
Fig. 2a, which can be ascribe to the state of proton of the hydroxyl
in CQP. Upon the addition of Cu2+, the fluorescence emission of
CQP–Cu2+ increased quickly in an acidic environment (from pH 2.2
to 6.4) and increased rapidly in an alkaline environment (pH > 7),
however, the fluorescence emission slightly decreased when
pH > 8 in Fig. 2b, possible because the hydroxyl could tautomerize
another 30 min. Upon cooling on ice,
a yellow solid was
precipitated from the solution, collected by filtration and washed
several times with water to give the desired compound as a yellow
solid (1.25 g, 68% yield). IR (KBr, cmꢀ1): 3431, 3045, 1742, 1700,
1659, 1595, 1480, 1445, 1304, 1236; MS (MALDI-TOF): m/z, calcd.
for (M+) 262.22; found 262.20; 1H NMR (400 MHz, DMSO-d6,
ppm):
d 1.28–1.31 (t, 3H, J = 6 Hz), 4.25–4.30 (q, 2H, J = 6.8 Hz),
7.01 (s, 1H), 8.06 (m, 1H), 8.75 (s, 1H), 10.40 (s, 1H), 12.50 (s, 1H).
Synthesis of (E)-ethyl-7-hydroxy-8-[[2-[2-(quinolin-8-yloxy)a-
cetyl] hydrazono] methyl]-coumarin-3-carboxylate (1, CQP): The
2-(quinolin-8-yloxy) acetohydrazide
3 was prepared by the
3.0
2.5
2.0
reported method [23]. Synthetic routes of CQP are depicted in
Scheme 1 and it was fully characterized by 1H NMR, 13C NMR, IR
and mass spectrum. Briefly, compound 3 was dissolved in 25 mL of
ethanol, followed by the addition of compound 2. Then the mixture
solution was stirred at 65 8C for 6 h. The cream-colored precipitate
formed was filtered and washed with ethanol and acetone and
dried under vacuum, then compound CQP was obtained (Scheme
CQP
+
Cu2+
1.5
1.0
0.5
0.0
CQP
1). Yield: 78%. 1H NMR (400 MHz, DMSO-d6)
d (ppm): 1.31 (t,
J = 4 Hz, 3H), 4.29 (m, 2H), 4.89 (s, 1H), 5.76 (s, 2H), 7.01 (dd, 1H,
J = 6 Hz), 7.36 (dd, 1H, J = 6 Hz), 7.48 (t, 1H, J = 8 Hz), 7.86 (m, 4H),
8.77 (s, 1H), 9.12 (s, 1H). 13C NMR (100 MHz, DMSO-d6): 169.8,
168.5, 167.7, 160.5, 159.8, 158.7, 154.7, 154.3, 148.4, 141.3, 138.3,
134.2, 131.9, 127.2, 126.4, 119.5, 117.0, 115.4, 110.2, 73.4, 66.1,
19.25. FI-IR (KBr, cmꢀ1): 3521, 3419, 3056, 2971, 1732, 1688, 1615,
1583, 1506, 1476, 1374, 1312, 1236, 1113, 1089, 1025, 952, 816,
755, 685, 643, 596, 523, 487. HRMS (ESI): m/z, calcd. for [M+H]+
250
300
350
400
450
500
Wavelength (nm)
Fig. 1. The absorption spectra of CQP and CQP + Cu2+. The red line represents only
the CQP (10
mol/L), the black line represents the subsequent addition of Cu2+
mol/L) to the solution. The buffer solution was Na2HPO4–citric acid buffer
+
C24H29N3O7 462.1301; found 462.1290.
m
Stock solutions of various ions (1.0 ꢁ 10ꢀ3 mol/L) were
Q3
(10
m
prepared in deionized water.
A stock solutions of CQP
(10 mmol/L, pH 7.4). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
(1 ꢁ 10ꢀ3 mol/L) were prepared in DMSO/methanol (1:5, v/v).
Please cite this article in press as: Y.-W. Duan, et al., The synthesis and study of the fluorescent probe for sensing Cu2+ based on a novel
coumarin Schiff-base, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.001

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Y.-W. Duan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
500
a
b
c
1600
1400
1200
1000
800
1400
1200
1000
800
600
400
200
0
400
300
200
100
0
CQP
2+
600
CQP
+ Cu
400
200
0
440
460
480
500
520
540
440
460
480
500
520
540
2
3
4
5
6
7
8
pH
Wavelength/nm
Wavelength (nm)
Fig. 2. (a) The effect of pH on the fluorescent intensity of CQP (10
(10
m
mol/L); (b) the effect of pH on the fluorescence intensity of CQP (10
m
mol/L) upon the addition of Cu2+
m
mol/L); (c) the effect of pH on the fluorescent intensity of CQP and CQP + Cu2+. The red line represents CQP only, the black line represents the subsequent addition of Cu2+
to the solution. The buffer solution was Na2HPO4–citric acid buffer (10 mmol/L, pH 7.4). (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
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to quinone to delay the electron transform. The slight hypochro-
matic shift can be seen in Fig. 2b, which may due to the state of
proton of the hydroxyl in CQP. The fluorescent intensity changes
with the different pH were shown in Fig. 2c. In order to obtain a
higher signal-to-noise ratio, Na2HPO4–citric acid buffer (pH 7.4,
10 mmol/L) was employed for the Cu2+ assay throughout the
experiment.
Cu2+ to the solution of CQP in the presence of Al3+, Mg2+, Sn2+, Ca2+
,
162
Fe3+, Hg2+, Mn2+, Cd2+, Cr3+, Zn2+, Co2+, Ni2+, Ag+, Pb2+ and Cu2+ in 163
Fig. 3b. It was observed that the presence of metal ions such as 164
Al3+, Mg2+, Sn2+, Ca2+, Fe3+, Hg2+, Mn2+, Cd2+, Cr3+, Zn2+, Co2+, Ni2+
,
165
Ag+, Pb2+ did not interfere with the CQP –Cu2+ decreased 166
fluorescence. However, the presence of Ca2+, Cr3+ and Zn2+ clearly 167
increase the fluorescence intensity, possible because the complex 168
formed between these metals and the probe was too stable to be 169
replaced by Cu2+, indicating that more attention should be paid to 170
To obtain insight into the fluorescent properties of CQP toward
metal ions, the emission changes were investigated with different
ions, such as Al3+, Mg2+, Sn2+, Ca2+, Fe3+, Hg2+, Mn2+, Cd2+, Cr3+, Zn2+
,
study the probe.
171
Co2+, Ni2+, Ag+, Pb2+and Cu2+ in Na2HPO4–citric buffer (10 mmol/L,
pH 7.4). The emission spectra were investigated by exciting CQP at
410 nm in Fig. 3a. From the emission spectra, it can be seen that
there was a great decrease in the fluorescence intensity of Cu2+
with CQP, which could be clearly distinguished from the other
metal ions in Fig. 3a. With the addition of Cu2+, compound CQP
could give rise to a visible colorless-to-yellow color change in
Fig. 4a and the prominent fluorescence changes of CQP were also
observable by a hand-gun UV lamp in Fig. 4b. The weak yellow-
green color that came from the solution of CQP faded away only by
the addition of Cu2+. The light blue fluorescence was only observed
with CQP–Cu2+. The combined information of fluorescence and
visible changes will be useful to discriminate Cu2+ from other
metal ions.
Further, the binding affinity of CQP for Cu2+ was examined 172
through a fluorescence titration experiment in which different 173
concentrations of Cu2+ were added to the solution of CQP 174
(10
L, pH 7.4,
m
mol/L) under the condition (Na2HPO4–citric acid, 10 mmol/ 175
ex = 410 nm). Upon the addition of Cu2+ to the solution 176
l
of CQP, fluorescence decrease occurred and the emission intensity 177
at 443 nm clearly decreased in Fig. 5a. Moreover, the linear 178
relationships of CQP toward Cu2+ at the low concentration range 179
(see insertion in Fig. 5a), showed that a reasonable linear 180
relationship could be built when the concentration of Cu2+ did 181
not exceed that of the CQP. To study the binding of CQP with Cu2+
the Job’s plot was tested by using a total concentration of 10 mol/ 183
L the CQP and Cu2+, and the results indicated that the probes 184
,
182
m
combined with Cu2+ with a 1:1 stoichiometry in Fig. 5b.
The binding ability of CQP toward Cu2+ was calculated following 186
the modified Benesi–Hildebrand equation (fluorescence method) 187
185
In order to validate the high sensitivity of CQP to Cu2+ in
practice, competition experiments were carried out by adding
1500
1600
a
b
1350
2+
Zn2+ C 3+
, r , Ca
1400
1200
1000
800
1200
1050
900
750
600
450
300
150
600
Cu2+
400
200
0
3+ Zn2+
2+
2+ Fe3+
Cu
+ Hg2+
Cd2+Ni2+Co2+Ag
2+ Mg Ca2+ Cr
Sn2+ Al3+Mn2+Pb
420
440
460
480
500
520
540
Wavelength (nm)
Metal ions
Fig. 3. (a) Fluorescence emission spectra of CQP (10
and Cu2+ in Na2HPO4–citric acid buffer (10 mmol/L, pH 7.4). (b) Metal ions selectivity of CQP (10
concentration of Cu2+ was 10
mol/L, and that of the other metal ions 15
m
mol/L) in the presence of different ions such as Al3+, Mg2+, Sn2+, Ca2+, Fe3+, Hg2+, Mn2+, Cd2+, Cr3+, Zn2+, Co2+, Ni2+, Ag+, Pb2+
mol/L) toward Cu2+ in Na2HPO4–citric acid buffer (10 mmol/L, pH 7.4). The
mol/L. Red bars represent the addition of the appropriate metal ions to the solution of CQP. Black
m
m
m
bars represent the subsequent addition of Cu2+ to the solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this article.)
Please cite this article in press as: Y.-W. Duan, et al., The synthesis and study of the fluorescent probe for sensing Cu2+ based on a novel
coumarin Schiff-base, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.001

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K = 1.75 106 mol/L
15
10
5
0
Fig. 4. (a) The color changes of CQP and CQP + Cu2+; (b) the color changes of CQP and
CQP + Cu2+ under the irradiation at 365 nm.
20000
40000
60000
80000
100000
1/[Cu2+] (L/mol)
Fig. 6. Determination of binding constant of CQP (10
m mmol/L)
mol/L) with Cu2+ (10
188
using Benesi–Hildebrand equation (fluorescent method).
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
1
1
1
1
¼
þ
n
D
F
D
Fmax
D
Fmax
K½Cꢃ
Table 1
Determination of Cu2+ concentrations in water samples.
18990
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192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
where
D
F = (Fx ꢀ F0),
D
Fmax = (Flim ꢀ F0) and F0, Fx, Flim are the
Sample
Cu2+ added
mol/L)
Cu2+ found
(mmol/L)
Recovery
(%)
emission intensities of CQP with adding Cu2+ at an intermediate
Cu2+ concentration, and at the concentration of complete interac-
tion, respectively. K is the binding constant between compound
CQP and Cu2+. C is the Cu2+ concentration and n is the number of
Cu2+ bound per CQP (here, n = 1). The value of K can be obtained
from the slope in Fig. 6. The results obtained were
KCQP = 1.75 ꢁ 106 mol/L, signifying that CQP may has a great
(m
Simulated aqueous sample (a)
Simulated aqueous sample (b)
0
5
0
5
1.01
5.98
0.52
5.48
101.0%
98.0%
104.0%
96.0%
banding affinity for Cu2+
.
water samples with good recovery results. Therefore, compound 1,
CQP, can be employed for Cu2+ assay in a water setting.
213
214
Simulated aqueous sample (a) obtained by deionized water,
mol/L CuSO4, 5.0 mol/L CdCl2, 20 mol/L NaCl, KCl, MgSO4,
and CaCl2 and simulated aqueous sample (b) obtained by deionized
water, 1.0 mol/L CuSO4, 5.0 mol/L Ni(NO3)2, 20 mol/L NaCl,
1.0
m
m
m
m
m
m
4. Conclusion
215
KCl, MgSO4, were analyzed by the proposed probe. Different
amount of simulated aqueous sample were added to the solution of
compound 1 in Na2HPO4–citric acid buffer (10 mmol/L, pH 7.4). As
shown in Fig. S1 in Supporting information, a linear relationship
between fluorescent intensity and the volume of simulated
aqueous sample was noted, thus indicated that the compound 1
is capable of detection Cu2+ in simulated aqueous sample.
Simulated aqueous sample were analyzed by compound 1 under
optimized conditions (Table 1). From the above results, it can be
seen that compound 1 can measure the concentration of Cu2+ in
In summary, on the basis of PET mechanism, we present a
Schiff-base fluorescent probe for the detection of Cu2+ based on
coumarin. The probe displayed an apparent response to Cu2+ via a
significant fluorescent change and a color change from colorless to
yellow. Moreover, other metal ions have no effect on its specific
response to Cu2+. The 1:1 binding mode was proposed based on the
Job’s plot. Also, the binding constant between CQP and Cu2+ was
calculated by Benesi–Hildebrand equation (fluorescent method).
The probe can be employed to measure Cu2+ in a water setting.
216
217
218
219
220
221
222
223
224
Fig. 5. (a) Fluorescent changes of CQP (10
(Na2HPO4–citric acid, pH 7.4) with an excitation at 410 nm. Insert figure: the linear relationships of CQP toward Cu2+ (below 10
concentration of CQP and Cu2+ was kept constant at 10
mol/L in Na2HPO4–citric acid buffer (10 mmol/L, pH 7.4). The excitation was at 410 nm (
m
mol/L) with different concentrations of Cu2+ (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
mol/L). (b) Job’s plot of CQP–Cu2+. The total
mol/L).
mmol/L, respectively) in buffer solution
m
m
m
Please cite this article in press as: Y.-W. Duan, et al., The synthesis and study of the fluorescent probe for sensing Cu2+ based on a novel
coumarin Schiff-base, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.001

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Special Foundation of the Education Committee of Shaanxi
Province (No. 12JK0580), the French Chinese Foundation for
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Please cite this article in press as: Y.-W. Duan, et al., The synthesis and study of the fluorescent probe for sensing Cu2+ based on a novel
coumarin Schiff-base, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.001