Coumarin-based novel fluorescent zinc ion probe in aqueous solution

Article abstract of DOI:10.1016/j.tet.2013.03.032

A novel coumarin-derived fluorescent probe, FCP, was designed for Zn ²⁺ quantification based on the photo-induced electron transfer (PET) mechanism. This probe selectively and sensitively detects Zn²⁺ in aqueous solution with wide pH

Full text of DOI:10.1016/j.tet.2013.03.032

Tetrahedron 69 (2013) 4743e4748
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Coumarin-based novel fluorescent zinc ion probe in aqueous
solution
y
y
*
Jun Li , Chun-Fang Zhang , Ze-Zhong Ming, Ge-Fei Hao, Wen-Chao Yang ,
*
Guang-Fu Yang
Key Laboratory of Pesticide & Chemical Biology, College of Chemistry, Ministry of Education, Central China Normal University, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
A novel coumarin-derived fluorescent probe, FCP, was designed for Zn2þ quantification based on the
photo-induced electron transfer (PET) mechanism. This probe selectively and sensitively detects Zn2þ in
aqueous solution with wide pH range. Large fluorescence enhancement (13-fold) was observed upon the
addition of Zn2þ. FCP also displays excellent cell permeability in HeLa cell model and very low cyto-
toxicity to HEK-293 cell model.
Article history:
Received 23 December 2012
Received in revised form 26 February 2013
Accepted 8 March 2013
Available online 14 March 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Coumarin
Zn2þ
Fluorescent probe
1. Introduction
biological systems. Other probes can not be applied to living sys-
tems due to poor cell permeability. Thus, efforts directed toward
As the second most abundant transition metal essential for
human health, zinc ion (Zn2þ) has drawn considerable interest due
to its broad biological functions, including modulation of catalytic
activity of hundreds of specific enzymes, regulation of gene tran-
scription, and involvement in brain pathology.1,2 Abnormal con-
centrations of Zn2þ are known to be closely related to several
diseases, including chronic diarrhea, growth failure, immune de-
ficiency, Alzheimer’s disease.3,4 In addition, with the development
of modern industry, the environment pollution caused by transi-
tion metals like Zn2þ has become more serious and pose a threat to
human heath and the environment.5 Therefore, the accurate mea-
surement of Zn2þ concentrations in either the clinical setting or for
environmental monitoring is of great importance.
Fluorescence emission spectrometry has emerged as one of the
most popular methods for specific measurement because of its high
sensitivity, simplicity, and real time monitoring without compli-
cated pretreatment.6e9 Numerous fluorescent probes for Zn2þ de-
tection have been developed by conjugating different kinds of
fluorophores.10e18 However, many of the existing Zn2þ probes
have poor water solubility, which prevents their applications in
the development of novel Zn2þ probes are ongoing.
A fluorescent probe usually consists of two structural units: the
receptor unit for specifically recognizing the target molecule or ion,
and the fluorophore unit for translating the hosteguest recognition
into fluorescence signal.19,20 Coumarins are widely used as effective
fluorophores due to their high fluorescence quantum yield.13,21,22
Furthermore, coumarin derivatives are usually easily synthesized
and possess good water solubility, low cytotoxicity, and good cell
permeability,23,24 making them powerful chemical tools for study-
ing biological systems. As one of the most commonly used chelators,
di-2-picolylamine (DPA) is a classical membrane-permeable chela-
tor with high selectivity for Zn2þ over other alkali and alkaline-earth
metal ions.25e27 Usually, electronegative atoms, such as N,28 O,29 S,30
were introduced to the fluorophore part of molecular probes as
additional electron donors with the aim to improve the chelator’s
affinity toward metal ions. To the best of our knowledge, few
coumarin-based DPA derivatives as fluorescent Zn2þ chemical
probes have been described until recently.31,32
Herein, based on the mechanism of photo-induced electron
transfer (PET),20 we designed and synthesized a coumarin-based
DPA fluorescent probe (FCP) with high sensitivity and selectivity
for zinc ion in aqueous solution (Scheme 1). We envisioned that
fluorescence intensity would be greatly enhanced when FCP’s PET
pathway was blocked upon the binding with Zn2þ, which might be
utilized to quantify Zn2þ levels conveniently for healthcare and
environmental monitoring.
* Corresponding authors. Tel.: þ86 27 6786 7706; fax: þ86 27 6786 7141; e-mail
addresses: tomyang@mail.ccnu.edu.cn (W.-C. Yang), gfyang@mail.ccnu.edu.cn (G.-F.
Yang).
y
These two authors made equal contributions.
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.03.032

4744
J. Li et al. / Tetrahedron 69 (2013) 4743e4748
CF3
CF3
O
CF3
O
a
b
B
r
Ts
N
O
O
NH
O
O
NH2
Ts
2
3
1
F
C
3
CF3
O
N
N
d
c
N
Br
N
O
O
O
N
H
H
4
FCP
Reagents and conditions: a, TsCl, pyridine, CH2Cl2, r.t., 4 h; b, Br(CH2)2Br, Cs2CO3, CH3CN,
reflux, 12 h; c, H2SO4 (conc.), 90 oC; d, DPA, K2CO3, KI, CH3CN, reflux, 12 h
Scheme 1. Synthesis of the new fluorescent probe FCP.
2. Experimental
(t, J¼7.2 Hz, 2H), 2.45 (s, 3H). EI-MS: m/z, calcd 490.29, found
490.91.
2.1. Reagents and instruments
2.2.3. Synthesis of 7-[N-(2-bromoethyl)amino]-4-trifluoromethyl-
coumarin. 7-(N-(2-Bromoethyl)-N-tosylamino)-4-trifluoromethyl-
coumarin (0.75 g, 1.53 mmol) was added to concentrated sulfuric
acid 5 mL and the solution was stirred for 4 h at 90 ꢀC. The reaction
mixture was cooled and carefully poured into water. The blue solid
was precipitated, followed by filtration and silica gel column
chromatography to obtain the compound 4 as yellow solid.33 Yield:
Unless otherwise noted, all chemical reagents were commer-
cially available and treated with standard methods before use. Silica
gel column chromatography (CC): silica gel (200e300 mesh);
Qingdao Makall Group Co., Ltd; Qingdao; China. Solvents were
dried in a routine way and redistilled. 1H NMR and 13C NMR spectra
were recorded in CDCl3 or CD3CN-d3 on a Varian Mercury 600 or
400 spectrometer and resonances (
d
) are given in parts per million
0.3 g (58%). Mp 151e152 ꢀC. 1H NMR (CDCl3, 400 MHz):
d 7.50 (d,
relative to tetramethylsilane (TMS). The following abbreviations
were used to designate chemical shift multiplicities: s¼singlet,
d¼doublet, t¼triplet, m¼multiplet, br¼broad. High resolution
mass spectra (HRMS) were acquired in positive mode on a WATERS
MALDI SYNAPT G2 HDMS (MA, USA).
J¼7.2 Hz, 1H), 6.61 (d, J¼8.8 Hz, 1H), 6.53 (s, 1H), 6.47 (s, 1H), 3.76 (t,
J¼5.6 Hz, 2H), 3.60 (t, J¼5.6 Hz, 2H). EI-MS: m/z, calcd 336.10, found
336.96.
2.2.4. Synthesis of probe FCP. DPA (199 mg, 1 mmol), K2CO3
(138 mg, 1 mmol), KI (166 mg, 1 mmol), intermediate 4 (168 mg,
0.5 mmol) were dissolved in distilled CH3CN, and the reaction
mixture was stirred under reflux for 12 h. After removing the sol-
vent under reduced pressure, the residue was then purified by silica
column chromatography using CH2Cl2/CH3OH (100/1, v/v) to afford
the final product as yellow oil. Yield: 91 mg (40%). IR (neat): 3392,
2.2. Synthesis and characterization of coumarin derivatives
2.2.1. Synthesis of 7-[N-(p-toluenesulfonyl)amino]-4-trifluoromethyl-
coumarin. 7-Amino-4-trifluoromethylcoumarin 3.71 g (16.2 mmol)
was dissolved in dichloromethane (20 mL). After slow addition
of the distilled pyridine (20 mL) and p-toluenesulfonyl chloride
(6.80 g, 35.7 mmol), the mixture was then stirred for 4 h at room
temperature. Then ethyl acetate 150 mL was added, followed
by wash of citric acid solution for three times and brine once.
The resulting solution was dried with sodium sulfate and evap-
orated, washed with a small portion of cooled dichloromethane
and dried in vacuo to give the compound 2 as white solid.33
2921, 2851, 1643, 1466, 1260, 1094 cmꢁ1
400 MHz):
;
1H NMR (CDCl3,
8.60 (d, J¼4.4 Hz, 2H), 7.66e7.62 (m, 2H), 7.44 (d,
d
J¼8.8 Hz, 1H), 7.37 (d, J¼8.0 Hz, 2H), 7.20e7.18 (m, 2H), 6.78 (s, 1H),
6.62 (dd, J1¼2.4 Hz, J2¼2.4 Hz, 1H), 6.41 (d, J¼2.4 Hz, 1H), 6.37 (s,
1H), 3.96 (s, 4H), 3.22 (t, J¼5.6HZ, 2H), 2.96 (t, J¼5.6HZ, 2H). 13C
NMR (100 MHz, CDCl3):
d 160.55, 158.59, 157.10, 152.75, 149.03,
141.98, 141.64, 136.61, 125.82, 123.32, 122.28, 111.42, 107.41, 103.03,
97.47, 59.92, 51.84, 40.74. HRMS (ESIþ) calcd for [FCPþH]þ: calcd
455.1695, found: 455.1690.
Yield: 5.64
400 MHz):
g
(91%). Mp 193e194 ꢀC. 1H NMR (CDCl3,
7.81 (d, J¼8.0 Hz, 2H), 7.57 (d, J¼8.8 Hz, 1H), 7.30 (d,
d
J¼8.0 Hz, 2H), 7.21 (d, J¼2.4 Hz, 1H), 7.08 (dd, J1¼2.0 Hz,
J2¼2.0 Hz, 1H), 6.67 (s, 1H), 2.40 (s, 3H). EI-MS: m/z, calcd 383.34,
found 383.02.
2.3. Determination of quantum yield
The quantum yields of fluorescence were determined by com-
2.2.2. Synthesis of 7-[N-(2-bromoethyl)-N-(p-toluenesulfonyl)amino]-
4-trifluoro -methyl coumarin. A mixture of 7-Tosylamino-4-tri-
fluoromethylcoumarin (3.2 g, 8.35 mmol), distilled acetonitrile
(30 mL), cesium carbonate (4.06 g, 12.50 mmol), and 1, 2-
dibromoethane (15.6 g, 83.5 mmol) was stirred for 12 h at 80 ꢀC.
After filtration, the solution was evaporated and purified by silica
gel column chromatography to afford the compound 3 as white
solid.33 Yield: 2.28 g (81%). Mp 80e81 ꢀC. 1H NMR (CDCl3,
parison of the integrated area of the corrected emission spectrum
of samples with a reference. Specifically, using fluorescein (
F
¼0.98,
0.1 M H2SO4) as reference, FCP (10 M) were prepared in 0.1 M
m
phosphate (pH 7.0) buffer and diluted to certain solution to make
their absorption less than 0.05. Then their UVevis absorption
spectrum was studied and the corresponding emission at relevant
wavelength of excitation was measured as well. After correction of
the refractive index of the different solvents determined by Abbe’s
refractometer, the quantum yields were calculated with the ex-
pression in following equation.
600 MHz):
d
7.71 (d, J¼7.8 Hz, 1H), 7.51e7.50 (m, 2H), 7.31e7.24
(m, 3H), 7.10 (s, 1H), 6.82 (s, 1H), 3.94 (t, J¼7.2 Hz, 2H), 3.43

J. Li et al. / Tetrahedron 69 (2013) 4743e4748
4745
the synthetic details are shown in Scheme 1 and the relevant spec-
trums are depicted in Supplementary data.
Fs$Ac
Fc$As
fs
¼
fc
The spectroscopic properties and the metal-binding behavior of
the newly synthesized probe were measured in 0.1 M phosphate
buffer (pH 7.0) at 30 ꢀC. The peak of UVevis absorption spectrum
for FCP was located at 390 nm (Fig. S1). Using 390 nm as the ex-
citation wavelength, FCP itself exhibited weak intensity of fluo-
2.4. Determination of association constant
Based on Job’s method, fluorescence spectroscopy was used to
determine the apparent association constant (Ka) of FCP with Zn2þ
.
As shown in Fig. 1b, two series of solutions containing certain Zn2þ
and FCP were prepared, in which their total concentrations are
constants. Fluorescence intensity of those mixtures of Zn2þ and FCP
in varying molar ratios (1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1) were
measured. Then the Ks can be calculated by fitting the data to the
following equation, where a1, b1, a2, b2 are the initial concentration
of metal ions and ligand concentration, X is the equilibrium con-
centration of complex.
rescence emission at 510 nm (
F
¼0.15, Fig. 1a). Upon addition of free
Zn2þ ion, the fluorescence intensity (F/F0) of FCP increased
promptly, and the maximum fluorescence intensity (F/F0) en-
hancement increased by 13-fold when 1 equiv of Zn2þ
(F
¼0.47)
was added. This resultant high quantum yield is quite comparable
with the existing Zn2þ fluorescent probes in aqueous solution.22,23
The titration curve in Fig 1a indicated that the binding stoi-
chiometry between FCP and Zn2þ is 1:1. To further understand the
12
10
7000
(a)
12000
(b)
8
1 mM
6
6000
5 mM
10000
8000
6000
4000
2000
0
4
5000
4000
3000
2000
1000
0
2
0
0
10
20
30
[
Zn2+] (
M)
450
500
550
600
0.0
0.2
0.4
0.6
0.8
1.0
Zinc molar fraction
Wavelength (nm)
Fig. 1. (a) Fluorescence spectra of FCP (lex¼390 nm, lem¼510 nm) in the presence of different concentrations of Zn2þ, and both the excitation and emission slit widths were 5 nm.
(b) The Job plots for binding between FCP and Zn2þ evaluated from the fluorescence with a total concentration of 1 mM and 5 mM, respectively. Inset in panel (a): Fluorescence
titration profile versus concentration of Zn2þ in 0.1 M phosphate buffer (pH 7.0).
mode of complexation, 1H NMR spectra studies were undertaken
X
X
using CH3CN-d6 as the solvent in the absence and presence of Zn2þ
(Fig. 2). The protons at the ortho position of the pyridines were
downfield shifted from 8.44 to 8.55 ppm by the N-metal co-
ordination effect upon the addition of Zn2þ. Similar shifts were
observed for the protons at the 2-, 3-, and 4-positions of pyridines,
and the H atoms at the 5-positon in the DPA moiety were split and
downfield shifted. In addition, density functional theory (DFT)
calculations were performed to gain further structural insight into
the mechanism. As depicted in Fig. 3, the HOMOeLUMO energy
gap decreased about 0.28 eV once FCP bound with Zn2þ whereas
the HOMOeLUMO energy gap increased slightly by approximately
0.03 eV for the coordination between FCP- and Zn2þ. DFT analysis
Ka
¼
¼
ða1 ꢁ XÞðb1 ꢁ XÞ
ða2 ꢁ XÞðb2 ꢁ XÞ
2.5. Imaging of HeLa cells
HeLa cells were cultured in DMEM media supplemented with
10% (v/v) fetal bovine Serum, 1% (v/v) 100 mM Sodium Pyruvate
(Gibco). The cells were seeded at 0.3ꢂ106 cells/ml in the culture
media, incubated at 37 ꢀC, 5% CO2. Then 20
mM ZnCl2 was added
into the cells and the cells were incubated for 12 h at 37 ꢀC. Later,
the samples were washed twice to remove the remaining zinc ions,
followed by the treatment of 10
containing 0.1% (v/v) DMSO. After 1 h incubation, the treated cells
were imaged by confocal laser scanning microscopy (CLSM).
mM FCP in the culture media
revealed that [Zn(FCP)2þ
]
preferentially occurred while
[Zn(FCP)þ] was not favorable, which is consistent with the ex-
perimental observations of PET phenomena other than internal
charge transfer (ICT). Furthermore, the detection limit (S/N¼3)
2.6. Quantum chemical calculation
was about 0.44
mM as based on the calculation of the titration
curve. According to the method described above, the apparent
Initial structures of FCP, FCPꢁ, [Zn(FCP)]2þ, [Zn(FCP)]þ were
constructed in SYBYL-X 1.1 software and optimized using Tripos
force field. The density functional theory (DFT) calculations were
carried out by using Gaussian 03 program. Ground state geometry
optimization were performed with the basis set of B3LYP 6-31þG
(d, p). The resulting geometries were used energy calculation of
HOMOs and LUMOs.
association constant (Ka) of FCP for Zn2þ was determined to be
about 4.10ꢂ105 Mꢁ1
.
After confirming the sensitivity of FCP, we also investigated the
interference of other bio-relevant metal ions, and the results were
summarized in Fig. 4a. Ca2þ, Mg2þ, Kþ, and Naþ, which are much
more abundant in the biosphere, did not trigger any fluorescence
enhancement even at very high concentrations (1 mM, 100 times
higher than the concentration of FCP). In addition, no significant
change of fluorescence emission intensity was observed upon the
addition of some first row heavy and transition metal (HTM) ions,
such as Mn2þ, Pb2þ, Cr3þ, and Fe3þ. Other first row transition metal
ions like Cu2þ, Co2þ, and Ni2þ quenched the fluorescence, probably
due to their empty d-orbitals when they bind with FCP.35 In con-
trast, Cd2þ and Hg2þ displayed obvious interference to the de-
tection of Zn2þ because they are in the same group of metals as
3. Results and discussion
3.1. Fluorescence detection in aqueous solution and related
mechanism
Starting with 7-amino-4-trifluoromethylcoumarin and DPA,12,33,34
the new sensor FCP was synthesized in a straightforward manner;

4746
J. Li et al. / Tetrahedron 69 (2013) 4743e4748
Fig. 2. 1H NMR spectra of FCP (1 mM) in the absence and presence of Zn2þ (1 equiv) in CD3CN-d3.
Fig. 3. The HOMOs and LUMOs of FCP, FCP, [Zn(FCP)2þ] and [Zn(FCP)þ].
FCP
8000
16
14
12
10
8
(a)
(b)
FCP+Zn
7000
6000
5000
4000
3000
2000
1000
0
6
4
2
0
2
4
6
8
10
pH
Fig. 4. (a) Fluorescence responses of 10
ions, Ca2þ, Mg2þ, Kþ, Naþ, and Liþ were added at 100 equiv and the rest were introduced at 10 equiv. (b) The pH dependence of FCP (10
(100 M).
m
M FCP to various metal ions in 0.1 M phosphate buffer (pH 7.0) at the excitation wavelength of 390 nm. Among those bio-relevant metals
m
M) in the absence and presence of Zn2þ
m
Zn2þ in the periodic table, but the fluorescence intensity upon their
addition was lower than that of Zn2þ at the same dosage. Consid-
ering their low concentrations in typical biological systems, FCP
should still be suitable for the accurate quantification of the con-
centration of Zn2þ in biological system.
The influence of pH on the fluorescence response of FCP was also
examined in different phosphate buffers (Fig. 4b). The resulting
fluorescence change of FCP alone was not especially dependent on
pH, which means both the chemical structure and fluorescence
property of FCP are relatively stable over a broad range from pH 2.0

J. Li et al. / Tetrahedron 69 (2013) 4743e4748
4747
to pH 10.0. As for the fluorescence emission of FCP in the presence of
10 equiv Zn2þ, the fluorescence was enhanced as the pH increased
and reached the maximum intensity at pH 6.0. The kinetic change of
the pH dependence of FCPeZn2þ complex can be attributed to: (1)
the amine group in DPA will be protonated in low pH solutions,
which would hinder the binding between FCP and Zn2þ; and (2) in
neutral solution or a high pH environment, the free amine group in
DPA can easily form strong interactions with Zn2þ. The above study
demonstrated that FCP could be applied to track the Zn2þ levels in
aqueous solutions with pH values ranging from 6.0 to 9.0.
solution, dimethyl sulfoxide was added to dissolve the formazan
crystals. The absorbance was measured at OD540 with microplate
reader. Each experiment was performed at least three times. As
depicted in Fig. 6, the cytotoxic effect of FCP was assessed through
the ratio of absorbances of FCP treated versus the control. It
revealed that the cells remained in good condition when treated
with FCP at a dosage of 20 mM for as long as 24 h.
4. Conclusion
In summary, we have successfully designed and synthesized
a novel ‘turn-on’ fluorescent probe FCP with excellent water solu-
bility by combining coumarin as fluorophore and DPA as chief
recognition unit. FCP displayed strong affinity to Zn2þ and pro-
duced great fluorescence enhancement due to the blocking of the
PET process upon the binding of Zn2þ. In addition, FCP possesses
much better quantum yield compared to most existing Zn2þ sen-
sors that have been used in 100% aqueous solution. Although in-
terference by other ions was observed, such as fluorescence
3.2. Living cell imaging
To further study the cell permeability and the in vivo co-
ordination capability, a preliminary study of FCP in living human
HeLa cells was performed using confocal laser scanning microscopy
(CLSM). As shown in Fig. 5, compared to the controls with either
FCP (10
was observed in cells with the sequential addition of both FCP
m mM), much stronger green fluorescence
M) or Zn2þ (20
(10
m
M) and Zn2þ (20
m
M). These results indicate that FCP has ex-
quenching by three bivalent metal cations (Co2þ, Ni2þ, and Cu2þ
)
cellent cell permeability and could efficiently bind with Zn2þ
and significant fluorescence enhancement by two bivalent metal
in vivo.
cations (Hg2þ and Cd2þ), these interferences could be ignored due
Fig. 5. (a) Brightfield image of HeLa cells. (b) CLSM image of HeLa cells only labeled with FCP (10
treatment with 20 M FCP. (d). Merged image of (a) and (c).
M Zn2þ and 10
mM) after incubation for 30 min. (c). CLSM image of HeLa cells after sequential
m
m
In addition, the cytotoxicity of FCP had been investigated in
normal human embryonic kidney (HEK-293) cells by the MTT assay.
HEK-293 cells were seeded at 1ꢂ105 cells per well in 24-well plates
and incubated for 24 h before treatment, followed by exposure to
to the very low concentrations of these ions in vivo. Furthermore,
both the distinguished cell permeability and low toxicity
demonstrated that FCP has great potential for reliable quantifica-
tion of Zn2þ in biological systems.
different concentrations (0
mM, 0.1 mM, 1 mM, 5 mM, 10 mM, 20 mM,
50 M, 100 M) of FCP for additional 24 h. Cell samples then were
incubated with MTT solution for 4 h, and after removal of the MTT
m
m
Acknowledgements
The research was supported in part by the National Basic
Research Program of China (No. 2010CB126103) and the NSFC
(No. 20925206, 21172091, and 21102052).
140
120
100
80
Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.03.032. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
60
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