A highly zinc(II)-selective fluorescent sensor based on 8-aminoquinoline and its application in biological imaging

Article abstract of DOI:10.1002/ejic.201100245

A fluorescent probe based on 8-aminoquinoline bearing an aminophenol unit (L1) was developed as a chemosensor for Zn²⁺. Sensor L1 exhibits a high selectivity and sensitivity towards Zn²⁺ over other common metal ions in the physiologi

Full text of DOI:10.1002/ejic.201100245

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100245
A Highly Zinc(II)-Selective Fluorescent Sensor Based on 8-Aminoquinoline and
Its Application in Biological Imaging
Guoqiang Xie,[a] Pinxian Xi,[a] Xiao Wang,[b] Xuefei Zhao,[c] Liang Huang,[a]
Fengjuan Chen,[a] Yongjie Wu,[b] Xiaojun Yao,[c] and Zhengzhi Zeng*[a]
Keywords: Sensors / Floresecent probes / Zinc / Aminoquinolines
A fluorescent probe based on 8-aminoquinoline bearing an
aminophenol unit (L1) was developed as a chemosensor for
Zn2+. Sensor L1 exhibits a high selectivity and sensitivity
towards Zn2+ over other common metal ions in the physiolog-
ical pH window and binds to Zn2+ through 1:1 binding mode.
Fluorescent imaging of Zn2+ in living cells is also successfully
demonstrated.
Introduction
that when Zn2+ interacts with the chemosensors.[10] There-
fore, there is a great need for the design and synthesis of
such chemosensors, which have a small molecule and high
sensitivity, and can exhibit real-time detection in biological
systems and discriminate between Zn2+ and Cd2+ at physio-
logical pH.
Herein we synthesized a new 8-aminoquinoline derivative
L1 bearing the ortho-aminophenol group as a receptor,
which shows high sensitivity and selectivity for Zn2+ over
other possible competitive cations, on the basis of an in-
ternal charge transfer (ICT) and chelation-enhanced fluo-
rescence (CHEF) mechanism. The introduction of a carb-
oxamido group is of advantage to the deprotonation of the
8-amino group.[11] Moreover, the ortho-aminophenol group
provides another two binding sites for coordination, which
enhances the high selectivity of the sensor towards Zn2+
over other metal ions.
The development of fluorescent chemosensors has at-
tracted increasing attention because of their simplicity, sen-
sitivity, specificity and instantaneous response.[1] It is highly
demanding to selectively sense transition-metal ions, such
as the zinc cation.[2] It is well known that zinc is the second-
most abundant transition metal in the human body after
iron and plays an important role in various pathological
processes, such as Alzheimer’s disease, epilepsy, ischemic
stroke and infantile diarrhoea, amongst others.[3] On the
other hand, Zn2+ does not give any spectroscopic or mag-
netic signals because of its 3d104s0 electronic configuration.
It is believed that the use of fluorescence is the most effec-
tive way to detect zinc in biological systems so far.
In the last decades, considerable efforts have been made
towards the design and synthesis of fluorescent chemosen-
sors for Zn2+ [4–6]
.
Among various fluorescent sensors, 8-
aminoquinoline and its derivatives were the first class of
probes to be developed for Zn2+ [7]
They exhibit good pho-
.
tostability, high affinity to metal ions, and satisfactory
Results and Discussion
membrane permeability.[8] Despite the availability of some
commercial fluorescent probes for Zn2+ [9]
the design of
,
Compound L1 was easily synthesized in 82% yield by a
two-step procedure.[12] The single crystal of L1 was ob-
tained by slow evaporation in dichloromethane (Figure 1)
and L1 was designed to chelate metal ions through two N
atoms of the 8-aminoquinoline unit and the N, O atoms of
the ortho-aminophenol group.
easy-to-synthesize and low-toxicity Zn2+-selective sensors is
still a challenging task. In addition, the Cd2+ ion usually
interferes with the Zn2+-sensing processes and causes spec-
tral changes after interaction with chemosensors similar to
[a] Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province,
The coordination of L1 with Zn2+ was investigated by
spectrophotometric titration in acetonitrile solution. As
shown in Figure 2, the absorbance of L1 at 240 and 296 nm
Lanzhou 730000, P. R. China
Fax: 86-931-8912582
E-mail: zengzhzh@yahoo.com.cn
decreases linearly with increasing concentration of Zn2+
.
zengzhzh@lzu.edu.cn
[b] Pharmacology Laboratory of Gansu Province, Key Laboratory
of Preclinical Study for New Traditional Chinese Medicine,
Lanzhou University,
Meanwhile, two new absorption peaks appear at 258 and
362 nm, and their intensity gradually increases upon ad-
dition of Zn2+. This is accompanied by three isosbestic
points at 248, 278 and 338 nm. These phenomena are ex-
Lanzhou 730000, P. R. China
[c] Department of Chemistry, Lanzhou University,
Lanzhou 730000, P. R. China
pected to correspond to the coordination of L1 with Zn2+
,
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100245.
which extends the conjugated system and results in the ap-
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Z. Zeng et al.
SHORT COMMUNICATION
curve by assuming a 1:1 stoichiometry (Figure S1, Support-
ing Information). This binding mode is also supported by
a Job’s plot (Figure S2).
Figure 1. Crystal structure of L1. All hydrogen atoms are omitted
for clarity (50% probability level for the thermal ellipsoids).
pearance of the new absorption in the long wavelength re-
gion.[13] According to the linear Benesi–Hildebrand expres-
sion,[14] the measured absorbance [1/(A–A0)] at 362 nm var-
ies as a function of 1/[Zn2+] with a linear relationship (R2
= 0.9947), which indicates a 1:1 stoichiometry between
Zn2+ and L1 (Figure 2, inset).
Figure 3. Fluorescence spectra of L1 (10 μm) excited at 367 nm in
acetonitrile solution in the presence of different concentrations of
Zn2+ (0–2 equiv.). Inset: visible emission observed from L1 (25 μm)
in the absence and presence of Zn2+ (50 μm) in CH3CN.
In order to understand the crucial role of the aminophe-
nol group, which provides additional binding sites for Zn2+
ions, the meta-aminophenol isomer L2 of L1 was synthe-
sized as a control. We tested the fluorescence changes in L1
and L2 upon addition of zinc ions (Figures S3 and 4). The
fluorescence change between L1/Zn2+ and L1 is 4.1 times
greater than that between L2/Zn2+ and L2 (2 equiv. of
1
Zn2+) in CH3CN. Moreover, the H NMR spectra of L1,
L2, L1/Zn2+ and L2/Zn2+ were recorded. The protons sig-
nals for the CONH group and the aromatic rings of the L1/
Zn2+ system all shift to a higher field relative to those of
the ligand L1 (Figure S4). However, for the L2/Zn2+ system,
no apparent changes in the proton signals are seen after the
addition of Zn2+ (Figure S5). This indicates that the nitro-
gen and oxygen atoms of the aminophenol unit play an im-
Figure 2. Absorption spectra of L1 (20 μm) in the presence of Zn2+
(0–2 equiv.) in CH3CN. Inset shows the measured absorbance
[1/(A–A0)] at 362 nm varied as a function of 1/[Zn2+].
portant role in the three-dimensional complexation of Zn2+
.
We propose a coordination geometry for Zn2+ that en-
compass three N atoms and one O atom.[17] Therefore, we
can deduce a possible coordination mode: three N atoms
Figure 3 shows gradual changes in the fluorescence spec-
tra of L1 upon addition of Zn2+. As expected, L1 shows
very weak fluorescence (F0 = 36.5, Φ0 = 0.006) in acetoni-
trile solution. Addition of Zn2+ (0–20 μm) to the solution
of L1 (10 μm) produces a new emission band centred at
502 nm with a 15.7-fold increase in the fluorescence and a
12.3-fold increase in quantum yield (F = 573, Φ = 0.074).
A greenish fluorescence emission of the solution was ob-
served by the naked eye (Figure 3, inset). These results can
be explained as follows. After 8-aminoquinoline binds with
Zn2+, its intramolecular hydrogen bond breaks and the in-
tramolecular electron-transfer process is forbidden, which
enhances the fluorescence emission.[15] Simultaneously, elec-
tron transfer from the nitrogen atom of the heterocycle to
the metal ion further enhances the ICT process.[16] Another
reason for fluorescence enhancement could be that a more
rigid structure is formed when Zn2+ binds with L1. The
association constant for Zn2+ is estimated to be
Figure 4. Fluorescence intensities of L1 (10 μm) and L2 (10 μm) in
the presence of various metal ions and ion mixtures in acetonitrile
solution. [K+] = [Na+] = [Ca2+] = [Mg2+] = 5 mm, [Zn2+] = [Mn2+
]
= [Cr3+] = [Pb2+] = [Cd2+] = [Co2+] = [Fe2+] = [Ni2+] = [Ag+] =
[Hg2+] = [Cu2+] = 20 μm. (λex,L1 = 367 nm, λem,L1 = 502 nm; λex,L2
3.2ϫ104 m–1 on the basis of linear fitting of the titration = 365 nm, λem,L2 = 494 nm).
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A Highly Zinc(II)-Selective Fluorescent Sensor
and an O atom participate in the coordination environment than 2 min. Therefore, this system could be used for real-
of Zn2+
.
time tracking of Zn2+ in cells and organisms.
An important property of chemosensors is their high
To understand the selectivity and the configuration of
selectivity towards the analyte over the other competitive L1-Zn2+, we carried out density functional theory (DFT)
metal ions. We therefore determined the fluorescence inten- calculations with B3LYP by using the Gaussian 09 package.
sity of L1 at 502 nm in the presence of various metal ions. The 6-31G basis set was used for the H, C, N, O atoms;
As shown in Figure 4, only Zn2+ gives rise to a prominent the exception was for the Zn atom, where the LANL2DZ
fluorescence enhancement of the acetonitrile solution of L1, effective core potential (ECP) was employed. The optimized
while the other metal ions such as K+, Na+, Ca2+, Mg2+
,
configuration is shown in Figure 6, which shows that Zn2+
which exist at high concentrations in human cells, do not ion binds to L1 very well through four coordination sites,
effect any significant colour or spectral changes. In particu- and the whole molecular system forms a nearly planar
lar, the addition of Cd2+, known as a typical competitive structure. The Zn–O bond length is 2.10 Å and Zn–N bond
ion for a Zn2+ sensor, does not effect such changes either. lengths are 2.08 Å (Zn–Nquinoline), 1.98 Å (Zn–Ncarboxamido
)
However, the fluorescence of the L1/Zn2+ system was and 2.15 Å (Zn–NH2), respectively. These data indicate that
quenched by Cu2+ and Hg2+, which may be due to nonradi- L1 could provide suitable space to better accommodate cor-
ative energy transition in the process of electron or energy responding ions.
transfer between the d orbital of the ions and the fluoro-
phore.[7,18] These results unambiguously demonstrate the
high specificity and selectivity of L1 for detecting Zn2+
.
Interestingly, L2 also exhibits good specificity and selectiv-
ity for Zn2+ in single and multicomponent systems. The
common metals such as K+, Na+, Ca2+, Mg2+, Mn2+, Cd2+
,
Co2+, Fe2+, Ni2+, Cr3+ and Ag+ have a negligible influence
on Zn2+ sensing. Only, addition of Pb2+ elicits tiny fluores-
cence enhancement, with respect to that for Zn2+, and Cu2+
and Hg2+ efficiently quench the fluorescence of L2/Zn2+
system.
The sensing ability of L1 toward Zn2+ at different pH
values was investigated (Figure 5). L1 shows no appreciable
fluorescence response to Zn2+ in an acidic environment be-
cause protonation of the amino group of L1 weakens the
Figure 6. Calculated energy-minimized structure of L1 with Zn2+
.
We then studied the Zn-sensing behaviour of L1 in living
cells. SCaBER cells (human bladder cancer cells) were incu-
bated with 20 μm L1 for 3 h at 37 °C and show very weak
intracellular fluorescence as determined by laser scanning
confocal microscopy (Figure 7a). The addition of Zn2+
(40 μm) to the treated cells in a culture medium over 0.5 h
at 37 °C leads to a significant increase in the fluorescence
(Figure 7b). The result indicates that L1 may be used as a
possible sensor to detect intracellular Zn2+ in living cells.
coordination ability of Zn2+ [19]
However, L1 exhibits satis-
.
factory sensing abilities when the pH is increased from 5.5
to 9.0. At pH ca. 7.0, the FL1+Zn/FL1 value reaches a maxi-
mum value of 152.5. These data indicate that L1 could act
as a fluorescent probe for Zn2+ under physiological pH con-
ditions.
Figure 7. Fluorescence microscope images of SCaBER cells loaded
with L1 incubated for 3 h, (a) without and (b) with the addition of
Zn2+ (2 equiv.).
Figure 5. Fluorescence intensities of L1 (10 μm) in the absence and
presence of Zn2+ (20 μm) at various pH values in CH3CN/H2O
(95:5, v/v) (λex = 367 nm, λem = 502 nm).
Conclusions
We also investigated the time evolution of the response
A new 8-aminoquinoline-based derivative was synthe-
of L1 to 2 equiv. of Zn2+ in CH3CN (Figure S6). We found sized for the detection of Zn2+ in living cells. It displays
that the interaction of L1 with Zn2+ was complete in less high selectivity, sensitivity and a fast time response to Zn2+
.
Eur. J. Inorg. Chem. 2011, 2927–2931
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Z. Zeng et al.
SHORT COMMUNICATION
L1 can discriminate between Zn2+ and Cd2+ in acetonitrile
solution in the physiological pH window and binds to Zn2+
through a 1:1 binding mode, which has been confirmed by
ESI-MS and DFT calculations. The fluorescent microscopy
measurements demonstrate that L1 is cell permeable. We
hope that such Zn2+-selective sensors would have great po-
tential in biomedical and environmental applications.
c, a = 15.022(6) Å, b = 5.256(2) Å, c = 20.586(7) Å, α = 90°, β =
117.58(2)°, γ = 90°, V = 1440.7(10) Å3, Z = 4, ρcalcd. = 1.352 gcm–3,
R1 = 0.0695 [I Ͼ 2σ(I)]. CCDC-795696 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Cell Incubation and Imaging: The SCaBER cells (human bladder
cancer cells) were provided by the Cells Bank of Chinese Academy
of Science (Shanghai, China). Cells were grown in H-DMEM (Dul-
becco’s Modified Eagle’s Medium, High Glucose) supplemented
with 10% FBS (Fetal Bovine Serum) in an atmosphere of 5% CO2,
95% air at 37 °C. Cells (5ϫ 108/L) were plated on 18-mm glass
coverslips and allowed to adhere for 24 h. Experiments to asses
Zn2+ uptake were performed in the same media supplemented with
40 μm Zn(NO3)2 for 0.5 h. Before the experiments, the cells were
washed with PBS buffer and then incubated with 20 μm L1 for 3 h
at 37 °C. Cell imaging was then carried out after washing the cells
with PBS. The cell images were then captured with a fluorescence-
inverted microscope (DMI4000 B, Leica Microsystem) with exci-
tation between 340–380 nm. The total magnification is 400ϫ.
Experimental Section
Preparation of 2-Chloro-N-(quinol-8-yl)acetamide: To a cooled,
stirred solution of 8-aminoquinoline (288 mg, 2.0 mmol) and pyr-
idine (0.23 mL, 2.8 mmol) was added dropwise a chloroform solu-
tion (10 mL) of 2-chloroacetyl chloride (461 mg, 2.0 mmol) over
1 h. After 2 h at room temperature, a brown–yellow solid was ob-
tained by removing the solvent under reduced pressure. The crude
product was purified by silica gel column chromatography by using
dichloromethane as eluent to afford 2-chloro-N-(quinol-8-yl)acet-
amide. Yield: 362 mg (80%). 1HNMR (CDCl3, 400 MHz, TMS): δ
= 10.92 (s, 1 H), 8.86–8.87 (dd, J = 4, J = 1.6 Hz, 1 H), 8.75–8.86
(m, 1 H), 8.17–8.20 (dd, J = 8, J = 1.6 Hz, 1 H), 7.47–7.59 (m, 3
H), 4.32 (s, 2 H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 164.4,
148.7, 138.7, 136.3, 133.6, 127.9, 127.2, 122.5, 121.8, 116.6,
43.3 ppm.
DFT Calculations: The geometry optimizations were carried out in
vacuo by using the hybrid density functional Becke-3-Lee–Yang–
Parr (B3LYP) potential in conjuration with a 6-31G basis set for
the H, C, N, O atoms and a LANL2DZ effective core potential
(ECP) basis set for the Zn atom. All the calculations were im-
plemented with the GAUSSIAN 09 software package. Frequency
calculations were also implemented for the optimized structure to
ensure that the optimized structure was the one which has the low-
est energy.
Preparation of L1: 2-Chloro-N-(quinol-8-yl)acetamide, (133 mg,
0.6 mmol), ortho-aminophenol (55 mg, 0.5 mmol) and potassium
iodide (8 mg) were added to an acetonitrile solution (30 mL). After
the solution was stirred and heated at reflux for 10 h under nitro-
gen, the mixture was cooled to room temperature, and the solvent
was removed under reduced pressure to obtain a pale yellow solid,
which was purified by silica gel column chromatography by using
ethyl acetate/petroleum (1:2, v/v) as eluent to afford L1. Yield:
123 mg (83.8%). 1H NMR (CDCl3, 400 MHz, TMS): δ = 11.12 (s,
1 H), 8.77–8.80 (m, 2 H), 8.17–8.20 (dd, J = 8, J = 1.6 Hz, 1 H),
7.55–7.58 (m, 1 H), 7.45–7.48 (dd, J = 8, J = 4 Hz, 1 H), 6.74–6.92
(m, 4 H), 4.75 (s, 1 H), 4.23 (br. s, 1 H) ppm. 13C NMR (CDCl3,
100 MHz): δ = 166.4, 148.5, 145.0, 138.7, 136.42, 136.45, 128.0,
127.3, 122.7, 122.2, 121.7, 118.6, 115.9, 112.4, 68.21 ppm. MS
(ESI): m/z = 294.0 [M + H+].
Supporting Information (see footnote on the first page of this arti-
cle): Materials and general methods, schematic molecular struc-
tures, experimental details and additional NMR and ESI-MS data
are presented.
Acknowledgments
This study was supported by the Foundation of State Key Labora-
tory of Applied Organic Chemistry and the NSFC (20171019).
Thanks to Shihui Shi for NMR measurements.
Preparation of L2: Analogous procedures starting with 2-chloro-N-
1
(quinol-8-yl)acetamide and meta-aminophenol gave L2. H NMR
(CDCl3, 400 MHz, TMS): δ = 10.94 (s, 1 H), 8.86–8.87 (dd, J = 4,
J = 1.6 Hz, 1 H), 8.81–8.83 (dd, J = 8, J = 1.6 Hz, 1 H), 8.15–8.18
(d, J = 1.6 Hz, 1 H), 7.54–7.56 (m, 2 H), 7.45–7.48 (dd, J = 8, J =
4 Hz, 1 H), 7.10–7.14 (m, 1 H), 6.37–6.53 (m, 3 H), 4.71 (s, 2 H),
3.72–3.75 (br., 2 H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 167.0,
158.6, 148.6, 148.0, 138.8, 136.2, 133.8, 130.4, 127.8, 127.2, 122.2,
121.7, 116.8, 109.2, 105.0, 102.1, 68.1 ppm. MS (ESI): m/z = 294.2
[M + H+].
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Received: March 10, 2011
Published Online: June 1, 2011
Eur. J. Inorg. Chem. 2011, 2927–2931
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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