Ratiometric and water-soluble fluorescent zinc sensor of carboxamidoquinoline with an alkoxyethylamino chain as receptor

Article abstract of DOI:10.1021/ol702869w

A novel "naked-eye" and ratiometric fluorescent zinc sensor (AQZ) of carboxamidoquinoline with an alkoxyethylamino chain as receptor was designed and synthesized. AQZ shows good water solubility and high selectivity for sensing; about an 8-fold increase in fluorescence quantum yield and a 75 nm red-shift of fluorescence emission upon binding Zn²⁺ in buffer aqueous solution are observed. Moreover, AQZ can enter yeast cells and signal the presence of Zn²⁺.

Full text of DOI:10.1021/ol702869w

ORGANIC
LETTERS
2
008
Vol. 10, No. 3
73-476
Ratiometric and Water-Soluble
Fluorescent Zinc Sensor of
4
Carboxamidoquinoline with an
Alkoxyethylamino Chain as Receptor
†,‡
,‡
‡
‡
,†,§
Yu Zhang, Xiangfeng Guo,* Wanxia Si, Lihua Jia, and Xuhong Qian*
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology,
Dalian 116012, China, Key Laboratory of Fine Chemicals (Qiqihar UniVersity),
College of Heilongjiang ProVince, Qiqihar 161006, China, and Shanghai Key
Laboratory of Chemical Biology, East China UniVersity of Science and Technology,
Shanghai 200237, China
xfguo@163.com; xhqian@ecust.edu.cn
Received November 27, 2007
ABSTRACT
A novel “naked-eye” and ratiometric fluorescent zinc sensor (AQZ) of carboxamidoquinoline with an alkoxyethylamino chain as receptor was
designed and synthesized. AQZ shows good water solubility and high selectivity for sensing; about an 8-fold increase in fluorescence quantum
2+
yield and a 75 nm red-shift of fluorescence emission upon binding Zn in buffer aqueous solution are observed. Moreover, AQZ can enter
yeast cells and signal the presence of Zn2
+
.
The design and synthesis of fluorescent sensors with high
selectivity and sensitivity is a vibrant field of supramolecular
chemistry. Especially of current, significant importance are
accomplished via measuring the metal-induced changes in
fluorescence intensity, which may be influenced by many
factors. Thus, these sensors are prone to be disturbed in
quantitative detection. Ratiometric fluorescent sensors permit
signal rationing to detect target molecules by measuring the
ratio of fluorescence intensities at two different wavelengths,5
1
sensors targeting heavy and transition metal ions, such as
4
2
2+
zinc cations. Despite having many commercial Zn sensors,
chemists continue endeavoring to design new ones and to
improve their sensitivity, selectivity, and reliability in order
to satisfy various needs that are due to the wide existence
which are autoemitted by sensors upon binding objects. The
autoreferential function of relative changes of two fluores-
cence intensities may avoid the influences of many nontarget
factors in the changes of monofluorescence intensity. There-
fore, the design of ratiometric fluorescent sensors is of great
2
+
3
of Zn in organisms and its extensive significance.
As is well-known, a majority of the reported Zn sensors
have poor water solubility, and recognition of metal ions is
2+
6
-8
†
‡
§
current interest.
Dalian University of Technology.
Qiqihar University.
East China University of Science and Technology.
1) (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
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1
0.1021/ol702869w CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/08/2008

The receptor moiety is the key for the design of ratiometric
sensors with good water solubility, except for aromatic
fluorophores with hydrophobicity. In the design and synthesis
electron-donating ability from the nitrogen atom of the
8-amino group to the quinoline ring. And the electron transfer
from the nitrogen atom of the heterocycle to the metal ion
further enhances the ICT process. As a result, a red-shift in
both emission and absorption wavelength could be observed.
Most importantly, the introduction of a 2-(2-hydroxyethoxy)-
ethylamino group provides not only another two metal-
coordination site (carboxamidoquinoline affords two sites)
but also a hydrophilic group.
AQZ was synthesized in a satisfactory yield by conjugat-
ing 2-(2-aminoethoxy)ethanol and 2-chloro-N-(quinol-8-yl)-
acetamide, which was prepared from 8-aminoquinoline and
2-chloroacetyl chloride (Scheme 1).
2+
of a ratiometric Zn fluoroionophore based on the ICT
mechanism, besides the famous di(2-picolyl)amine (DPA)
as a specific neutral receptor, well-known sulfonamidoquino-
9
line as a traditional receptor is widely used. We notice that
few reports on the Zn2 sensor of carboxamidoquinoline have
+
been published, except for Liu’s report on a novel supramo-
10
lecular system formed by 8-carboxamidoquinolyl-modifed
2+
â-cyclodextrin and 1-adamantaneacetic acid as a Zn sensor.
This system coordinated Zn2 through a cyclodextrin/
substrate/metal triple recognition mode. We wondered if a
+
2+
simple, small-molecule, ratiometric, and water-soluble Zn
sensor could be obtained based on carboxamidoquinoline,
that would not need the third component for recognition,
would reduce the complexity of measurement, and would
delete possible disturbing effects from Cd , Cu , and Ni ,
etc.
Scheme 1. Synthesis of AQZ
2+
2+
2+
Bearing this in mind, we synthesized a fluorescent sensor
of carboxamidoquinoline with an alkoxyethylamino chain
as receptor (AQZ) starting from 8-aminoquinoline. Here, the
introduction of a carboxamido group is of advantage to the
11
deprotonation of the 8-amino group. After binding metal
ions, the intramolecular hydrogen bond of 8-aminoquinoline
is broken, and the intramolecular electron-transfer process
is forbidden,3
a,12
thus enhancing fluorescence emission.
Simultaneously, the deprotonation process strengthens the
As expected, AQZ showed a very weak fluorescence (Φ
0.008, λmax(em) ) 440 nm) in tris-HCl (0.01 M) solution,
and its fluorescence was slightly influenced by the addition
0
(
6) (a) Taki, M.; Wolford, J. L.; O’Halloran, T. V. J. Am. Chem. Soc.
)
2
004, 126, 712-713. (b) Woodroofe, C. C.; Won, A. C.; Lippard, S. J.
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2+
+
2+
2+
2+
+
+
3+
2+
of Cd , Ag , Ca , Mg , Pb , Na , K , Fe , and Hg
Figure 1). A fluorescence quenching was detected upon the
(
1
1889. (f) Komatsu, K.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem.
Soc. 2007, 129, 13447-13454.
(
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S. J. J. Am. Chem. Soc. 2007, 129, 5910-5918.
(
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1
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3
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E. J. Neurosci. Methods 1987, 20, 91-103. (b) Zalewski, P. D.; Forbes, I.
J.; Betts, W. H. Biochem. J. 1993, 296, 403-408. (c) Fahrni, C. J.;
O’Halloran, T. V. J. Am. Chem. Soc. 1999, 121, 11448-11458. (d) Kimber,
M. C.; Mahadevan, I. B.; Lincoln, S. F.; Ward, A. D.; Tiekink, E. R. T. J.
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N. K.; Savage, P. B.; Izatt, R. M.; Prodi, L.; Montalti, M.; Zaccheroni, N.
Tetrahedron 2002, 58, 4809-4815. (f) Jiang, P.; Chen, L.; Lin, J.; Liu, Q.;
Ding, J.; Gao, X.; Guo, Z. Chem. Commun. 2002, 1424-1425. (g)
Hendrickson, K. M.; Geue, J. P.; Wyness, O.; Lincoln, S. F.; Ward, A. D.
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Arduini, M.; Mancin, F.; Tecilla, P.; Tonellato, U. Chem. Eur. J. 2007, 13,
Figure 1. Fluorescence spectra of AQZ (10 µM) in tris-HCl (0.01
M) solution (methanol/water ) 1:9, v/v, pH ) 7.22) in the presence
of different metal ions (5 equiv), and nearly no response to some
2+
+
2+
2+
2+
+
+
3+
other metal ions (Cd , Ag , Ca , Mg , Pb , Na , K , Fe
,
2
4
1
238-2245.
10) Chen, Y.; Han, K.; Liu, Y. Bioorg. Med. Chem. 2007, 15, 4537-
542.
2+
Hg ). (Inset) Visible emission observed from AQZ in the absence
(
2+
and presence of Zn (5 equiv).
(11) (a) Hiratani, K.; Hirose, T.; Kasuga, K.; Saito, K. J. Org. Chem.
992, 57, 7083-7087. (b) Yang, T.; Tu, C.; Zhang, J.; Lin, L.; Zhang, X.;
Liu, Q.; Ding, J.; Xu, Q.; Guo, Z. Dalton Trans. 2003, 3419-3424.
addition of Cu2 (Φ/Φ
+
) 0.32, λmax(em) ) 442 nm), Co
2+
0
(12) Meervelt, L. V.; Goethals, M.; Leroux, N.; Zeegers-Huyskens, T.
2
+
J. Phys. Org. Chem. 1997, 10, 680-686.
(Φ/Φ
0
) 0.22, λmax(em) ) 442 nm), and Ni (Φ/Φ
0
) 0.22,
474
Org. Lett., Vol. 10, No. 3, 2008

λ
max(em) ) 442 nm). However, a fluorescence enhancement
Φ/Φ ) 7.96, λmax(em) ) 515 nm) and a remarkable red-
shift of 75 nm were observed for AQZ upon binding Zn
(
0
2
+
by comparison with that of only AQZ in the solution.
Actually, an obviously blue-green emission of the solution
can easily be observed by the naked eye.
The signal response of AQZ toward Zn2 was recorded
+
in aqueous buffer solution in both emission and absorption
2
+
spectra. Upon the addition of increasing amounts of Zn ,
about an 8-fold increase in fluorescence quantum yield and
a 75 nm red-shift from 440 to 515 nm of fluorescence
emission were observed (Figure 2). Its intensity ratio at 515
Figure 3. Absorption spectra of AQZ (10 µM) in tris-HCl (0.01
M) solution (methanol/water ) 1:9, v/v, pH ) 7.22) in the presence
of different concentrations of Zn2 (0-3 equiv). (Inset:) Ratiometric
calibration curve A344 nm/A305 nm as a function of Zn2 concentration.
+
+
ing that AQZ possessed the highest sensing ability under
the physiological pH window.
Figure 2. Fluorescence spectra of AQZ (10 µM) in tris-HCl (0.01
M) solution (methanol/water ) 1:9, v/v, pH ) 7.22) in the presence
of different concentrations of Zn2 (0-3 equiv). (Inset) Ratiometric
calibration curve I515 nm/I440 nm as a function of Zn2 concentration.
+
+
and 440 nm (I515 nm/I440 nm) increased linearly with the
2+
concentration of Zn (0-1 equiv, linearly dependent coef-
2
2+
ficient: R ) 0.9887) up to a mole ratio (AQZ/Zn ) of 1:1,
and there it remained. Meanwhile, there was a 39 nm red-
shift from 305 to 344 nm of absorption wavelength with three
isosbestic points at 242, 280, and 324 nm, respectively
Figure 4. Fluorescence intensity of AQZ (10 µM) at various pH
values in methanol/water (1:9, v/v) solution in the absence and
presence of Zn2 (1 equiv).
+
(
A
(
Figure 3). Its absorbance ratio at 344 and 305 nm (A344 nm/
2+
305 nm) also increased linearly with the concentration of Zn
2
2+
0-1 equiv, linearly dependent coefficient: R ) 0.9896)
To further explore the selectivity of AQZ for Zn , the
2+
up to a mole ratio (AQZ/Zn ) of 1:1, and there it remained.
competition experiments were conducted in the presence of
2
+
2+
+
2+
2+
2+
+
+
These imply the formation of a complex with 1:1 stoichi-
ometry of AQZ and Zn , and the association constant is
Zn mixed with Cd , Ag , Ca , Mg , Pb , Na , K ,
2+
3+
2+
2+
2+
2+
Fe , Hg , Ni , Co , and Cu , respectively. While a range
of metal ions bound to the sensor, the addition of 1 equiv of
6
-1 13
6
.7 × 10 M . Moreover, a Job’s plot, which exhibits a
2+
2+
maximum at 0.5 M fraction of Zn , further indicates that
only a 1:1 complex is formed (Figure S1, Supporting
Information).
Moreover, the Zn -sensing ability of AQZ at different
pH values was also investigated. As shown in Figure 4, AQZ
had no fluorescence response to Zn2 in the acidic environ-
ment due to the protonation of the amino group of AQZ
Zn outcompeted most (Figure S2, Supporting Information).
2
+
2+
2+
The metal ions Co and Cu , especially Cu , remained
bound, and thus fluorescence emission of AQZ had a small
enhancement. In addition, these free cations would have little
influence in vivo since they exist at a very low concentra-
2+
+
14
tion.
2
+
A preliminary study on the Zn -sensing behaviors of
AQZ in living yeast cells (Saccharomyces cereVisiae) was
carried out by fluorescence microscopy. After incubation with
2+ 9h
15
leading to a weak coordination ability of Zn ; however,
satisfactory Zn -sensing abilities were exhibited when the
2+
pH was increased from 6.1 to 11.8. At pH ) 7.22, the IAQZ
AQZ at 37 °C for 1 h, the cells displayed very weak
+
Zn(II)/IAQZ value reached its maximum value of 5.7, indicat-
(
14) Maruyama, S.; Kikuchi, K.; Hirano, T.; Urano, Y.; Nagano, T. J.
(
13) (a) Valeur, B.; Pouget, J.; Bouson, J. J. Phys. Chem. 1992, 96, 6545-
Am. Chem. Soc. 2002, 124, 10650-10651.
(15) Devirgiliis, C.; Murgia, C.; Danscher, G.; Perozzi, G. Biochem.
Biophys. Res. Commun. 2004, 323, 58-64.
6
4
549. (b) Bouson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552-
557.
Org. Lett., Vol. 10, No. 3, 2008
475

intracellular straining, demonstrating that AQZ is cell
permeable (Figure 5). The cells also exhibited strong blue-
In conclusion, we have developed a new class of “naked-
2+
eye” and ratiometric fluorescent sensor AQZ for Zn in
2
+
aqueous solution. A highly Zn -selective fluorescence-
enhancing property in conjunction with a remarkable red-
shift of fluorescence emission was observed. Moreover, AQZ
2
+
can be used as an imaging reagent of Zn in living tissue
or in cells. It might be the first simple, small-molecule,
2
+
ratiometric, and water-soluble Zn fluorescent sensor de-
rived from carboxamidoquinoline, and we hope that car-
boxamidoquinoline could become a popular moiety for the
2
+
design of Zn sensors in the future, just as toluenesulfona-
midoquinoline has done.
Acknowledgment. This work was partially supported by
the National Natural Science Foundation of China (20572056,
Figure 5. Phase contrast (left) and fluorescence (right) microscopy
images of yeast cells incubated for 1 h with AQZ (40 µM), without
20536010), the Major Program of the Ministry of Education
top) and with (bottom) the addition of Zn2 (1 equiv).
+
of China (205040, 207028), the Natural Science Foundation
of Heilongjiang Province of China (E200530), and the
Overseas Chinese Scholars Program of Heilongjiang Prov-
ince (1151hz023, 1152hz20).
(
2+
green fluorescence with the addition of Zn . These results
indicate that AQZ may be used as a possible sensor to detect
Zn released from stimulated cells.
Supporting Information Available: Synthetic details,
characteristics, and spectroscopic data of AQZ. This material
is available free of charge via the Internet at http://pubs.acs.org.
2+
16
(16) Gee, K. R.; Zhou, Z. L.; Qian, W. J.; Kenney, R. J. Am. Chem.
Soc. 2002, 124, 776-778.
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