A novel sensitive turn-on fluorescent Zn2+ chemosensor based on an easy to prepare C 3-symmetric schiff-base derivative in 100% aqueous solution

Article abstract of DOI:10.1021/ol2034417

A C₃-symmetric Schiff-base example of the new simple, low cost, highly water soluble, and sensitive turn-on fluorescent Zn²⁺ chemosensor is described. The sensor was successfully applied to the detection of intracellular Zn²⁺. Moreover, the sensor could also serve as a potential recyclable component in sensing materials. Notably, the color change is so obvious that all of the recycling process can be seen clearly by the naked eye.

Full text of DOI:10.1021/ol2034417

ORGANIC
LETTERS
2
012
Vol. 14, No. 5
214–1217
A Novel Sensitive Turn-on Fluorescent
Zn Chemosensor Based on an Easy
2
þ
1
To Prepare C -Symmetric Schiff-Base
3
Derivative in 100% Aqueous Solution
†
†
,†
†
Ying Zhou, Zhan-Xian Li, Shuang-Quan Zang,* Yan-Yan Zhu, Hong-Yan Zhang,*
,‡
†
Hong-Wei Hou, and Thomas C. W. Mak
†,§
The College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450001, People’s Republic of China, Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing, People’s Republic of China, and Department of
Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong,
Shatin, New Territories, Hong Kong SAR, People’s Republic of China
zangsqzg@zzu.edu.cn; zhanghongyan@mail.ipc.ac.cn
Received December 24, 2011
ABSTRACT
2þ
A C -symmetric Schiff-base example of the new simple, low cost, highly water soluble, and sensitive turn-on fluorescent Zn chemosensor is
3
2
þ
described. The sensor was successfully applied to the detection of intracellular Zn . Moreover, the sensor could also serve as a potential
recyclable component in sensing materials. Notably, the color change is so obvious that all of the recycling process can be seen clearly by the
naked eye.
A chemosensor is the type of compound that renders a
significant change in magnetic, electronic, electrical, or
optical properties when it binds to a specific guest molecule
or ion. Among various types of sensors, fluorescent che-
mosensors for metal ions have attracted much attention
due to their convenient use and high sensitivity, and they
have been employed to clarify the real-time dynamics and
various biological functions of targeted metal cations in
†
Zhengzhou University.
1
‡
living cells. The zinc ion, asthe second most abundant and
Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences.
essential trace element in the human body behind iron,
plays a significant role in various fundamental biological
processes, such as cellular metabolism, gene expression,
apoptosis, neurotransmission, regulation of metalloenzymes,
§
The Chinese University of Hong Kong.
1) (a) Meng, Q.-T.; Zhang, X.-L.; He, C.; He, G.-J.; Zhou, P.; Duan,
(
C.-Y. Adv. Funct. Mater. 2010, 20, 1903. (b) Ishida, M.; Naruta, Y.;
Tani, F. Angew. Chem., Int. Ed. 2010, 49, 91. (c) He, G.-J.; Zhang, X.-L.;
He, C.; Zhao, X.-W.; Duan, C.-Y. Tetrahedron 2010, 66, 9762. (d) Jung,
H. S.; Kwon, P. S.; Lee, J. W.; Kim, J. I.; Hong, C. S.; Kim, J. W.; Yan,
S.; Lee, J. Y.; Lee, J. H.; Joo, T.; Kim, J. S. J. Am. Chem. Soc. 2009, 131,
(2) (a) Zhou, X.-Y.; Yu, B.-R.; Guo, Y.-L.; Tang, X.-L.; Zhang, H.-
H.; Liu, W.-S. Inorg. Chem. 2010, 49, 4002. (b) Qian, F.; Zhang, C.-L.;
Zhang, Y.-M.; He, W.-J.; Gao, X.; Hu, P.; Guo, Z.-J. J. Am. Chem. Soc.
2009, 131, 1460. (c) Berg, J. M.; Shi, Y. Science 1996, 271, 1081. (d)
Vallee, B. L.; Falchuk, K. H. Physiol. Rev. 1993, 73, 79.
2
008. (e) Suresh, M.; Shrivastav, A.; Mishra, S.; Suresh, E.; Das, A. Org.
Lett. 2008, 10, 3013. (f) Suresh, M.; Ghosh, A.; Das, A. Chem. Commun.
008, 3906. (g) Numata, M.; Li, C.; Bae, A.-H.; Kaneko, K.; Sakurai, K.;
Shinkai, S. Chem. Commun. 2005, 4655.
2
1
0.1021/ol2034417 r 2012 American Chemical Society
Published on Web 02/17/2012

2
and DNA binding or recognition. Thus far, a number of
fluorescence-enhancement-based chemosensors for Zn(II)-
selective detection were reported and have been used with
3
some success in biological applications. Nevertheless, most
of the reported fluorescent as well as colorimetric sensors for
2þ
Zn function in either pure organic or organicꢀaqueous
solutions. This limits their practical applications in a physio-
logical environment. Moreover, their preparations require
laborious multistep organic synthesis, which renders their
discovery processes slow and causes prohibitively high cost.
In this context, the design and synthesis of simple, facile, and
2þ
2a
efficient Zn probes remains of great interest, particularly
for those with high solubility in a solely water system and
selectivity and photostability in the spectral visible region.
Figure 1. X-ray crystal structure of the zinc complex of L: (a)
trinuclear unit and (b) side view of the hexanuclear Zn(II)
complex. All hydrogen atoms and solvent molecules were
omitted for clarity. Symmetry code: (#1) ꢀx þ 1, y, ꢀz þ 0.5.
Scheme 1. Synthesis of Fluorescent Sensor L
properties, as compared to those of the free compound L.
Figure 1 shows the single-crystal X-ray structure of the
2þ
hexanuclear Zn(II) complex (L-Zn ), which confirms the
formation of the quite stable drumlike structure of this
2þ
complex. The solid-state structure of the L-Zn complex
consists of two planar trinuclear units bridged by three
chlorine atoms. We have investigated the sensor’s chemo-
dosimetric properties, which can provide a sensitive mea-
2þ
surement of Zn in an absolute aqueous environment.
Furthermore, for presenting its application in biological
samples, the application of L to cultured A549 lung adeno-
carcinoma cells was implemented and discussed.
To our knowledge, C -symmetric ligands can be regarded
3
as triangular building blocks. This type of ligand can adopt
different conformations and bind in a number of coordina-
tion modes, according to the geometric requirements of the
metal ions, and may offer avenues for the construction of
organometallic frameworks with a variety of structures.
Herein, we report a new fluorescent chemosensor, tris(2-
4
pyridinecarboxaldehyde)triaminoguanidinium chloride (L),
which is the first C -symmetric Schiff-base example that
3
shows an excellent solubility in 100% aqueous media with
a highly sensitive fluorescence turn-on property upon binding
2þ
to Zn . More importantly, compound L can be readily
prepared by a simple and low-cost Schiff-base reaction of
picolinaldehyde with triaminoguanidinium chloride (Scheme
1; details are available in the Supporting Information).
The organic framework of L possesses an approximate
planar structure with three equivalent cavities capable of
chelating Zn ions. The formation of a Zn(II) complex
can enhance the coplanarity of the conjugated system, lead-
ing to a rather rigid structure with quite strong fluorescence
Figure 2. UVꢀvis spectra of L (50 μM) upon the titration of
2þ
2
þ
Zn (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, and 3.0
equiv) in buffer solution (10 mM TrisꢀHCl, pH 7.54) with an
excitation of 370 nm.
(3) (a) Peng, X.; Wu, T.; Fan, J.; Wang, J.; Zhang, S.; Song, F.; Sun, S.
Angew. Chem., Int. Ed. 2011, 50, 4180. (b) Xu, Z.; Yoon, J.; Spring, D. R.
Chem. Soc. Rev. 2010, 39, 1996. (c) Guo, Z.-Q.; Zhu, W.-H.; Zhu, M.-M.;
Wu, X.-M.; Tian, H. Chem. Eur. J. 2010, 16, 14424. (d) Liu,
Z.-P.; Zhang, C.-L.; Li, Y.-L.; Wu, Z.-Y.; Qian, F.; Yang, X.-L.; He,
W.-J.; Gao, X.; Guo, Z.-J. Org. Lett. 2009, 11, 795. (e) Felton, C. E.;
Harding, L. P.;Jones, J. E.;Kariuki,B. M.;Pope, S. J. A.;Rice,C. R.Chem.
Commun. 2008, 6185. (f) Wang, H.-H.; Gan, Q.; Wang, X.-J.; Xue, L.; Liu,
S.-H.; Jiang, H. Org. Lett. 2007, 9, 4995.
To determine the practical applications, the spectro-
scopic properties of L were measured under simulated
physiological conditions (10 mM TrisꢀHCl aqueous buf-
fer at pH 7.54). As shown in Figure 2, the UVꢀvis spec-
trum of L exhibits an absorption maximum at 325 nm (ε =
4
ꢀ1
ꢀ1
2
.12 ꢁ 10 M cm ). During photometric titrations of L
(4) (a) Hu, X.-Y.; Wang, J.; Zhu, X.; Dong, D.-P.; Zhang, X.-L.; Wu,
2þ
with Zn (0ꢀ3 equiv), a gradual decrease of the absor-
bance at 325 nm and a concomitant gradual increase of a
new absorption band centered at 396 nm were observed
with a distinct isosbestic point at 371 nm. The latter
S.-O.; Duan, C.-Y. Chem. Commun. 2011, 11507. (b) Zeng, X.; Dong, L.;
Wu, C.; Mu, L.; Xue, S.-F.; Tao, Z. Sens. Actuators B: Chem. 2009, 141,
5
1
06. (c) Hu, X. L.; Rodriguez, I. C.; Meyer, K. J. Am. Chem. Soc. 2004,
26, 13464. (d) Bretonni ꢀe re, Y.; Mazzanti, M.; Wietzke, R.; P ꢁe caut, J.
Chem. Commun. 2000, 1543.
Org. Lett., Vol. 14, No. 5, 2012
1215

indicates the formation of only one UV-active zinc com-
plex. Meanwhile, two new absorption peaks appeared at
2
maximum absorbance at 396 nm was observed when the
45 and 288 nm with some changes in pattern, and a
2þ
Zn concentration was increased to 3 equiv. Subse-
quently, the absorption spectrum remained at a plateau
2þ
upon further addition of Zn (>3 equiv). These observa-
tions imply the undoubted conversion of free compound L
to the corresponding zinc complex.
Figure 3 shows the change of fluorescence spectra of L
2þ
upon addition of Zn . Compound L alone displayed a
weak, single fluorescence emission band at 483 nm with a
negligible fluorescence quantum yield (<0.002) when it
2þ
was excited at 370 nm. A Zn titration experiment led to a
prominent fluorescence enhancement, accompanied by a
perceived color change from colorless to dark yellow,
ꢀ
5
Figure 3. Emission spectra of L (5 ꢁ 10 M) obtained in
2þ
TrisꢀHCl buffer (10 mM, pH 7.54) when titrated with Zn
2þ
2þ
(λex 370 nm). The final ratio of Zn ion to L is 12 equiv.
indicating a Zn -selective OFFꢀON fluorescent signal-
ing behavior. The increased in emission intensity may be
2þ
attributed to the formation of the L-Zn complex, in which
induced an emission enhancement to a certain extent, this
enhancement was less pronounced as compared to that of
Zn . Also, it should be noted that addition of Cd induced
a red shift of emission from 483 to 532 nm, which is quite
5
the CdN isomerization and proton transfer (ESPT) are
6
2þ
2þ
2þ
inhibited. Meanwhile, upon binding Zn , L exhibited a
large emission bathochromic shift of 97 nm from 483 to
2þ
2þ
2þ
580 nm, indicating that chemosensor Lrecognizes Zn par-
tially based on the chelation-enhanced fluorescence (CHEF)
different from that of Zn . Fortunately, Cd as a highly
toxic heavy-metal ion in vivo has a very low concentration.
This small interference does not affect the probe’s applica-
tion in physiological detection. As is well-known, the phy-
7
mechanism. In comparison to that of free L in aqueous
buffer a ∼21-fold significant fluorescence enhancement
2þ
þ
þ
2þ
2þ
could be observed when 3.0 equiv of Zn was present. Both
the Job plot and BensiꢀHildebrand analysis were imple-
siologically important metal ions Na , K , Mg , and Ca
exist at high concentration in living cells. In our case, they
were measured at a concentration as high as 1 mM. As
expected, these cations exerted a negligible effect on the
fluorescence response for L. Similar phenomena were also
2þ
mented, demonstrating a 1/3 stoichiometry for the L-Zn
complexation, which is in good agreement with results from
single-crystal X-ray structural analysis (Figures S6 and S7,
Supporting Information). The association constants of L
2þ
3þ
3þ
2þ
2þ
observed with the addition of Hg , Cr , Fe , Pb , Ni ,
2þ
5
2þ
with Zn were calculated as K = 1.9 ꢁ 10 , K = 1.5 ꢁ
and Co , respectively. Only Cu might form a complex
9
Zn1
3
Zn2
4
1
0 , and K = 6.1 ꢁ 10 , respectively, by literature
with L and obviously quenched the fluorescence.
2þ
Zn3
8
methods. The fluorescent titration profile of L with Zn
2þ
The selectivity of L for Zn over other transition-metal
ions has been examined. Results of our studies have revealed
2þ
(
Figure S8) demonstrated that the detection limit of Zn
ꢀ
6
2þ
is 2.5 ꢁ 10 M under the experimental conditions used
that all potentially competitive metal cations (except Cu )
exerted no or little influence on the fluorescence detection of
Zn in aqueous buffer solutions. In order to understand the
2þ
here. After Zn was added into a buffer solution of L, the
fluorescence decay curve was fitted to double-exponential
decay (Figure S9).
2þ
2þ
strong quenching behavior of Cu , we have measured the
2þ
2þ
Achieving a higher selectivity toward a specific analyte
over other potentially competing species is a necessity for a
change in emission intensity of L-Zn with increasing Cu
ion concentrations (Figure S10). The emission intensity of
3
c
2þ
fluorescence chemosensor. We evaluated the emission
response of Lagainst different transition-metal ions in buffer
L-Zn at ∼580 nm showed a gradual decrease, indicating
2þ
2þ
2þ
that Cu can displace Zn to form a L-Cu complex. In
2þ
2þ
solutions and found that only Zn caused a significant ratio
this regard, the complex L-Zn can be considered as a good
2þ
2þ
signal output (Figure 4). Although the addition of Cd also
ONꢀOFF chemosensor candidate for Cu . In addition,
2þ
the effects of counterions on the selective properties of Zn
2þ
demonstrated that Zn salts such as Zn(NO ) , Zn(ClO ) ,
3
2
4 2
(
5) (a) Wu, J.-S.; Liu, W.-M.; Ge., J.-C.; Zhang, H.-Y.; Wang., P.-F.
Chem. Soc. Rev. 2011, 40, 3483. (b) Suresh, M.; Mandal, A. K.; Saha, S.;
Suresh, E.; Mandoli, A.; Liddo, R. D.; Parnigotto, P. P.; Das, A. Org.
Lett. 2010, 12, 5406. (c) Ray, D.; Bharadwaj, P. K. Inorg. Chem. 2008, 47,
ZnBr , Zn(SO ) , and Zn(CH COO) showed almost simi-
2 4 2 3 2
lar spectral changes of L-ZnCl (Figures S11 and S12), which
2
suggests that the counteranion effects are negligibly small.
In order to obtain a better understanding of the interactions
2
252. (d) Sheng, J.-R.; Feng, F.; Qiang, Y.; Liang, F.-G.; Sen, L.; Wei,
F.-H. Anal. Lett. 2008, 41, 2203.
6) (a) Zhang, X.; Guo, L.; Wu, F.-Y.; Jiang, Y.-B. Org. Lett. 2003, 5,
667. (b) Sengupta, P. K.; Kasha, M. Chem. Phys. Lett. 1979, 68, 382.
7) (a) Ma, T.-H.; Dong, M.; Dong, Y.-M.; Wang, Y.-W.; Peng, Y.
Chem. Eur. J. 2010, 16, 10313. (b) Xue, L.; Liu, Q.; Jiang, H. Org. Lett.
(
2þ
between L and Zn , we studied the chemical reversibility of
2
(
2
009, 11, 3454. (c) Jiang, P.-J.; Guo, Z.-J. Coord. Chem. Rev. 2004, 248,
(9) (a) Li, Z.-X.; Yu, M.-M.; Zhang, L.-F.; Yu, M.; Liu, J.-X.; Wei,
L.-H.; Zhang, H.-Y. Chem. Commun. 2010, 46, 7169. (b) Peng, X.-J.; Du,
J.-J.; Fan, J.-L.; Wang, J.-Y.; Wu, Y.-K.; Zhao, J.-Z.; Sun, S.-G.; Xu, T.
J. Am. Chem. Soc. 2007, 129, 1500. (c) Zheng, Y.; Orbulescu, J.; Ji, X.;
Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. J. Am. Chem. Soc.
2003, 125, 2680.
205. (d) Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. J. Am. Chem.
Soc. 1990, 112, 3590.
8) (a) Zhao, Q.; Li, R.-F.; Xing, S.-K.; Liu, X.-M.; Hu, T.-L.; Bu,
X.-H. Inorg. Chem. 2011, 50, 10041. (b) Valeur, B. Molecular Fluorescence:
Principles and Applications; Wiley-VCH: Weinheim, Germany, 2001.
(
1
216
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2
þ
Figure 5. Imaging of intracellular Zn in A549 cells at 37 °C
with use of confocal microscopy: (a) cells incubated with L
ꢀ
ꢀ
2
(
10 μmol) for 1 h; (b) cells incubated with ZnCl
2
(3 ꢁ
2
1
PBS, then incubated for another 1 h after being treated with L
0
μmol) aqueous solution for 10 min and washed with
Figure 4. Fluorescence responses of L to various metal ions
gray bars) and fluorescence change of the mixture of L and
Zn after addition of an excess of the appropriate metal ions
(
ꢀ
2
2þ
(
Emission was collected at a 600/75 nm detector channel upon
10 μmol); (c) bright field image of A549 cells in parts a and b.
2
þ
2þ
þ
(
K
black bars), The equiv ratio of L to each of Ca , Mg , Na ,
þ
excitation at 408 nm by a diode laser.
is 1/20, and the equiv ratio of L to each of the other
transition-metal ions is 4. Mix: Zn þ Na /K /Mg /Ca
ex 370 nm.
2þ
þ
þ
2þ
2þ
,
λ
2þ
L-Zn , density functional theory (DFT) and time-depen-
dent DFT (TDDFT) calculations were carried out. The
optimized structures and the contribution of each electronic
oscillator (orbital transition) to the lowest energy transition
are all shown in the Supporting Information (Figures S17
and S18, Table S1). The calculated dihedral angles of
2
þ
the binding of L to Zn in buffer solutions. As seen by the
2þ
naked eye, the yellow color of a solution of L-Zn gradually
disappeared upon the addition of excess disodium EDTA
Figures S13 and S14). The effects of pH on the fluorescence
of the free compound L and its complex L-Zn are shown in
Figures S15 and S16, respectively. Over the pH range we
tested, compound L alone was not sensitive to pH. However,
the fluorescence intensity of L-Zn displayed a strong pH
dependence, as illustrated by its fluorescence intensity at
2þ
(
L-Zn are in good agreement with the experimental crystal
structure (Table S2). From the results of these calculations,
2þ
2þ
the introduction of Zn onto the group can distinctly affect
the steric hindrance and electronic structure of the imine
when CdN isomerization takes place. Such localization of
the electronic densities and increase of the structural rigidity
2þ
2þ
580 nm. The resulting sigmoidal curve (Figure S15) gives
a pK value of 6.5. An intense and stable fluorescence of
of the sensor L upon Zn complexation could be attribu-
table to both the emission enhancement and red shift.
In conclusion, we have presented a simple, water-solu-
a
2
þ
L-Zn in the pH range 7.0ꢀ10.0 warrants its application in
2þ
monitoring intracellular Zn without being affected by
changes in physiological pH values.
ble, and C -symmetric Schiff-base example of a highly
3
2þ
selective fluorescent chemosensor for Zn . The complex
2þ
The ability of biosensing molecules to selectively moni-
tor guest species in living systems is of great importance for
d
biological applications. Wehavemeasured the sensitivity
of L to Zn in human lung adenocarcinoma epithelial
cells (A549) by using laser scanning confocal microscopy
features a large Zn -induced red emission shift (97 nm)
and a distinguishable color change from colorless to dark
yellow under complete aqueous physiological conditions
1
2þ
2þ
when Zn was present. Notably, the chemically reversible
2þ
binding of L to Zn showed L could serve as a potential
recyclable component in sensing materials. In addition, the
(Nikon C1Si). Fluorescence images were recorded with a
Spinhole aperture, 100% gain of detector, and an oil objec-
2þ
complex L-Zn could be considered as a good ONꢀOFF
2þ
tive with 60ꢁ magnification and 1.40 NA. After A549 cells
Cu sensor candidate. Specifically, the new probe’s water
solubility and membrane permeability are favorable for its
ꢀ
2
were incubated with L (10 μmol) and PBS (1 mL) for 1 h
at 37 °C, no obvious fluorescence could be imaged (Figure
2þ
potential application in intracellular imaging of Zn
.
5
a). However, when the cells were supplemented with
ꢀ
2
exogenous ZnCl (3 ꢁ 10 μmol) in the growth medium
2
Acknowledgment. We gratefully acknowledge financial
support by the National Natural Science Foundation of
China (No. 20901070) and Zhengzhou University (No.
for 10 min at 37 °C, washed with 1 mL of PBS, and then
loaded with L and incubated for another 1 h under the
same experimental conditions, a significant fluorescence
increase from the intracellular region was clearly observed,
although the zinc complex has a relatively lower quantum
000001307106).
Supporting Information Available. Text, figures, tables,
and a CIF file giving experimental procedures, character-
ization data, including supplementary spectral data,
yield (Φ = 0.01) (Figure 5b). The strong fluorescence
f
2þ
resulted from the reaction of L and intracellular Zn ions.
The bright field transmission image of these A549 cells in
Figure 5c is exactly the same as the fluorescence image in
Figure 5b, confirming an intracellular fluorescence image.
To gain an additional understanding of the mechanism of
the fluorescence behavior of L and its chelation complex
1
13
H/ C NMR and mass spectra, and crystallographic
2þ
data for L-Zn . This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 5, 2012
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