An 2-(2′-aminophenyl)benzoxazole-based OFF-ON fluorescent chemosensor for Zn2+ in aqueous solution

Article abstract of DOI:10.1039/c0ob00983k

A water-soluble fluorescent sensor, 1, based on the "receptor-spacer- fluorophore" [2-(2′-aminophenyl)benzoxazole-amide-2-picolylamine] sensor platform, demonstrates the high sensitivity for Zn²⁺ with a 25-fold fluorescence enhancement upon che

Full text of DOI:10.1039/c0ob00983k

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PAPER
An 2-(2¢-aminophenyl)benzoxazole-based OFF–ON fluorescent chemosensor
for Zn2+ in aqueous solution†
Maliang Chen, Xin Lv, Yunlong Liu, Yun Zhao, Jing Liu, Pi Wang and Wei Guo*
Received 5th November 2010, Accepted 5th January 2011
DOI: 10.1039/c0ob00983k
A water-soluble fluorescent sensor, 1, based on the “receptor-spacer-fluorophore”
[2-(2¢-aminophenyl)benzoxazole-amide-2-picolylamine] sensor platform, demonstrates the high
sensitivity for Zn2+ with a 25-fold fluorescence enhancement upon chelation to Zn2+ and also exhibits
high selectivity to Zn2+ over other metal ions. X-ray crystal structure of Zn2+ complex reveals that the
amide oxygen (O2) cooperates with 2-picolylamine unit (N3, N4) as a receptor bind Zn2+.
Numerous scientific endeavours have been engaged in the
Introduction
development of fluorescent chemosensors for the in vitro
and in vivo detection of Zn2+.8–17 Quinoline-based fluores-
Detection and quantification of transition metal ions found in
biological systems and in the environment remains an active area
cent sensor molecules are the most widely used fluores-
cent Zn2+ chemosensors.8 Recently, several fluorescent probes
for Zn2+ have also been proposed based on different fluo-
rophores such as boradizazindacene (bodipy),9 rhodamine,10
Bis(benzoxazole),11 anthracene,12 bipyridine,13 spirobenzopyran,14
naphthalimide,15 tricarbocyanine,16 and 4-amino-7-nitro-2,1,3-
benzoxadiazole (ANBD).17 However, some of them have poor
water solubility, thus various solvent mixtures are used in charac-
terization, which inevitably hinders their applications. In addition,
many available Zn2+ sensors have difficulty in distinguishing Zn2+
and Cd2+, since Cd2+ is in the same group of the periodic table
and has similar properties with Zn2+ which results in similar
fluorescence changes when Zn2+ and Cd2+ are coordinated with
fluorescent sensors. Therefore, the exploration of new fluorescent
turn-on probes for analyzing Zn2+ with appropriate selectivity,
high sensitivity, rapid response, and good water solubility remains
a challenge.
In this paper, we report the design and synthesis of a highly
selective “off–on” fluorescent sensor 1 for Zn2+ in aqueous solu-
tion. Our approach is based on the “receptor-spacer-fluorophore”
sensor platform to promote the fluorescence “off–on” changes.
The choice of 2-(2¢-aminophenyl)benzoxazole derivative as the
fluorophore is based on the excellent photophysical properties
of benzoxazole derivatives, which are extensively used as sensor
molecules for a series of cations and anions.18 The choice of 2-
picolylamine as Zn2+-chelator was based on the consideration
that this unit and its derivatives are known to have an strong
affinity for Zn2+.16,18b,19 In addition, a rigid amide group was
introduced into the sensor not only to link two moieties of the
2-(2¢-aminophenyl)benzoxazole fluorophore and 2-picolylamine
chelator but also to provide another one metal-coordination site
(amide carbonyl).
of research, as these ions are either quite beneficial or toxic to
human health.1 Among the various transition metal ions, Zn2+
is the second most abundant transition metal in human body,
and plays crucial roles in many important biological processes
in acting as the structural and catalytic cofactors, neural signal
transmitters or modulators, and regulators of gene expression,
and apoptosis.2,3 Failure to maintain zinc homeostasis has been
implicated in a number of severe neurological diseases such as
Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS),
Guam ALS-Parkinsonism-dementia Parkinson’s disease, hypoxia
ischemia, and epilepsy.4–6 Therefore, highly selective and sensitive
detection of Zn2+ in various samples is of toxicological and
environmental concern.
Sensors based on the chemical species induced changes in
fluorescence appear to be particularly attractive due to the
highly sensitive, quick, simple and real time monitoring of the
fluorescence.7 Moreover, fluorescence-based chemosensors can be
used to readily detect intracellular ion levels without the need for
sophisticated instrumentation or time-consuming sample prepara-
tion. For sensitivity reasons, chemosensors exhibiting fluorescence
enhancement (turn-on) are favored over those showing fluores-
cence quenching (turn-off). Therefore, a crucial issue is controlling
the emission of a fluoroionophore so that it is nonfluorescent in
the absence of metal ion, whereas highly fluorescent in its metal
complex.
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan,
030006, China. E-mail: guow@sxu.edu.cn; Tel: +86 351 7011600
† Electronic supplementary information (ESI) available: 1H NMR, 13C
NMR, ESI-MS and additional spectroscopic data of 1. CCDC reference
number 799368. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0ob00983k
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added rigidity to the complex, thus resulting in a nearly planar
molecule conformation. The rigid molecular structure appears
to be favorable for fluorescence emission of the complex due to
the decreased nonradiative decay through rotatory motion of the
amide group or others.
Results and discussion
The synthesis of
1 is shown in Scheme 1. The 2-(2¢-
aminophenyl)benzoxazole (3) was prepared in 65% yield accord-
ing to a literature method,20 then condensation of 3 with 2-
chloroacetyl chloride to produce 2 in 70% yield. Reaction of 2
with 2-picolylamine under basic conditions gives 1 in 65% yield.
The metal-binding behavior of 1 has been determined by
fluorescence spectroscopic studies. Sensor 1 showed a very weak
fluorescence in Tri-HCl buffer (10 mM, pH 7.2) solution. Upon
addition of Zn2+, the fluorescence intensity increased remarkably
and a fluorescence enhancement factor at 445 nm of approximately
25-fold was estimated (Fig. 2a). These changes in the fluorescence
spectrum stopped and the emission intensities became constant
when the amount of Zn2+ added reached 3 equiv, and a clear blue
fluorescence could be observed. The association constant for Zn2+
was estimated to be 4.3 ¥ 105 M-1 (R2 = 0.9969) on the basis of
nonlinear fitting of the titration curve assuming 1 : 1 stoichiometry
(Fig. 2b). Job plot analysis of the fluorescence titrations revealed
a maximum at about 0.5 mole fraction (Fig. 2b, inset), in accord
with the proposed 1 : 1 binding stoichiometry. The solid evidence
of the 1 : 1 binding mode comes from the ESI-MS spectra of the
complex of 1 with Zn2+, in which the peaks at m/z 421.0635 (calcd =
421.0643) corresponding to [1+Zn2+–H+]+ was clearly observed
(Fig. S1, ESI†).
Scheme 1 Synthesis of fluorescent sensor 1.
To obtain the clear insight on the 1-Zn2+ interaction, we
firstly performed the X-ray diffraction study of 1-Zn2+ complex.
The single crystal of 1-Zn2+ complex was obtained by slow
evaporation of the MeCN solution of 1 and equivalent ZnCl2,
and characterized using X-ray crystallography.21 As shown in
Fig. 1, X-ray structural analysis of 1-Zn2+ complex reveals the
formation of a 1 : 1 metal/ligand mononuclear complex. The
amide oxygen (O2) cooperates with the 2-picolylamine unit (N3
and N4) and two chloride atoms as a receptor to bind Zn2+.
The bond lengths of Zn(1)–O(2), Zn(1)–N(3), and Zn(1)–N(4) are
˚
˚
˚
2.522 A, 2.113 A, and 2.129 A, respectively. Notably, the crystal
structure of 1-Zn2+ complex exhibits an intramolecular N–H ◊ ◊ ◊ N
hydrogen bond (N2 . . . N1, 2.683 A; H . . . N1, 1.905 A) and a
Zn2+-ligand chelation interaction, both of which should provide
˚
˚
Fig. 2 (a) Fluorescence sprctra of 1 (10 mM) in Tris-HCl buffer (10 mM,
pH = 7.2) in the presence of different amounts of Zn2+. Inset: fluorescence
changes of 1 (10 mM) upon addition of 3 equiv of Zn2+. (b) Fluorescence
titration profile vs. concentration of Zn2+ in Tris-HCl buffer (10 mM, pH =
Fig. 1 (a) Crystal structure of 1-Zn2+ complex, face-on view, with
displacement atomic ellipsoids drawn at the 50% probability level. (b) Side
view of the structure in (a) showing nearly planar molecular conformation.
7.2) for 1. Inset: Job’s plot for determining the stoichiometry of 1 and Zn2+
.
Slit: 5 nm/5 nm.
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When 1 was employed at 5 mM and the slit was adjusted
to 10 nm/10 nm, the linear dynamic response concentration
range for Zn2+ covers from 1.0 ¥ 10-7 to 1.0 ¥ 10-6 M (Fig. 3).
The detection limit (LOD) and quantification limit (LOQ) were
An important feature of 1 is its high selectivity toward the Zn2+
over the other competitive species. Changes of fluorescence spectra
of 1 (10 mM) caused by Zn2+ (3 equiv) and miscellaneous cations
including Na+, K+, Mg2+, Ca2+ (30 equiv); Cd2+, Cr3+, Mn2+, Hg2+,
Ag+, Pb2+, Fe3+, Ni2+, Co2+, Cu2+ (3 equiv), in Tris-HCl buffer
(10 mM, pH = 7.2) are recorded in Fig. 5. Most of tested metal
cations including Cd2+ did not induce any distinct emission shift
and enhancement. Moreover, the presence of these competitive
cations, Zn2+ ion still resulted in the similar fluorescence changes
(Fig. 6). However, some paramagnetic metal ions, such as Cu2+,
Co2+, and Ni2+, obviously quench the fluorescence, which is always
encountered in the other metal ion sensors.
measured to be 3.66 ¥ 10-8 M (3s/slope) and 1.22 ¥ 10-7
M
(10s/slope), respectively. In addition, the interaction of 1 with
Zn2+ was completed in a few seconds (Fig. S2, ESI†), therefore 1
could be used for the real-time and real-space analysis of Zn2+ ions
in cells and organisms.
Fig. 3 Fluorescence response of 1 (5 mM) to Zn2+ in Tris-HCl buffer
(10 mM, pH = 7.2). lex/lem = 350/445 nm. Slit: 10 nm/10 nm.
Fig. 5 The fluorescence spectra of 1 (10 mM) upon addition of 3 equiv
of Zn2+ and various other metal ions, including of Na+, K+, Mg2+, Ca2+
(30 equiv); Cd2+, Cr3+, Mn2+, Hg2+, Ag+, Pb2+, Fe3+, Ni2+, Co2+, Cu2+ (3
equiv), in Tris-HCl buffer (10 mM, pH = 7.2). Inset: Fluorescence responses
of 1 towards Zn2+ to various cations. Intensity was recorded at 445 nm,
excitation at 350 nm.
To apply 1 in more complicated systems, like in an organism
and the environment, the influence of pH on the fluorescence of
1 was examined by fluorescence titration in water (Fig. 4). The
fluorescence spectrum of 1 exhibits a weak emission band with
a maximum at 445 nm, and remained unaffected with increasing
acidity from pH 13.7 to 2.0. Upon addition of Zn2+, 1 responds
stably to Zn2+ between pH 5.2 and 12.1 without any interference by
protons, indicating that 1 successfully reacts with Zn2+ and allows
Zn2+ detection in a wide pH range.
Fig. 6 Fluorescence responses of 1 (10 mM) to Zn2+ (3 equiv) containing
various metal ions including of Na+, K+, Mg2+, Ca2+ (30 equiv); Cd2+, Cr3+
,
Mn2+, Hg2+, Ag+, Pb2+, Fe3+, Ni2+, Co2+, Cu2+ (3 equiv), in Tris-HCl buffer
(10 mM, pH = 7.2). Intensity was recorded at 445 nm, excitation at 350 nm.
Several factors can be listed for rationalizing the observed
emission enhancement. The weak fluorescence of 1 in the absence
of Zn2+ might be attributed to radiationless channels from the np*
state.22 In the presence of Zn2+ that coordinates with the lone pair
of the carbonyl oxygen (O2), the energy of the np* state would
Fig. 4 Changes in fluorescence intensity (lex/lem = 350 nm/445 nm) of 1
(10 mM) in water measured with and without Zn2+ (3.0 equiv) as a function
of pH.
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◦
1
be raised so that the pp* state becomes the lowest excited state,
leading a substantial increase in the fluorescence intensity.23 In
addition, Zn2+ binding to 1 could not only induce a conformation
restriction24 but also block PET quenching of the singlet excited
state of the 2-(2¢-aminophenyl)benzoxazole moiety, thus, a big
fluorescence enhancement could be observed (Fig. 7).
135–136 C. H NMR (300 MHz, CDCl3): d 12.78 (b, 1H), 8.86
(d, J = 8.4, 1H), 8.28 (d, J = 7.8, 1H), 7.34–7.81 (m, 6H), 4.36 (s,
2H).
Synthesis of compound 1
Compound 2 (0.143 g, 0.5 mmol), 2-picolylamine (0.065 g,
0.6 mmol), N, N-diisopropylethylamine (DIEA, 0.8 mL), and
potassium iodide (30 mg) were added to acetonitrile (50 mL).
After stirring and refluxing for 10 h under nitrogen atmosphere,
the mixture was cooled to room temperature, and was evaporated
to dryness under reduced pressure. The residue was purified by
silica gel column chromatography using CHCl3–MeOH (200 : 1)
as eluent to afford compound 1. Yield: 0.12 g (65%). Mp: 85–
◦
1
86 C. H NMR (300 MHz, DMSO-d6): d 8.82 (d, J = 8.7, 1H),
8.52 (d, J = 4.5, 1H), 8.20 (d, J = 7.8, 1H), 7.79 (d, J = 8.1, 1H),
7.58–7.66 (m, 2H), 7.51 (d, J = 7.8, 1H), 7.45 (m, 1H), 7.24–7.35
(m, 4H), 3.98 (s, 1H), 3.46 (s, 1H). 13C NMR (75 MHz, CDCl3):
171.8, 161.9, 159.1, 149.6, 141.3, 138.8, 136.7, 132.8, 128.7, 125.6,
124.7, 123.3, 122.7, 122.4, 120.9, 119.8, 113.8, 110.7, 55.2, 53.8.
IR (KBr) cm-1: 3434, 3178, 2931, 1676, 1618, 1589, 1548, 1531,
1438. HRMS (ESI) calcd. for (M–H)- 357.1351, found 357.1354.
Fig. 7 The proposed sensing mechanisms of 1 for Zn2+
.
Conclusions
In summary, we have successfully prepared a water-soluble and
small molecular fluorescent sensor for Zn2+. The sensor displays
an obvious fluorescence enhancement up to 25-fold with Zn2+ in
aqueous solution. Moreover, the sensor shows good selectivity
for Zn2+ over other metal ions. The binding mode of the sensor
with Zn2+ was probed by X-ray diffraction study. We expect
that the method will serve as practical tool for environmental
samples analysis and biological studies. Further investigations on
its applications in life science are underway in our lab.
Crystallography
Intensity data of 1-Zn2+ complex were collected on a Siemens
SMART-CCD diffractometer with graphite-monochromated Mo-
˚
Ka (l = 0.71073 A) using the SMART and SAINT programs.
Crystal data for 1-Zn2+, C21H18Cl2N4O2Zn, M = 494.66, Triclinic,
¯
˚
˚
˚
space group P1, a = 8.6993(◦17) A, b = 10.744(2) A, c = 11.593(2) A,
◦
◦
3
˚
a = 75.65(3) , b = 74.28(3) , g = 86.28(3) , V = 1010.6(3) A , m =
1.506 mm-1, Z = 2, T = 113(2) K, cyrstal dimensions 0.20 ¥ 0.18
¥ 0.10 mm3, q range 1.88–25.01◦. 9125 reflections were collected
of which 3541 reflections were unique (Rint = 0.0382). Final R
indices [I ≥ 2s (I)]: R1 = 0.0319, wR2 = 0.0946, R indices (all data):
R1 = 0.0380, wR2 = 0.1052, goodness of fit on F2 = 1.022. CCDC:
799368.
Experimental section
Materials and general methods
2-(2¢-Aminophenyl)benzoxazole (3) were prepared by literature
procedures.20 All other reagents and solvents were purchased
from commercial sources and were of the highest grade. Solvents
were dried according to standard procedures. All reactions were
magnetically stirred and monitored by thin-layer chromatography
(TLC). Flash chromatography (FC) was performed using silica gel
60 (230–400 mesh). Absorption spectra were taken on an Agilent
8453 spectrophotometer. Fluorescence spectra were taken on
Procedures of ions sensing
Deionized water was used throughout all experiments. Solutions
of Ca2+ were prepared from their chloride salts; solutions of Na+,
K+, Ag+, Mg2+, Cu2+, Cd2+, Co2+, Mn2+, Ni2+, Zn2+, Hg2+, Pb2+,
Cr3+, and Fe3+ were prepared from their nitrate salts. A stock
solution of 1 (25 mM) was prepared in MeCN. The stock solution
of 1 was then diluted to the corresponding concentration (10 mM
and 5 mM) with the buffer solution (10 mM Tris-HCl, pH 7.2].
The Zn2+ stock solution of 1.0 ¥ 10-1 M was diluted to 1.0 ¥ 10-2
M, 1.0 ¥ 10-3 M and 1.0 ¥ 10-4 M with deionized water for spectra
titration studies. In the titration experiments, a 2.0 mL solution of
1 (10 mM and 5 mM) was poured into a quartz optical cell of 1 cm
optical path length each time, and Zn2+ solution was added into
the quartz optical cell gradually by using a micro-pipette. Spectral
data were recorded in an indicated time after the addition.
1
HITACHI F-2500 fluorescence spectrometer. The H NMR and
13C NMR spectra were recorded at 300 and 75 MHz, respectively.
The following abbreviations were used to explain the multiplicities:
s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet; br =
broad. High resolution mass spectra were obtained on a Varian
QFT-ESI mass spectrometer.
Synthesis of compound 2
A solution of 0.34 g (3 mmol) of 2-chloroacetyl chloride in
5 mL of dry CH2Cl2 was added dropwise to a solution of 0.42 g
(2 mmol) 2-(2¢-aminophenyl)benzoxazole (3) and 0.5 g (4.1 mmol)
4-dimethylaminopyridine (DMAP) in 10 mL of dry CH2Cl2 stirred
in an ice bath. After stirred 2 h at room temperature, the mixture
was evaporated under reduced pressure, and the residues were
purified by silica gel column chromatography using PE/CH2Cl2
(2 : 1) as eluent to afford compound 2. Yield: 0.4 g (70%). Mp:
Acknowledgements
We would like to thank the Natural Science Foundation of
China (NSFC No. 20772073 and 21072121) and the Natural
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Science Foundation of Shanxi Province (2008011015) for financial
support.
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