A simple pincer-type chemosensor for reversible fluorescence turn-on detection of zinc ion at physiological pH range

Article abstract of DOI:10.1039/c5nj00023h

A new, highly selective and sensitive pincer-type chemosensor (Y) for zinc ion with turn-on fluorescence behavior in physiological pH range solution was developed. Y is based on bi-hydrazone derivatives, which contain pyridine as the fluorescent group and a hydrazone bond as recognition site. The selectivity mechanism of Y for zinc is based on the combined effects of the inhibition of excited-state intramolecular proton transfer (ESIPT) and -HC=N- isomerization, as well as chelation-enhanced fluorescence. The entire process takes less than 15 seconds. The minimum detection limit of Y for zinc reaches 1.39 × 10^-8 M. Moreover, Y can conveniently detect zinc in test strip form, and it can be recycled.

Full text of DOI:10.1039/c5nj00023h

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A simple pincer-type chemosensor for reversible
fluorescence turn-on detection of zinc ion at
physiological pH range†
Cite this:DOI: 10.1039/c5nj00023h
Qi Lin,* Yi Cai, Qiao Li, Jing Chang, Hong Yao, You-Ming Zhang and Tai-Bao Wei*
A new, highly selective and sensitive pincer-type chemosensor (Y) for zinc ion with turn-on fluorescence
behavior in physiological pH range solution was developed. Y is based on bi-hydrazone derivatives, which
contain pyridine as the fluorescent group and a hydrazone bond as recognition site. The selectivity
mechanism of Y for zinc is based on the combined effects of the inhibition of excited-state intramolecular
proton transfer (ESIPT) and –HCQN– isomerization, as well as chelation-enhanced fluorescence. The entire
process takes less than 15 seconds. The minimum detection limit of Y for zinc reaches 1.39 Â 10À8 M.
Moreover, Y can conveniently detect zinc in test strip form, and it can be recycled.
Received (in Montpellier, France)
4th January 2015,
Accepted 16th March 2015
DOI: 10.1039/c5nj00023h
www.rsc.org/njc
increase the electronic density and stabilize the coordination
sphere of a transition metal (TM). In particular, the pincer-type
Introduction
The development of chemical probes for sensing metal ions1–6 motif featuring a tridentate, meridional coordinating ligand
and anions7–11 has received considerable attention due to their framework offers a myriad of opportunities for controlling the
fundamental applications in chemical, environmental and steric and electronic properties of TM complexes.39 Generally,
biological fields. For example, despite the fact that zinc has the side arms of a pincer ligand consist of neutral, two-electron
significant roles in catalytic centers and serves as structural Lewis donor moieties, which are connected through a linker
cofactor of many Zn2+- containing enzymes and DNA-binding group to a neutral or monoanionic anchoring site. Herein, we
proteins,12–17 an unregulated zinc level in the body may lead to report the synthesis and sensing properties of a new hydrazone-
a number of severe neurological diseases (e.g. Alzheimer’s disease, based chemosensor Y, which is pincer-type, with both the
cerebral ischemia, and epilepsy), developmental defects, and hydrazone and an imine functional group. Our approach to
malfunctions.18–28 Therefore, considerable effort has been designing this fluorescence-based, bi-functional chemosensor
devoted to the development of efficient and selective methods relies on the strong coordination capability of the hydrazone
to detect Zn2+. Numerous approaches, such as inductively coupled base with metal ions. Receptor Y showed a reversible, intense
plasma atomic emission spectrometry,29 atomic absorption spectro- fluorescence enhancement in the presence of zinc ions in aqueous
scopy,30 and electrochemical methods,31 have been employed to solutions at physiological pH range. Additionally, Y could also
detect both the metal ions and anions. However, most of these conveniently detect Zn2+ in the form of test strips.
methods require sophisticated instrumentation, tedious sample
preparation procedures, and trained operators. By contrast, fluores-
cence technology provides a convenient and easy method for
Experimental section
Materials and physical methods
monitoring the target ions.32–35 Methods using fluorescence have,
therefore, attracted considerable attention in the detection of metal
ions or anions, including Zn2+. Recently, simple chemosensors Fresh doubly distilled water was used throughout the experiment.
have become very popular among analysts because of their fast All other reagents and solvents were commercially available at
detection time and cost reduction.36–38 Meanwhile, multidentate analytical grade and were used without further purification.
donor ligands have the advantage that they can simultaneously 1H NMR and 13C NMR spectra were recorded on a Mercury-400BB
1
spectrometer at 400 MHz for H. The chemical shifts, reported in
ppm downfield from tetra spectra, were recorded with a Mercury-
400BB spectrometer at 400 MHz (TMS, d scale with the solvent
resonances as internal standards). Electrospray ionization mass
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education of China, Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: linqi2004@126.com,
spectra (ESI-MS) were measured on an Agilent 1100 LC-MSD-Trap-
weitaibao@126.com; Tel: +86 931 7970394
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj00023h VL system. UV-visible spectra were recorded on a Shimadzu UV-2550
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spectrometer. The photoluminescence spectra were performed on a Results and discussion
Shimadzu RF-5301 fluorescence spectrophotometer. The melting
Fluorescence spectroscopic studies of Y toward Zn2+ ions
points were measured on an X-4 digital melting-point apparatus
(uncorrected). The infrared spectra were performed on a Digilab
FTS-3000 FT-IR spectrophotometer.
To gain insight into the fluorescent properties of receptor Y
toward metal ions, the emission changes were measured with
various metal ions in DMSO/H2O (v/v = 9/1) HEPES buffer
solutions at pH = 7.2. When excited at 355 nm, Y exhibited a
weak fluorescence, which was much lower than that in the
presence of Zn2+ (Fig. 1). By contrast, upon addition of other
metal ions such as Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+,
Pb2+, Cr3+ and Mg2+, either no increase or a slight increase in
intensity was observed. To this end, it is noteworthy that Y can act as
a ‘‘turn-on’’ sensor for Zn2+ and differentiate Zn2+ from Cd2+, which
has almost always been a major problem in the past.40–43 This
preferential fluorescence enhancement for Zn2+ might be due to the
formation of a chelate complex (rigid system) between Y and
the Zn2+ ion, leading to the chelation-enhanced fluorescence
(CHEF) effect.44 Additionally, receptor Y is a poor fluorescent due
to the following combined effects: (a) isomerization of the –HCQN–
double bonds in the excited state45 and (b) excited-state intra-
molecular proton transfer (ESIPT), involving the hydroxy proton of
the 4-hydroxyisophthalaldehyde moiety (Scheme 2).46 Upon stable
Synthesis of sensor molecule Y
Compound Y can be readily prepared by a simple and low-
cost Schiff base reaction of 4-hydroxyisophthalaldehyde and
2-hydrazinylpyridine (Scheme 1). 4-Hydroxyisophthalaldehyde
(0.7500 g, 5 mmol), 2-hydrazinylpyridine (0.5995 g, 5.5 mmol)
and a catalytic amount of acetic acid (AcOH) were combined in
hot absolute EtOH (30 mL). The solution was stirred under
reflux for 4 hours. After cooling to room temperature, the dark
yellow precipitate was filtered, washed three times with hot
absolute ethanol, then recrystallized with EtOH to obtain
a yellowy powdered product Y (4.31 mmol) in 86.2% yield
(m.p. 4300 1C). 1H NMR (DMSO-d6, 400 MHz) d: 11.01
(s, 1H, O–H); d: 10.80 (s, 1H, N–H); d: 10.72 (s, 1H, N–H); d:
8.31 (s, 1H, CHQN). (Fig. S1, ESI†) IR (KBr) n: 1622 cmÀ1
(–HCQN–), 3422 cmÀ1 (N–H). (Fig. S8, ESI†) ESI-MS calcd for
[C18H16N6O + H+]+ 333.3592. Found 333.1727. (Fig. S2, ESI†)
General procedure for spectroscopy
All the UV-vis experiments were carried out in DMSO solution on a
Shimadzu UV-2550 spectrometer. Any changes in the UV-vis spectra
of the synthesized compound were recorded upon the addition of
perchlorate salts while keeping the ligand concentration constant
(2.0 Â 10À5 M) in all experiments. The perchlorate salts of Fe3+
,
Hg2+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, and Mg2+
were used for the UV-vis experiments.
Fluorescence spectroscopy was carried out in a DMSO solution
on a Shimadzu RF-5301 spectrometer. Any changes in the fluores-
cence spectra of the synthesized compound were recorded upon
the addition of perchlorate salts while keeping the ligand concen-
tration constant (2.0 Â 10À5 M) in all experiments.
The perchlorate salts of the ions (Fe3+, Hg2+, Ca2+, Cu2+, Co2+,
Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, and Mg2+) were used for the fluores-
1
cence experiments. For H NMR titrations, the solution of Y was Fig. 1 Bar graph shows the relative emission intensity of Y (c = 2 Â
10À5 M) in the presence of different metal ions (10 equiv.) such as Fe3+
,
prepared in DMSO-d6, and the appropriate concentration of test
solutions was prepared in doubly distilled water. Aliquots of the
two solutions were mixed directly in the NMR tubes.
Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+ and Mg2+ with an
excitation of 355 nm in DMSO/H2O (v/v = 9/1) HEPES buffer solutions at
pH = 7.2 and 524 nm.
Scheme 1 Synthesis of the sensor molecule Y.
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Scheme 2 A possible sensing mechanism of the sensor Y to Zn2+
.
chelation with the zinc ion, –HCQN– isomerization and ESIPT
might be inhibited, leading to enhanced fluorescence.
To further investigate the sensing properties of Y, fluorimetric
titration of Y was performed with the Zn2+ ion. As shown in Fig. 2,
the emission intensity of Y at 524 nm steadily increased until the
amount of Zn2+ reached 2.6 equiv. The Job plot47 showed a 1 : 1
complexation stoichiometry between Zn2+ and Y (Fig. S3, ESI†),
which was further confirmed by 1H NMR titration analysis (Fig. S4,
ESI†). Based on the Job plot analysis, we propose the structure of
the Zn2+–Y complex, as shown in Scheme 2. From the fluorescence
titration data, the association constant for Zn2+–Y complexation
was determined to be 2.4 Â 105 MÀ2 from Li’s equations.48
Meanwhile, the fluorescence quantum yield (F) increases from
0.2 to 0.87.49,50 These results indicate that the pincer-type
chemosensor Y reacts with Zn2+ to form a complex. In the
corresponding UV-vis spectra, after adding Zn2+ to Y, the maximum
absorption peak red shifts obviously, which also supports our
conjecture (Fig. S5, ESI†).
Fig. 2 (a) Fluorescence spectral changes of Y (c = 2 Â 10À5 M) in the
presence of different concentrations of Zn2+ ions in DMSO/H2O (v/v = 9/1)
HEPES buffer solutions at pH = 7.2. (b) Fluorescence intensity at 524 nm
versus the number of equiv. of Zn2+ added.
The detection limit of Y for Zn2+ calculated on the basis of
3sB/S51 (Fig. S6, ESI†) is 1.39 Â 10À8 M for fluorescence spectra,
which is far lower than the WHO guidelines for drinking
water (76 mM).52,53 Meanwhile, the fluorimetric detection limit
of Zn2+ by the naked eye for sensor Y was also tested. As
is shown in Fig. 3, the minimum concentration of Zn2+ for
fluorescence color change under an UV lamp at 360 nm
observed by the naked eye is 2.0 Â 10À6 M.
To further check the practical applicability of receptor Y as a
Zn2+-selective fluorescent sensor, we carried out competition
experiments. Y was treated with 10 equiv. of Zn2+ in the
presence of other metal ions (Fe3+, Hg2+, Ag+, Ca2+, Cu2+,
Co2+, Ni2+, Cd3+, Pb2+, Cr3+, Mg2+) of the same concentration.
Fig. 3 Naked-eye detection limit under an UV lamp at 365 nm. From left to
right, the concentration of Zn2+ is 2.0 Â 10À6 mol LÀ1, 2.0 Â 10À5 mol LÀ1
,
2.0 Â 10À4 mol LÀ1, and 2.0 Â 10À3 mol LÀ1
.
As shown in Fig. S7 (ESI†), other background metal ions However, it is worth noting that cadmium ions hardly inhibited
had little or no obvious interference with the detection of the fluorescence intensity of the Zn2+–Y complex. Hence, these
Zn2+ ions, except for Cu2+, Fe3+ and Hg2+, after a long time. results suggest that Y could be a good sensor for Zn2+ and in
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Fig. 5 Stepwise complexation–decomplexation cycles carried out in
DMSO/H2O (v/v 9/1). Emission intensity changes at 524 nm of Y upon
addition of 10 equiv. Zn2+ before and after treatment of 10 equiv. EDTA.
Fig. 4 pH value affects the charge distribution of receptor Y and Y + Zn2+
.
particular, distinguish Zn2+ from Cd2+, which have many common
properties.54
Since the pH value affects the charge distribution of receptor Y
and may change its inherent fluorescence properties, the
effect of pH on the emission bands of Y in DMSO/H2O
(v/v = 9/1) HEPES buffer solution at pH = 7.2 was also studied
(Fig. 4). The Zn2+–Y complex showed a significant fluorescence
response between pH 5 and 8, which includes the physio-
logically relevant range of pH 7.0–8.4.55 These results indicate
that Zn2+ could be clearly detected by the fluorescence spectral
measurement using Y within the physiological pH range
(pH = 7.0–8.4).
Fig. 6 Fluorescence change of the test strips of Y (c = 1 Â 10À3 M) with
Zn2+ in DMSO/H2O (v/v = 9/1) HEPES buffer solution at pH = 7.2, under
irradiation at 365 nm.
To further examine the binding mode of Y with Zn2+, we
performed 1H NMR titrations in DMSO-d6 (Fig. S4, ESI†). Upon
addition of 0.1 equiv. of Zn2+, protons of the hydroxyl (H1)
started to disappear, and protons of hydrazone (H2, H3) were
downshifted. At 1.0 equiv. of Zn2+, H1 was deprotonated, and determine the suitability of a ‘dip-stick’ method for the detection
H3 took the proton transfer from 10.70 ppm to 5.78 ppm. of Zn2+, similar to that commonly used for pH measurement.
This indicated that Zn2+ coordinated with the –O: of hydroxyl When the test strips coated with Y (c = 1 Â 10À3 M) were
and –N: of hydrazine. Importantly, the proton of one pyridine immersed into the pure water solutions of Zn2+, obvious
(H16) underwent a tiny chemical shift, and another pyridine Kelly fluorescence appeared (Fig. 6). The development of such
(H14) underwent no obvious chemical shift, indicating that a ‘dip-stick’ approach is extremely attractive for ‘in-the-field’
only one of the two pyridine groups participates in the coordi- measurements that do not require any additional equipment.
nation procession. Therefore, the 1H NMR titration results Therefore, the test strips of Y have excellent application value in
support the structure of a 1 : 1 complex of Y and Zn2+ proposed the detection of Zn2+.
by the Job plot analysis. Furthermore, in the IR spectra (Fig. S8,
ESI†), the peak at 3422 cmÀ1 (O–H) disappeared, which also of long response time. In our case, the detection course of Zn2+
supports this inference. using Y was found to be relatively faster (Fig. 7). After adding the
As is well known, typical chemosensors always have the problem
Fig. 5 reveals the reversibility of Zn2+ (Y + Zn2+) binding to Y zinc ion, the fluorescence emission intensity of Y increased at
(EDTA). This process was repeated several times without a large 524 nm and reached the plateau region in less than 15 seconds,
loss in the emission intensity. The switching was probably then remained quite stable, suggesting that the entire process of
caused by the EDTA reduction of Zn2+ to [EDTA + Zn2+]2+ to recognition might be completed instantly, and the chemosensor
regenerate Y.
has rapid detection ability. Meanwhile, when the water ratio
Motivated by the favorable features of sensor Y in solution, increased, the response time did not change a lot, and all reactions
we prepared test strips by immersing filter papers (3 Â 1 cm2) were completed within less than 1 min (Fig. S9, ESI†).
into the DMSO/H2O (v/v = 9/1) HEPES buffer solution at pH = 7.2
All the above changes indicated that the pyridine derivative-
of sensor Y (c = 1 Â 10À3 M), and then dried them in air to based, bis-schiff-base pincer-type Y can selectively detect Zn2+
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Fig. 7 Fluorescence intensity at 524 nm for Y (c = 2.0 Â 10À5 M) in a
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Conclusions
We have synthesized a pincer-type fluorescence sensor based on
the combination of 4-hydroxyisophthalaldehyde and 2-hydra-
zinylpyridine. The receptor Y exhibited a fluorescence enhance-
ment upon binding to Zn2+ in aqueous solution of physiological
pH range due to the combined effects of the inhibition of ESIPT
and –HCQN– isomerization, as well as chelation-enhanced
fluorescence (CHEF). The binding of the receptor Y and Zn2+
was also utilized in test strip form for convenience. Moreover, Y
can be recycled, and the minimum detection limit of Y for zinc
reaches 1.39 Â 10À8 M.
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Acknowledgements
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (No. 21064006; 21161018; 21262032), the
Natural Science Foundation of Gansu Province (1308RJZA221) and
the Program for Changjiang Scholars and innovative Research
Team at University of Ministry of Education of China (IRT1177).
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