A simple fluorescent probe for Zn(II) based on the aggregation-induced emission

Article abstract of DOI:10.1016/j.dyepig.2012.10.007

A aggregation-induced emission-based fluorescent probe 1 for Zn ²⁺ was designed and simply synthesized by condensation of salicylaldehyde with aqueous hydrazine. The experimental conditions were first optimized. It was found that N, N-Dimethylformamide (DMF) was the best solvent for the Zn²⁺-triggered aggregation of compound 1 compared with other solvents. The emission intensity was gradually increased, accompanied by the simultaneous red shift of the maximum emission peak with increasing Zn ²⁺ concentrations. A red shift about 45 nm was achieved when Zn ²⁺ concentration is 100 μM. Compared with other Zn²⁺ fluorescent sensors based on aggregation-induced emission (AIE), compound 1 can detect a lower concentration of Zn²⁺ with a detection limit of 0.1 μM. Compound 1 also exhibited good selectivity toward Zn²⁺. The aggregation was verified by the dynamic light scattering (DLS) results, with a Zn²⁺ concentration-dependent size observed. It was also directly confirmed by TEM analyses.

Full text of DOI:10.1016/j.dyepig.2012.10.007

Dyes and Pigments 96 (2013) 495e499
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A simple fluorescent probe for Zn(II) based on the aggregation-induced emission
,
,
a,
De-Xun Xie a, Zhao-Jin Ran a b, Zhen Jin a, Xiao-Bing Zhang a , De-Lie An
*
*
a State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
b College of Sciences, China Agricultural University, Beijing 100193, PR China
a r t i c l e i n f o
a b s t r a c t
A aggregation-induced emission-based fluorescent probe 1 for Zn2þ was designed and simply synthe-
sized by condensation of salicylaldehyde with aqueous hydrazine. The experimental conditions were first
optimized. It was found that N, N-Dimethylformamide (DMF) was the best solvent for the Zn2þ-triggered
aggregation of compound 1 compared with other solvents. The emission intensity was gradually
increased, accompanied by the simultaneous red shift of the maximum emission peak with increasing
Article history:
Received 5 September 2012
Received in revised form
7 October 2012
Accepted 8 October 2012
Available online 17 October 2012
Zn2þ concentrations. A red shift about 45 nm was achieved when Zn2þ concentration is 100
Compared with other Zn2þ fluorescent sensors based on aggregation-induced emission (AIE), compound
mM.
Keywords:
1 can detect a lower concentration of Zn2þ with a detection limit of 0.1
mM. Compound 1 also exhibited
Fluorescent probe
Salicylaldehyde
Hydrazine
good selectivity toward Zn2þ. The aggregation was verified by the dynamic light scattering (DLS) results,
with a Zn2þ concentration-dependent size observed. It was also directly confirmed by TEM analyses.
Ó 2012 Elsevier Ltd. All rights reserved.
Zn2þ detection
Red shift
Aggregation-induced emission (AIE)
1. Introduction
which are based on photoinduced electron transfer (PET) [11e14],
internal charge transfer (ICT) [13,15e17], and fluorescence reso-
Following iron, zinc ranks the second in abundance among the
transition metals in the human body, and plays vital roles in many
biological processes such as cellular metabolism, gene expression,
and immune function [1e3]. Zn2þ is also believed to be an essential
factor in some pathological processes, including Alzheimer’s
disease, epilepsy, and ischemic stroke [4,5]. Therefore, it is impor-
tant to monitor Zn2þ in many scientific fields and clinical situations.
In the past few years, many traditional analytical techniques
such as atomic absorption spectrometry [6], flame atomic absorp-
tion spectroscopy [7], inductively coupled plasmaeatomic emission
spectrometry [8], mass spectrometry [9], and cyclic voltammetry
methods [10] have been employed to detect the concentration of
Zn2þ. Though these techniques are sensitive, selective, and accurate
for Zn2þ detection, most of them are rather complicated, time-
consuming, and expensive as well as inadequate for on-line
monitoring. Owing to the advantages of simplicity and inexpen-
sive instrumentation, there are many methods such as spectro-
photometric and fluorophotometric methods reported for the
determination of trace amounts of zinc. Up to now, a variety of
Zn2þ-selective fluorescent probes have been reported, most of
nance energy transfer (FRET) [18e22]. In recent years, various
aggregation-induced emission (AIE) active compounds have been
synthesized [23,24,27], and several Zn2þ-selective fluorescent turn-
on probes based on aggregation-induced emission have also been
developed [24,25].
Although the synthesis and optical property of 1,2-bis(2-
hydroxybenzylidene) hydrazine have been previously reported
[26], its analytical recognition characteristics are not investigated
up to date. It was found that it is practically nonluminescent in the
solution state, but becomes highly emissive as nanoparticle
suspensions in poor solvents, demonstrating a novel phenomenon
of AIE [27]. By taking advantage of the AIE feature of 1,2-bis(2-
hydroxybenzylidene) hydrazine 1 (Scheme 1), herein we report
a new fluorescence turn-on probe 1 for Zn2þ. Compound 1
exhibited good selectivity and sensitivity toward Zn2þ
.
2. Experimental
2.1. General
Chemicals were purchased from commercial suppliers and used
without further purification. Double-distilled water was used
throughout all experiments. Thin layer chromatography (TLC) was
carried out using silica gel HSGF254, which were obtained from the
Qingdao Ocean Chemicals (Qingdao, China). NMR spectra were
* Corresponding authors.
E-mail addresses: xbzhang@hnu.edu.cn, xiaobingzhang89@hotmail.com
(X.-B. Zhang), deliean@hnu.edu.cn (D.-L. An).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.10.007

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D.-X. Xie et al. / Dyes and Pigments 96 (2013) 495e499
2.2. Synthesis of compound 1
Compound 1 was synthesized according to a reported method
(Scheme 1) [28]. To a solution of aqueous hydrazine (13 mg,
2.25 mmol) in methanol (20 mL) was added salicylaldehyde
(549 mg, 4.50 mol) over a period of 1 h. Then the mixture was
refluxed for 4 h. The solution was concentrated to give crude solid,
which was recrystallized methanol and dried in vacuo to get
421 mg (78%) of compound 1 as a pale yellow solid. 1H NMR
(400 Hz, CDCl3): 6.97 (t, J ¼ 7.2 Hz, 2H), 7.04 (d, J ¼ 8.0 Hz, 2H),
7.35e7.42 (m, 4H), 8.72 (s, 2H), 11.34 (brs, 2H); 13C NMR (100 Hz,
Scheme 1. Synthetic routes of compound 1.
recorded on a Bruker DRX-400 spectrometer operating at 400 MHz.
LC-MS analyses were performed using an Agilent 1100 HPLC/MSD
spectrometer. All fluorescence measurements were carried out on
a Hitachi-F4500 fluorescence spectrometer with excitation slit set
at 5 nm and emission at 5 nm. Dynamic light scattering (DLS)
characterization was performed using a Zetasizer Nano ZS90 DLS
system. The morphology and particle size distribution of particles
were characterized using a Jeol JEM-2010 transmission electron
microscopy.
CDCl3): d 117.16,119.72,132.55,133.44,159.81,164.70; ESI-MS: calcd
for C14H12N2O2 m/z 240.3, found (M þ H)þ: 241.1.
2.3. Procedures for metal ion sensing
A 11.11 mM stock solution of 1 was prepared by dissolving 1 in
DMF. A standard stock solution of Zn2þ (10 mM) was prepared by
dissolving an appropriate amount of zinc nitrate in water and
Fig. 1. Fluorescence emission spectra of the fluorescence chemosensor exposed to various solvents (dotted line: compound 1; solid line: compound 1 with Zn2þ): a) DMF; b) DMSO;
c) CH3OH; d) THF; e) C2H5OH; f) CH3CN. lex ¼ 400 nm.

D.-X. Xie et al. / Dyes and Pigments 96 (2013) 495e499
497
Fig. 4. Photographs of 1 in DMF treated with different metal ions at 1.0 ꢀ 10ꢁ4
M
(Cd2þ, Co2þ, Cr3þ, Cu2þ, Mn2þ, Ni2þ, Pb2þ, Hg2þ, Fe2þ, Fe3þ and Zn2þ, from left to right).
best sensitivity with a wavelength red shift among the chosen
water-solubility organic solvents. So we chose DMF as the solvent
thorough our subsequent experiment.
3.2. Analytical performance characteristics of compound 1
Fig. 2. Fluorescence emission spectra of the fluorescence chemosensor exposed to
Fig. 2 shows the fluorescence spectra of 1 in water/DMF (1:9, v/
v) after mixing with aqueous solutions containing different
concentrations of Zn2þ. They were recorded with an excitation at
400.0 nm and emission wavelength of 410e550 nm. In the absence
of Zn2þ, Compound 1 in water/DMF (1:9, v/v) did not emit any
obvious and characteristic fluorescence. With the introduction of
Zn2þ of various concentrations: 0, 1.0 ꢀ 10ꢁ7, 2.0 ꢀ 10ꢁ7, 4.0 ꢀ 10ꢁ7, 6.0 ꢀ 10ꢁ7
,
8.0 ꢀ 10ꢁ7, 1.0 ꢀ 10ꢁ6, 2.0 ꢀ 10ꢁ6, 4.0 ꢀ 10ꢁ6, 6.0 ꢀ 10ꢁ6, 8.0 ꢀ 10ꢁ6, 1.0 ꢀ 10ꢁ5
,
2.0 ꢀ 10ꢁ5, 4.0 ꢀ 10ꢁ5, 6.0 ꢀ 10ꢁ5, 8.0 ꢀ 10ꢁ5, 1.0 ꢀ 10ꢁ4 mol/L from bottom to top.
These spectra were measured in water/DMF (1:9, v/v) solution. lex ¼ 400 nm.
Zn2þ
, a new emission band with the maximum emission
adjusting the volume to 500 mL in a volumetric flask. This was
further diluted to 1.0 ꢀ 10ꢁ3e1.0 ꢀ 10ꢁ6 M stepwise. The complex
wavelength at 445 nm appeared. Moreover, the emission
intensity was gradually increased with the increasing Zn2þ
concentration (Fig. 2). Meanwhile, the maximum emission band
was red-shifted with increasing Zn2þ concentrations, with an
emission red shift of 40 nm observed when Zn2þ concentration
reached 1.0 ꢀ 10ꢁ4 M. This provides the basis for the detection of
Zn2þ with fluorescent probe, as used in our following study. A
detection limit of 1.0 ꢀ 10ꢁ7 M (3 times the standard deviation in
the blank solution) for Zn2þ is established under current experi-
mental conditions, lower than the other Zn2þ fluorescent sensors
based on tetraphenylethylene with aggregation-induced emission
(AIE) [24] (Fig. 3 inset).
solution of Zn2þ/1 was prepared by adding 900
mL of the stock
solution of 1 and 100
resultant solution, the concentrations were 10
0.1 mMe0.1
m
L of the stock solution of Zn2þ. In the
M of 1 and
M of Zn2þ. Blank solution of 1 was prepared under the
m
m
same conditions without Zn2þ. Stock solutions of other metal ions
were prepared in water with a similar procedure. The fluorescence
emission spectra were recorded at an excitation wavelength of
400.0 nm with an emission wavelength range from 410 nm to
550 nm with excitation slit set at 5 nm and emission slit set at 5 nm.
3. Results and discussion
3.1. Sensitivity of compound 1 in different solvents
3.3. Selectivity of compound 1
In order to achieve the best sensitivity of the probe, compound 1
was dissolved in DMF, DMSO, CH3OH, THF, C2H5OH, CH3CN,
respectively. As shown in Fig. 1, Compound 1 in DMF showed the
Achieving highly selective response to analytes of interest over
other potentially competing species coexisting in the sample is
a necessity for a probe with potential application for practical use.
Fig. 3. Fluorescence titration curve of compound 1 with Zn2þ in water/DMF (1:9, v/v)
solution. Inset shows the fluorescence responses at low Zn2þ concentrations.
lex ¼ 400 nm, lem ¼ 485 nm.
Fig. 5. Fluorescence response of 10
m
M 1 to1.0 ꢀ 10ꢁ5 M of Zn2þ or other metal ions
(the black bar portion) and to the mixture of 10 mM of other metal ions with 10 mM of
Zn2þ (the gray bar portion), lex ¼ 400 nm, lem ¼ 485 nm.

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D.-X. Xie et al. / Dyes and Pigments 96 (2013) 495e499
Fig. 6. Job’s plot for 1 and Zn2þ. The total concentration of 1 and Zn2þ kept at
1.0 ꢀ 10ꢁ5 mol/L. lex ¼ 400 nm, lem ¼ 485 nm.
Fig. 7. DLS profiles (from left to right) of 1 (10 mM DMF solution) and in the presence of
1 þ 10ꢁ6 M Zn2þ, 1 þ 10ꢁ5 M Zn2þ, and 1 þ 10ꢁ4 M Zn2þ, respectively.
Therefore, the selectivity and competition experiments were
extended to a variety of other metal ions, such as common transi-
tion metal ions (Co2þ, Cu2þ, Mn2þ Ni2þ, Fe2þ and Fe3þ), and envi-
ronmentally relevant heavy metal ions (Cd2þ, Pb2þ, Cr3þ and Hg2þ).
The compound 1 emitted strong fluorescence near 485 nm
the PET within 1, but this alone still cannot turn-on the fluores-
cence of compound 1. Possible coordination modes of 1 with Zn2þ
may exist as following (Scheme 2): One Zn2þ ion coordinates with
two compound 1, leading to coordination oligomers and even
polymers. These coordination oligomers and polymers may exhibit
lower solubility in water/DMF solution, and accordingly the fluo-
rescence from 1 will emerge due to AIE. The intermolecular coor-
([Zn2þ] ¼ 100
mM, excited at 400 nm) (Fig. 4). However, amongst
other heavy metal ions (Cd2þ, Cr3þ, Co2þ, Cu2þ, Mn2þ, Ni2þ Pb2þ
,
Hg2þ, Fe2þ and Fe3þ) that usually show an interfering effect on Zn2þ
ion assays, only Cr3þ Hg2þ and Fe2þ showed fluorescence
,
dination may become dominant at high concentration of Zn2þ
,
quenching after they were respectively added to compound 1
(Fig. 5). This clearly indicated that our proposed probe exhibits high
selectivity to Zn2þ over Cd2þ and other competing metal ions.
which was verified by the dynamic light scattering (DLS) results. As
depicted in Fig. 7, the initial DLS signal corresponds to species of
3.4. The schematic illustration of the coordination modes between 1
and Zn2þ
To determine the binding stoichiometry of 1 and Zn2þ, Job’s
method for the emission was employed. The concentrations of 1
and Zn2þ were varied, while the sum of the two concentrations
kept constant at 1.0 ꢀ 10ꢁ5 mol/L. The change of the fluorescence
intensity at 485 nm with a serial of concentration ratios of 1 to Zn2þ
is shown in Fig. 6. When the molecular fraction of Zn2þ was closed
to 50%, the complex of 1 and Zn2þ exhibited a maximum fluores-
cence emission at 485 nm. This indicated that a 1:1 stoichiometry is
possible for the binding mode of 1 and Zn2þ. The schematic illus-
tration of the coordination modes between 1 and Zn2þ was shown
in Scheme 2.
3.5. Confirmation the aggregation of compound 1
These spectrum changes should be attributed to the coordina-
tion of Zn2þ with N and O groups. Such coordination could inhibit
Fig. 8. TEM images of compound 1(10 mM) in water/DMF (1:9, v/v) before (upper) and
Scheme 2. Schematic illustration of the coordination modes between 1 and Zn2þ
.
after (lower) addition of 1.0 ꢀ 10ꢁ5 M Zn2þ
.

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499
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A new fluorescent sensor for Zn2þ has been developed by taking
advantage of the AIE feature of compound 1. This sensor for
Zn2þ was unique in terms of the following points: (1) the
sensing mechanism was different from most previously reported;
(2) Compound 1 was able to be synthesized with relative ease;
(3) Compound 1 exhibited high sensitivity and good selectivity
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This work was supported by the National Key Scientific Program
of China (2011CB911000), NSFC (Grants 20975034, 21177036), the
National Key Natural Science Foundation of China (21135001),
National Instrumentation Program (2011YQ030124), the Ministry
of Education of China (20100161110011), Hunan Provincial Natural
Science Foundation (Grant 11JJ1002), and the National Natural
Science Foundation of China (21072052).
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