A highly selective turn-on fluorescent chemosensor for zinc ion in aqueous Media

Article abstract of DOI:10.1007/s10895-013-1255-1

In the paper, a novel rhodamine6G based fluorescent chemosensor bearing 3-carbaldehyde chromone was designed and synthesized. According to the fluorescence behavior toward several metal ions, it showed highly selectivity and sensitivity to Zn(II) over other commonly coexistent metal ions (Cu(II), Cd(II), Hg(II), Mg(II), K(I), Pb(II), Fe(III) and Cr(III)) in aqueous environment (pH = 7.4). Meanwhile the binding constant between Zn(II) and chemosensor achieved 6.21 × 10¹¹ M^-1 in aqueous media. Moreover, according to the Job plot, 1:1 stoichiometry between Zn(II) and sensor was deduced in aqueous media (pH = 7.4). The good selectivity and sensitivity in aqueous media effectively enhanced the application value of the fluorescent chemosensor for Zn(II).

Full text of DOI:10.1007/s10895-013-1255-1

J Fluoresc
DOI 10.1007/s10895-013-1255-1
ORIGINAL PAPER
A Highly Selective Turn-On Fluorescent Chemosensor
for Zinc Ion in Aqueous Media
Zeng-chen Liu & Zheng-yin Yang & Yan-xia Li & Bao-dui Wang &
Yong Li & Tian-rong Li & Yong-jie Ding
Received: 29 March 2013 /Accepted: 24 June 2013
#
Springer Science+Business Media New York 2013
Abstract In the paper, a novel rhodamine6G based fluores-
cent chemosensor bearing 3-carbaldehyde chromone was
designed and synthesized. According to the fluorescence be-
havior toward several metal ions, it showed highly selectivity
and sensitivity to Zn(II) over other commonly coexistent metal
ions (Cu(II), Cd(II), Hg(II), Mg(II), K(I), Pb(II), Fe(III) and
Cr(III)) in aqueous environment (pH=7.4). Meanwhile the
binding constant between Zn(II) and chemosensor achieved
6.21×1011 M−1 in aqueous media. Moreover, according to the
Job plot, 1:1 stoichiometry between Zn(II) and sensor was
deduced in aqueous media (pH=7.4). The good selectivity
and sensitivity in aqueous media effectively enhanced the
application value of the fluorescent chemosensor for Zn(II).
Introduction
The highly selective and sensitive chemosensors toward metal
ions in aqueous environment are of great importance in study-
ing and tracking the chemical and physiological functions in
the wide range of chemical and biological processes [1–5].
Because the analysis of relevant metal elements like copper,
zinc, iron and cobalt et al. in living organism can provide some
valuable information about our physical state. At present, ow-
ing to its particular structural property, the rhodamine frame-
work is an available mode to construct OFF-ON fluorescent
chemosensors, consequently, considerable efforts have been
devoted to developing fluorescent chemosensors based on
rhodamine derivates toward corresponding metal ions due to
their simplicity, high selectivity and sensitivity [6–10]. The
Zinc is a biologically essential element and plays an important
role in living organism. Zinc ions serves as key structural
components of a large of proteins and also performs catalytic
roles in enzymes, which is relevant to many cellular processes.
Moreover, in the physiological processes such as brain, prostate
and intestine locations, abnormal level of zinc ions even can
cause some relevant diseases like Alzheimer’s disease (AD),
amyotrophic lateral sclerosis (ALS) and parkinson’s disease
[11–14]. Additionally, in environment, an excess of zinc is
regarded as a pollutant, which may lead to the death of plants
and are harmful to useful soil [15, 16]. But the incompletely
understood of zinc ions remains active topics of research. Thus,
there is a considerable need to design highly selective fluores-
cent chemosensors, which are capable of detecting the presence
of zinc in biological and environmental system in aqueous
media at a physiological pH value condition. In addition, it is
also a challenging task for designing the chemosensor for
Zn(II) via chelation enhanced fluorescence (CHEF) [17, 18].
To date, some selective fluorescent chemosensors for Zn(II)
.
.
Keywords Chemosensor Zn(II) selectivity
3-carbaldehyde chromone-rhodamine6G hydrazone Turn
.
.
on Binding constant
:
:
Z.<c. Liu (*) Y.<x. Li Y.<j. Ding
College of Chemistry, Zhoukou Normal University, Zhoukou
466001, People’s Republic of China
e-mail: liuzch07@lzu.edu.cn
:
:
Z.<c. Liu Y.<x. Li Y.<j. Ding
The Key Laboratory of Rare Earth Functional Materials and
Applications, Zhoukou 466001, People’s Republic of China
Y. Li
Faculty of Material Science and Chemistry Engineering, China
University of Geosciences, Wuhan 430074, People’s Republic of
China
:
:
Z.<y. Yang B.<d. Wang T.<r. Li
College of Chemistry and Chemical Engineering and State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, People’s Republic of China

J Fluoresc
Scheme 1 The synthesis and probable complexation mechanism of corresponding compounds
have been achieved. However, to the best of our knowledge,
very few fluorescent sensors in aqueous media have been
designed.
When aiming at the rational development of specific fluo-
rescent chemosensors for targeting zinc ion (Zn(II)), the choice
of binding motif is a critical factor, because in the complicated
environment containing, many metal ions such as Cu(II),
Fe(III), Hg(II) and Cd(II) et al. can interfere with zinc binding
[19, 20]. It is well known that the chelating group such as
-C=N- and -C=O- exhibits a high affinity to transition metal
cations, but less binding affinity toward alkali and alkaline
earth metal cations [21–23]. Thus, it is significative to design
Fig. 1 a The changes in the fluorescence spectra (excitation at 525 nm, Slit: excitation/emission=3/3) of L (1.0×10−5 M) in the presence of different
metal ions (4.0×10−5 M) in ethanol-HEPES buffer solution (9:1, pH=7.4). b the histogram of selectivity for various metal ions

J Fluoresc
Fig. 2 The fluorescence image
of L (2.0×10−5 M) and the
fluorescence images of L with
addition of various metal ions
(4.0×10−5 M)
relevant fluorescent chemosensors containing the functional
group. In the present investigation, novel 3-carbaldehyde
chromone-rhodamine6G hydrazone architecture is developed
to increase the selectivity and sensitivity for detecting Zn(II).
According to the spectra analysis, 3-carbaldehyde chromone-
rhodamine6G hydrazone based fluorescent sensor exhibits
highly selectivity toward Zn(II) over other metal ions in aque-
ous buffer media (pH=7.4).
An ethanol solution (20 mL) of rhodamine6G hydrazine
(0.1 mol, 0.428 g) was added to another ethanol (50 mL)
containing 3-carbaldehyde chromone (0.1 mol, 0.1174 g),
Then the solution was reflux for 12 h and cooled to room
temperature. The mixture was filtered and dried under vacu-
um. Recrystallization from MeOH/H2O (V:V=1:1) gave 3-
carbaldehyde chromone-rhodamine6G hydrazone (L), which
was dried under vacuum. Yield, 75 %. m.p: 283–285 °C. 1H-
NMR (DMSO–d6 400 MHz): δ 8.55 (1H, s, –C3–H), δ 8.46
(1H, s, –C11H=N–), δ 8.3–8.46 (1H, d, –C8–H), δ 8.00–8.03
(1H, m, –C12–H), δ 7.56–7.60 (1H, m, –C7–H), δ 7.44–7.49
(2H, m, –C13,14–H), δ 7.37–7.39 (1H, d, –C5–H), δ 7.31–7.34
(1H, m, –C6–H), δ 7.04–7.06 (1H, m, –C15–H), δ 6.44 (2H, s,
–C20,21–H), δ 6.31 (2H, s, –C16,17–H), δ 3.71–3.72 (2H, m, –
C22,23–H), δ 3.19–3.20 (4H, d, –C24,25–H), δ 1.869 (6H, s, –
C18,19–H), δ 1.28–1.31 (6H, m, –C26,27–H).
Experimental Sections
Materials
All the chemicals containing solvents were of reagent grade and
were used without further purification.
All spectroscopic measurements were performed in ethanol-
HEPES (9:1) buffer solution (pH=7.4).
Synthesis of 3-Carbaldehyde Chromone-Rhodamine6G
Hydrazone (L)
Stock solutions (1.0×10−3 M) of metal ions (metal chlo-
ride) were prepared in two-distilled water. The stock solution
of 1.0×10−3 M 3-carbaldehyde chromone -rhodamine6G
hydrazone (L) was prepared in ethanol. In titration experi-
ments, each time 2 mL aqueous solution containing 20 μL
Rhodamine6G hydrazine and 3-carbaldehyde chromone was
prepared by the reported methods [24–26]. Their synthesis
routine was shown in Scheme 1.
Fig. 3 a Fluorescence spectrum of L (1.0×10−5 M) upon addition of
Zn(II) (0–8 equiv) in ethanol-HEPES solution (pH=7.4). Excitation at
525 nm. b Fitting of Fluorescence titration curve of L in ethanol-
HEPES solution (pH=7.4). The linear equation is Y=0.0161X-
3.3975E-4, the linear range of concentration of Zn(II) is from
1.0×10−7 M to 4.0×10−5
M

J Fluoresc
I0 is the fluorescence intensity of L in the absence of Zn(II).
On the basis of the plot of 1/(I-I0) versus 1/[Zn(II)], the
stability constant can be obtained.
Physical Measurement
1H-NMR spectra were recorded on a Varian VR400-MHz
spectrometer with TMS as an internal standard. The melting
points of the compound were determined on a Beijing XT4-
100X microscopic melting point apparatus. The UV–vis spec-
tra were recorded on a Perkin-Elmer Lambda-35 UV–vis
spectrophotometer. Fluorescence spectra were obtained on a
Shimadzu RF-5301 spectrophotometer at room temperature.
Fig. 4 Job’s plot according to the method for continuous variations,
indicating the 1:1 stoichiometry for L-Zn(II) (the total concentration of
L and Zn(II) is 1.0×10-5 M). (λex=525 nm, Slit: excitation/emission=3/3)
Results and Discussions
Spectra Investigation
solution of L (1.0×10−3 M) was filled in a quartz optical cell
of 1 cm optical path length. Then equal amount of Zn(II) stock
solution (5 μL) was added to the compound solution with
micro-pippet. Spectral data was recorded at 2 min after
the addition. In selectivity experiment, the test samples were
prepared by placing appropriate amounts of metal ion stock
solution into 2 mL aqueous solution of L (1.0×10−5 M). For
fluorescence measurements, excitation wavelength is at 525 nm.
The binding constant between L and Zn(II) was calculated
by the linear Benesi-Hildebrand expression [27, 28]
To investigate the fluorescence selectivity toward special met-
al ion, the fluorescence selectivity experiments of various
metal (Zn(II), Cu(II), Cd(II), Hg(II), Mg(II), K(I), Pb(II),
Fe(III), Cr(III)) were conducted in ethanol-HEPES (9:1) buff-
er solution at pH=7.4. As Shown in Fig. 1, in the absence
of metal ions, the 3-carbaldehyde chromone-rhodamine6G
hydrazone exhibited a rather weak fluorescence signal in the
range from 520 nm to 700 nm in ethanol-HEPES medium
(pH=7.4). However, with addition of four equivalent various
metal ions, only the addition of Zn(II) lead to approximately
15-fold fluorescence enhancement in ethanol-HEPES media
(pH=7.4). Except for weak fluorescence increase for Cd(II),
other metal ions did not cause obvious fluorescence changes
of L. It’s deduced that the coordination between Zn(II) and L
aroused the opening of rhodamine6G ring and resulted in the
formation of Zn(II)–L with enhanced fluorescence intensity.
1
1
1
1
¼
þ
⋅
I−I0 ½Lꢀ KS ½Lꢀ½Mꢀ
Where I is the change of fluorescence intensity in the
presence of Zn at 545 nm, Ks is the stability constant, and
[L] and [M] are the concentration of L and Zn(II), respectively.
Fig. 5 The selectivity of L for Zn(II) in the presence of other metal ions
(Cu(II), Cd(II), Hg(II), Mg(II), K(I), Pb(II), Fe(III), Cr(III)) in ethanol-
HEPES media (pH=7.4). Excitation at 525 nm. The response is normalized
with respect to fluorescence intensity of the free L(1.0×10−5 M) with
addition of Zn(II) (4.0×10−5 M). Then other metal ions were added
(4.0×10−5 M)

J Fluoresc
The fluorescence behavior preliminarily indicated 3-carbaldehyde
chromone -rhodamine6G hydrazone could act as an fluores-
cence probe for Zn(II) under physiological pH conditions. In
addition, according to the histogram, the highly selectivity of
L for Zn(II) over other metal ions were indicated obviously.
To illustrate the selectivity of L toward Zn(II) in aqueous
media, the fluorescence images under UV light were also
made (Fig. 2). Upon addition of two equivalent metal ion,
only the solution of L with Zn(II) showed remarkable green
fluorescence under UV light, which indicated 3-carbaldehyde
chromone -rhodamine6G hydrazone could act as a highly
selective turn-on fluorescent chemosensor for Zn(II) in aque-
ous media (pH=7.4).
Fluorescence titration experiment (Fig. 3a) of L with Zn(II)
was performed in aqueous buffer media (pH=7.4) at room
temperature. Upon addition of Zn(II), the fluorescence signal
at 545 nm significantly enhanced. It was explicit that the
binding between 3-carbaldehyde chromone-rhodamine6G
hydrazone and Zn(II) induced the open ring of spirolactam
of chemosensor, which was responsible for the fluorescence
intensity change. It was also clear that the OFF-ON process of
chemosenor, which was switched on by coordinative Zn(II)
as the excitation at 525 nm, resulted in the emission of
rhodamine6G fluorophore with a maximum at 545 nm. As
shown in Fig. 3b, the association constant between L and
Zn(II) was estimated to be 6.21×1011 M−1 in aqueous media
(pH=7.4) by fitting the data to the Benesi-Hildebrand expres-
sion with a good linear relationship. And it exhibited wide
linear range of concentration of Zn(II), which is from
1.0×10−7 M to 4.0×10−5 M. Additionally, by the Job plot
shown in Fig. 4, the binding mode between L and Zn(II) was
investigated systemically. We could deduce that there was a
1:1 stoichiometry between them. Moreover, in accordance
with the 1:1 stoichiometry, the turn-on fluorescent chemosenor
Fig. 7 Fluorescence spectra (excitation at 525 nm, Slit: excitation/
emission=5/5) of L (1 μM) with addition of Zn(II) (0.1 μM) in
ethanol-HEPES media (pH=7.4). Red line represents L
was most likely to chelate with zinc ion via its carbonyl O,
imino N and 3-carbaldehyde chromone O atoms.
Furthermore, to validate the high selectivity of L for Zn(II),
the fluorescence competitive experiments of Zn(II) with other
cations were also investigated. Four equivalent Zn(II) was
added to the aqueous solution of L (1.0×10−5 M), then equiv-
alent amount of other metal ions (Cu(II), Cd(II), Hg(II),
Mg(II), K(I), Pb(II), Fe(III) and Cr(III)) were also inserted
into the solution. Their fluorescence intensities were recorded,
respectively. The histogram of fluorescence changes were
listed in Fig. 5. As shown in Fig. 5, no significant variation
in the fluorescence emission of L-Zn(II) was observed by
comparison with the fluorescence property with addition of
other metal ions. All the experimental results indicated the
high selectivity of L toward Zn(II) over other co-existent
metal ions in aqueous media (pH=7.4).
As shown in Fig. 6, the absorption spectrum of L (1.0×
10−5 M) in aqueous media (pH=7.4) exhibited a very weak
Fig. 6 The absorption spectra of L (1.0×10−5 M) in ethanol-HEPES
media in the presence of different amounts of Zn(II) (0–4 equiv). Inset:
the color change of chemosenor solution with addition of Zn(II)
Fig. 8 Time course of the response of L (1×10−5 M) to 1 equiv Zn(II)
in ethanol-HEPES solution (pH=7.4)

J Fluoresc
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peak at the range 500–560 nm, which was ascribed to the
closed spricolactam of chemosensor molecule L. With addition
of Zn(II), the absorption spectrum changed significantly with
the solution turning from colorness to yellow, clearly indicat-
ing the formation of ring-opened amide form of L with
Zn(II) binding (Fig. 6 inset). Additionally, several new bands
appeared obviously at the range 400–450 nm upon addition of
Zn(II), which also implied that the coordination between L
and Zn(II) lead to the fluorescence emission of chromone
fluorophore. To evaluate the sensitivity of chemosensor L for
Zn(II) in aqueous media, the detection limit of L in recognizing
Zn(II) was also tested using fluorescence spectra (Fig. 7). The
fluorescence titration profile of L (1.0×10−6 M) with Zn(II)
demonstrated the detection of Zn(II) in aqueous media
(pH=7.4) was at the part 1.0×10−7 M. Under the conditions,
the fluorescence intensities of chemosensor L solution was still
proportional to the amount of Zn(II). Simultaneously, the time
course of response of L to equal Zn(II) in aqueous media
(pH=7.4) was also investigated (Fig. 8). The experiment dem-
onstrated that the interaction between L and Zn(II) was com-
pleted in less than 90 s. Therefore, the fluorescent chemosensor
could be applied in real-time tracking of Zn(II) in organism.
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Conclusions
In summary, we have developed a novel rhodamine6G based
turn-on fluorescent chemosensor for Zn(II). It exhibits high
selectivity and sensitivity toward Zn(II) over other metal ions
in aqueous media (pH=7.4). The closed spirolactam ring
of 3-carbaldehyde chromone-rhodamine6G hydrazone is
opened with addition of Zn(II) in aqueous media, then in-
tense green fluorescence appears under the combined action
of rhodamine6G and chromone fluorophores. Moreover,
according to the investigation, 1:1 stiochiometry complex
between L and Zn(II) is formed. The excellent selectivity
of chemosensor 3-carbaldehyde chromone-rhodamine6G
hydrazone for Zn(II) in aqueous media (pH=7.4) indicates
its potential application value in the biological monitoring
and tracking of zinc ions.
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Acknowledgments This work is supported by the Research Start Funds
Sponsored Program of Zhoukou Normal University (zksybscx201201) and
the National Natural Science Foundation of China (81171337, 21271093).
21. Yoon S, Albers AE, Wong AP, Chang CJ (2005) Screening mercury
levels in fish with a selective fluorescent chemosensor. J Am Chem
Soc 127:16030–16031
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