A novel and resumable Schiff-base fluorescent chemosensor for Zn(II)

Article abstract of DOI:10.1016/j.tetlet.2016.05.079

A new resumable Schiff-base fluorescent probe 7-methoxychromone-3-carbaldehyde-(3′-hydroxy-2′-naphthaleneformyl) hydrazone (L) was designed and synthesized for selective recognition Zn²⁺. With the titration and the ESI-MS spectra data, we reached the conclusion that the binding ratio between L and Zn²⁺ was determined to be 1. The sensor showed a strong fluorescence enhancement in ethanol system of Zn²⁺ (excitation 430 nm and emission 498 nm). And the enhancement may be due to C=N isomerization and Photoinduced Electron Transfer (PET).

Full text of DOI:10.1016/j.tetlet.2016.05.079

Accepted Manuscript
A novel and resumable Schiff-base fluorescent chemosensor for Zn(II)
Jun Yan, Long Fan, Jing-can Qin, Chao-rui Li, Zheng-yin Yang
PII:
S0040-4039(16)30612-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.05.079
TETL 47698
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
22 March 2016
15 May 2016
20 May 2016
Please cite this article as: Yan, J., Fan, L., Qin, J-c., Li, C-r., Yang, Z-y., A novel and resumable Schiff-base
fluorescent chemosensor for Zn(II), Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.05.079
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A novel and resumable Schiff-base fluorescent chemosensor for
Zn(II)
Jun Yan, Long Fan, Jing-can Qin, Chao-rui Li, Zheng-yin Yang*
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, P.R. China.
**Corresponding author. Tel.: +86 931 8913515; Fax: +86 931 8912582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)
Abstract:
A
new
resumable
Schiff-base
fluorescent
probe
7
-methoxychromone-3-carbaldehyde-(3’-hydroxy-2’-naphthaleneformyl) hydrazone (L) was
2
designed and synthesized for selective recognition Zn . With the titration and the ESI-MS
+
2
spectra data, we reached the conclusion that the binding ratio between L and Zn was
+
determined to be 1. The sensor showed a strong fluorescence enhancement in ethanol system of
2
Zn (excitation 430 nm and emission 498 nm). And the enhancement may be due to C=N
+
isomerization and Photoinduced Electron Transfer (PET).
2
Keywords: Fluorescent sensor; Reversibility; PET; C=N isomerization; Zn .
+
1

1. Introduction
In recent years, with the increasingly deterioration of the environment, the toxic heavy
1
metal ions are becoming more and more threatening to human health. So it is of great
-5
significance to recognize heavy metal ions quickly and efficiently. Zinc ion, one of the eight
6
most common heavy-metal pollution, causes many severe diseases such as Parkinson’s disease,
7
-10
Alzheimer’s disease, and epilepsy if human ingest excessive amounts of it.
However, we
cannot stop using Zinc in that it is also one of the most abundant transition-metal ions present in
living cells and plays a very important role in variety of physiological and pathological processes
such as regulation of metalloenzymes, gene expression, cell apoptosis and neural signal
1
1-13
transmission.
Hence it is very important to develop highly selective and sensitive
2
chemosensors for the detection of Zn in production and our life.
+
1
4-19
Over the years there were many reported methods
to detect transition heavy metal ions,
2
+
especially Zn ions. Among these methods, fluorescent chemosensors have been paid much
attention to detect metal ions because of their high response speed, good selectivity, high
2
0-25
sensitive and easy operation character when mixed with these ions.
The design of fluorescent
chemosensors for metal ions has attracted considerable attention in environmental researches and
analytical fields. Schiff-base compounds and their metal complexes have been widely used in
medicine, analytical chemistry, and the field of photochromic effect. What’s more, as good
ligands, Schiff-bases can be used to identify the content of metal ions and the quantitative
2
6-29
analysis of metal ions.
In our study, 7-methoxychromone-3-carbaldehyde and
3
-hydroxy-2-naphthoic acid hydrazide were selected as reactants to get the Schiff-base for both
30-32
of them were widely used to synthesise chemosensors.
This Schiff-base was considered as a
2

potential fluorescent probe for the determination of metal cations.
2
+
Herein, we designed a new Schiff-base chemosensor L that allows the Zn to be
differentiated on the basis of distinct fluorescence responses in ethanol. The free L emitted very
weak fluorescence due to the C=N isomerization and PET phenomena; while when bound with
2
Zn , the C=N isomerization and PET effects were inhibited and the molecular structure changed
+
3
3-35
from flexible to rigid, which dramatically increased the fluorescence intensity.
2. Experimental
2.1. Materials and instrumentation
All chemicals were obtained from commercial suppliers and used without further
purification. Fluorescence spectra were recorded on a Hitachi RF-4500 spectrophotometer
equipped with quartz cuvettes of 1 cm path length. ESI-MS were determined on a Bruker esquire
6000 spectrometer. UV-Vis absorption spectra were determined on a Shimadzu UV-240
1
spectrophotometer. H NMR spectra were measured on the Bruker 400 MHz instruments using
TMS as an internal standard.
2.2. Synthesis
3
6
7
-methoxychromone-3-carbaldehyde was obtained according to the literature procedures.
Synthesis of L was based on the following method (Scheme 1): an ethanol solution (12 mL) of
F
H
3
CO
OH
H
3
CO
H
3
CO
O
O
_
B
F
POCl ,DMF
3
(
CH CO)
3
2
O,BF
3
Et
2
O
100
， 1h
O
+
100
， 1h
CHO
O
O
O
H
N
CH O
3
NH2
H
O
O
OH
EtOH reflux 6h
O
O
N
N
H
(
L)
3

3
-hydroxy-2-naphthoic acid hydrazide (0.101 g, 0.5 mmol) was added dropwise to a solution (25
mL) of 7-methoxychromone-3-carbaldehyde (0.102 g, 0.5 mmol) in ethanol. Then the solution
was refluxed for 6 hrs under stirring and some white precipitant appeared. The mixture was
filtered
and
dried
under
vacuum.
Recrystallization
from
ethanol
gave
7
-methoxychromone-3-carbaldehyde-(3’-hydroxy-2’-naphthaleneformyl) hydrazone (L) which
o
1
was dried under vacuum. Yield, 71.4%, m.p: 226–227 C. H NMR (DMSO-d , 400 MHz): δ
6
1
2.03 (s, 1H, -NH-), 11.22 (s, 1H, -OH), 8.75 (s, 1H, H ), 8.55 (s, 1H, H ), 8.40 (s, 1H, H ),
1
2
22
1
5
21
.00 (d, J = 8.0 Hz, 1H, H ), 7.86 (d, J = 8.0 Hz, 1H, H ), 7.72 (d, J = 8.0 Hz, 1H, H ), 7.48 (t,
18
8
1
9
20
8
17
J = 8.0 Hz, 1H, H ), 7.33 (t, J = 8.0 Hz, 1H, H ), 7.27 (s, 1H, H ), 7.19 (s, 1H, H ), 7.09 (d, J =
6
+
8
3
.0 Hz, 1H, H ), 3.87 (s, 3H, -CH ) (Figure S1). MS (ESI) m/z 389.1 [M+H] , (Figure S2).
2.3. Analysis
3
+
2+
2+
2+
Stock solutions (5.0 mM) of the nitrate salts of Al , Ba , Ca , Cu , Hg , Co , K , Mg ,
2+
2+
+
2+
2
+
+
3+
+
2+
2+
+
3+
2+
Mn , Na , Cr , Li , Pb , Cd , Ag , Fe and Zn in ethanol were prepared. Stock solutions of
L (5.0 mM) were prepared in ethanol. Test solutions were prepared by placing 20 µL of the
probe stock solution into cuvettes, adding an appropriate aliquot of each ions stock, and diluting
the solution to 2 mL with ethanol solutions. Both the excitation and emission slit widths were 3.0
nm.
3. Results and discussion
3.1. Absorption studies
2
The binding properties of L with Zn were studied by UV-Vis titration in the above
+
2
+
solution at room temperature (Figure 1). Upon addition of Zn ions (0-1.3 equiv.), the
absorbance bands at 335 nm and 430 nm enhanced gradually while those at 268 nm lowered by
4

degrees. Moreover, there was clearly an isosbestic point at 307 nm, and the presence of the
3
7-38
isosbestic point for the sensor suggested the formation of a complex.
.2. Emission studies
A fluorescence study was further used to determine the selectivity and sensitivity of L in the
3
3
+
2+
2+
2+
2+
2+
+
2+
2+
+
presence of various metal ions, such as Al , Ba , Ca , Cu , Hg , Co , K , Mg , Mn , Na ,
3
+
+
2+
2+
+
3+
2+
2+
Cr , Li , Pb , Cd , Ag , Fe and Zn . Fascinatingly, the treatment of L (50.0 µM) with Zn
(
50.0 µM) resulted in a strong increase in the fluorescence intensity in the emission spectrum at
498 nm upon excitation at 430 nm (Figure 2). And interestingly, no such effect was observed
2
+
when L was treated with other cations (50.0 µM). The probe exhibited high selectivity for Zn
over other metal ions.
5

To explore the possibility of using L as a practical ion selective fluorescent chemosensor
2
for Zn , competition experiments were carried out. For this purpose, L was treated with Zn
+
2+
3
+
2+
2+
2+
2+
(
50.0 µM) in the presence of other metal ions (50.0 µM), such as Al , Ba , Ca , Cu , Hg ,
2
+
+
2+
2+
+
3+
+
2+
Co , K , Mg , Mn , Na , Cr , Li , Pb , Cd , Ag and Fe . As shown in Figure 3, good
2+
+
3+
3
+
selectivity of L was displayed in the presence of most metal ions, except for two metal ions Fe
2
+
and Cu that are well known as the strong quenchers of fluorescence. The fluorescence of the
2
L-Zn system was quenched, which might be due to an energy or electron transfer. As
+
39
40-41
3+
it was possibly because the newly formed complex L-Fe or
reported in the literature,
6

2
L-Cu was too stable to be replaced by Zn .
+
2+
In Figure 4, the free L (50.0 µM, λex = 430 nm) emitted very weak fluorescence due to the
2
C=N isomerization and PET phenomena. Upon addition of Zn , a strong fluorescence
+
2
+
enhancement was observed at 498 nm. And the stoichiometry of L-Zn complex was
2
+
determined by changes in the fluorogenic response in the presence of varying Zn
2
concentrations. After the addition of 1.0 equiv. of Zn , the fluorescence spectrum reached a
+
2
+
saturation level. The saturation behavior of L with Zn indicated that the L-Zn complex has a
2+
1
:1 stoichiometry.
3.3. Binding studies
2
In order to obtain more evidence to determine the stoichiometry between L and Zn , the
+
4
2
method of continuous variation (Job's plot) was used. The total concentration of the L and Zn
2+
2
+
was constant (50.0 µM), with a continuous variable molar fraction of guest ([L] / [L] + [Zn ]).
2
Figure 5 shows the Job's plot of L with Zn (at 498 nm) and reveals that the L-Zn complex
+
2+
7

2
concentration approaches a maximum when the molar fraction of Zn is 0.5, which means L and
+
2
Zn forms a 1:1 complex. In addition, the formation of a 1:1 complex between L and Zn was
+
2+
2
further conformed by the appearance of a peak at m/z 452.5 that is assignable to [L + Zn -H ]
+
+ +
in the ESI-MS spectra (Figure S3).
2+ 1
NMR studies provided additional evidence of the interaction between L and Zn . H NMR
2
+
6
spectra of L were recorded in DMSO-d upon the addition of Zn . Significant spectral changes
were observed as shown in Figure 6. Peaks for the protons of -NH- and -OH gradually
disappeared, which indicated the enolization and deprotonation of the ligand in the presence of
2+ 43-44
Zn .
5
11
17
22
The signals of H , H , H and H were downfield shifted 0.11, 0.22, 0.07, 0.08
respectively, while signals of the other protons remained nearly unchanged. These phenomena
2
+
indicated that the hydroxyl group, carbonyl group and CN group coordinated with Zn , which
changed the electron distribution in the chemosensor. Furthermore, there were a number of small
signals for the aryl protons coming into being, such as the signal peaks of δ 8.86, δ 8.16, δ 7.84,
2
+
1
δ 6.50 and so on, which showed a slight change for the system upon the addition of Zn . The H
2
NMR data gave the conclusion that the binding of L to Zn generated a rigid structure through a
+
8

chelating reaction so that the C=N isomerization and PET effects were inhibited (Figure 7).
a
As shown in Figure 8, the association constant K of the complex was then calculated to be
9

4
-1
45
2
.0×10 M with a linear relationship by Benesi-Hildebrand equation (Eq. (1)):
ͥ
ͥ
ͥ
Ɣ ꢁʚꢀ
ƍ ꢀ
ĠĔīͯꢀĠĜġ
(1)
ĠĔīͯꢀĠĜġʛʞΎΜv~
ꢀ
ͯꢀĠĜġ
ʟ
2
where Fmax, F and Fmin are fluorescence intensities of L in the presence of Zn at saturation, any
+
2
intermediate Zn concentration, and free L.
+
2
The detection limit of L for the analysis of Zn was also calculated based on the
+
fluorescence titration. As shown in Figure S4, according to the following equation: detection
4
6
-7
limit=3σ/k, the detection limit reached at 1.73×10 M. Where σ was the standard deviation of
blank measurements, and k was the slope between fluorescence intensity versus sample
concentration.
Reversibility is a prerequisite in developing novel chemosensors for practical application.
2
The reversibility of the recognition process of L was performed by adding Zn bonding agent
+
2
+
and Na
2 2
EDTA. The addition of Na EDTA to a mixture of L and Zn resulted in diminution of
the fluorescence intensity, which indicated the regeneration of the free chemosensor L. Upon
2
addition of Zn again, the fluorescence was recovered. After several cycles, the chemosensor
+
still showed excellent restorability with little fluorescent efficiency loss (Figure 9). Such
2
reversibility and regeneration are important for the fabrication of devices to sense the Zn ions.
+
10

4. Conclusion
In summary, we have developed a novel chemosensor L based on the C=N isomerization
2
and PET mechanism. The probe exhibited high selectivity for Zn over other metal ions with a
+
7
−
strong fluorescence enhancement and high sensitivity with the detection limit reaching at 10 M
level in ethanol. This discovery of the dramatically enhanced fluorescence is probably due to the
2
formation of a 1:1 complex L-Zn in which the C=N isomerization and PET effects are
+
inhibited. Moreover, the chemosensor still showed excellent restorability. Thus, we believe L
2
has the ability to serve as a potential sensor for Zn detection. This strategy may provide a
+
11

general way for designing new sensors to detect other environmentally and biologically relevant
species.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (81171337) and
Gansu NSF (1308RJZA115).
12

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15

Figure captions
Figure 1. UV-Vis absorption spectra of L in EtOH (50.0 µM) upon the addition of Zn(NO
3
)
2
2
+
(
0-1.2 equiv.). Inset: Absorption intensity of L at 430 nm as a function of (0-1.3 equiv.) Zn
concentration.
Figure 2. Fluorescence responses of L (50.0 µM) in EtOH with 50.0 µM of Al , Ba , Ca ,
3
+
2+
2+
2
+
2+
2+
+
2+
2+
+
3+
+
2+
2+
+
3+
2+
Cu , Hg , Co , K , Mg , Mn , Na , Cr , Li , Pb , Cd , Ag , Fe and Zn (λex= 430 nm).
Inset: fluorescence emission color and intensity changes of the sensor (50.0 µM) in the absence
2
and presence of Zn (50.0 µM).
+
2
+
Figure 3. Relative fluorescence of L and its complexation with Zn in the presence of various
3
+
2+
2+
2+
2+
metal ions in EtOH. Black bar: L (50.0 µM) and L with 1.0 equiv. of Al , Ba , Ca , Cu , Hg ,
2
+
+
2+
2+
+
3+
+
2+
2+
+
3+
Co , K , Mg , Mn , Na , Cr , Li , Pb , Cd , Ag , Fe stated. Red bar: 50.0 µM of L and
2
+
1
.0 equiv. of Zn in the presence of 1.0 equiv. of other metal ions stated (λex= 430 nm).
2
Figure 4. Fluorescence spectra of L (50.0 µM) with addition of increasing amount of Zn (0-1.5
+
equiv.) in EtOH (λex =430 nm, slit: 3 nm / 3 nm). Inset: Fluorescence emission intensity of L at
2
98 nm as a function of (0-1.5 equiv.) Zn concentration.
+
4
Figure 5. Job’s plot of L in EtOH showing 1:1 stoichiometry of the complex between L and
2
+
2+
Zn . The total concentration of L and Zn is 50.0 µM. Fluorescence emission intensity is
recorded at 498 nm.
1
Figure 6. H NMR spectra of L with Zn(NO
3
)
2
·6H
2
O in DMSO-d
6
: (I) L (bottom); (II) L with
2
+
Zn (top).
Figure 7. Proposed binding mode for interaction of Zn with L.
Figure 8. Benesi-Hildebrand analysis of the emission changes for the complexation between L
2
+
16

2
+
and Zn .
Figure 9. (a) Fluorescence spectra of L (50.0 µM) upon the sequential addition of Zn and
Na EDTA (1.0 equiv.) in ethanol. (b) Reversible changes in fluorescence intensity of L (50.0 µM)
at 498 nm after the sequential addition of Zn and Na
2
+
2
2
+
2
EDTA (1.0 equiv.).
Scheme1. Synthesis of 7-methoxychromone-3-carbaldehyde-(3’-hydroxy-2’-naphthaleneformyl)
hydrazone (L).
17

2
Proposed binding mode for interaction of Zn with L.
+
18

•
•
•
•
An easy-to-make Schiff-base fluorescent sensor was designed and synthesized.
2
+
The probe exhibited high selectivity and sensitivity for Zn over other metal ions.
The chemosensor showed excellent restorability.
C=N isomerization and PET can be used to explain the high selectivity of the probe.
19