Novel ‘naked-eye’ Bis-Schiff base fluorescent chemosensors for sensitive detection of Zn2+ and bio-imaging in living cells

Article abstract of DOI:10.1016/j.tet.2020.131108

Novel chemosensors based on excited state intramolecular proton transfer chemosensors (F1, F3, F5) and reference dyes (F2, F4, F6) are presented in this work. Target dyes F1, F3 and F5 display high selectivity and sensitivity toward Zn²⁺ in solvents (such as EtOH, EtOH/PBS buffer) in the presence of competitive ions (K+, Na+, Ni2+, Pb2+, Mn2+, Mg2+, Hg2+, Cu2+,Cd2+, Cr3+, Fe3+). Sensing property for Zn²⁺ was studied by UV–Visible, fluorescence spectrophotometric analyses and Job's plot analyses. F1, F3 and F5 for Zn²⁺ detection revealed extremely low detection limit (67.2 nM), high anti-interference ability, and the immediate response. Furthermore, inverted fluorescence microscopy imaging experiment were also investigated, the results demonstrated that the probes have well cell membrane permeability and hypotoxicity. Simultaneously, due to the ability to adapt a wider range of pH, the probes could also image Zn²⁺ in live cells with remarkable fluorescence variation and provide a new method to visualize monitoring in live cells.

Full text of DOI:10.1016/j.tet.2020.131108

Tetrahedron 76 (2020) 131108
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Novel ‘naked-eye’ Bis-Schiff base fluorescent chemosensors for
sensitive detection of Zn2þ and bio-imaging in living cells
,
**
,
, *,
Xinchao Wang a 1, Ge Ding b 1, Yuanke Duan a, Min Wang a, Guangshi Zhu a,
Xiujuan Li a ***, Yanfen Zhang a ****, Xiaozhuan Qin c
,
,
, *****
a College of Pharmacy, Heze University, Heze, Shandong Province, China
b Chongqing Key Laboratory of Environmental Materials & Remediation Technologies, College of Chemistry and Environmental Engineering, Chongqing
University of Arts and Sciences, Yongchuan, 402160, China
c Zhengzhou Institute of Technology, School of Chemical Engineering & Food Science, Henan, Zhengzhou, 450044, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel chemosensors based on excited state intramolecular proton transfer chemosensors (F1, F3, F5) and
reference dyes (F2, F4, F6) are presented in this work. Target dyes F1, F3 and F5 display high selectivity
and sensitivity toward Zn2þ in solvents (such as EtOH, EtOH/PBS buffer) in the presence of competitive
ions (Kþ, Naþ, Ni2þ, Pb2þ, Mn2þ, Mg2þ, Hg2þ, Cu2þ,Cd2þ, Cr3þ, Fe3þ). Sensing property for Zn2þ
was studied by UVeVisible, fluorescence spectrophotometric analyses and Job's plot analyses. F1, F3 and
F5 for Zn2þ detection revealed extremely low detection limit (67.2 nM), high anti-interference ability, and
the immediate response. Furthermore, inverted fluorescence microscopy imaging experiment were also
investigated, the results demonstrated that the probes have well cell membrane permeability and
hypotoxicity. Simultaneously, due to the ability to adapt a wider range of pH, the probes could also image
Zn2þ in live cells with remarkable fluorescence variation and provide a new method to visualize
monitoring in live cells.
Received 5 January 2020
Received in revised form
4 March 2020
Accepted 6 March 2020
Available online 17 March 2020
Keywords:
ESIPT
Organic chromophore
Zinc ion sensors
Naked-eye emission
Cell imaging
© 2020 Elsevier Ltd. All rights reserved.
1. Introduction
Simultaneously, Zinc deficiency is one of the main causes of genetic
defects in children's growth period [10]. Hence, it is one of our main
In the last two decades, fluorescent probes or chemosensors for
selective recognition of metal ions have drawn considerable in-
terests due to its great application potentials in a variety of fields
including analytical chemistry, medicine, biology, environment
processes and so on, which is based on various superior advantages
of possessing high sensitivity, selectivity, simplicity and instanta-
neous response [1e5]. Zn2þ is one of the important trace elements
in living organisms, and it is widely distributed in living organisms
[6]. It plays a vital role in physiological processes such as enzyme
catalysis [7], protein sequencing [8], and gene expression [9].
concerns to development of artificial chemosensors for recognition
of Zn2þ ion in environmental as well as biological samples.
Recently, several zinc sensors have been developed based on
different fluorophores such as quinoline [11], fluorescein [12],
coumarin [13] and pyrene [14]. Fluorophores with excited state
intramolecular proton transfer (ESIPT) properties are a kind of
substance with good biological activity, such as the complex
formed by Schiff base and metal ion exhibits good biological ac-
tivity in bacteriostatic, bactericidal, antitumor and fluorescence
detection [15e18]. Photophysical properties of excited state intra-
molecular proton transfer (ESIPT) processes in fluorescence spec-
troscopy have generated considerable interest for both
fundamental investigations and applications of organic molecules
[19e23]. Therefore, the improvement or inhibition of the ESIPT
process may have a significant effect on the normal fluorescence
* Corresponding author.
** Corresponding author.
*** Corresponding author.
**** Corresponding author.
emission peak, especially the Schiff base containing
a tri-
***** Corresponding author.
fluoromethylaryl group, which is widely used in the fields of
bioluminescence detection and medical analysis. Organic mole-
cules exhibiting ESIPT properties are one of the powerful potential
E-mail addresses: wxc198566@126.com (X. Wang), dingge1989cqu@126.com
(G. Ding), 1103361352@qq.com (Y. Duan).
1
Xinchao Wang and Ge Ding are designated as the co-first authors, who make
equal contribution to this study.
https://doi.org/10.1016/j.tet.2020.131108
0040-4020/© 2020 Elsevier Ltd. All rights reserved.

2
X. Wang et al. / Tetrahedron 76 (2020) 131108
candidates as the fluorescent chemosensors due to the sensitive
decay in the excited singlet states, and ESIPT fluorophores have an
unusually large Stokes shift (~200 nm) when compared to tradi-
tional fluorophores (fluorescein, rhodamine, etc.). This helps avoid
unwanted self-reabsorption and inner-filter effects. Furthermore,
ESIPT compounds including N, O and S donor atoms are within the
oldest ligands in coordination chemistry, and they can form stable
complexes with some metal ions easily [24]. While so far, most
ESIPT chemosensors are limited to benzotriazole derivatives
[25e27], and very few ESIPT chemosensors contains tri-
fluoromethyl have been reported up to date [28,29]. Furthermore,
for Zn2þ sensors, most of them have the drawback of inadequate
selectivity towards Zn2þ due to special interference [30].
Taking into account concerns mentioned above, a series new
bis-Schiff base fluorescent chemosensors (F1/F3/F5 as the target
dyes and F2/F4/F6 as the reference dyes shown in Scheme 1) con-
tains trifluoromethyl groups and different ether backbone are
designed and synthesized, in which the pendant group -OH and-
C]N act as the recognition moiety.
It is demonstrated that the target compounds with ESIPT
properties show good photochromic properties and exhibit “naked-
eye” recognition and a notable fluorescence “turn-on” in the
presence of Zn2þ with high selectivity and sensitivity in many
solvents (such as Ethanol、Ethanol/PBS buffer, DMF、ACN). In
contrast, the reference dyes (F2/F4/F6) lack of -OH group only show
normal emission properties. Furthermore, fluorescence imaging for
Zn2þ in living SW620 cells proves its value of potential application
in biological systems.
maximum absorption wavelength of the target dyes increase
gradually (F3>F1> F5) due to the increase in conjugation, and the
partners F1/F2 and F5/F6 also show the similar absorption spectral
properties as F3/F4 (typically shown in Fig. S1, also see Tables S1
and SI).
The different emission spectra between the target and the
reference dyes are presented in Fig. 1 (b). The target dyes F1/F3/F5
present well-separated dual emission peaks in various organic
solvents including the first emission band peaked around 400 nm
with the normal Stokes shift (approximate 50 nm in ethanol,
typically depicted in Fig. 1 (b)), and the second emission band
presents obvious red shift comparing to the first emission band
(such as F1, 169 nm, F3, 171 nm, in ethanol). By contrast, reference
dyes F2, F4 and F6 show only a single emission band peaked at
400 nm. The results suggest that the first emission bands of the
fluorescent chemosensors are ascribed to the normal enol emission
decay, while the second emission bands of F3 could be assigned to
keto tautomer produced by intramolecular proton transfer in the
excited state (ESIPT) through a representative four-level cycle
photo-process (Scheme S1, SI). The partners F1/F2 and F5/F6 also
show the similar fluorescence spectral properties as F3/F4 (typi-
cally shown in Fig. S2, also see Tables S1 and SI).
Spectral properties of the target molecules induced by various
metal ions in ethanol are measured at room temperature (Fig. 2,
Fig. S3 & Fig. S4, SI). Take F3 and the reference F4 as example, Fig. 2
(a/b) shows the UVeVis absorption spectra and fluorescence
spectra of F3 respectively upon addition Zn2þ (C ¼ 1 ꢀ 10-
7 mol.Le1) and various other metal ions (including Kþ, Naþ, Ni2þ,
Pb2þ, Mn2þ, Mg2þ, Hg2þ, Cu2þ,Cd2þ, Cr3þ, Fe3þ, C ¼ 1 ꢀ 10-
6 mol.Le1). It could be clearly seen that none of these metal ions led
to significant enhancement in the fluorescence intensity or obvi-
ously changes in the UVeVis absorption spectrum of F3 except for
Zn2þ, which result in a more than 40 folds (F3, 0.0032, in EtOH;
0.128, after Zn2þ addition) fluorescence enhancement and a
shoulder peaked at 410 nm of absorption spectrum. It must be
pointed out that with the addition of Zn2þ, the ESIPT peak at
530 nm disappears in ethanol, and a new peak with significant
fluorescence enhancement appears at 500 nm, which may be due
to the chelation of Zn2þ with the target compound F3 and restrict
C]N isomerization mechanism. Comparing to other target dyes, it
also results in a almost 35 folds (F1, 0.0030, in EtOH; 0.105; F5,
0.0028, in EtOH; 0.1008, after Zn2þ addition) fluorescence
enhancement and a shoulder peaked at 408 nm of absorption
2. Results and discussions
The UVeVis absorption spectra and fluorescence spectra of the
target and reference molecules in various solvents are determined.
The typical absorption spectra of F3 and F4 in pure ethanol are
shown in Fig. 1 (a). The UV spectra show that F3 and F4 display a
little difference in spectral shape, while the absorption wavelength
maximum of F3 exhibits a red shift (22 nm, F3, 346 nm, F4, 324 nm)
comparing with F4. Furthermore, the absorbance intensity of F4 is
higher than that of F3 at range of 250e340 nm (also see Tables S1
and SI). Those phenomena shows that the red-shift in the absorp-
tion maximum of F3 with respect to that of F4 is attributed to the
presence of intramolecular H-bonding effect between OeH and
CH]N groups and increase in molecular conjugation in F3, the
Scheme 1. Chemical structures of the molecules studied in this work.

X. Wang et al. / Tetrahedron 76 (2020) 131108
3
Fig. 1. UVeVis absorbance spectrum (a) and fluorescence emission spectrum (b) of F3 and F4 in ethanol (C ¼ 5.0 ꢀ 10ꢁ6 mol. Lꢁ1), excitation wavelength, 320 nm.
Fig. 2. (a) UVeVis absorption spectrum and (b) Fluorescence emission spectrum changes of F3 induced by different metal ions in Ethanol (CF3 ¼ 5.0 ꢀ 10ꢁ6 mol. Lꢁ1
,
CIons ¼ 1.0 ꢀ 10ꢁ7 mol. Lꢁ1), excitation wavelength, 320 nm.
spectrum, which means that the bigger in molecular conjugation
the better in zinc recognition. At the same time, the spectral
properties of the reference molecule F4, F2 and F6 show no any
significant change in the addition of the above ions (Figure S5 &
Figure S6, SI). It is worth mentioning that the similar phenome-
non is also found in other organic solution (such as DMSO, CAN,
Ethanol/PBS buffer). Hence, it could be considered that the chela-
tion of Zn2þ ions does greatly suppress the occurrence of ESIPT.
In addition to the interference experiment in the presence of
different ions, we also analyzed whether the different pH value had
an effect on the identification of zinc ions for sensors. Take prober
F3 for example (Figure S7, SI), it could be seen clearly that UVeVis
spectrum of F3 almost kept the same as in neutral environment
when zinc ions was added under the different pH environment
(value of pH from 2.0 to 10.0), this result proved an evidence that a
wide range of pH changes has no obvious effect on the basic
characteristics of probe sensing. Thus, this phenomenon opens up
the possibility for zinc ions detection in living cell or cell imaging.
It is acceptable that the Zn2þ ion has four sp3 hybrid orbitals
with a regular coordination tetrahedral geometry [31]. The target
probe molecule F3 has -C]N、-OH groups which containing lone
pairs of electrons can chelate with Zn2þ to form a conjugated large
Pi bond, thereby the fluorescence intensity has a remarkable
enhancement. Other ions, such as Hg2þ ions, are characterized by
three coordinated sp2 hybrid orbitals with flat triangular chelation
geometry, so that the target compound cannot be effectively
complexed with Hg2þ. It is worth noting that the target molecule
has a bis-Schiff base structure, so Zn2þ could coordinated with C]
N and -OH groups (Scheme 2) [32e34].
Chemical shifts of probe were identified by the 1H-NMR titra-
tion analysis through addition Zn2þ and without Zn2þ (Fig. 3).
When probe F3 was added different concentration of Zn2þ ions
gradually (from 0 to 1.0 equiv.), the hydroxyl peak at 12.874 ppm
gradually decreased and totally disappeared on addition of 1.0
equiv. of Zn2þ. Furthermore, small downfield shifts were found in
the aromatic region. These shifts clearly indicated that the F3-Zn2þ
complex were formed successfully.
In order to further determine the higher selectivity of target
dyes to zinc ion, the competition experiments were also carried out
by monitoring the change in fluorescence emission intensity at
500 nm upon addition of Zn2þ (1 ꢀ 10ꢁ7 mol. Lꢁ1) and different
metal ions (5 ꢀ 10ꢁ6 mol. Lꢁ1) to a solution of target dyes in ethanol
solution, Zn2þ could be recognized obviously through a dramati-
cally fluorescence emission enhancement in ethanol aqueous so-
lution, which could be observed distinctly through “naked-eye”
color change from colorless to bluish green due to OFF-ON process
under the UV lamp (typically shown in Fig. 4 (a)). It could be seen
that a slight or negligible variation could be observed as the addi-
tion of other metal ions aqueous solutions. At the same time, as the
conjugation extent of the target molecules (F3>F1>F5) increase,
the fluorescence emission intensity increases accordingly (also see
in Figure S8, SI). However, the reference molecules do not show any
significant changes under the UV lamp (Figure S9, SI). Further
investigation shows that the presence of the other interfered ions
does not exhibit remarkable influence on the sensing response of
the substrates to Zn2þ in ethanol aqueous solution, which are

4
X. Wang et al. / Tetrahedron 76 (2020) 131108
Scheme 2. Fluorescence emission variation of F3 induced by Zn2þ addition and the proposed binding mode.
Fig. 3. 1H-NMR spectra of F3 and F3-Zn2þ
.
Fig. 4. (a) Fluorescence emission intensity changes of F3 induced by different metal ions in ethanol under UV lamp. (b) Metal ions competition analysis of F3 (5 ꢀ 10ꢁ6 mol. Lꢁ1) in
EtOH. The black bars represent fluorescence emission of F3 after addition of other metal ions (1 ꢀ 10ꢁ7 mol. Lꢁ1). The red bars represent fluorescence changes that occur upon
addition of 1 ꢀ 10ꢁ7 mol. Lꢁ1 of other metal ions to solution containing F3 and Zn2þ (1 ꢀ 10ꢁ7 mol. Lꢁ1).

X. Wang et al. / Tetrahedron 76 (2020) 131108
5
shown in Fig. 4 (b) and Fig. S10 (SI). These results further indicate
that the target fluorescent probes can strongly coordinate with
Zn2þ and restricted the occurrence of C]N isomerization, which
blocks ESIPT reaction.
stoichiometric ratio was 1: 1 for the probes (F1/F3/F5)-Zn2þ com-
plex (see Scheme 2, Fig. 7 & Fig. S14,SI). The fluorescence titration
test of probe under the existence of different Zn2þ concentrations
was performed in EtOH solution. Keeping the total concentration of
the probe and Zn2þ for 5.0 ꢀ 10ꢁ6 mol/L, and the concentration
ratio of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1 between probe
and Zn2þ was titrated.
We further detected the fluorescence intensity of probes with
Zn2þ in different volume ratio of PBS/EtOH (from 0: 10 to 10: 0, v/v,
pH ¼ 7.20) system and the fluorescence titration curve of target
dyes with Zn2þ(Fig. 5, Fig. S11 & Fig. S12, SI) in pure ethanol and in
ethanol/PBS buffer (1/1, v/v, pH ¼ 7.20) mixed solution, respec-
tively, take F3 for example. It can be clearly seen that the emission
peak generated by Zn2þ chelation at 500 nm with a sharp
enhancement in peak intensity as Zn2þ was gradually added
(1.0 ꢀ 10-7 mol. Le1 to 1.0 ꢀ 10-4 mol. Le1), and further increasing
the amount of Zn2þ only led to small changes in the fluorescence
intensity. Simultaneously, Fig. 5 and Fig. S11 & Fig. S12 (SI) indicated
that the fluorescence titration curve in pure ethanol and in ethanol/
buffer mixed solution all displayed the almost similar emission
variation. Meanwhile, as the PBS volume increased, the formed
Zn2þ complex can still maintain a strong fluorescence emission.
The coordination constant was determined by the measurement
of the fluorescence spectra changes according to Benesi-Hildebrand
equation. The equation is described as followings:
To ensure F3 could be used for detecting Zn2þ in living cells,
toxicity assay was also performed by adding a series of concen-
trations (0, 2, 4, 25, 50, 100 mM) of probe F3 in living human colonic
cancer cells line (SW620), the cells survival rate higher than 96% in
every concentration gradient, which indicates that probe F3 has no
apparent toxicity to living SW620 cells. The hypotoxicity of probe
F3 clearly demonstrates that probe has low cytotoxicity to living
cells.
To further explore the potential biological application, the probe
was applied in SW620 cancer cells by fluorescence imaging
experiment shown in Fig. 8. Firstly, SW620 cells incubation with 1%
DMSO-containing F3 for 0.5 h at 37 ꢃC, and no obviously fluores-
cence can be observed under the fluorescence microscope (Fig. 8
(b)). However, the green fluorescence was observed under the
.
.
.
.
.
F0 ðF0ꢁFÞ¼F0 ðF0ꢁFcomplexÞþF0 ðF0ꢁFcomplexÞꢀ1 Kꢀ1 ½Mꢂ
(1)
wherein K represents the binding constant, F0 is the integrated
fluorescence intensity of free ligand, F shows the observed inte-
grated fluorescence intensities, Fcomplex is the emission in-
tensities of the ligand-metal complex, and [M] shows the
concentration of Zn ions. The binding constant is given by the ratio
intercept/slope from the plot of F0/F0-F versus 1/[M]. The curve
fitting of the fluorescence intensity of F3 to the reciprocal of the
Zn2þ concentration further demonstrates the linear characteristics
of complex (Fig. 6). From the fluorescence titration data, sensor F3
for the detection of Zn2þ revealed extremely low detection limit
(67.2 nM), the binding constants (Ka) of the probe-Zn2þ complex
were determined as 1.37 ꢀ 106 L/mol (F1), 1.42 ꢀ 106 L/mol (F3)
and 1.13 ꢀ 106 L/mol (F5), respectively (Fig. S13, SI). The larger
binding constant (Ka) means probe-Zn2þcomplex possesses much
more stabilities.
Fig. 6. Fluorescence emission spectra of F3 (CF3 ¼ 5.0 ꢀ 10ꢁ6 mol. Lꢁ1) upon the titra-
tion of Zn2þ (CIons ¼ 0e1.0 ꢀ 10ꢁ4 mol. Lꢁ1) in EtOH solution.
Job's plot analysis of the fluorescence data was shown that the
Fig. 5. Fluorescence spectra of ligand F3 measured in ethanol upon addition Zn2þ and
after the addition of increasing amounts of Zn2þ (CF3 ¼ 5.0 ꢀ 10ꢁ6 mol. Lꢁ1
,
CIons ¼ 0e1.0 ꢀ 10ꢁ4 mol. Lꢁ1).
Fig. 7. Job's plot for F3 with Zn2þ ions in EtOH solution.

6
X. Wang et al. / Tetrahedron 76 (2020) 131108
Fig. 8. Confocal fluorescence images of SW620 cells. (aec) Cells cultured with F3 (1 ꢀ 10ꢁ6 mol.Lꢁ1) for 0.5 h; (def) Cells treated with F3 (1 ꢀ 10ꢁ6 mol.Lꢁ1) and then incubated with
5 ꢀ 10ꢁ6 mol.Lꢁ1 Zn2þ ions for another 0.5 h. Excitation wavelength, 320 nm, emission wavelength, 500 nm.
fluorescence microscope after incubation of the probe F3 treated
cells with Zn2þ for another 0.5 h at the same conditions (Fig. 8 (e)).
The experiments indicated F3 could be served as an efficient
candidate for monitoring changes in the intracellular Zn2þ under
biological conditions.
molecular structural characterization of the intermediates and the
final molecules were described in “supporting information (SI)”.
3.4. The determination of DL
The detection limit (DL) was determined according to formula
(2) by emission data of probes upon gradual addition of Zn2þ
.
3. Experimental section
DL ¼ 3
d
=S
(2)
3.1. Material and characterization
where
d was the standard deviation of blank ample, S represented
The organic solvents were purchased by the Sigma-Aldrich
Corporation and were further processed by the standard labora-
tory method. The starting materials for the preparation of new
studied chemosensors in our own laboratory were supplied by the
Acros Corporation.
the absolute value of the slope between fluorescence intensity and
Zn2þ concentration.
3.5. Bioimaging experiment
The 1H and 13C nuclear magnetic resonance (NMR) spectra of the
studied dyes molecules were measured by the Bruker apparatus
(400 MHz) in standard NMR tubes at the room temperature. The 1H
and 13C NMR peak chemical shifts of the target molecules were
calculated by an internal reference tetramethylsilane (TMS). A
Beijing Fukai melting point apparatus was employed to determine
the melting points of the samples. The elemental analysis of the
prepared dye molecules was acquired by a CE440 elemental anal-
ysis meter from Exeter Analytical Inc.
SW620 cells were cultured in DMEM (containing 10% FBS) in a
humidified incubator at 37 ꢃC and 5% CO2. The cells were seeded
2.5 ꢀ 105 per well in six-well plates. After 18 h proliferation, the
compound F3 at the final concentration of 1 ꢀ 10ꢁ6 mol/L (con-
taining 1% DMSO) were exposed to the cells and incubated for an
additional 30 min at 37 ꢃC. Then, the medium was discarded, and
the cells were washed once with phosphate buffer saline (PBS).
After that, the Zn2þ solution was added to the wells with a final
concentration of 5 ꢀ 10ꢁ6 mol/L, and the cells were incubated at
37 ꢃC for another 0.5 h. The cells were washed with cold PBS for
three times and fixed with 4% of paraformaldehyde PBS solution at
4 ꢃC for 15 min, then PBS was used to rinse cells for three times.
Finally, the cells were observed with a confocal microscope.
3.2. UV/visible absorption spectral determination and fluorescence
emission spectral measurement
The UV/visible absorption spectra and fluorescence emission
spectra of the samples were surveyed by using the spectral grade
solvents. A TU1901 spectrophotometer from Beijing PUXI General
Equipment Limited Corporation was employed to determine the
UV/visible absorption spectra of the samples. The spectro-
fluorophotonmeter (Cary eclipse) was used to determine the fluo-
rescence emission spectra of the samples. The fluorescence
quantum yields of the organic dyes in the solvents were measured
4. Conclusions
In conclusion, a series of novel ‘naked-eye’ Bis-Schiff base
fluorescent chemosensors are successfully synthesized. The results
of ultraviolet visible absorption spectra and fluorescent emission
spectra indicate that the target compounds have a unique selec-
tivity and highly sensitivity for the ‘naked-eye’ detection of Zn2þ
,
by using quinine sulfate in 0.5 mol/L H2SO4 (F, 0.546) as reference
[35].
and the bigger in molecular conjugation the better in zinc recog-
nition. Furthermore, for the good acid-base adaptation, the bio-
imaging experiment has successfully conducted as a fluorescent
probe to detect the Zn2þ in SW620 cells. This work provides a useful
design strategy for constructing fluorescent sensors for the recog-
nition of specific metal ions by rational structural design.
3.3. Synthesis of the studied target dyes
The synthesis routes of the new studied dyes F1-F6 were pre-
sented in Scheme 1. The preparation details of these target dyes,

X. Wang et al. / Tetrahedron 76 (2020) 131108
7
[14] Y. Yang, C. Ma, Y. Zhang, Q. Xue, H. Guo, A highly selective “turn-on” fluo-
rescent sensor for zinc ion based on a cinnamyl pyrazoline derivative and its
imaging in live cells, Anal. Methods 16 (2018) 1833e1841.
[15] A.C. Sedgwick, L.L. Wu, H.H. Han, S.D. Bull, X.P. He, T.D. James, J.L. Sessler,
B.Z. Tang, H. Tian, J.Y. Yoon, Excited-state intramolecular proton-transfer
(ESIPT) based fluorescence sensors and imaging agents, Chem. Soc. Rev. 47
(2018) 8842e8880.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
[16] J. Massue, D. Jacquemin, G. Ulrich, Molecular engineering of excited-state
intramolecular proton transfer (ESIPT) dual and triple emitters, Chem. Lett.
47 (2018) 1083e1089.
[17] L.L. Wu, Y. Wang, M. Weber, L.Y. Liu, A.C. Sedgwick, S.D. Bull, C.S. Huang,
T.D. James, ESIPT-based ratiometric fluorescence probe for the intracellular
imaging of peroxynitrite, Chem. Commun. 71 (2018) 9953e9956.
[18] X.C. Wang, H.Y. Ma, G. Ding, Y. Lu, Y.L. Gong, S.T. Zhang, Z.P. Luo, H.R. Li, F. Gao,
New armed near-IR two-photon organic chemosensors undergoing ESIPT
and‘naked eye’fluorescence sensing to zinc ions, Tetrahedron Lett. 56 (2015)
2758e2763.
We thank warm encouragement of the HeZe University PhD
Fund Planning (No.XY16BS33), Heze University Scientific Research
Foundation
(No.XY17KJ10,NO.XY18PY02,NO.XY18PY08).
G.D.
thanks a lot for the financial support of Chongqing Science and
Technology Commission (NO. cstc2018jcyjAX0446) and Chongqing
University of Arts and Sciences Scientific Research Foundation (NO.
2017RCH03) and X.Q. thanks the financial support of the Depart-
ment of Science and Technology of Henan province (No.
192102210201).
[19] S. Sinha, B. Chowdhury, P.A. Ghosh, Highly sensitive ESIPT-based ratiometric
fluorescence sensor for selective detection of Al3þ, Inorg. Chem. 55 (2016)
9212e9220.
[20] L. Chen, D. Wu, J.M. Kim, J. Yoon, An ESIPT-based fluorescence probe for
colorimetric, ratiometric, and selective detection of phosgene in solutions and
the gas phase, Anal. Chem. 89 (2017) 12596e12601.
Appendix A. Supplementary data
[21] I.J. Chang, K.S. Hwang, S.K. Chang, Selective Hg2þ signaling via dithiane to
aldehyde conversion of an ESIPT fluorophore, Dyes Pigments 137 (2017)
69e74.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2020.131108.
[22] J. Zhou, R. Shi, J. Liu, R. Wang, Y. Xu, X. Qian, An ESIPT-based fluorescent probe
for sensitive detection of hydrazine in aqueous solution, Org. Biomol. Chem.
13 (2015) 5344e5348.
References
[23] Y. Deng, X.Y. Min, L.L. Liu, R. Zhong, X. Xiao, J.M. Luo, X.B. Luo, New insight on
the adsorption capacity of metallogels for antimonite and antimonate
removal: from experimental to theoretical study, J. Hazard Mater. 36 (2018)
218e225.
[24] I. Nemec, R. Boca, M. Gembicky, L. Dlhan, R. Herchel, F. Renz, High-spin Schiff-
base dinuclear iron (III) complexes bridged by N-oxide ligands, Inorg. Chim.
Acta. 362 (2009) 4754e4759.
[25] Y. Xu, L. Xiao, S. Sun, Z. Pei, Y. Pei, Y. Pang, Switchable and selective detection
of Zn2þ or Cd2þ in living cells based on 3’-o-substitutedarrangement of
benzoxazole-derived fluorescent probes, Chem. Commun. 50 (2014)
7514e7516.
[26] A. Mahapatra, P. Karmakar, S. Manna, K. Maiti, D. Mandal, Benzthiazole-
derived chromogenic, fluorogenic and ratiometric probes for detection of
hydrazine in environmental samples and living cells, J. Photochem. Photobiol.
Chem. 334 (2017) 1e12.
[1] G. Kasirajan, V. Krishnaswamy, R. Nandhakumar, M. Mahalingam,
M. Sadasivam, M.P. Subramaniam, S. Ramasamy, New pyrazolo-quinoline
scaffold as a reversible colorimetric fluorescent probe for selective detection
of Zn2þ ions and its imaging in live cells, J. Photochem. Photobiol. Chem. 341
(2017) 136e145.
[2] G. Zhao, B.Y. Guo, G. Wei, S.Y. Guang, Z.Y. Gu, H.Y. Xu, A novel dual-channel
Schiff base fluorescent chemo-sensor for Zn2þ and Ca2þ recognition: synthe-
sis, mechanism and application, Dyes Pigments 170 (2019) 107614.
[3] F. Wang, J.H. Moon, R. Nandhakumar, B. Kang, D. Kim, K.M. Kim, J.Y. Lee,
J. Yoon, Zn2þ-induced conformational changes in a binaphthyl-pyrene deriv-
ative monitored by using fluorescence and CD spectroscopy, Chem. Commun.
49 (2013) 7228e7230.
[4] Z. Kowser, U. Rayhan, S. Rahman, P.E. Georghiou, T. Yamato, A fluorescence
“turn-on” sensor for multiple analytes: OAc- and F- triggered fluorogenic
detection of Zn2þ in
a co-operative fashion, Tetrahedron 73 (2017)
[27] Y. Gong, Z. Wang, S. Zhang, Z. Luo, F. Gao, H. Li, New ESIPT-inspired photo-
stabilizers of two-photon absorption coumarin-benzotriazole dyads: from
experiments to molecular modeling, Ind. Eng. Chem. Res. 55 (2016)
5223e5230.
[28] S.N. Yu, Z.C. Li, Y.Z. Hu, D.X. Yin, Synthesis of bis-schiff base compound with
trifluoromethyl, Chin. J. Synth. Chem. 2 (2010) 226e228.
5418e5424.
[5] C.R. Li, S.L. Li, Z.Y. Yang, Development of a coumarin-furan conjugate as Zn2þ
ratiometric fluorescent probe in ethanol-water system, Spectrochim. Acta
Mol. Biomol. Spectrosc. 174 (2017) 214e222.
[6] E.L. Que, D.W. Domaille, C.J. Chang, Metals in neurobiology: probing their
chemistry and biology with molecular imaging, Chem. Rev. 108 (2008)
1517e1549.
[7] J.H. Viles, Metal ions and amyloid fiber formation in neurodegenerative dis-
eases. Copper, zinc and iron in Alzheimer's, Parkinson's and prion diseases,
Coord. Chem. Rev. 256 (2012) 2271e2284.
[29] A. Maliakal, G. Lem, N.J. Turro, R. Ravichandran, J.C. Suhadolnik, A.D. DeBellis,
M.G. Wood, J. Lau, Twisted intramolecular charge transfer states in 2-
arylbenzotriazoles: fluorescence deactivation via intramolecular electron
transfer rather than proton transfer, J. Phys. Chem. A. 106 (2002) 7680e7689.
€
[30] A.O. Eseola, H. Gorls, M. Bangesh, W. Plass, ESIPT-capable 2,6-di (1H-imidazol-
[8] T. Jiang, X. Xiong, S.Y. Luo, Q. Fei, A. Yu, Z. Zhu, Direct mass spectrometric
analysis of zinc and cadmium in water by microwave plasma torch coupled
with a linear ion trap mass spectrometer, Int. J. Mass Spectrom. 399 (2016)
33e39.
2-yl) phenols with very strong fluorescent sensing signals towards Cr (iii), Zn
(ii), and Cd (ii): molecular variation effects on turn-on efficiency, New J. Chem.
42 (2018) 7884e7900.
[31] A.B. Davis,
A Library of Coumarin-Enaminone Chemodosimeters for the
[9] N. Pourhassanali, S. Roshan-Milani, F. Kheradmand, M. Motazakker,
M. Bagheri, E. Saboory, Zinc attenuates ethanol-induced Sertoli cell toxicity
and apoptosis through caspase-3 mediated pathways, Reprod. Toxicol. 61
(2016) 97e103.
[10] D.G. Alison, J.S. Kerry, P.S. Christine, P.W.J. Keith, C. Parul, Micronutrient de-
ficiencies in pregnancy worldwide: health effects and prevention, Nat. Rev.
Endocrinol. 12 (2016) 274e289.
[11] K.S. Hwa, Y.O. Seung, P. Chulhwan, K. Young, Colorimetric detection of Co2þ
ion using silver nanoparticles with spherical, plate, and rod shapes, Langmuir
29 (2013) 8978e8982.
[12] Y.C. Chen, C.C. Zhu, J.J. Cen, Y. Bai, W.J. He, Z.J. Guo, Ratiometric detection of pH
fluctuation in mitochondria with a new fluorescein/cyanine hybrid sensor,
Chem. Sci. 6 (2015) 3187e3194.
Detection of Analytes, Dissertations, 2016, p. 402.
[32] R. Chandra, A.K. Manna, M. Sahu, K. Rout, G.K. Patra, Simple salicylaldimine-
functionalized dipodal bis Schiff base chromogenic and fluorogenic chemo-
sensors for selective and sensitive detection of Al3þ and Cr3þ, Inorg. Chim.
Acta. 499 (2019) 119192.
[33] S. Mondal, S.M. Mandal, D. Ojha, D. Chattopadhyay, C. Sinha, Water soluble
sulfaguanidine based Schiff base as a ‘‘Turn-on” fluorescent probe for intra-
cellular recognition of Zn2þ in living cells and exploration for biological ac-
tivities, Polyhedron 172 (2019) 28e38.
[34] V. Venkatesan, K.R. Selva, S.K.A. Kumar, S.K. Sahoo, Highly selective turn-on
fluorogenic chemosensor for Zn2þ based on chelation enhanced fluores-
cence, Inorg. Chem. Commun. 102 (2019) 171e179.
[35] L. Ilies, Y. Sato, C. Mitsui, H. Tsuji, E. Nakamura, Modular synthesis of poly-
benzo, [b]silole compounds for hole-blocking material in phosphorescent
organic light emitting diodes, Chem. Asian J. 5 (2010) 1376e1381.
[13] D. Maity, T. Govindaraju, A differentially selective sensor with fluorescence
turn-on response to Zn2þ and dual-mode ratiometric response to Al3þ in
aqueous media, Chem. Commun. 48 (2012) 1039e1041.