Preparation, regulation and biological application of a Schiff base fluorescence probe

Article abstract of DOI:10.1016/j.saa.2015.07.107

A facile fluorescence switch with Schiff base units was designed and achieved by nucleophilic addition and dehydration reaction. The fluorescence of the probe can be regulated by metal ions (Al^{3 +} and Cu^{2 +}). The whole process shows that the weak fluorescence of the probe enhances with the addition of Al^{3 +}, and then the strong fluorescence of the probe/Al^{3 +} ensemble reduces by introducing Cu^{2 +}. Meanwhile, the solution color changes of the probe with metal ions can be observed under 365 nm UV-vis light from weak light, pale green, green, pale green to weak light. Noticeably, the photo regulation processes of the probe by metal ions can be realized in the biological system and applied in cells imaging. The work provides a new strategy for designing facile regulation probe and develops a new application for Schiff base derivatives.

Full text of DOI:10.1016/j.saa.2015.07.107

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 1–5
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Preparation, regulation and biological application of a Schiff base
fluorescence probe
Ninghua Yin a,1, Haipeng Diao b,1, Wen Liu , Jingru Wang , Liheng Feng ⁎
b
a
a,
a
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
Department of Chemistry, Shanxi Medical University, Taiyuan 030001, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2015
Received in revised form 17 July 2015
Accepted 29 July 2015
Available online 8 August 2015
A facile fluorescence switch with Schiff base units was designed and achieved by nucleophilic addition and
3+
2+
dehydration reaction. The fluorescence of the probe can be regulated by metal ions (Al
and Cu ). The
3+
whole process shows that the weak fluorescence of the probe enhances with the addition of Al , and then
3+
2+
the strong fluorescence of the probe/Al
ensemble reduces by introducing Cu . Meanwhile, the solution
color changes of the probe with metal ions can be observed under 365 nm UV–vis light from weak light, pale
green, green, pale green to weak light. Noticeably, the photo regulation processes of the probe by metal ions
can be realized in the biological system and applied in cells imaging. The work provides a new strategy for design-
ing facile regulation probe and develops a new application for Schiff base derivatives.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Fluorescence probe
Regulation switch
Schiff base
Metal ions
Cells imaging
1
. Introduction
Fluorescence probes based on Schiff base units have been developed
and sensitivity for Al3+. For the speculated binding action, they think
that the hydroxyl groups in 4-position of benzene ring do not take
part in combining with Al3 . However, we design and synthesize a sim-
ilar probe MBSB (Scheme 1), which is only a difference in hydroxyl
group changing to methoxy group for 4-position of benzene ring. Unex-
pectedly, the MBSB probe has no response for Al3 detection. Based on
the fact, we think that all of the hydroxyl groups of HBSB participate in
+
and applied in molecular recognition field in recent years [1–5]. Many
researches display that an ideal fluorescence probe depends on the
facile design and matched bonding space for various analytes [6–10].
Compared with transition metal ions, to realize excellent detection for
Al ion is hard because of its poor coordination ability and spectroscopic
characteristics [11–14]. Fortunately, the development and application of
Schiff base compounds with N and O as hard-base donor sites provides a
facile pathway for determining Al with high sensitivity and selectivity.
To date, many well-property probes with Schiff bases units for Al3 have
been obtained and applied in analysis and testing fields [15–19]. These
studies mainly focused on improving the parameter values of sensitivity
and selectivity of the probes for Al3 . Meanwhile, a more meaningful as-
pect for these probes was ignored. We believe that the development of a
probe with regulation and control property through different metal ions
will be more important and meaningful because of its wide applied range
and facile switch function in biological field.
+
3
+
3+
the binding action with Al . And, multiple heteroatoms (N or O) in the
3
+
probe are favorable for occurring effective bonding with Al . Herein,
we design and synthesize a new Schiff base probe (HMBSB, Scheme 1)
with two = N–NH bonds, in which –N–H groups are used to bind
3
+
+
3+
Al . As expected, due to forming six-membered ring and then leading
to excited-state intramolecular proton transfer, the probe has very weak
fluorescence [20–22]. With introduction of Al3 to the probe, it shows
an obvious fluorescence “turn-on” response. It's worth noting that the
+
+
3
+
bright fluorescence of HMBSB/Al
ensemble can be quenched by
2
+
3+
2+
Cu . In this case, a facile regulation probe by Al
realized. The probe not only acts as a chemosensor for Al , but also is
a photo regulation switch, which is desired for applying in cells imaging.
and Cu
ions is
3
+
In order to achieve regulation probes by metal ions, it is indispens-
able for designing and preparing a probe with high recognition property
for one metal ion. In the aspect, Liu and coworkers reported a simple
Schiff base probe for Al3 in 2011 [19]. Though the probe (HBSB,
Scheme 1) with a very simple structure, it displays higher selectivity
2. Experimental
+
2.1. Materials and instruments
Unless otherwise stated, all chemical reagents were obtained
from commercial suppliers and used without further purification.
Solvents used were purified and dried by standard methods prior
to use. Terephthaloyl dichloride, hydrazine (85%), 2-hydroxy-4-
⁎
Corresponding author.
E-mail address: lhfeng@sxu.edu.cn (L. Feng).
The authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.saa.2015.07.107
386-1425/© 2015 Elsevier B.V. All rights reserved.
1

2
N. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 1–5
Scheme 1. The chemical structures of four Schiff base compounds.
methoxybenzaldehyde and 2-chloro-4-methoxybenzaldehyde were pur-
chased from Aldrich (Steinheim, Germany). Metal ions both were nitrates
The detail synthesis routes of these compounds were showed in
Scheme S1. The MBSB compound was prepared according to the report-
ed literature [23]. The CMBSB was synthesized according to the HMBSB
method and these characterization data were displayed in supplemen-
tary data.
1
13
and were provided from Alfa Aesar (Tianjin, China). H NMR and
C
NMR were measured on a Bruker ARX400 spectrometer with chemical
shifts reported as ppm (TMS as an internal standard). High-resolution
mass spectra (HRMS) were acquired on an Agilent 6510 Q-TOF LC/MS in-
strument (Agilent Technologies, Palo Alto, CA) equipped with an
electrospray ionization (ESI) source. Fluorescence spectra were acquired
with a Hitachi F-4600 fluorescence spectrophotometer, the excitation
and emission slit widths both were 5.0 nm. Confocal laser scanning mi-
croscopy (CLSM) imaging was taken on a confocal laser scanning biolog-
ical microscope (FV1000-IX81, Olympus, Japan).
2.4. Cells culture and imaging
2
Under a humidified atmosphere containing 5% CO , SiHa cells were
grown in DMEM medium containing 10% FBS routinely, then harvested
for subculture using trypsin (0.05%, Gibco/Invitrogen) at 37 °C. SiHa
cells were subculture onto a 35 mm × 35 mm Petri dish with a glass
bottom, then allowed to grow for 24 h for attachment, after which
1
mL of DMEM medium containing 10% 20 μM HMBSB was used to
2
.2. Fluorescence measurements
incubate the SiHa cells at 37 °C for 5 h. The medium was replaced and
phosphate-buffered saline (PBS, pH = 7.4) was used to wash the cells
thrice. And different equivalent metal ions in PBS buffer solution were
added into the dish and the cells were cultured at 37 °C for 1 h. The
medium was replaced and phosphate-buffered saline (PBS, pH 7.4)
was used to wash the cells thrice. Then fresh medium with cytoplasm
located dye (Lyso tracker red) was added and incubated. After washing
thrice with PBS, the images of the cells were recorded on confocal laser
scanning microscopy.
2
All experiments were in mixture solvents (DMSO/H O = 6/4, v/v).
−
3
The stock solutions (1.0 × 10 M) of these compounds were diluted
in 1.0 L measuring flask with DMSO/H O to afford the working solution
M). The stock solutions of metal ions were both
M. The standard stock solutions of lower concentrations
2
−
5
(
1
1.0 × 10
−
3
.0 × 10
were prepared by suitable dilution of the stock solution. All spectra
analysis studies were carried out in a quartz cuvette with 1 cm path.
The total volume of working solutions is 2.0 mL. The excitation wave-
length was set in 375 nm according to experimental requirements. All
of the experiments were performed at room temperature.
3. Results and discussion
3
.1. The fluorescence properties and bonding processes of the probes with
3
+
2
.3. Synthesis of the compounds
Al ion
A solution of 2.03 g terephthaloyl dichloride (0.01 mol) in 20 mL THF
Based on the previously reported result [19], the type probes usually
was slowly added to 20 mL THF with 10 mL hydrazine (85%) at room
temperature. The mixture was stirred for 5 h at the room temperature.
Then, the mixture was poured into 200 mL iced water and extracted
three times with dichloromethane (3 × 100 mL). The obtained organic
phase was washed by redistilled water (2 × 100 mL) and dried by anhy-
drous sodium sulfate. The organic solvent was removed and obtained
terephthalohydrazide. Then, 0.76 g 2-hydroxy-4-methoxybenzaldehyde
was added to a solution of 0.97 g the above terephthalohydrazide
have low fluorescence emission because of the six-ring formation and
excited-state intramolecular proton transfer. Hence, the excitation and
emission spectra of HMBSB are 310 nm and 420 nm with low fluores-
cence intensity, respectively (Fig. 1). When introducing Al3+ to
2
HMSBS in DMSO/H O, the fluorescence excitation and emission spectra
take obvious changes. An obvious red-shift from 420 nm to 495 nm of
emission peak was observed with the increase of Al3 (Fig. S1). Corre-
sponding, a new maximum excitation peak at 375 nm also was investi-
gated (Fig. 1). From these results, we believe that the complex of HMBSB
and Al3 has been formed and the maximum excitation and emission
peaks are 375 nm and 495 nm, respectively. In order to verify the
+
(
6
0.05 mol) in 20 mL ethanol and the mixture was stirred for 12 h at
0 °C. The separated solid was filtered, washed and dried and then
+
1
4
gave the product N ,N -bis[(E)-2-hydroxy-4-methoxybenzylidene]-
1
3+
terephthalohydrazide (HMBSB) in 58.6% yield. H NMR (d-DMSO,
combination way of HMBSB with Al , other two compounds (MBSB
4
8
00 MHz) δ 12.31 (b, 1H), 11.97 (b, 1H), 10.82 (s, 2H), 8.87 (s, 2H),
and CMBSB, Scheme 1) were synthesized and studied in the interaction
.11 (m, 4H), 7.57 (d, 2H), 6.58 (m, 4H), 3.80 (s, 6H); 13C NMR (d-
with Al in the same condition. The results show that the fluorescence
3+
3
+
DMSO, 100 MHz) δ 168.65, 165.92, 164.87, 137.51, 134.82, 132.97,
16.59, 112.34, 106.18, 60.69; EI-MS (C24 , 462, m/z) 463
M + 1].
2
has no change with the adding of Al to MBSB or CMBSB in DMSO/H O.
1
[
H
22
N
4
O
6
Compared to the structure with previously reported HBSB (Scheme 1),
the only difference of MBSM lies in hydroxyl group in 4-position

N. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 1–5
3
3
+
3
.2. Sensitivity and selectivity of HMBSB to Al ion
The detail interaction of HMBSB with Al3+ was investigated by
fluorescence spectra in DMSO/H O (Fig. 2). As can been seen from
Fig. 2, the emission intensities at 495 nm gradually enhance with the in-
2
crease of Al3 . When the concentration of Al reaches one equivalent
of HMBSB, a 15-fold increase of fluorescence intensity is observed. And,
no obvious enhancement of fluorescence intensity is achieved with the
concentration of Al3 up to four equivalent of HMBSB (Fig. S2). The
whole binding process of HMBSB with Al3 displays a curve change.
However, the relative fluorescence intensity as the function to the con-
+
3+
+
+
centration of Al3 between 0 and 1 × 10
+
−5
M exists a good linearity
(
Fig. S3). Correspondingly, the solution color from weak light to strong
blue-green light can be observed by 365 nm UV–vis illumination
(Scheme 2). These results mean that the fluorescence of HMBSB may
be induced by Al3 , and the HMBSB can be used as a highly “turn-on”
fluorescent chemosensor for Al3 . Additionally, the selectivity of
HMBSB toward other metal ions also was determined by fluorescence
3
+
Fig. 1. Fluorescence spectra of HMBSB and HMBSB/Al ensemble. The concentrations of
HMBSB and Al were both 1.0 × 10 mol/L.
+
3
+
−5
+
2
spectra in DMSO/H O (Fig. 3). From Fig. 3, we can receive that the
substituted by methoxy group. Thus, we can obtain the fact that the
fluorescence intensity of HMBSB has only a little change with the two
3
+
3+
hydroxyl group in 4-position of HBSB has an interaction with Al
.
equivalent metal ions except for Al , respectively. The high selectivity
Additionally, based on the result of CMBSB with Al3 , another fact can
be obtained that the hydroxyl group in 2-position of HBSB or HMBSB
also takes part in bonding with Al3 . In view of the above results and
+
3+
of HMBSB to Al
lies in the matched space of HMBSB and efficient
3
+
binding way of HMBSB with Al . In a word, the fluorescence of
HMBSB can be induced by only Al , and the fluorescence intensities
of HMBSB generally enhance with the increase of Al
+
3+
3
+
3+
analysis, we guess that the binding way of HMBSB with Al is showed
in Scheme 2. With the introduction of Al3 , the enhancement of HMBSB
fluorescence mainly lies in destroying the intramolecular hydrogen
bond between the phenolic O–H and the nitrogen of the imine group
that undergoes excited-state intramolecular proton transfer (ESIPT)
.
+
2
+
3+
3.3. Regulation process of Cu ion to HMBSB/Al ensemble
2
+
Usually, for organic fluorescence probes, Cu ion is a quencher and
[21–23].
leads to the intensity reduction of these probes. In the work, upon
Scheme 2. The bonding ways of HMBSB with Al3+ and Cu2+ ions, and the color changes of HMBSB (1.0 × 10−4 mol/L) solution by introduction of Al3+ followed by Cu2+ in DMSO/H
solution. The solutions were irradiated by λ365 nm UV–vis light and daylight lamp.
2
O (6/4,V/V)

4
N. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 1–5
3
+
−5
Fig. 2. Characteristic fluorescence response by the introduction of Al to the HMBSB in
Fig. 4. Characteristic fluorescence response of the HMBSB (1.0 × 10
mol/L) and
O. The concentration of HMBSB was 1.0 × 10− mol/L.
5
3+
−5
2+
DMSO/H
2
Al
(2.0 × 10
mol/L) by introduction of Cu
in DMSO/H
2
O solution.
introduction the Cu2+ ion into the HMBSB/Al3+ ensemble (high fluo-
rescence), the expectant reduction of fluorescence intensity is observed
and the reduction extent of fluorescence intensity depends on the
the SiHa cells were cultured with HMBSB at 37 °C for 5 h, and then, the
3+
various equivalent Al
were added in the system and sequentially
2+
cultured the cells for 1 h. Lastly, the different concentrations of Cu
were introduced in the above system with culturing of 1 h at 37 °C.
The fluorescence imaging was taken, where Lyso tracker red was used
as a cytoplasm located dye. As shown in Fig. 5, the cytoplasm regions
of SiHa cells are stained by HMBSB and well overlayed with cytoplasm
2
+
quantity of Cu
(Fig. 4). A curve change of fluorescence intensity is
2
+
displayed with the addition of Cu (Fig. S4). And, when the concentra-
2
+
tion of Cu
reaches ten equivalent of HMBSB, a 6-fold reduction in
fluorescence intensity is observed. It's worth noting that the obvious
changes of fluorescence intensity are not observed when adding other
located dye (Lyso tracker red). Obviously, the cells can be illumed
3
+
metal ions to HMBSB/Al
ensemble. So, a desired photo regulation
3+
by introduction of Al
to SiHa cells with HMBSB, and the stained
3
+
2+
switch was realized by metal ions (Al and Cu ). In the regulation
3+
brightness enhances with the increase of Al . It is worth noting that
the illumed cells can be restrained by Cu
3
+
2+
switch, Al
acts as an illumining fluorescence role, and Cu
is an
2+
ion. So, the regulation
extinguishing function for the fluorescence. Additionally, due to the in-
tensity change of fluorescence in the regulation process, the solution
color also took place obvious change (Scheme 2). The detailed processes
are from weak light, pale green to green color changes with the addition
of Al3 . On the contrary, the change process from green, pale green to
weak light is observed under 365 nm light.
3+
2+
photo switch (HMBSB) by metal ions (Al and Cu ) was realized on
cell level. The ensemble may be used as a fluorescence switch probe
for cell imaging with a facile regulation function. This result provides a
new strategy to fluorescence probe design for regulation imaging of
living cells.
+
4. Conclusion
3
.4. Cell imaging in vitro
In summary, a new regulation and control probe by Al3+ and Cu2+
Compared with current reported fluorescence imaging reagents, the
ions was successfully synthesized by simple two steps. The probe
probe with regulation function by metal ions has many advantages.
First, due to the fluorescence intensity not depending on the quantity
of the reagent, the cytotoxicity that comes from the reagent can be
decreased in cell imaging. Second, the metal ions that act as a regulating
role are compatible and existing for many organisms, which is facile and
favorable for developing the probe applications in biological aspect.
Hence, biological application of the regulation probe for cells imaging
was investigated by confocal laser scanning microscope (CLSM). Firstly,
3
+
HMBSB displays a fluorescence “turn-on” response for Al with high
3
+
sensitivity and selectivity. The “turn-on” fluorescence of HMBSB/Al
ensemble can be quenched by Cu2 . In the whole action processes,
+
the intensity of fluorescence of the probe can be regulated with the
additional measure of Al3 and Cu
+
2+
ions. Compared with previous
reported probe, the work has the following advantages: (1) the probe
3
+
has higher sensitivity and selectivity for Al and may be used to deter-
3
+
3+
mine Al ; (2) the probe is a regulation switch of fluorescence by Al
and Cu2 ; (3) the regulation processes of the probe by metal ions can be
observed by 365 nm light with the changes of solution color (from weak
light, pale green, green, pale green to weak light); and (4) the regulation
processes of the probe by metal ions may be realized in cell imaging. In
view of above points, the work not only obtains a facile regulation probe
+
3
+
2+
and an efficient chemosensor for Al
and Cu , but also provides a
new strategy for designing regulation probes and a new development
direction of ions recognition.
Acknowledgments
The work described in this paper was supported by the National
Nature Science Foundation (No. 21371110 and 21201024), the Youth
Science Foundation of Shanxi Province (No. 2014021006) and the
Program for the Outstanding Innovative Teams of Higher Learning Insti-
tutions of Shanxi (2013) (2013070803).
−
5
Fig. 3. Binding characteristics of HMBSB (1.0 × 10
mol/L) and metal ions
−
5
(
2.0 × 10
2
mol/L) in DMSO/H O, respectively.

N. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 1–5
5
Fig. 5. Fluorescence imaging of SiHa cells with HMBSB. The first line is bright images; the second line is fluorescence images of HMBSB, Al and Cu2+ ions; the third line is fluorescence
3
+
3
+
2+
images of cytoplasm located dye (Lyso tracker red); the last line is merged images of cells, HMBSB, Al , Cu , and located dye. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
[
11] Y. Zhao, Z. Lin, H. Liao, C. Duan, Q. Meng, A highly selective fluorescent chemosensor for
3+
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applications, Anal. Chem. 59 (1987) 629–636.
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