A New Fluorescent Probe for Selective Cd2+ Detection and Cell Imaging

Article abstract of DOI:10.1002/zaac.201800454

A new highly selective fluorescent probe, (E)-4-(4-([2,2′:6′,2′′-terpyridin]-4′-yl)styryl)-1-octadecylpyridin-1-ium bromide (ZC-F8), was designed and synthesized for cadmium detection and cell imaging. The fluorescence spectra of ZC-F8 exhibited its excel

Full text of DOI:10.1002/zaac.201800454

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800454
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
A New Fluorescent Probe for Selective Cd2+ Detection and Cell
Imaging
Wenxin Lin,*[a] Xiaochun Xie,[a] Yijia Wang,[a] and Jianjun Chen[a]
Abstract.
A new highly selective fluorescent probe, (E)-4-(4- aggregation induced emission effect. The cell-imaging experiment was
([2,2Ј:6Ј,2ЈЈ-terpyridin]-4Ј-yl)styryl)-1-octadecylpyridin-1-ium brom-
ide (ZC-F8), was designed and synthesized for cadmium detection and
cell imaging. The fluorescence spectra of ZC-F8 exhibited its excellent
carried out to estimate the actual biological application of ZC-F8. The
probe displayed ideal membrane permeable and labeled property for
cadmium, indicating its promising application for metal ions detection
response towards Cd2+ via intramolecular charge transfer effect and and tracing in living cells.
exhibits differential response to Zn2+ and Cd2+.[19] Ng et al.
Introduction
synthesized two fluorescent probes for distinct detection of
Zn2+ and Cd2+ by BODIPY.[20] Besides, Das et al. obtained a
fluorescent probe to detect three metal ions (Zn2+, Cd2+ and
Pb2+) and to distinguish one from another via chelation-
enhanced fluorescence mechanism, resulting from the chela-
tion induced the internal charge transfer process.[21] However,
most of the probes are not applicable in the cell imaging for
the sake of their high cytotoxicity.
Cadmium, as one of the important resources, has been
widely used in many fields including electroplating, industry
and agriculture. However, the toxic heavy metal Cd2+ may
cause acute or chronic poisoning resulting in cancers and other
diseases.[1] Inevitably, rapid detection and efficient instan-
taneous monitoring of cadmium both in environment and bio-
logical systems are highly desirable and necessary.
Recently, fluorescent probes for detecting metal ions have
been interested by researchers for their advantages, just like
high sensitivity, briefness, and real-time detection.[2–5] So far,
many fluorescent sensors are designed and synthesized for the
Cd2+ detection.[6–11] However, most of them bear the problem
of selectivity due to other metal ions with similar chemical
structures, such as Zn2+, which interfere the recognition. Gen-
erally, the photo-induced electron transfer (PET) effect, which
adopts di-2-picolylamine (DPA) as ionophore for binding tar-
get ions, is employed for the design of Zn2+ and Cd2+ detection
sensors. The fluorophore linking on the DPA unit can be
quenched by the PET effect and the fluorescence is then re-
covered when specific metal ions coordinate with DPA to
break the PET effect.[12–14] Therefore, the recognition capacity
of sensors depend on the selective coordination of the iono-
phore with the target metal ions.[15,16] Nevertheless, the de-
ficiency of selectivity may present on some metal pairs due to
species with similar binding ability leading to the interruption
by each other. In general, this phenomenon not only appears
in Zn2+/Cd2+, but also in K+/Na+, Ag+/Hg2+ and Ca2+/Mg2+
pairs.
The new sensitive and selective fluorescent probes for sens-
ing metal ions has attracted our attentions for several years.[22–
25]
Especially, the fluorescent sensors recognize Cd2+ from
Zn2+ via the intramolecular charge transfer (ICT) effect.[26–28]
However, some defects remain in these probes restricting their
application in the biological field. The poor solubility of ZC-
F1 limits its biological application, and the broadband emis-
sion range of ZC-F2 causes serious signal overlay in Zn2+/
Cd2+ pair. Therefore, the synthesis of satisfactory fluorescent
probe is desired to improve its bioapplication.
In this work, a new fluorescent probe (E)-4-(4-([2,2Ј:6Ј,2ЈЈ-
terpyridin]-4Ј-yl)styryl)-1-octadecylpyridin-1-ium
bromide
(ZC-F8, Figure 1) is designed and synthesized to solve the
problems mentioned above. Terpydine, a good receptor for
Cd2+ and Zn2+, is conjugated with pyridinium by the styryl
group, establishing a conjugated system in the molecule.
Meantime, the pyridinium group can improve solubility to ad-
vance the hydrophilia of the fluorescent sensor. Besides, an
Till now, the development of sensors for tracing cadmium
in cells with high sensitivity and selectivity still remains chal-
lenging. Jiang et al. introduced the carbonyl group to a fluores-
cent molecule for improving the sensing efficiency between
Zn2+ and Cd2+.[17,18] Lin et al. have reported a sensor, which
* Dr. W. Lin
E-Mail: linwx@zstu.edu.cn
[a] College of Materials and Textiles
Zhejiang Sci-Tech University
Hangzhou 310018, P. R. China
Figure 1. Structure of probe ZC-F8.
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alkane with sixteen carbon atoms is introduced into the mol-
ecule, which is found to be efficient for labeling cell mem-
brane. It is supposed that the probe can localize on the surface
of the cell membrane when it is injected in the living cells.
After combination with Cd2+, this probe may leave the mem-
brane surface with different fluorescent emission colors, due
to the change on molecular electrical properties. Thus, a probe
for labeling Cd2+ in cells can be expected.
Results and Discussion
Figure 2. (a) Fluorescence (FL) spectra of ZC-F8 (10 μM) in the
As shown in Scheme 1, the molecule ZC-F8 was synthe-
sized from commercially available chemicals through several
steps. The detailed synthetic route and the structure characteri-
zation of ZC-F8 were described in the Experimental Section.
To check the aggregation induced emission (AIE) effect of
obtained ZC-F8, the fluorescence spectra of probe were inves-
tigated in DMSO/H2O system (Figure 2). Generally, aggre-
gates would form when a suitable amount of nonsolvent was
added, which was a common method to estimate AIE behavior
in solvent/nonsolvent mixtures. In DMSO solution, ZC-F8
emitted weak fluorescence. The active intramolecular rotations
of multiple pyridinium rings exhausted the excited state en-
ergy, resulting in fluorescence quenching. Finally the fluores-
cence intensity decreased with the water fraction (fw, by vol-
ume%) increased, because of the twisted intramolecular
charge-transfer process of probe with D-A structure. When fw
was over 80%, the probe molecules aggregated and the intra-
molecular rotations were limited, and the excited states de-
cayed to the corresponding ground states via strong fluorescent
emission. Therefore, ZC-F8 emitted bright fluorescence and
the emission peak red shifted. The fluorescence was intensified
with the incremental addition of water, indicating the AIE ef-
fect of ZC-F8.
DMSO/H2O system with different water fraction (fw) excited at
375 nm. (b) Plot of the changes of the peak FL intensity of ZC-F8
with variation of fw. (I0 = the peak FL intensity of ZC-F8 when fw is
zero.).
was dried on the silicon sheet. This phenomenon could be as-
cribed to the AIE property of ZC-F8. The probe molecules
dissipated excited state energy by motions in solution, while
their motions were limited when they are solid state resulting
in fluorescent emission to dissipate the excited state energy.
The fluorescence spectra of ZC-F8 and its response towards
Zn2+ and Cd2+ were taken out in solution (water/DMSO =
99:1) at 1 μM (Figure 3). Weak fluorescent emissions were
gained before and after the titration of either Zn2+ or Cd2+ due
to the high polarity of water.[29–31] Nevertheless, bright blue-
Figure 3. Fluorescence spectra of ZC-F8 (1 μM) excited at 375 nm
upon addition of Zn2+ (0 and 1.2 μM) or Cd2+ (0 and 1.2 μM). Insets:
the luminescence photograph of ZC-F8 before (b) and after addition
green fluorescent emission could be observed when the probe of Zn2+ (a) and Cd2+ (c).
Scheme 1. Synthesis of probe ZC-F8.
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ZC-F8 exhibited a broadband emission band peaked at shown in Figure 5b and e, the fluorescent probe ZC-F8 gath-
480 nm, which might be resulted from the strong electronic ered in cell membrane due to the alkane part of probe mol-
push-pull effect, indicating the strong ICT effect in the mol- ecule. The fluorescent molecules passed through cell mem-
ecule. The initial fluorescence band of ZC-F8 disappeared and brane and labeled Cd2+ in cell nucleus after the addition of
a new emission peak at 530 nm was observed after titrated Cd2+. These remarkable alterations indicated that ZC-F8 could
with Cd2+. Meanwhile, the fluorescence spectrum of ZC-F8 pass through the cell membrane and detect Cd2+ in living cells.
had no obvious variation after addition of Zn2+, which was It could be used as a helpful probe for observing the distribu-
known as one of the most important interference of Cd2+, re- tion of Cd2+ in living cells and investigating the influence of
vealing that the probe ZC-F8 could be applied for distinguish- cadmium on the cells.
ing Cd2+ from Zn2+. The selective fluorescence responses
might be ascribed to the combination of sensor ZC-F8 and the
metal ions.[21,32] The different sized, electronic behavior, and
charge density of metal ions resulting in the distinct ICT effect,
leading to the change of emission colors. The probe ZC-F8
exhibited differential detection of Zn2+/Cd2+.
To further determine the selectivity of ZC-F8 in the practical
environment, some other metal ions (such as Hg2+, Ag+, Fe3+,
Na+, Ca2+, Pb2+, Al3+, Co2+, Cu2+, and K+) were also utilized
to assess their interference on the selectivity of ZC-F8 for de-
tecting Cd2+. As shown in Figure 4, the introduction of main
group metal ions exhibited no influence on the fluorescence
intensity when ZC-F8 was coordinated with Cd2+, whereas
Fe3+, Co2+, Cu2+, and Pb2+ quenched the intensity in a minor
degree. In addition, Hg2+ and its common interference Ag+
displayed no interruption, although Hg lied in the same group
as Zn and Cd.
Figure 5. Fluorescence images of HeLa cells cultured with ZC-F8
(10 μM) and CdCl2 [0 μM (a–c); 5 μM (d–f)]. Panels (a) and (d) show
differential interference contrast (DIC) images, Panels (b) and (e) dis-
play the corresponding confocal fluorescence images collected at 460–
500 nm and 510–600 nm, respectively. Panel (c) is the overlap of (a)
and (b). Panel (f) is the image of HeLa cells cultured with both ZC-
F8 and Hoechest 33342. The wavelength for excitation is 405 nm. The
scale bar is 10 μm.
Conclusions
A new highly selective fluorescent probe for Cd2+ detection
in environment and living cells was obtained. This probe
shows potential usage in the detection of Cd2+. By utilizing
metal ions as a part of the conjugated system, the sensor can
response to Cd2+ at aggregated state via ICT and AIE effects.
In addition, the cell-imaging experiment shows the desired cell
membrane permeable and satisfactory tracing property for
Cd2+ of ZC-F8. It is an interesting probe capable of detecting
and labeling Cd2+ in cell, indicating the design of such mol-
ecules can be applied in future researches on tracing metal ions
in living cells.
Figure 4. Fluorescence spectra of ZC-F8 (1 μM) dropped on silicon
sheets before and after addition of Cd2+ and various interferences
(5 μM for K+, Ca2+, Na+ and Al3+ and 1.2 μM for others) upon exci-
tation at 395 nm.
Considering the excellent sensing properties for Cd2+ in vi-
tro, the investigation of whether ZC-F8 could recognize Cd2+
in living cells was taken out by labeling HeLa cells. As con-
trolled, the cells were cultured with ZC-F8 (10 μM) for
30 min, which emitted strong fluorescence on the cell mem-
brane in the range of 460–500 nm. Other cells were treated
with ZC-F8 (10 μM) for 15 min, and then were cultured in
DMEM solution (CdCl2, 5 μM) for another 15 min. After that,
the cells were rinsed to remove excess CdCl2 and the fluores-
cence (510–600 nm) was observed obviously in cell nucleus
(Figure 5). The bright-field images (Figure 5a and d) showed
the cells were viable in the imaging experiments, indicating
Experimental Section
General: In this work, all the chemical reagents were gained from
commercial source and used without further purification. 1H NMR
spectra were performed with a Bruker Avance DMX500 spectrometer
using tetramethysilane (TMS) as an internal standard. Elemental analy-
sis was carried out with a Thermo Finnigan Flash EA1112 microele-
mental analyzer. Fluorescence spectra and excitation spectra were
taken out with a Hitachi F4600 fluorescence spectrophotometer. Fluo-
rescence images were gained with an Olympus FV-1000 confocal laser
scanning microscope.
4Ј-(p-Methylphenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine (1): NaOH (0.22 g,
there was no effect of treatment with ZC-F8 and Cd2+. As 5.4 mmol) and NH4OH (30 mL) were introduced into a methanol solu-
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tion (120 mL) of 4-methylbenzaldehyde (0.65 g, 5.4 mmol) and 2-acet-
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Keywords: Fluorescent probe; Selectivity; Imaging agents;
Cadmium; Cd2+ labeling; Cell nucleus
Received: October 18, 2018
Published Online: ᭿
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W. Lin,* X. Xie, Y. Wang, J. Chen .................................... 1–6
A New Fluorescent Probe for Selective Cd2+ Detection and
Cell Imaging
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