Highly selective and sensitive near-infrared fluorescent sensors for cadmium in aqueous solution

Article abstract of DOI:10.1021/ol102692p

On the basis of tricabocyanine, two near-infrared fluorescent sensors CYP-1 and CYP-2 have been designed and synthesized. Both of them can selectively and sensitively recognize Cd²⁺ from other metal ions, especially the CYP-2, which can disting

Full text of DOI:10.1021/ol102692p

ORGANIC
LETTERS
2011
Vol. 13, No. 2
264-267
Highly Selective and Sensitive
Near-Infrared Fluorescent Sensors for
Cadmium in Aqueous Solution
Yangyang Yang, Tanyu Cheng, Weiping Zhu, Yufang Xu,* and Xuhong Qian*
Shanghai Key Laboratory of Chemical Biology, State Key Laboratory of Bioreactor
Engineering, School of Pharmacy, East China UniVersity of Science and Technology,
Shanghai 200237, China
yfxu@ecust.edu.cn; xhqian@ecust.edu.cn
Received November 6, 2010
ABSTRACT
On the basis of tricabocyanine, two near-infrared fluorescent sensors CYP-1 and CYP-2 have been designed and synthesized. Both of them
can selectively and sensitively recognize Cd2+ from other metal ions, especially the CYP-2, which can distinguish Cd2+ in neutral buffer
solution.
Wide use of cadmium, such as in fertilizers and batteries,1
makes it a severe environmental hazard but also a serious
health threat to humans.2 Either short-term or long-term
human exposures to environmental cadmium may promote
the chances of getting lung, prostate, breast, or endometrial
cancer.3 Therefore developing excellent methods to monitor
cadmium both in the environment and in vivo settings is in
urgent demand.
temporal resolution, which makes it a most popular method
to detect the analytes of interest. Recently, many fluorescent
sensors for cadmium have been reported in the literature.4
Most of them exhibited good selectivity against zinc and
rendered soluble in aqueous conditions. But some limitations
existed for in vivo detection of cadmium. Most of these
sensors have to be excited by UV-vis light (<600 nm) which
would cause damage to the living cells and does not penetrate
biological tissues very well. Additionally, short wavelength
excitation induces relatively strong autofluorescence, which
limits the detection sensitivity.5 For in vivo applications, it
is desirable to have both the excitation and emission
wavelengths of the probe in the range of 650-900 nm.6
Compared with other short-wavelength fluorophores, tricar-
bocyanine dyes are superior due to its strong near-infrared
(NIR) absorption and emission,7 and are commonly utilized
in designing NIR fluorescent probes and in vivo imaging.8
To the best of our knowledge, there is no report of fluorescent
Fluorescent sensing technology displays paramount sen-
sitivity and affords quick responses in a high spatial and
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10.1021/ol102692p  2011 American Chemical Society
Published on Web 12/08/2010

sensors for cadmium with the excitation and emission
wavelengths beyond 650 nm.
and selectivity in aqueous solution and have the potential to
be applied for in vivo cadmium imaging in biological
systems.
The sensors (CYP-1 and CYP-2) were synthesized ac-
cording to Scheme 1. The water-soluble tetraamide 1 has
For the metal ion fluorescent sensors, one of the most
commonly employed signaling mechanisms is photoinduced
electron transfer (PET).9 These sensors usually consist of
three moieties: an ion selective receptor and a fluorophore
tethered by a covalent linker.10 Herein, we report two novel
NIR fluorescent sensors based on the PET mechanism for
cadmium, CYP-1 and CYP-2, which display high sensitivity
Scheme 1. Synthesis of CYP-1 and CYP-2
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been shown to be an excellent receptor for Cd2+.4m The
CYP-1 was designed as a cell-permeable probe and the
CYP-2 was functioalized with two sulfonate groups to
increase the solubility and to reduce the self-aggregation in
water.
Nagano et al. reported an NIR fluorescent probe for NO,
whose fluorescence was largely quenched by the electron-
rich o-phenylenediamine moiety through PET.9e Our Cd2+
probes were expected to behave in a similar fashion. Upon
addition of Cd2+, metal-ligand coordination will inhibit the
PET process and the NIR fluorescence of the tricabocyanine
fluorophore was recovered (Scheme 2).
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Scheme 2
.
The Predicted Coordination between Sensors and
Cd2+
We then examined the spectral properties of our sensors.
We first evaluated the effect of pH on the fluorescence
properties of CYPs (Figures S1 and S2, Supporting Informa-
tion) and find that the CYPs are inert to pH in the range of
6.5-8.0. Therefore CYPs work well under physiological pH
condition. So the spectroscopic properties of CYP-2 were
studied in neutral buffer solution (12.5 mM Tris-HCl solution
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Org. Lett., Vol. 13, No. 2, 2011
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containing 0.05 mM sodium phosphate, pH 7.2). The
UV-vis and fluorescent spectra of the CYP-2 (see Figure 1
nonlinear least-squares analysis of fluorescence intensity
versus Cd2+ ion concentration to be 8.8 × 103 and 1.9 ×
105 (Table S2, Supporting Information). And at the same
time, the CYP-1 yielded similar results (Table S1 and Figures
S3 and S4, Supporting Information), which could selectively
recognize cadmium from other metal ions in Tris-HCl (12.5
mM) solution (acetonitrile/water ) 9/1, v/v, containing 0.05
mM sodium phosphate, pH 7.2), with some interference of
Zn2+ and Pb2+. The fluorescence emission spectra were
reversed by adding an excess of EDTA when the sensors
were saturated with Cd2+, indicating that the sensing action
was reversible (Figure S5, Supporting Information). And at
the same time the detection limit of CYPs was found to be
3.1 µM for CYP-1 and 2.3 µM for CYP-2 (Figure S6,
Supporting Information).
The selectivity of CYP-2 toward different metal ions was
then examined. As shown in Figure 2a, there is no obvious
Figure 1. (a) Fluorescence emission spectra (λex ) 771 nm) of
CYP-2 (3 µM) in Tris-HCl (12.5 mM) solution (containing 0.05
mM sodium phosphate, pH 7.2) upon addition of Cd2+ (0 - 400
µM). Inset: Binding isotherm between CYP-2 and Cd2+ with
emission intensity at 793 nm. (b) Fluorescence intensity of CYP-2
at 793 nm as a function of lg[Cd2+] (8 - 400 µM) in the same
condition of the Cd2+ titration.
Figure 2. (a) Fluorescence intensity of CYP-2 (3 µM) at 793 nm
and Table S1, Supporting Information) exhibited a maximum
absorption at 771 nm and an emission at 793 nm.
in the presence of different metal ions (40 µM) in Tris-HCl (12.5
mM) solution (containing 0.05 mM sodium phosphate, pH 7.2).
(b) The fluorescence intensity of CYP-2 at 793 nm with 40 µM
The cadmium titration experiment was then carried out
with CYP-2. As shown in Figure 1a, the fluorescent intensity
increased as the Cd2+ concentration increased. There is a
only minor spectra shift (<5 nm) that occurred, which
indicated that the lone pair of two aniline nitrogen atoms
becomes involved in Cd2+ coordination and the PET process
from nitrogen atoms to tricarbocyanine was removed. The
fluorescence quantum yield of CYP-2 increased from 0.0065
to 0.0145 in this process (see Table S1, Supporting Informa-
tion).11 The enhancement of fluorescence intensity of CYP-2
and the corresponding lg[Cd2+] (8-400 µM) (Figure 1b)
showed good linear relationship (R2 ) 0.99175). The
association constants K11 and K21 were determined by a
M
n+, followed by 40 µM Cd2+
.
change of fluorescent emission with Pb2+, Hg2+, Ag+, Zn2+,
Cu2+, Mn2+, Co2+, Ca2+, Fe3+, Li+, Ni2+, Ba2+, Cr3+, K+,
Fe2+, and Mg2+ while an obvious fluorescence enhancement
was observed when adding the same number of equivalents
of Cd2+ in the same condition, which demonstrated that
CYP-2 can distinguish Cd2+ from other metal ions in neutral
buffer solution. The results of competition experiment were
shown in Figure 2b. The fluorescence was increased when
Cd2+ was added into the solution that already contained the
different metal ions and CYP-2 (3 µM). The stoichiometry
of Cd2+:CYP-2 was determined by the Job’s plot (Figure
3). An abrupt discontinuity indicated a 2:1 species.
(11) Velapoldi, R. A.; Tønnesen, H. H. J. Fluoresc. 2004, 14, 465–
472.
266
Org. Lett., Vol. 13, No. 2, 2011

Figure 4.
Fluorescent images of Cd2+ in HeLa cells for sensor
CYP-1 (3 µM): (a) bright-field transmission image of HeLa cells
incubated with CYP-1 after adding Cd2+(500 µM); (b) fluorescence
image of HeLa cells incubated with CYP-1 after adding Cd2+
.
Figure 3. .
Job’s plot showing the 1:2 binding of CYP-2 with Cd2+
The total concentration of the sensor and Cd2+ is 10 µM in Tris-
HCl (12.5 mM) solution (containing 0.05 mM sodium phosphate,
pH 7.2).
CYP-2 can also discriminate Cd2+ from other metal ions in
Tris-HCl (12.5 mM) solution (containing 0.05 mM sodium
phosphate, pH 7.2). Both the excitation and emission
wavelengths are in the NIR region. The living cell image
experiments further demonstrate its value in the practical
applications of biological systems.
To further demonstrate the practical application of the
probe, we carried out experiments in living cells. The HeLa
cells was incubated with CYP-1 (10 µM) for 0.5 h at 37 °C
and washed once. And then Cd2+ (500 µM) was added for
another 30 min, which was washed three times. The
fluorescence images were taken, as shown in Figure 4. The
results suggest that CYP-1 can penetrate the cell membrane
and can be used for imaging of Cd2+ in living cells and in
vivo potentially. However, we did not get the fluorescence
images of CYP-2 since it was hard to penetrate into the cell
membrane.
In conclusion, we have designed and synthesized two
selective, sensitive NIR fluorescent sensors for cadmium
based on a PET mechanism. The CYP-1 can distinguish Cd2+
in Tris-HCl (12.5 mM,) solution (acetonitrile/water ) 9/1,
v/v, containing 0.05 mM sodium phosphate, pH 7.2), and
Acknowledgment. This work was financially supported
by the Key New Drug Creation and Manufacturing Program
(2009ZX09103-102), the China 111 Project (grant B07023),
and the Shanghai Leading Academic Discipline Project
(B507). We appreciate Professor Youjun Yang (School of
Pharmacy, East China University of Science and Technology)
for the improvements of this paper.
Supporting Information Available: Synthesis, experi-
mental details, and additional spectroscopic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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