Red-emission fluorescent probe sensing cadmium and pyrophosphate selectively in aqueous solution

Article abstract of DOI:10.1021/ol201305d

A fluorescent sensor for cadmium (CS) based on the BODIPY fluorophore exploiting the PET (Photoinduced Electron Transfer) mechanism was prepared. CS exhibited high selectivity and sensitivity for detecting cadmium in aqueous buffer solution. In addition,

Full text of DOI:10.1021/ol201305d

ORGANIC
LETTERS
2011
Vol. 13, No. 14
3656–3659
Red-Emission Fluorescent Probe Sensing
Cadmium and Pyrophosphate Selectively
in Aqueous Solution
Tanyu Cheng, Tao Wang, Weiping Zhu, Xinlei Chen, Youjun Yang, 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 May 15, 2011
ABSTRACT
A fluorescent sensor for cadmium (CS) based on the BODIPY fluorophore exploiting the PET (Photoinduced Electron Transfer) mechanism was
prepared. CS exhibited high selectivity and sensitivity for detecting cadmium in aqueous buffer solution. In addition, the complex of CS with
cadmium could detect pyrophosphate (PPi) selectively and sensitively.
Heavy metal contamination has become an increasingly
serious threat to human health and ecology.1 Cadmium is an
important heavy metal and widely used in industry and agri-
culture including the production of various metal alloys,
batteries, and also phosphate fertilizers.2 Cadmium is a
verytoxic element and easilyabsorbedand accumulatedby
plants andotherorganisms.3 It causes many serious diseases
such as lung, prostatic, and renal cancers, even at very low
concentrations.4
Fluorescence basedtechniques are robust yet convenient
methods for sensing a wide range of analytes5 includ-
ing cations, anions, and neutral molecules. The photoin-
duced electron transfer (PET) mechanism is a practical
control over the signaling of a fluorophore and has yielded
numerous sensitive OFF-ON type probes.6
Due to the toxicity of cadmium, it drew the close attention
of scientists. Many fluorescent sensors for cadmium have
been reported in past years. However, only a few probes7
displayed high selectivity to cadmium over zinc due to their
similarity in physical and chemical properties.
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Pyrophosphates (PPi) play very important roles in bio-
logical processes. For example, the products of ATP
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r
10.1021/ol201305d
Published on Web 06/20/2011
2011 American Chemical Society

arteriosclerosis (MA).9 Therefore, PPi sensing has received
attention in recent years, and many PPi fluorescent probes
have been reported. Many receptors for PPi were found,
and most of them are based on metal cations.10 Only a few
probes had good selectivity to distinguish PPi from analo-
gues such as ATP and AMP.10c,e,l,m,11
In this work, a long-wavelength and water-soluble cad-
mium sensor (CS, Figure 1) was designed and synthesized
(Supporting Information). The sensor is based on the
BODIPY fluorophore, which has excellent spectral prop-
erties, such as sharp absorption and emission bands, high
stability against photobleaching, and high molar absorptivity
and fluorescence quantum yield. To attain NIR and sensitive
sensors, a water dissoluble polyamide receptor which selec-
tively binds Cd2þ was chosen and two of them were con-
jugated to BODIPY. CS showed good selectivity for detecting
cadmium in a buffer solution, and the complex of CS with
Cd2þ provided excellent selectivity toward PPi in water.
remained stable with the pH above 2. This indicates that
CS could be used for sensing Cd2þ in a wide range of pH.
We chose a neutral Tris-HCl (0.02 M) solution (10%
DMSO, containing 0.1 mM sodium phosphate, pH 7.5)
as a testing system.
Cadmium titration was conducted by addition of an aliquot
of Cd2þ stock solutions to the aformentioned buffer solu-
tion containing 5 μM of CS. As shown in Figure 2, free CS
had an absorption maximum at 665 nm with a shoulder
peak at 614 nm. And it showed very weak fluorescence
when it was excited at 620 nm, because of the efficient
PET quenching from two diaminobenzene moieties to the
BODIPY fluorophore. Upon addition of Cd2þ, two blue-
shifted peaks appeared around 627 and 580 nm and the
peak at 665 nm diminished gradually. The blue shift indi-
cated the coordination of four anilinal nitrogen atoms
to Cd2þ and hence the intermolecular charge transfer
(ICT) process of the sensor was affected. Two isoabsorptic
points at 654 and 640 nm were shown. This may be attrib-
uted to the successive association of Cd2þ ions to the two
binding cavities presented on the probe. The appearance of
two peaks was presumably caused by fluorophore aggre-
gation in aqueous media.12 With the addition of Cd2þ, the
fluorescence intensity at 638 nm increased significantly.
And the quantum yield increased from 0 (<0.001) to 0.3.
A Job plot indicated that CS chelated a Cd2þ ion with 1:4
stoichiometry (Figure S2).
AsshowninFigureS4, CSshowed nofluorescencein the
buffer solution. Only Pb2þ, Agþ, Zn2þ, and Mn2þ caused
minimal fluorescence intensity increase with representative
5 equiv metal ion (Ni2þ, Pb2þ, Ca2þ, Liþ, Ba2þ, Agþ,
Hg2þ, Cr3þ, Co2þ, Zn2þ, Kþ, Csþ, Naþ, Cu2þ, Mg2þ
,
Mn2þ, Fe2þ, Fe3þ) addition. However, when 5 equiv of
Cd2þ were added into the CS solution, the fluorescence
intensity enhanced significantlyunderthe sameconditions.
This indicated that CS had good selectivity in the buffer
solution. Competition experiments were also conducted.
When 3 equiv of Cd2þ were added into the solution of CS
Figure 1. Structure of fluorescent sensor of CS.
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The effect of pH on the fluorescence properties of probe
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Figure 3. Time course of the response of CS to Hg2þ and Cd2þ
.
The increasing of fluorescent intensity at 639 nm was monitored
at time intervals after Hg2þ or Cd2þ addition. Experiments were
conducted in Tris-HCl (0.02 M) solution (10% DMSO, contain-
ing 0.1 mM sodium phosphate, pH 7.5). The concentration of
CS was 5 μM. Slit widths were 5 nm.
with Cd2þ only. This may be because the structure of the
receptor changed somewhat in the presence of Hg2þ, and
that change was good for Cd2þ binding. Thus we could
confirm if there was Hg2þ present with the time element
taken into consideration.
The complex of CS with Cd2þ (CS-Cd) was chosen as a
fluorescent probe for PPi. Free probe of CS-Cd showed
strong fluorescence in water with 0.5% CH3CN (v/v). As
shown in Figure S5, the fluorescence intensity of CS-Cd
remained stable when the pH was in the range of 5.5 to 8.5.
The fluorescence intensity of CS-Cd increased gradually
when the pH went below 5.5. This was likely because CS-
Cd was protonated, and the PET process was blocked. On
the other hand, when the solution was very alkaline, the
fluorescence intensity of CS-Cd decreased. This may be
because excess hydroxyl bound with Cd2þ and the PET
process recovered.
Figure 2. Absorption (a) and emission (b) spectra of CS in Tris-
HCl (20 mM) solution (10% DMSO, pH = 7.5, containing
0.1 mM sodium phosphate) in the presence of increasing concen-
tration of Cd2þ. The concentration of CS was 5 μM. The samples
were excited at 620 nm with excitation and emission slit widths
set at 5 and 2.5 nm respectively. Inset: The curve of fluorescence
intensity at 639 nm versus increasing Cd2þ concentration in Tris-
HCl (20 mM) solution (10% DMSO, pH = 7.5, containing
0.1 mM sodium phosphate) (Figure S3).
Experiments focused toward sensing PPi were con-
ducted in water with 0.5% CH3CN (v/v). As shown in
Figure 4, the absorption wavelength shifted to red with PPi
added into the solution. However, the absorption spectra
did not recover to the original intensity of CS, which may
be because PPi did not lead to the dissociation of CS-Cd.
Meanwhile, the fluorescence intensity around 637 nm
decreased obviously and stabilized gradually after 60 μM
PPi addition.
Then the selectivity of CS-Cd was determined, as shown
in Figure 5. Nearly no fluorescence changes were observed
with the addition of other anions (such as (2) AMP,
(3) ADP, (4) ATP, (5) HCO3ꢀ, (6) Fꢀ, (7) Iꢀ, (8) Brꢀ,
(9) SO42ꢀ, (10) CF3SO3ꢀ, (11) NO3ꢀ, (12) AcOꢀ, (13)
H2PO4ꢀ, (14) HSO4ꢀ, (15) Clꢀ) added into the solution.
Therefore, CS-Cd could distinguish ADP and ATP from PPi.
In conclusion, a red emitting fluorescent probe (CS) for
cadmium basedonthe BODIPYfluorophorewas designed
in presence of 5 equiv of other metals, the presence of most
other metal ions except Hg2þ had not induced any notice-
able spectral change compared to Cd2þ alone. The fluor-
escence intensity with Hg2þ presence was much stronger
than that with Cd2þ
.
Because Hg2þ caused additional enhancement of the
fluorescence intensity, we investigated the change of emis-
sion with both Cd2þ and Hg2þ present. The time course of
the fluorescence of CS (5 μM) in the presence of Hg2þ
,
Cd2þ, and Hg2þ and Cd2þ is shown in Figure 3. A very
slight change of fluorescence intensity was observed in the
presence of 10 equiv of Hg2þ in 10 min, and the fluores-
cence intensity at 639 nm did not change after 20 s in the
presence of 10 equiv of Cd2þ. However, the fluorescence
intensity greatly enhances within 10 min in the presence
of 10 equiv of both Hg2þ and Cd2þ compared to that
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Org. Lett., Vol. 13, No. 14, 2011

Figure 5. Fluorescence intensity change at 637 nm in water (0.5%
CH3CN) with 10 equiv of anions presence. The concentration of
CS-Cd was 10 μM, excitation wavelength was 620 nm. (1) CS-Cd,
(2) AMP, (3) ADP, (4) ATP, (5) HCO3ꢀ, (6) Fꢀ, (7) Iꢀ, (8) Brꢀꢀ,
(9) SO42ꢀ, (10) CF3SO3ꢀ, (11) NO3ꢀ, (12) AcOꢀ, (13) H2PO4
(14) HSO4ꢀ, (15) Clꢀ, (16) PPi.
,
sensitively in water. Both ATP and ADP did not cause
the spectra change of CS-Cd. When PPi was added into the
solution of CS-Cd, a red shift of the absorption wavelength
was observed and the fluorescence intensity around 637
nm diminished.
Acknowledgment. This work is financially supported by
the National Basic Research Program of China (973
Program, 2010CB126100), the National High Technology
Research and Development Program of China (863 Pro-
gram 2011AA10A207), the National Natural Science
Foundation of China (Grants 21076077), the China 111
Project (Grant B07023), the Shanghai Leading Academic
Discipline Project (B507), and the Fundamental Research
Funds for the Central Universities.
Figure 4. Absorption (a) and emission (b) spectra of CS-Cd2þ in
water (0.5% CH3CN) in the presence of increasing concentration
of PPi (0ꢀ100 μM). The concentration of CS-Cd was 10 μM. The
samples were excited at 620 nm. Inset: Curve of fluorescent inten-
sity at 637 nm versus increasing PPi concentration (Figure S6).
and synthesized and showed good selectivity toward Cd2þ
.
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.
Cd2þ induced a blue shift of the absorption and a dramatic
enhancement of the emission intensity. The complex of CS
with Cd2þ (CS-Cd) could sense PPi selectively and
Org. Lett., Vol. 13, No. 14, 2011
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