Design of quinoline-based fluorescent probe for the ratiometric detection of cadmium in aqueous media

Article abstract of DOI:10.1016/j.cclet.2013.04.002

A new fluorescent probe, DQCd2, based on 4-piperidinoquinoline has been synthesized as a fluorescent probe for Cd²⁺. It can ratiometrically respond to Cd²⁺ in PBS buffer by a remarkable emission intensity enhancement (88-fold) and wavelength shift (70 nm).

Full text of DOI:10.1016/j.cclet.2013.04.002

Chinese Chemical Letters 24 (2013) 479–482
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Design of quinoline-based fluorescent probe for the ratiometric detection of
cadmium in aqueous media
a,
Qing Liu a,b, Guo-Ping Li a,b, Dong-Jian Zhu a,b, Lin Xue a, , Hua Jiang
*
*
a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 China
b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A new fluorescent probe, DQCd2, based on 4-piperidinoquinoline has been synthesized as a fluorescent
probe for Cd2+. It can ratiometrically respond to Cd2+ in PBS buffer by a remarkable emission intensity
enhancement (88-fold) and wavelength shift (70 nm).
Received 17 February 2013
Received in revised form 21 March 2013
Accepted 25 March 2013
Available online 28 April 2013
ß 2013 Lin Xue and Hua Jiang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Keywords:
Ratiometric
Probe
Fluorescence
Cadmium
Quinoline
1. Introduction
can, in principle, address the issue of accuracy and quantitative
measurements [7].
With the development of industrial production, many highly
toxic heavy metal ions have entered the environment and the
human food chain. Among them, cadmium has caught growing
attention because of its bioaccumulation and pathogenicity [1].
Hence, both the EPA (United States Environmental Protection
Agency) and the WHO (World Health Organization) have provided
strict safety standards for drinking water [2]. It is rather essential
to develop facile and efficient analytical methods to detect and
monitor Cd level in drinking water or other environmental
samples.
Owing to the simplicity, high sensitivity and real-time
detection, fluorescent sensors/probes have been regarded as
excellent candidates to selectively recognize and quantify metal
ions [3]. Up to now, only a few Cd2+-selective fluorescent probes
have been realized [4], partially because the detection of Cd2+
could be seriously interfered by Zn2+, which has similar chemical
properties to Cd2+ [5]. Moreover, most of these probes report a
sensing event merely based on the variation of emission intensity,
which might be affected by sample environment, dye concentra-
tion, photo-bleaching and so forth [6]. In contrast, ratiometric
fluorescent probes, which have a built-in calibration based on the
ratio of two excitation/emission wavelengths for extrinsic factors,
In our previous work [8], we have developed a quinoline
scaffold for the design of ratiometric metal ion-selective probes,
such as DQCd1, DQZn1-4, and DQAg, in aqueous solutions based on
inhibition of resonance charge transfer. Although DQCd1 exhibits
remarkable ratiometric response to Cd2+ with high sensitivity, it
still requires a masking agent, nitrilotriacetic acid (NTA), to
improve the specificity for Cd2+. On the other hand, Qian et al. have
recently reported that the water soluble polyamide is an efficient
Cd2+ receptor to develop fluorescent Cd2+-selective probes [9].
These results prompted us to design a highly selective ratiometric
probe for Cd2+ by combining our ratiometric quinoline-based
sensing scaffold with this polyamide Cd2+-receptor. Therefore, in
this contribution, we report the synthesis and photophysical
evaluation of DQCd2 as a ratiometric Cd2+-probe in aqueous
solution (Scheme 1).
2. Experimental
Unless otherwise noted, the chemicals were purchased from
Alfa Aesar (China) and used as received. All solvents were purified
and dried by standard methods prior to use. Pure water (18.2
V)
was used to prepare all aqueous solutions. 1H NMR and 13C NMR
spectra were recorded on a Bruker AVANCE-400 spectrometer. All
chemical shifts are reported in the standard notation of parts per
million using residual solvent protons as an internal standard.
Mass spectra (ESI) were obtained on a Bruker Apex IV Fourier
* Corresponding authors.
E-mail addresses: zinclin@iccas.ac.cn (L. Xue), hjiang@iccas.ac.cn (H. Jiang).
1001-8417/$ – see front matter ß 2013 Lin Xue and Hua Jiang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.04.002

[
Q. Liu et al. / Chinese Chemical Letters 24 (2013) 479–482
O
Br
O
O
O2N
O2N
HN
H2N
MeO C
a
b
c
d
2
N
H
COOMe
N
COOMe
NO2
NO2
MeO C
2
O
O
1
2
3
N
N
N
N
N
N
O2N
H2N
N
O
N
e
f
g
h
OH
OH
COOMe
COOMe
N
COOMe
N
O
O
O
O
4
5
6
7
N
N
N
N
N
HN
N
N
O
N
i
j
O
O
O
Cl
N
N
O
N
O
O
HN
O
OH
8
9
DQCd2
Scheme 1. Synthesis of probe DQCd2. Reagents and conditions: (a) Ethyl acetoacetate, MeOH, reflux, 64%. (b) Diphenyl oxide, 250 8C, 68%. (c) POBr3, THF, 60 8C, 75%. (d)
Piperidine, dioxane, 80 8C, 80%. (e) H2, Pd/C, EA, 95%. (f) Paraformaldehyde, NaBH3(CN), AcOH, r.t., 92%. (g) NaBH4, MeOH/THF, 65 8C, 88%. (h) SOCl2, DCM, 0 8C. (i) Diethyl
iminodiacetate, K2CO3, KI, acetonitrile, r.t. (j) Ethanolamine, acetonitrile, reflux, 48%.
transform mass spectrometer. UV–vis absorption spectra were
obtained using a Shimadzu UV-2550 spectrometer. Fluorescence
spectra were obtained using a Hitachi F-4600 spectrometer. The
path length was 1 cm with a cell volume of 3.0 mL. All of the
spectroscopic measurements were carried out in PBS buffer
(10 mmol/L phosphate, 8 g/L NaCl, 0.2 g/L KCl, pH 7.0) at 25 8C.
The solutions were allowed to equilibrate at 25 8C for 3 min after
each addition. Quinine sulfate in 0.1 mol/L H2SO4 (F = 0.54) was
used as the standard for quantum yield measurements [10].
The synthetic procedures are outlined in Scheme 1. Compound
ranges, from which two apparent pKa values of 7.19 and 2.66
were obtained (Fig. S1). According to previous reports, [8,11] we
reasoned that the quinolinic nitrogen, methoxyl oxygen and
tertiary nitrogen constitute a proton binding pocket, leading to the
combined pKa1 of 7.19 and a partial reduction of the photo-induced
electron transfer (PeT) quenching by the tertiary nitrogen from ion
binding receptor. Hence, the pKa2 of 2.66 is assigned to the
dimethylamino nitrogen atom. These data suggest DQCd2 can be
well protonated in neutral aqueous media. Therefore, a phosphate
buffered saline solution (PBS) at pH 7.0 containing 10 mmol/L
phosphate, 8 g/L NaCl, 0.2 g/L KCl was chosen for measurements.
In PBS solution, free DQCd2 showed a clear charge-transfer
band around 380–450 nm in the absorption spectra resulting from
7
was prepared according to our previous procedures [8].
Chlorination of 7 gave compound 8, which was further reacted
with diethyl iminodiacetate under basic conditions to give
compound 9. The probe DQCd2 was then obtained by the
aminolysis of the ethyl ester 9.
its mono-protonated species (Fig. 1). Upon the addition of Cd2+
,
this band gradually decreased with the appearance of a new
absorption band around 270–370 nm and four distinct isosbestic
points at 380, 358, 278, 265 nm. These spectral changes can be well
3. Results and discussion
[F(ig._2)TD$FIG]
As a routine procedure, we initially measured the fluorescence
spectra of DQCd2 in the aqueous solution over pH 2.5–10. The
fluorimetric titration curve shows two distinct pH changing
Fig. 2. Fluorescence spectra (
Cd2+ (0–170
mol/L) in PBS buffer. Inset: ratio (F502nm/F572nm) changes as a function
of the Cd2+ concentration.
lex = 402 nm) of 5 mmol/L DQCd2 upon titration of
Fig. 1. Absorption spectra of 5
m mmol/L)
mol/L DQCd2 upon titration of Cd2+ (0–150
m
in PBS buffer.

[
Q. Liu et al. / Chinese Chemical Letters 24 (2013) 479–482
481
Fig. 3. (a) Fluorescence spectra of DQCd2 (5
and Mg2+) in PBS buffer,
ex = 402 nm. (b) Gray bars represent the fluorescence intensity (top) and ratio calibration (bottom) responses of DQCd2 (5
various metal ions. Black bars represent the responses of DQCd2 in the presence of the indicated metal ions, followed by 100
mol/L Cd2+
m
mol/L) in the presence of various metal ions (100
m
mol/L Cd2+, Ag+, Co2+, Cr3+, Cu2+, Hg2+, Mn2+, Ni2+, Pb2+, Zn2+, Fe2+, Fe3+, Ca2+
mol/L) in the presence of
l
m
m
.
explained as a result of the Cd-coordination induced liberation of
the proton at quinolinic site and subsequent inhibition of
resonance [8]. As shown in Fig. 2, DQCd2 displayed relatively
Program of China (Nos. 2009CB930802, 2011CB935800), and the
State Key Laboratory of Fine Chemicals, Department of Chemical
Engineering, Dalian University of Technology for financial supports.
weak emission at 572 nm in the absence of Cd2+
(
F
0 = 0.07). Upon
the addition of Cd2+, the emission peak gradually shifted to 502 nm
with remarkable fluorescence enhancements Cd = 0.49; F/
Appendix A. Supplementary data
(
F
F0 = 88, F measured at 502 nm), also indicating that the Cd-
induced inhibition of resonance occurred. Meanwhile, the ratios
(F502/F572nm) of the fluorescence intensities at 502 nm and 572 nm
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.04.002.
varied from 0.2 to 3.8 with the continuous addition of Cd2+
.
Further, Job’s plot suggested that DQCd1 chelated Cd2+ with 1:1
binding stoichiometry (Fig. S2). By measuring the ratios of
emission intensity (F502/F572nm) at different concentrations of
Cd2+ in PBS buffer, the dissociation constant (Kd) of DQCd2 was
determined to be 1.2 Â 10À5 mol/L (Fig. 2, inset). These data clearly
indicate that DQCd2 is an excellent turn-on and ratiometric
fluorescent Cd2+-selective probe (F/F0 = 88, and R/R0 = 19).
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