Aminoquinoline-based fluorescent probe for detection of Cu2 + and hydrogen sulfide

Article abstract of DOI:10.1016/j.inoche.2013.07.005

Diethyl 2,2′-(2-oxo-2-(quinolin-8-ylamino)ethylazanediyl) diacetate (DOQED), an aminoquinoline-based fluorescent probe, was synthesized, and successfully applied in detection of Cu^{2 +} and S^{2 -} in an aqueous buffered solution. The emission signal of DOQED was selectively quenched by Cu^{2 +}, and was exclusively recovered by subsequent addition of S^{2 -}. The formation of a [Cu(DOQED)]^{2 +} complex was confirmed by X-ray crystallography.

Full text of DOI:10.1016/j.inoche.2013.07.005

Inorganic Chemistry Communications 35 (2013) 311–314
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Aminoquinoline-based fluorescent probe for detection of Cu2+ and
hydrogen sulfide
⁎
Liangwei Zhang, Jinyu Sun, Shudi Liu, Xuemei Cui, Weishuang Li, Jianguo Fang
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Diethyl 2,2′-(2-oxo-2-(quinolin-8-ylamino)ethylazanediyl) diacetate (DOQED), an aminoquinoline-based
fluorescent probe, was synthesized, and successfully applied in detection of Cu2+ and S2− in an aqueous buffered
solution. The emission signal of DOQED was selectively quenched by Cu2+, and was exclusively recovered by sub-
sequent addition of S2−. The formation of a [Cu(DOQED)]2+ complex was confirmed by X-ray crystallography.
© 2013 Elsevier B.V. All rights reserved.
Received 21 March 2013
Accepted 20 July 2013
Available online 27 July 2013
Keywords:
Fluorescence probe
Aminoquinoline
Copper (II)
Hydrogen sulfide
Exploring functions of biomolecules is critically contingent upon the
development of novel assay techniques. Among the various detection
methods, fluorescence sensing, due to its high sensitivity, operational
simplicity, and biocompatibility, is one of the most powerful and popu-
lar tools to visualize complicated biological processes [1]. And thus, the
past years have witnessed a burst of interest in developing diverse
fluorescent probes in reconigizing biomolecules of interest [2]. Copper,
an essential trace element in humans, is a pivotal co-factor for many
enzymes, and participates in numerous physiological processes [3]. Its de-
ficiency or over-accumulation may lead to various diseases [4]. Sulfide
anion, a traditional toxic pollutant, is widespread in the environment
where it is generated from industrial processes or biological metabolism
[5]. Recent studies have also demonstrated that protonated sulfide is
involved in various physiological processes [6]. For example, together
with NO and CO, H2S has been recognized as the third gaseous transmit-
ter and shown to regulate a variety of physiological/pathophysiological
processes. Although, a large number of fluorescent probes have
been developed for sensing Cu2+ [7] and S2− [8], the probes simulta-
neously sensing for both species are quite rare [9]. In this work, an
aminoquinoline-based fluorescence probe DOQED was synthesized
(Scheme 1) and fully characterized (Figs. S1–S9). The emission signal
was selectively quenched by Cu2+ via forming a [Cu(DOQED)]2+
complex, and was exclusively recovered followed by an addition of
S2−. This “on–off–on” response provides a convenient and practical
way in detection of both Cu2+ and S2− in biological samples.
(10 μΜ) displayed two peaks at 235 nm and 310 nm with molar absorp-
tion coefficients of 14,458 cm−1 M−1 and 3153 cm−1 M−1, respectively,
the typical absorbance bands of aminoquinoline. Both bands were red-
shifted with the addition of Cu2+ (0–1.2 equiv.) in the absorption spec-
trum, accompanying three isosbestic points at 240, 285 and 320 nm,
supporting intermediate complex formation.
In order to understand the binding modes of probe DOQED towards
Cu2+, fluorescence titration experiments were carried out. In Fig. 1B
with the increasing addition of Cu2+ (0–1.5 equiv.) to a solution
of 10 μΜ probe DOQED in a PBS buffer (10 mM, pH 7.4), a gradual de-
crease in the fluorescence intensity at 420 nm was observed. Quenching
of fluorescence was due to the coordination of Cu2+ to probe DOQED
which decreased the electron-donating of nitrogen atom of quinoline
and then inhibited the ICT process. Furthermore, the paramagnetic
nature of Cu2+ commonly leads to fluorescence quenching. There was a
good linear correlation between the fluorescence intensity and the con-
centration of Cu2+ in the range from 0 μΜ to 10 μΜ, and the fluorescence
intensity was retained even if more Cu2+ was added (R2 = 0.998, inset
As shown in Fig. 1A, the response of probe DOQED to Cu2+ was eval-
uated by UV–vis absorption titration at first. The free probe DOQED
⁎
Corresponding author. Tel.: +86 931 8912500; fax: +86 931 8915557.
E-mail address: fangjg@lzu.edu.cn (J. Fang).
Scheme 1. Synthesis of probe DOQED.
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.07.005

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L. Zhang et al. / Inorganic Chemistry Communications 35 (2013) 311–314
Fig. 2. The crystal structure of [Cu(DOQED)]2+ complex (thermal ellipsoid at the 30%
probability level). ClO−4 and H atoms are omitted for clarity.
atom (N1) from acetonitrile. Selected bond lengths and bond angles for
[Cu(DOQED)]2+ complex are given in Table S1. The absolute quantum
yield of DOQED and its Cu2+ complex is determined as 0.042 and
0.037. The detection limit of probe DOQED for Cu2+ was estimated to
be 1.44 × 10−7 according to the reported method [11]. The dissociation
constant, Kd, of the [Cu(DOQED)]2+ complex was obtained to be
8.0 × 10−5 M from the fluorescence titration by theoretical nonlinear
fitting assuming 1:1 stoichiometry [12] (Fig. S11). In addition, the fluores-
cence intensity of probe DOQED in the absence and presence of Cu2+ at
various pH conditions was measured for exploitation of practical applica-
tions of the probe. The fluorescence responses towards Cu2+ can be
employed in a wide pH range from 5.0 to 12.0 (Fig. S12).
The fluorescence titration of probe DOQED towards representative
metal ions and its selectivity for Cu2+ were investigated (Fig. 1C).
When probe DOQED was treated with a variety of metal ions, none of
them, except Cu(ClO4)2, caused prominent fluorescence quenching at
420 nm. Furthermore, the competition experiment was carried out by
adding 1 equiv. of Cu2+ to the solution of probe DOQED (10 μΜ) in
the presence of 10 equiv. of other metal ions. There was little interfer-
ence from other ions for Cu2+ to quench the emission. Taken together,
probe DOQED is a highly selective probe for Cu2+
.
Since sulfide is known to react with copper ions to form a very
stable CuS species, which has a low-solubility product constant
Ksp = 6.3 × 10−36 [13]. We subsequently examined the fluorescence
response of [Cu(DOQED)]2+ complex to S2− in a PBS buffer. As shown
in Fig. 3A, upon addition of S2− (0–5 equiv.) to a solution of probe
DOQED (10 μM) in the presence of 10 μM Cu2+, the fluorescence inten-
sity was gradually recovered. The fluorescence intensity was enhanced
by S2− due to the fact that S2− can coordinate with Cu2+ to form very
stable CuS, and thus release the free probe. We also determined the
fluorescence signal of the probe with different anions. As shown in
Fig. 3B, complex [Cu(DOQED)]2+ (10 μM) showed highly selectivity
for S2− (20 μM) over other common anions (100 μM), such as for NaF,
NaCl, KBr, KI, Na2SO3, NaHSO3, Na2S2O3, NaHSO4, Na2SO4, NaAc, KNO3,
Na2CO3, NaHCO3, NaNO2, Na3PO4, KH2PO4, NaH2PO2, Na2HPO4, KClO4,
KBrO3. In addition, complex [Cu(DOQED)]2+ could detect S2− without
interference even in the presence of these common anions. All these
result suggest that the [Cu(DOQED)]2+ complex is a practical probe
for detection of S2− with high selectivity.
Fig. 1. (A) UV–vis spectra changes of the probe DOQED (10 μΜ) upon addition of Cu2+
(0–12 μΜ); (B) the fluorescence changes of the probe DOQED (10 μΜ) upon addition of
Cu2+ (0–15 μΜ), Inset: A linear correlation between fluorescence intensity and concen-
trations of Cu2+; (C) The fluorescence responses of DOQED to various cations. The black
bars represent the fluorescence intensity of DOQED (10 μΜ) in the presence of various
cations (100 μΜ), the red bars represent the changes of fluorescence intensity that occur
upon the subsequent addition of Cu2+ (10 μΜ) to above solution.
in Fig. 1B). The results indicated that probe DOQED coordinated
with Cu2+ with 1:1 stoichiometry, which is consistent with the result
from the Job’s plot (Fig. S10). Moreover, the single crystal of the
[Cu(DOQED)]2+ complex [10] was obtained to understand the coordina-
tive geometry between probe DOQED and Cu2+. In Fig. 2, it can be
clearly seen that the probe complexes with Cu2+ in a 1:1 binding
mode, and Cu(II) is hexa-coordinated with two oxygen atoms (O6, O9),
three nitrogen atoms (N2, N3, N4) from DOQED, and other nitrogen
In conclusion, the probe DOQED shows high selectivity towards
Cu2+ via forming a [Cu(DOQED)]2+ complex, which can be subse-
quently used in recognition of S2− without suffering from other anion
interference. Since all of our tests were performed in pure aqueous

L. Zhang et al. / Inorganic Chemistry Communications 35 (2013) 311–314
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We thank the National Natural Science Found of China (21002047,
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