A novel intramolecular charge transfer fluorescent chemosensor highly selective for Cu2+ in neutral aqueous solutions

Article abstract of DOI:10.1007/s11458-010-0114-8

A selective and sensitive intramolecular charge transfer (ICT) fluorescent chemosensor was designed for Cu²⁺ in neutral aqueous solutions of pH 7. 0. The design of this totally water-soluble fluorescent chemosensor was based on the binding motif of Cu²⁺ to aminoacid, which is coupled to an ICT fluorophore bearing a 1,3,4-thiodiazole moiety in the electron acceptor. The formation of a 1:1 complex of Cu²⁺ to 2 was suggested to lead to fluorescence quenching. The quenching obeyed Stern-Volmer theory in neutral aqueous solution of pH 7. 0 for Cu²⁺ over 5. 0 × 10^-7 to 3. 0 × 10^-5 mol·L^-1, with a quenching constant of 1. 8 × 10⁵ L·mol^-1 and a detection limit of 2. 0 × 10^-7 mol·L^-1. The binding of Cu²⁺ to 2 can be fully reversed by addition of chelator EDTA, affording a reversible sensing performance.

Full text of DOI:10.1007/s11458-010-0114-8

Front. Chem. China 2010, 5(2): 178–183
DOI 10.1007/s11458-010-0114-8
RESEARCH ARTICLE
applications to environmental and biologic systems, chemo-
sensors are expected to exhibit high selectivity and sensitivity
in aqueous solutions at neutral pH with no organic cosolvents
[27–41]. In recent years, we have seen several such Cu2+
chemosensors by using polymeric microspheres, quantum
dots, and nanomaterials instead of convenient small organic
compounds [27–30]. The reported fluorescent organic che-
mosensors for Cu2+ under physiologic conditions are mostly
based on peptidyl structures [31–35] or following hydrolysis
reactions [36]. The latter can be traced back to the first
example of so-called chemodosimeter for Cu2+ released by
Czarnik et al. that underwent a hydrolysis reaction in the
presence of Cu2+ [4]. These chemodosimeters provided good
Cu2+ selectivity, whereas required a reaction-duration. As the
chemical reaction is involved, the fluorescent signaling in
many cases is irreversible. Small-molecule chemosensors that
operate under physiologic conditions remain rare [37–41]. For
real-time monitoring of Cu2+, a selective yet reversible
small-molecule based fluorescent chemosensor would be
preferred.
The design of chelating ligands for selective complexation
of metal ions has developed very well due to the exciting
developments of supramolecular chemistry. It is known that
a-amino acid is a good chelating ligand for Cu2+, we
therefore designed an intramolecular charge transfer (ICT)
fluorescent chemosensor (2 in Scheme 1) bearing an
aminodiacetate moiety. The latter is linked to the electron
acceptor of the ICT fluorophore [42] by a 1,3,4-thiodiazole in
which S atom was expected to participate in coordination with
Cu2+, considering the thiophilic character of Cu2+. Previously
we have reported that 1 (Scheme 1), the precursor of 2,
showed a highly selective response in its emission toward
Hg2+ [42]. By introducing two acetate terminals into 1, 2 is
totally soluble in water and shows a highly sensitive and
selective response toward Cu2+ in neutral buffered media in a
fluorescence quenching mode. The binding of Cu2+ to 2 and
the accompanied fluorescence signaling are fast. These
processes can be fully reversed by using chelator EDTA,
implying a similar binding motif of 2 to that of EDTA toward
Cu2+.
A novel intramolecular
charge transfer fluorescent
chemosensor highly
selective for Cu2+ in neutral
aqueous solutions
Qingxian LIAO1,2, Aifang LI1, Zhao LI1,
Yibin RUAN1 and Yunbao JIANG (✉)1
A selective and sensitive intramolecular charge transfer
(ICT) fluorescent chemosensor was designed for Cu2+ in
neutral aqueous solutions of pH 7.0. The design of this
totally water-soluble fluorescent chemosensor was
based on the binding motif of Cu2+ to aminoacid,
which is coupled to an ICT fluorophore bearing a 1,3,4-
thiodiazole moiety in the electron acceptor. The
formation of a 1:1 complex of Cu2+ to 2 was suggested
to lead to fluorescence quenching. The quenching
obeyed Stern-Volmer theory in neutral aqueous
solution of pH 7.0 for Cu2+ over 5.0 ꢀ 10–7 to 3.0 ꢀ
10–5 mol$L–1, with a quenching constant of 1.8 ꢀ
105 L$mol–1 and a detection limit of 2.0 ꢀ 10–7 mol$L–1.
The binding of Cu2+ to 2 can be fully reversed by
addition of chelator EDTA, affording a reversible
sensing performance.
Keywords fluorescent chemosensor, Cu2+, fluorescence
quenching, intramolecular charge transfer, 1, 3, 4-thiodia-
zole
1 Introduction
Copper is one of the essential trace elements for human health,
playing a vital role in human metabolism process [1]. Facile
determination of copper under physiologic conditions is
therefore highly demanded. There are many chromogenic and
fluorometric approaches reported for the determination of
copper [2]. Most of them operated in aqueous solutions
containing organic cosolvents [3–26]. To extend the
2 Experimental
2.1 Apparatus
All fluorescence measurements were carried out on a Hitachi
F-4500 spectrofluorometer with excitation and emission slits
set at 5.0 nm. Fluorescence quantum yield was measured
using quinine sulfate as a standard (Ff = 0.546 in 0.5 N
H2SO4) [43]. Absorption spectra were taken on a Varian
CARY-300 UV-Vis spectrophotometer. IR spectra were
Received February 25, 2010; accepted March 15, 2010
1. Department of Chemistry, College of Chemistry and Chemical
Engineering, and Key Laboratory of Analytical Sciences, Xiamen
University, Xiamen 361005, China
2. Xiamen Water Group Co. Ltd., Xiamen 361008, China
E-mail: ybjiang@xmu.edu.cn
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2010

A novel intramolecular charge transfer fluorescent chemosensor highly selective for Cu2+ in neutral aqueous solutions
179
Scheme 1 Synthetic procedures for 2.
obtained using an Nicolet AVATAR FT-IR360 spectrophot- 2.4 Absorption and fluorescence measurements
1
ometer (KBr disks). H NMR and 13C NMR spectra in D2O
Absorption and fluorescence titrations were performed
by adding aliquot Cu2+ to a 2 mL solution containing
10 mmol$L–1 2 and 10 mmol$L–1 Tris-HCl buffer of pH 7.0.
Absorption and fluorescence spectra were recorded after well
were recorded on a Varian Unity 400 MHz NMR spectro-
meter. Mass spectra were obtained from a Bruker Micromass-
LCT spectrometer. Solution pH was monitored by a Mettler
Toledo 320 pH meter calibrated by standard solutions. All of
the experiments were carried out at room temperature (25°C).
mixed. Fluorescence intensity was measured at lex / lem
=
355 nm / 428 nm.
2.2 Reagents
3 Results and discussion
Solvents and reagents used for the synthesis of the
chemosensor were commercially available at AR (analytical
reagent) grade. Deionized water was used throughout the
experiment. Inorganic nitrate salts were of the highest
available purity and were used for preparing their aqueous
solutions.
3.1 Absorption spectra
The absorption spectra of 2 in the presence of various
concentration of Cu2+ are shown in Figure 1. It was found that
the major absorption peak of 2 was blue-shifted from 350 to
339 nm upon the addition of Cu2+, while a tail extending
beyond 400 nm was developed. In the titration traces, two
clear isosbestic points were observed at 355 and 396 nm,
2.3 Synthesis
Chemosensor 2 was synthesized according to Scheme 1
[42,44].
Sodium 2,2′-5-(4-(dimethylamino)phenyl)-1,3,4-thiodia-
zol-2-ylazanediyl)diacetate (2). KI (0.67 g, 4 mmol) and
K2CO3 (0.71 g, 4 mmol) were added into 1 (0.22 g, 2 mmol)
[42] in acetonitrile (CH3CN, 15 mL) . The resulting mixture
was stirred at room temperature while 2 equivalents of ethyl
bromoacetate (0.67 g, 4 mmol) were slowly added into the
mixture. Refluxing at 70°C was allowed for 4 h. After thin
layer chromatography (TLC) indication of the completion of
reaction, the mixture was filtered and solvent was evaporated
under reduced pressure. Into the residue dissolved in 50 mL
ethanol, NaOH (0.3 g, 7.5 mmol) in 2 mL water was added.
The resulting solution was refluxed for 3 h. Upon cooling,
pale yellow solid was precipitated and was washed, affording
1
2 (0.3 g) in 80% yield. H NMR (D2O, 400 MHz), d (ppm):
Figure 1 UV-Vis absorption spectra of 2 upon the addition of
increasing concentration of Cu2+ in 10 mmol$L–1 Tris-HCl buffer
(pH 7.0). [2] = 1.0ꢀ10–5 mol$L–1. Cu2+ ion concentrations were
(1) 0 (2) 5.0ꢀ10–6 mol$L–1, (3) 1.0ꢀ10–5 mol$L–1, (4) 2.0ꢀ
10–5 mol$L–1, (5) 4.0ꢀ10–5 mol$L–1, (6) 7.5ꢀ10–5 mol$L–1, and
(7) 1.0ꢀ10–4 mol$L–1, respectively.
2.87 (s, 6H), 3.77 (s, 2H), 4.48 (s, 2H), 6.85 (d, J = 8.8 Hz,
2H), 7.50 (d, J = 8.8 Hz, 2H). 13C NMR (D2O), d (ppm):
40.0,51.0, 59.6, 113.3, 118.5, 127.2, 148.4, 152.5, 162.4,
175.2, 178.0. HRMS(ESI): calcd for [C14H17N4 O4S+] m/z
337.0971, found (M + H+, H2O) 337.0976.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

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Qingxian LIAO, Aifang LI, Zhao LI, Yibin RUAN and Yunbao JIANG
respectively. This is an indication of the formation of well-
defined complex between Cu2+ and 2. On the basis of the
structure of 2, the appearance of red-shifted absorption is
indicative of an increase in the electron accepting ability of the
electron acceptor. It is understandable as Cu2+ would bind at
its aminodiacetate moiety among others and would agree with
the observed red-shift in the absorption spectrum of 1 in the
presence of Hg2+ [42].
3.2 Fluorescence properties
Fluorescence of 2 was investigated at a physiologic pH 7.0 in
Tris-HCl buffer. In the absence of Cu2+, the fluorescence
quantum yield of 2 was determined to be 0.0572, which is
lower than that of 1 [42], suggesting the occurrence of ICT in
2 in aqueous solution. The ICT character of the emissive state
of 2 was confirmed by a solvatochromism measurement.
Figure 2 shows that the fluorescence spectrum of 2
continuously shifts to the red in CH3CN-water binary mixture
when water content is increased, being 408 nm in CH3CN and
428 nm in Tris-HCl buffer solution. In the presence of Cu2+,
fluorescence of 2 in Tris-HCl buffer (pH 7.0) was found
quenched (Figure 3). No band shift was observed, which
means that the formation of a nonfluorescent complex of 2
with Cu2+ is responsible for the quenching.
Figure 3 Fluorescence spectra of 2 in the presence of Cu2+ in
10 mmol$L–1 pH 7.0 Tris-HCl buffer. Cu2+ ion concentrations
were (1) 0, (2) 5.0ꢀ10–7 mol$L–1, (3) 1.0ꢀ10–6 mol$L–1, (4)
2.0ꢀ10–6 mol$L–1, (5) 3.0ꢀ10–6 mol$L–1, (6) 4.0ꢀ10–6 mol$L–1
,
(7) 6.0ꢀ10–6 mol$L–1
,
(8) 7.0ꢀ10–6 mol$L–1
,
(9) 8.0ꢀ
10–6 mol$L–1, (10) 1.0ꢀ10–5 mol$L–1, (11) 1.2ꢀ10–5 mol$L–1
,
(12) 1.5ꢀ10–5 mol$L–1, (13) 2.0ꢀ10–5 mol$L–1, (14) 3.0ꢀ
10–5 mol$L–1, (15) 5.0ꢀ10–5 mol$L–1, (16) 1.0ꢀ10–4 mol$L–1
,
(17) 1.25ꢀ10–4 mol$L–1, (18) 1.5ꢀ10–4 mol$L–1, (19) 2.0ꢀ
10–4 mol$L–1, and (20) 2.5ꢀ10–4 mol$L–1, respectively. [2] =
1.0ꢀ10–5 mol$L–1. Excitation was chosen at an isosbestic
wavelength of 355 nm.
Figure 4 Plots of I0/I of 2 versus Cu2þ concentration in aqueous
solution of varied pH. I0 and I are fluorescence intensities of 2 in
the absence and presence of Cu2þ, respectively.
Figure 2 Normalized fluorescence spectra of 2 in CH3CN and
Tris-HCl buffer binary mixture.
3.3 Linear range and selectivity
optimal results at pH 7.0. Tris-HCl buffer of pH 7.0 was
therefore employed. The interaction of 2 with Cu2+ was found
instant and the fluorescence intensity remained strictly
unaltered within 1 h. The concentration of 2 was varied to
achieve a credible and wide dynamic range in which Stern-
Volmer quenching can be observed. It can found that 2 at 1.0
ꢀ10–5 mol$L–1 was the optimal choice. Fluorescence response
In order to optimize experimental conditions, several possible
factors affecting the measurement of Cu2+ were examined in
terms of chemosensor concentration, buffer solution pH, and
reaction time. Experiments showed that the extent of quench-
ing was highest at pH 7.0 (Figure 4). Tested buffer systems
such as HEPES, Tris-HCl, and phosphates all produced
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A novel intramolecular charge transfer fluorescent chemosensor highly selective for Cu2+ in neutral aqueous solutions
181
of 2 toward Cu2+ in terms of quenching factor was found
linear to Cu2+ concentration ranging from 5.0 ꢀ 10–7 to 3.0 ꢀ
10–5 mol$L–1 (I0/I = 0.85 + 1.81 ꢀ 105 ꢀ [Cu2+], R = 0.9989,
Figure 3). The detection limit of the chemosensor is defined
by the concentration of Cu2+ giving a signal equal to the sum
of blank signal and three standard deviation of the blank
[45,46]. It was 2.0 ꢀ 10–7 mol$L–1. The tested foreign metal
cations (except Ni2+) showed practically no interference
(Figure 5). This observation suggests that 2 is a highly
sensitive and selective fluorescent chemosensor for Cu2+ in
neutral buffered solutions.
Figure 6 The Job plot for Cu2+ binding to 2 in Tris-HCl
buffer (pH 7.0). Total concentration of 2 and Cu2+ was 2.0ꢀ
10–5 mol$L–1
.
consequence, quenching of the fluorescence of 2 would be
expected due to a substantially enhanced ICT in the 2-Cu2+
complex, as observed experimentally (Figure 3). Since Cu2+
is a heavy transition metal cation, quenching of the
fluorescence of 2 by Cu2+ can also be due to a heavy atom
effect. Although this seems to be less likely in current case as
the other heavier Hg2+ does not quench the fluorescence of 2
under the same conditions (Figure 5), at this stage we are
unable to completely rule out this possibility for Cu2+. It was
interesting to note that the binding of Cu2+ to 2 and the
resulting fluorescence quenching can be fully reversed by
adding EDTA (Figure 7). Again this is indicative of the
formation of a nonfluorescent complex of 2-Cu2+, which is
responsible for the fluorescence quenching and a similar
binding motif of 2 to EDTA toward Cu2+. This reversibility
also renders the sensing system reversible. It is significant to
Figure 5 Quenching factor I0/I of 2 of individual metal ion in
pH 7.0 Tris-HCl buffer solution. Cu2+ (1.0ꢀ10–5 mol$L–1),
Ca2+, Mg2+, Ba2+ (4.0ꢀ10–4 mol$L–1), Co2+, Hg2+, Zn2+
(2.0ꢀ10–5 mol$L–1), Cd2+ (5.0ꢀ10–5 mol$L–1), Ni2+ (1.0ꢀ
10–5 mol$L–1). I and I0 are fluorescence intensities of 2 in the
presence and absence of metal ion, respectively. [2] = 1.0ꢀ
10–5 mol$L–1
.
3.4 Quenching mechanisms
The observation of isosbestic points in the absorption titration
traces suggested the formation of a well-defined complex
between 2 and Cu2+, which we confirmed from the Job plot to
be of 1∶1 stoichiometry (Figure 6). To achieve this 1∶1
binding stoichiometry, carbonyl O and amino N of 2 are the
most possible binding sites for Cu2+. Actually, IR measure-
ments showed that the carbonyl stretching band shifted from
1608 cm–1 in 2 to 1598 cm–1 in 2 + Cu2+, which was in
agreement with the expected coordination of aminodiacetate
moiety with Cu2+ [37,38,41]. With 1, it was suggested that
both the 2-amino N atom and the S atom were involved in its
coordination with Hg2+ [42]. Considering a similar thiophilic
character of Cu2+ to Hg2+, S atom in 2 was assumed to be
coordinating with Cu2+. This would strengthen the electron
acceptor of 2 upon the Cu2+ binding, the red-shifted
absorption tail might be the result of this enhancement. As a
Figure 7 (a) Absorption and (b) fluorescence spectra of 2 in the
absence and presence of Cu2+ and Cu2+ plus EDTA.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

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Qingxian LIAO, Aifang LI, Zhao LI, Yibin RUAN and Yunbao JIANG
point out that by slight modification of the structure of 1 into
2, we successfully tune the response selectivity from Hg2+ to
Cu2+. As there are both N and S coordination centers in the
1,3,4-thiodiazole electron acceptor in the structural frame-
work of 1 and 2, diverse structural motifs can be created to act
as fluorescent chemosensors for given metal cations. This
shall be the subject of continuing effort.
supramolecular fluorescent chemosensing and biointermolecular
interactions.
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