Polymer-based fluorescent sensor incorporating 2,2′-bipyridyl and benzo[2,1,3]thiadiazole moieties for Cu2+ detection

Article abstract of DOI:10.1016/j.inoche.2011.07.014

The polymer was synthesized by the polymerization of 4,4′-((4- iodophenoxy)methyl)-2,2′-bipyridine (M-1) with 4,7-diethynylbenzo[2,1,3]- thiadiazole (M-2) via Pd-catalyzed Sonogashira reaction. The fluorescence responses of the polymer towards various tra

Full text of DOI:10.1016/j.inoche.2011.07.014

Inorganic Chemistry Communications 14 (2011) 1719–1722
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Polymer-based fluorescent sensor incorporating 2,2′-bipyridyl and
benzo[2,1,3]thiadiazole moieties for Cu2+ detection
a
Yu Dong a, Biyhaliy Koken b, Xiao Ma a, Lu Wang a, Yixiang Cheng a, , Chengjian Zhu
⁎
a
Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
b
Deaprtment of Enviromental and Life Resounce, Ili Normal University, Ili, Xinjiang 833200, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The polymer was synthesized by the polymerization of 4,4′-((4-iodophenoxy)methyl)-2,2′-bipyridine (M-1)
with 4,7-diethynylbenzo[2,1,3]-thiadiazole (M-2) via Pd-catalyzed Sonogashira reaction. The fluorescence
responses of the polymer towards various transition metal ions were investigated by fluorescence spectra and
UV–Vis spectra. The polymer shows green fluorescence under ultraviolet lamp, which can be quenched by Cu2+
completely due to the heavy metal effect. The results suggested that the polymer can be used as a Cu2+-selective
sensor via a naked eyes detecting mode.
Received 21 June 2011
Accepted 18 July 2011
Available online 22 July 2011
Keywords:
Polymer
Fluorescent sensor
Benzo[2,1,3]thiadiazole
Cu2+ detection
© 2011 Elsevier B.V. All rights reserved.
Cu2+ ranks the third in abundance among the essential heavy
metals and is a vital trace element in human bodies. It plays a crucial
role in various fundamental physiologic processes ranging from
haemopoiesis to enzyme-catalyzed reactions and redox processes
[1]. Disorders of Copper metabolism could lead to an increasing risk of
various neurodegenerative diseases, such as Alzheimer's, Parkinson's,
Menke's and Wilson's diseases [2]. On the other hand, Cu2+ is known
as one of the most important ecological pollutions around the world
[3]. Therefore, an intense effort has been placed on design and
synthesis of highly sensitive chemosensors for Cu2+[4], such as laser
ablation inductively coupled plasma mass spectrometry [4d], UV
Spectroscopy [4f], Atomic Emission Spectroscopy [4g], potentiometry
[4h,i], and so on. Of these sensors, fluorescent chemosensors have
distinct advantages over other types due to their simplicity, high
sensitivity and fast response time [5].
Due to the electron-withdrawing properties and high fluorescence
quantum yield both in solution and in the solid state, benzo[2,1,3]
thiadiazole (BTD) has been used for developing optoelectronic materials
during the past few years [6], such as constructing D–π–A–π–D-type
fluorescent dyes, developing liquid crystal displays (LCDs) and light-
emitting diodes [7]. More recently, molecules incorporating BTD moieties
have found their applications in the field of fluorescence probing and
imaging due to their attractive and tunable fluorescence properties [8],
and several BTD-based polymers were designed and applied as metal-ion
sensor and biosensor [9]. Pyridyl and bipyridyl ligands have been
extensively used as an excellent metal chelating ligand in a variety of
approaches dealing with structural coordination chemistry or functional
systems based on bipyridine containing metal complexes because of their
redox active behavior, luminescent properties, and supramolecule
forming reactivity [10]. Dennis [11a] and Zhang [11b] recently reported
two 2,2′-bipyridine based fluorescence sensors for Zn2+ detection. Some
fluorescent sensors have limitation of distinguishing Cu2+ from Hg2+
under no external stimuli [4k]. To our best knowledge, very few polymer-
based fluorescence sensor incorporating bipyridyl moieties as binding site
was reported [10c, d].
An advantage of fluorescent polymers over small molecules is that
signal amplification occurs from electronic communication along the
polymer backbone [12]. As a result, a single polymer provides
enhanced optical response relative to one of its monomer units.
During the last few years, our interest has been focused on the
designing of fluorescence polymer sensors and their applications in
metal ions detection and chiral molecule recognition [13]. In this
paper, we described the synthesis of a non-conjugated fluorescence
polymer incorporating 2,2′-bipyridyl and benzo [2,1,3] thiadiazole
moieties in the polymer main chain backbone by Pd-catalyzed
Sonogashira coupling reaction. The responsive optical properties of
the polymer sensor on various metal ions were investigated by
fluorescence and UV–Vis spectra. The results show that the polymer
exhibits bright green under ultraviolet lamp, which could be
quenched completely by Cu2+, rather than other cations (Na+, K+
,
Mg2+, Ca2+, Cr3+, Fe3+, Zn2+, Ag+, Cd2+, Hg2+, and Pb2+). More
importantly, the color changing process could be easily detected by
naked eyes under UV lamp. The results also indicate that this kind of
non-conjugated polymer incorporating 2,2′-bipyridyl moiety as a
recognition site could exhibit high sensitivity and selectivity for Cu2+
detection.
⁎
Corresponding author.: Tel.: +86 83686509; fax: +86 25 83317761.
E-mail address: yxcheng@nju.edu.cn (Y. Cheng).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.07.014

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Y. Dong et al. / Inorganic Chemistry Communications 14 (2011) 1719–1722
100
90
80
70
60
50
4,4′-((4-iodophenoxy)methyl)-2,2′-bipyridine (M-1) was synthe-
sized from the starting material 4,4′-methyl-2,2′-bipyridine by a two-
step reaction according to reported literatures [14]. 4,7-bis(ethynyl)-
2,1,3-benzothiadiazole (M-2) was prepared through a two-step
reaction from the starting material 5-bromosalicylaldehyde according
to a reported method [15]. The non-conjugated polymer incorporat-
ing 2,2′-bipyridyl and benzo[2,1,3]thiadiazole Moieties was obtained
by a typical Sonogashira reaction between M-1 and M-2 as a brown
powder in 63.0% yield (Scheme 1). Relative molecular weight of
polymer was determined by GPC with Waters-244 HPLC pump and
THF was used as solvent and polystyrene as standards. Mw, Mn and
PDI of the polymer are 6710, 5590 and 1.20, respectively. The non-
conjugated polymer is an air stable powder with yellow color and
shows good solubility in common organic solvents, such as toluene,
acetone, ethyl acetate, THF, CHCl3 and CH2Cl2, which facilitates its
application in the sensory system. Meanwhile, the 2,2′-bipyridyl
moieties can form stable complexes with various metal ions due to the
potentially non-hindering bidentate N2 donor.
60 120 180 240 300 360 420 480 540 600 660
Temperature (oC)
According to thermal analysis of the polymer (Fig. 1), it has a high
thermal stability with 5% loss of weight from 20 to 325 °C. An
accelerated degradation was observed at the temperature ranging
from 325 °C to 570 °C, and then the polymer tends to complete
decomposition at 680 °C with a total loss of about 48%. Therefore, the
chiral polymer is stable enough and can provide desirable thermal
property for practical application as a fluorescence sensor.
Fig. 2 illustrates the absorption spectra (a) and emission spectra
(b) of polymer, model compounds 1 and 2, respectively. UV spectrum
of model compound 1 shows a maximum absorption at 280 nm, and
UV spectrum of model compound 2 exhibits two absorptions at
311 nm and 440 nm; compared to model compound 2, the two
absorption peaks of polymer are blue shifted to 294 nm and 430 nm
Fig. 1. TGA of the polymer sensor.
due to the electric drawing character of bipyridine group. The
emission spectrum of polymer (515 nm) also shows a blue shift
compared with model compound 2 (534 nm).
As shown in Fig. 3, the UV spectrum of polymer shows a maximal
absorption at 294 nm and a broad band centered at 430 nm before
titrations in THF. With the increasing amount of Cu2+, UV–Vis spectra at
430 nm remain unchanged, but new peaks arise at 311 nm. This
phenomenon may be attributed to 2,2′-bipyridyl moiety in the backbone
of the non-conjugated polymer, where the binding event with Cu2+
happens. To make it more clearly, the UV-titration experiment of model
Scheme 1. Synthesis procedures of model compounds 1, 2 and chiral polymer sensor.

Y. Dong et al. / Inorganic Chemistry Communications 14 (2011) 1719–1722
1721
1.0
a
311nm
polymer
Model compound 2
Model compound 1
294nm
280nm
0.3
0.2
0.1
0.0
0.8
0.6
440nm
0.4
430nm
0.2
0.0
+
2+
+
2+ 3+
Ca Cr
3+
Fe
2+ 2+
Cu Zn
2+
Ag
2+ 2+ 2+
Hg Cd Pb
Na Mg
K
Fig. 4. Fluorescence quenching efficiencies of the polymer in the presence of various
transition metal ions (2:1).
250
300
350
400
450
500
Wavelength(nm)
compound 1 shows a similar increase as Fig. 3 at 310 nm due to the
coordination between 2,2′-bipyridyl moiety and Cu2+
.
b
900
800
700
600
500
400
300
200
100
0
515nm
The fluorescence titration of the polymer with various metal ions
was conducted to check its selectivity. Fig. 4 shows the quenching
efficiencies of various metal ions for the polymer sensor. Addition of
1.0 eq Na+, Mg2+, K+, Ca2+, Cr3+, Fe3+, Ag2+, Hg2+, Cd2+, and Pb2+
causes fluorescence quenching less than 0.2 (1−F/F0), Zn2+ leads to
a fluorescence quenching efficiency of 0.23. However, in the presence
of 0.5 eq Cu2+, a completely quenching is observed. Fluorescence
titration of model compound 2 was also conducted under the same
condition (SI, Fig. S3), and no obvious fluorescence response can be
observed from the titration spectra of model compound 2, which also
proves that benzo[2,1,3]thiadiazole unit does not bind Cu2+ in this
experiment. In addition, water has a quenching effect on the
fluorescence of this polymer sensor (10 μM in THF), along with a
red shift from 516 to 539 nm due to the hydrogen bond formation
between water and the polymer (SI, Fig. S4).
534nm
Polymer
Model compound 2
We further studied the fluorescence response of polymer to an
increasing amount of Cu2+. The polymer shows a strong fluorescence
emission centered at 515 nm. With the addition of Cu2+ from 0.05 to
0.50 molar ratio, the fluorescence intensity was quenched steadily
with almost no wavelength changed due to the heavy metal effect
(Fig. 5). The result indicates that binding ratio between ligand unit
480 500 520 540 560 580 600 620 640
Wavelength(nm)
Fig. 2. (a): UV spectra of polymer, model compounds 1 and 2; (b): emission spectra of
polymer and model compound 2 (10 μM in THF).
compounds 1 and 2 was conducted (SI, Figs. S1 and S2). Almost no change
is observed in UV spectra in the absence and presence Cu2+ for model
compound 2, which is in accordance with Fig. 3; UV titration of model
900
800
Cu2+
0.35
700
Blank
0.05 eq
0.10 eq
0.15 eq
600
500
400
300
200
100
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.20 eq
0.25 eq
0.30 eq
0.35 eq
0.40 eq
0.45 eq
0.50 eq
0.60 eq
0.70 eq
0
460 480 500 520 540 560 580 600 620 640
Wavelength (nm)
250
300
350
400
450
500
Fig. 5. Fluorescence spectra of the polymer (1.0×10−5 mol/L) with increasing amounts
of Cu2+ (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 7.0×10−6 mol/L)
(λex =449 nm); Inset: visible emission changes observed: 10 μM polymer solution in
THF, left; in the presence of Cu2+, right.
Wavelength (nm)
Fig. 3. UV–Vis spectra of the chiral polymer (1.0×10−5 mol/L) with increasing amounts
of Cu2+ (0.05–0.70 equiv.).

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,
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Nos. 20832001 and 21074054), National Basic
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at doi:10.
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