Polymer-based fluorescence sensor incorporating triazole moieties for Hg2+ detection via click reaction

Article abstract of DOI:10.1016/j.polymer.2010.05.001

The polymer could be obtained by the polymerization of 1,4-dibutoxy-2,5-diethynylbenzene (M-1) with 1,4-diazidobenzene (M-2) via click reaction. The polymer show blue fluorescence. The responsive optical properties of the polymer on various transition metal ions were investigated by fluorescence spectra. Compared with other cations, such as Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺ and Zn²⁺, Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer due to triazole moiety in the polymer main chain as the metal binding ligand. The results indicate this kind of conjugated polymer with triazole moiety synthesized by click reaction can be used as a selective fluorescence sensor for Hg²⁺ detection.

Full text of DOI:10.1016/j.polymer.2010.05.001

Polymer 51 (2010) 3064e3067
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Polymer-based fluorescence sensor incorporating triazole moieties for Hg2þ
detection via click reaction
,
b,
**
Xiaobo Huang a, Jie Meng a, Yu Dong 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 State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The polymer could be obtained by the polymerization of 1,4-dibutoxy-2,5-diethynylbenzene (M-1) with
1,4-diazidobenzene (M-2) via click reaction. The polymer show blue fluorescence. The responsive optical
properties of the polymer on various transition metal ions were investigated by fluorescence spectra.
Compared with other cations, such as Co2+, Ni2+, Ag+, Cd2+, Cu2+ and Zn2+, Hg2+ can exhibit the most
pronounced fluorescence response of the polymer Hg2+ can exhibit the most pronounced fluorescence
response of the polymer due to triazole moiety in the polymer main chain as the metal binding ligand.
The results indicate this kind of conjugated polymer with triazole moiety synthesized by click reaction
can be used as a selective fluorescence sensor for Hg2+ detection.
Received 31 January 2010
Received in revised form
21 April 2010
Accepted 4 May 2010
Available online 11 May 2010
Keywords:
Click reaction
fluorescence sensor
mercury detection
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Click reaction established by Sharpless and co-workers is an
efficient tool for the preparation of wide-ranging organic
Conjugated polymers (CPs) used as fluorescence sensors have
been gained increased attention due to their high sensitivity and
ease of measurement. The high sensitivity of the conjugated
polymers originates from the sensory signal amplification which
attributed to energy migration along the polymer backbone upon
light excitations. An additional advantage of polymer-based fluo-
rescence sensors is that the structure and sequence of the conju-
gated polymers can be systematically modified to suit for diverse
targets by the introduction of the functional groups based on
steric and electronic property at well-defined molecular level [1,2].
The research on the polymer-based fluorescence sensor for heavy
and transition metal (HTM) ions detection is emerging as an area
of current interest in the recent years [3e5]. Among them,
mercury and its derivatives are dangerous and widespread global
pollutants, and they have caused serious environmental and
health problems [6e9]. Many fluorescence sensors for mercury
detection have been reported, but most of them are based on
small molecules [3,10], and polymer-based chemosensors are very
few [11e19]. Therefore, the development of highly selective and
sensitive polymer-based chemosensors for HTM ions detection is
still attractive.
compounds [20,21]. The Copper-catalyzed azide-alkyne click
reaction (also termed CuAAC) has had enormous impact on the field
of polymer science due to the high efficiency, mild reaction
conditions and technical simplicity of the reaction in the last five
years. Polymer chemists have widely employed click chemistry in
material science and polymer bioconjugation [22e25]. Although
a triazole unit which may be used as a potential transition metal
coordinating ligand is formed in CuAAC [26], very few polymer-
based conjugated fluorescence sensors incorporating triazole
moieties examples have been reported. In this paper, we report the
synthesis of the conjugated polymer incorporating triazole moie-
ties used as fluorescence sensor for transition metal ions detection
via click reaction. Compared with other cations, such as Co2þ, Ni2þ
,
Agþ, Cd2þ, Cu2þ and Zn2þ, Hg2þ can exhibit the most pronounced
fluorescence response of the polymer using triazole moiety as the
metal binding sites. The results indicate this kind of polymer with
triazole moiety based on click reaction can be used as a selective
fluorescence sensor for Hg2þ detection.
2. Experimental part
2.1. Materials
* Corresponding author. Tel.: þ86 25 83685199; fax: þ86 25 83317761.
** Corresponding author.
All solvents and reagents were commercially available and analyt-
E-mail addresses: yxcheng@nju.edu.cn (Y. Cheng), cjzhu@nju.edu.cn (C. Zhu).
ical-reagent-grade. para-Phenylenediol and para-phenylenediamine
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.05.001

X. Huang et al. / Polymer 51 (2010) 3064e3067
3065
were purchased from Aldrich and directly used without purification.
1,4-Diazidobenzene (M-2) could be synthesized from para-phenyl-
enediamine by a 2-step reaction according to reported literature [27].
2.4. Preparation of the conjugated polymer (Scheme 1)
mixture of M-1 (108.0 mg, 0.40 mmol), M-2 (64.0 mg,
A
0.40 mmol), 10 mol% sodium ascorbate (7.92 mg, 0.040 mmol) and
5 mol% Cu2SO4.5H2O (4.99 mg, 0.020 mmol) was dissolved in the
mixed solvents of 10 mL THF, 10 mL t-BuOH, and 10 mL H2O. The
solution was stirred at 30e36 ꢀC for 2 days under N2. The solvents
were removed under reduced pressure and the residue was
extracted with CHCl3 (2 ꢁ 50 mL). The organic layer was washed
with an aqueous NH4OH solution, water and then dried over
anhydrous Na2SO4. After the solution was removed, the resulting
polymer was precipitated into methanol, and then filtered and
washed with methanol several times. Further purification could be
conducted by dissolving the polymer in CHCl3 to precipitate in
methanol again. The polymer was dried in vacuum to give 138.0 mg
as a yellow solid in 80.2% yield. Mw ¼ 8400, Mn ¼ 7080, PDI ¼ 1.2. 1H
2.2. Measurements
NMR spectra were obtained using a 300-Bruker spectrometer
300 MHz for 1H NMR and 75 MHz for 13C NMR and reported as
parts per million (ppm) from the internal standard TMS. FT-IR
spectra were taken on a Nexus 870 FT-IR spectrometer. UVevis
spectra were obtained from a PerkineElmer Lambda 25 spec-
trometer. Fluorescence spectra were obtained from a RF-5301PC
spectrometer. Thermogravimetric analyses (TGA) was performed
on a PerkineElmer Pyris-1 instrument under N2 atmosphere. MS
was determined on a Micromass GCT. C, H and N of elemental
analyses were performed on an Elementar Vario MICRO analyzer.
Molecular weight was determined by GPC with Waters-244 HPLC
pump and THF was used as solvent and relative to polystyrene
standards. All solvents and reagents were commercially available A.
R. grade.
NMR (300 Hz, DMSO-d6):
d 8.97 (s, 1H), 8.84 (s, 1H), 8.22e7.97 (m,
2H), 7.83e7.22 (m, 4H), 4.35e4.10 (m, 4H), 1.94e1.87 (m, 4H),
1.55e1.46 (m, 4H),1.02e0.94 (m, 4H). FT-IR (KBr, cmꢂ1): 2956, 2931,
2869, 2093, 1525, 1243, 1029. Anal. calcd for C24H26N6O2: C, 66.96;
H, 6.09; N, 19.52. Found: C, 66.76; H, 6.21; N, 19.43.
2.3. Preparation of M-1 (Scheme 1)
2.5. Metal ion titration
1,4-Dibutoxybenzene was synthesized according to a reported
method [28].
A
solution of 1,4-dibutoxybenzene (6.60 g,
Each metal ion titration experiment was started with a 3.0 mL
29.7 mmol), KIO3 (2.55 g, 11.9 mmol) and I2 (8.35 g, 32.9 mmol) in
50 mL acetic acid, 0.9 mL sulphuric acid and 4 mL water was stirred
at 80 ꢀC for 24 h. After cooling, solution of Na2S2O3 was added until
the purple color disappeared, then the reaction mixture was
poured into 100 mL water and extracted with petroleum ether
(3 ꢁ 50 mL). The organic layer was washed with water, and then
dried over anhydrous Na2SO4. The solution was evaporated under
reduced pressure to give 1,4-dibutoxy-2,5-diiodobenzene (10.3 g,
polymer in CHCl3 solution with
a
known concentration
(1.0 ꢁ 10ꢂ5 mol Lꢂ1). Mercury perchlorate salt and other various
metal salts (nitrate, 1.0 ꢁ 10ꢂ3 mol Lꢂ1, CH3CN) were used for the
titration. Polymeremetal complexes were produced by adding
aliquots of a solution of the selected metal salt to a CHCl3 solution of
the polymer. All kinds of measurements were monitored 2 h after
addition of the metal salt to the polymer solutions.
73.6%) as a white solid. 1H NMR (CDCl3, 300 MHz):
d 7.20 (s, 2H),
3. Results and discussion
3.96 (t, 4H, J ¼ 6.6 Hz), 1.83e1.76 (m, 4H), 1.59e1.52 (m, 4H), 1.00 (t,
6H, J ¼ 7.5 Hz).
3.1. Synthesis and feature of the conjugated polymer
A
mixture of 1,4-dibutoxy-2,5-diiodobenzene (3.00 g,
6.33 mmol), Pd(PPh3)4 (366 mg, 0.32 mmol), CuI (241 mg,
1.27 mmol) and trimethylsilyl acetylene (3.58 mL, 25.3 mmol) was
dissolved in 15 mL Et3N and 40 mL THF. The reaction mixture was
stirred at 40 ꢀC for 48 h under a N2 atmosphere. The solution was
cooled to room temperature, and then the solvent was removed
under reduced pressure. The residue was extracted with CH2Cl2
(2 ꢁ 50 mL). The organic layer was washed with cool water, and
then dried over anhydrous Na2SO4. The solution was evaporated
under reduced pressure, and the residue was purified by silica gel
column chromatography (petroleum ether/ethyl acetate) (50:1, v/v)
to give 2,5-bis(trimethylsilylethynyl)-1,4-dibutoxybenzene (2.30 g,
The synthesis procedures of the monomers 1,4-dibutoxy-2,5-
diethynylbenzene (M-1), 1,4-diazidobenzene (M-2) and the
conjugated polymer are outlined in Scheme 1. The monomer M-1
could be obtained by a 4-step reaction from the starting material p-
dihydroxybenzene [28]. M-1 and M-2 could be served as the
monomers for the synthesis of the target polymer. In this paper,
a typical click reaction condition was applied to the synthesis of the
polymer [21,29]. The polymerization could be carried out under
mild reaction conditions in THF/t-BuOH (1:1, v/v) solution in the
presence of a catalytic amount of sodium ascorbate (10%mol) and
CuSO4$5H2O (5%mol) with a good yield as high as 80.2%. The
number-average molecular weight (Mn) and the weight-average
87.8%) as a white solid. 1H NMR (CDCl3, 300 MHz):
d 6.91 (s, 2H),
3.97 (t, 4H, J ¼ 6.3 Hz), 1.84e1.75 (m, 4H), 1.62e1.49 (m, 4H), 0.99 (t,
6H, J ¼ 7.5 Hz). 0.27 (s, 18H). A mixture of 2,5-bis(trimethylsilyle-
thynyl)-1,4-dibutoxybenzene (0.85 g, 2.05 mmol) in 1 mol Lꢂ1 KOH
methanol solution (50 mL) was stirred at room temperature for 1 h.
The completion of the reaction was determined by TLC. The reac-
tion mixture was poured into water (20 mL) and extracted with
CHCl3 (2 ꢁ 30 mL). The combined organic layers were dried over
anhydrous Na2SO4, and then evaporated in vacuo to dryness to give
M-1 (0.50 g, 90.9%) as a pale yellow solid. 1H NMR (CDCl3,
1) n-C H Br, K CO
3
OC H -n
4
9
2
4
9
2) KIO , I , HAc, H SO
4
3
2
2
HO
OH
H
H
3)
H
SiMe , Pd(Ph ) , CuI, Et N
3 3 4 3
4) KOH, CH OH
3
n-C H O
4 9
M-1
1) NaBF , HCl
4
H N
2
NH
N
N
3
2
3
2) NaN
3
300 MHz):
d
6.97 (s, 2H), 4.00 (t, 4H, J ¼ 6.6 Hz), 3.35 (s, 2H),
M-2
1.83e1.78 (m, 4H), 1.56e1.51 (m, 4H), 0.99 (t, 6H, J ¼ 7.2 Hz); 13C
OC H -n
4
9
NMR (75 MHz, CDCl3):
d
153.9, 117.7, 113.2, 82.3, 79.7, 69.2, 31.1, 19.1,
N
N
N
N
CuSO 5H O, NaAscorbate
4
2
13.7. FT-IR (KBr, cmꢂ1): 3270, 2962, 2921, 2861, 1498, 1465, 1402,
1384,1221,1063,1025. MS (EI, m/z): 270 (29%), 214 (6%),158 (100%),
102 (11%), 84 (14%), 49 (23%). Anal. calcd for C18H22O2: C, 79.96; H,
8.20. Found: C, 79.90; H, 8.25.
N
N
M-1
M-2
+
THF/t-BuOH(1:1)
n
n-C H O
4
9
Scheme 1. Synthesis procedures of the polymer sensor.

3066
X. Huang et al. / Polymer 51 (2010) 3064e3067
60000
molecular weight (Mw) of the polymer are 7080 and 8400 with
polydispersity index of 1.2. The GPC result of the polymer shows the
moderate molecular weight. The resulting polymer shows
moderate solubility in common solvents, such as toluene, THF,
CHCl3, and CH2Cl2, which can be attributed to the flexible n-butyl
group substitutents on phenyl units as side chains of the polymer.
TGA of the polymer was carried out under a N2 atmosphere at
a heating rate of 10 ꢀC/min (See Supporting Information, Fig. S1).
The results show the polymer has high thermal stability without
loss weight before 240 ꢀC.
50000
40000
30000
20000
10000
3.2. The selective and sensitive recognition of the polymer sensor on
Hg2þ
0
1
2
3
4
5
[Hg2+] 10-5M
The absorption spectra of the polymer in CHCl3 solution display
two absorption peaks at 273 and 339 nm and the fluorescent
wavelength of the polymer appears at 377 nm with the excitation at
339 nm (See Supporting Information, Fig. S2). The photo-
luminescence efficiency (FPL) of polymer is 0.24 with reference to
quinine sulfate in 0.5 mol Lꢂ1 H2SO4 solution. The effects of the
fluorescence response behaviors of the polymer on various transi-
tion metal ions have been investigated. Fig.1 shows the fluorescence
spectra of the polymer (1.0 ꢁ 10ꢂ5 mol Lꢂ1 in CHCl3) upon the
addition of Hg2þ in CH3CN solution. As is evident from Fig.1, obvious
fluorescence quenching could be observed upon the addition of
Hg2þ. Hg2þ leads to about 74.3% fluorescence quenching for the
polymer at a concentration of 4:1 molar ratio. Moreover, the fluo-
rescent wavelength of the metal-chelated polymer appears the
obvious red shift upon the molar ratio additions from 1:2 to 1:3,
which may be attributed to the conjugation effect between triazole
moiety and phenyl group while triazole moiety in the polymer main
chain coordinates with Hg2þ. As shown in Fig. 2, it can be also found
the fluorescence intensity of the polymer shows gradual decrease
with the concentration mole ratio of Hg2þ from 0 to 5.0. A curve
equation is got in the range of Hg2þ concentrations from 0 to
2.0 ꢁ 10ꢂ5 mol Lꢂ1 is F ¼ 5.65 ꢁ104e9.80 ꢁ 108 [Hg2þ], R ¼ ꢂ0.997,
N ¼ 7 (See Supporting Information, Fig. S3). According to the equa-
tion and the standard deviation of the blank, the fluorescence
detection limit of the polymer solution for Hg2þ was determined to
be as low as 4.6 ꢁ 10ꢂ7 mol Lꢂ1. Fluorescence spectra of the polymer
(1.0 ꢁ 10ꢂ5 mol Lꢂ1) in CHCl3 with increasing amounts of Hg2þ in
CH3CN (5.0 ꢁ 10ꢂ5 mol Lꢂ1) at different time have been investigated
in order to get the response time of the fluorescence sensor to Hg2þ
(See Supporting Information, Fig. S4). The response time of the
polymer sensor to Hg2þ is 2 h.
Fig. 2. Fluorescence intensity of the conjugated polymer vs the increasing concen-
tration of Hg2þ from 1:0 to 1:5 molar ratios.
condition as Hg2þ ion determination (Fig. 3). The fluorescence
intensities of the polymer are not almost affected by the addition
of Cd2þ, Co2þ, Ni2þ, Zn2þ and Cu2þ. Although the fluorescence
quenching of the polymer detected upon adding Agþ ion, the
value of the fluorescence quenching efficiency of the polymer for
Hg2þ ion is approximately 2.8 times higher than that for Agþ ion.
These results indicate that the polymer possesses specific recog-
nition ability for Hg2þ ion. Normally, the excellent fluorescence
sensor with highly sensitive and selective feature need meet one
of the essential requirements which is minor or no interference
from other metal ions. In this paper, we further investigated the
utility of the polymer as an ion-selective fluorescence sensor for
Hg2þ. Herein, we subjected the polymer (1.0 ꢁ 10ꢂ5 mol Lꢂ1) to
a mixture of Hg2þ (4.0 ꢁ 10ꢂ5 mol Lꢂ1) and each of the metals
(4.0 ꢁ 10ꢂ5 mol Lꢂ1) as shown in Fig. 4. The deviations from other
transition metals’ interference are less than 8% in the coexisted
metal experiment. Most importantly, while the fluorescence
response was carried out by the Hg2þ titrations in presence of the
mixed metal ions, such as Co2þ, Ni2þ, Agþ, Cd2þ, Cu2þ and Zn2þ, no
obvious interference was observed in the fluorescence of the
polymer. Herein, the polymer is specifically sensitive toward Hg2þ
,
not toward other metals, which can be arisen from several factors,
such as the structural rigidity of the triazole unit, the larger radius
of the Hg2þ ion and coordination diversification of Hg2þ, its soft
acid property, the binding ability of triazole unit and strong HgeN
binding [17]. Because of its 5d106s2 electronic configuration and
lack of ligand field stabilization energy, Hg(II) can accommodate
The fluorescence response behaviors of the polymer on other
transition metal ions were also examined under the same
Hg2+
80
70
60
50
40
30
20
10
0
800
600
400
200
0
350
400
450
500
550
Wavelength (nm)
Co
Ni
Hg
Ag
Cd
Cu
Zn
Fig. 1. Fluorescence spectra of the polymer (1.0 ꢁ 10ꢂ5 mol Lꢂ1
)
in CHCl3 with
increasing amounts of Hg2þ in CH3CN (0, 0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 3.0, 4.0,
Fig. 3. Fluorescence quenching efficiencies of the polymer (1.0 ꢁ 10ꢂ5 mol Lꢂ1) in the
presence of various transition metal ions (each 4.0 ꢁ 10ꢂ5 mol Lꢂ1).
5.0 ꢁ 10ꢂ5 mol Lꢂ1) (lex ¼ 339 nm).

X. Huang et al. / Polymer 51 (2010) 3064e3067
3067
Acknowledgements
90
80
70
60
50
40
30
20
10
0
This work was supported by the National Natural Science
Foundation of China (No. 20774042) and the National Basic
Research Program of China (2010CB923303).
Appendix. Supporting information
Supplementary data associated with this article can be found in
the online version, at doi: 10.1016/j.polymer.2010.05.001.
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4. Conclusion
The polymer sensor incorporating triazole unit could be
synthesized by click reaction. The responsive properties of the
polymer on various transition metal ions were investigated by
fluorescence spectra. Compared with other cations, such as Co2þ
,
Ni2þ, Agþ, Cd2þ, Cu2þ and Zn2þ, Hg2þ can exhibit the most
pronounced fluorescence response of the polymer due to tri-
azole moiety in the polymer main chain as the metal binding
ligand. The results indicate this kind of polymer with triazole
moiety can be used as a selective fluorescence sensor for Hg2þ
detection. This work can further expand the application of click
reaction in design and synthesis of the novel fluorescence sensor
incorporating triazole unit for metal ion detection.
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