A highly selective and sensitive polymer-based fluorescence sensor for Hg2+-ion detection via click reaction

Article abstract of DOI:10.1002/asia.201100534

Mercury rising: A Highly Selective and sensitive polymer-based fluorescence sensor synthesized using click reactions affords the most-pronounced fluorescence response to Hg²⁺ ions.

Full text of DOI:10.1002/asia.201100534

DOI: 10.1002/asia.201100534
A Highly Selective and Sensitive Polymer-based Fluorescence Sensor for
Hg2+-Ion Detection via Click Reaction
Yuanzhao Wu, Yu Dong, Junfeng Li, Xiaobo Huang, Yixiang Cheng,* and
Chengjian Zhu[a]
On the occasion of the 10th anniversary of click chemistry
Fluorescence sensors have witnessed a tremendous rise in
the number of monitoring biologically relevant species such
as metal ions in the past few years. In particular, numerous
efforts have been devoted to the design of chemosensory
systems with unique electrical and optical properties that
are capable of detecting metal ions in both a real-time and
reversible fashion. Conjugated polymers, when modified by
hydrophilic and functional pendant substituents with fine
structural control on the conjugated main chain, have
showed great potential applications in many fields.[1] Due to
their high sensitivity and selectivity of detection measure-
ment, conjugated polymers (CPs) as fluorescence sensors
have recently attracted increased attentions.[2] Compared to
the original monomers, CPs can amplify fluorescence by
many folds, which is attributed to the fast energy transfer of
excitons along the conjugated backbones. Swager reported
that the delocalizable p-electronic conjugated “molecular
wire” polymer can greatly amplify the fluorescence response
signal due to facile energy migration along the polymer
backbone upon light excitations.[3]
As is known to all, mercury and its derivatives are danger-
ous and widespread, and they can cause serious environmen-
tal and health problems.[4] The most abundant ionic form of
this element is the mercury ion (Hg2+), which can be accu-
mulated in the organs of human or animal bodies through
the food chain.[5] Therefore, the development of highly se-
lective and sensitive chemosensors for the mercury detection
with minor or no interference from other metal ions is of
great interest. However, most of sensing detectors can only
work in organic solvents which prevent their potential appli-
cations in environment and biological systems. Although
mercury ion is relatively easy to be chelated and detected in
organic solvents, it is rather difficult to recognize directly in
aqueous environments due to their strong hydrations. There-
fore, to develop highly selective chemosensors for mercury
ion detection in aqueous solution is especially necessary.
There were many works on fluorescence sensors for mercury
ion detection,[6] but there were few reports on water-soluble
Hg2+ sensors. To the best of our knowledge, there have
been very few reports on polymer-based fluorescence sen-
sors for mercury ion detection in aqueous solution.[7]
Due to the high efficiency, mild reaction condition and
technical simplicity of the reaction, the copper-catalyzed
azide-alkyne click reaction (CuAAC) had enormous impacts
on the polymer science in last ten years. Click chemistry had
been also widely employed in material science and polymer
bio-conjugation applications.[8] The click-generated triazole
unit can act as a covalent linker as well as a coordinating
ligand for metal ions.[9] In our previous work we firstly de-
signed several polymer-based fluorescence sensors incorpo-
rating triazole unit via click reaction for Hg2+ detection.[6p–q]
Even though there were some literatures on fluorescence
sensor based on triazole unit as receptor for metal ion de-
tection,[10] there has been no report on polymer-based fluo-
rescence sensor incorporating triazole unit for Hg2+ detec-
tion in aqueous solution. Herein, a novel conjugated poly-
mer sensor was synthesized by the polymerization of tri
{1,4-diethynyl-2,5-bis(2-(2-methoxyethoxy)-ethoxy)}-benzene
(M-1) and 1, 4-diazidobenzene (M-2) via click reaction. The
fluorescence response behaviors of the polymer sensor on
various metal ions were investigated by fluorescence spectra
in aqueous solution. The result indicated that the polymer
sensor incorporating triazole unit can exhibit the most pro-
nounced fluorescence quenching response to Hg2+ with high
selectivity and sensitivity in aqueous solution (Scheme 1).
The synthesis procedure of the monomers (M-1), 1, 4-dia-
zidobenzene (M-2) and the polymer sensor was depicted in
Scheme 1. The compound 1 was synthesized by a 3-step re-
action from the starting material p-dihydroxybenzene. The
[a] Y. Wu, Y. Dong, J. Li, X. Huang, Y. Cheng, C. Zhu
Key Lab of Mesoscopic Chemistry of MOE
School of Chemistry and Chemical Engineering
Nanjing University
Nanjing 210093 (P.R. China)
E-mail: yxcheng@nju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100534.
Chem. Asian J. 2011, 6, 2725 – 2729
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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COMMUNICATION
well-defined spatial arrange-
ment in the polymer sensor
backbone and directly coordi-
nate with various metal ions to
form metal-polymer complexes.
Thermogravimetric
analyses
(TGA) of the polymer sensor
was carried out under a N2 at-
mosphere at a heating rate of
108CminÀ1. According to the
Thermogravimetric
(Figure 1), the polymer sensor
had high thermal stability
analysis
a
without loss weight before
3008C. An apparently one-step
degradation was observed at
the temperature ranging from
3008C to 6508C. The polymer
sensor tended to complete de-
composition at 7008C. A total
loss of about 46% was ob-
served when it was heated to
7008C, which can provide desir-
able thermal property for prac-
tical application as a fluores-
cence sensor (Figure 1).
Scheme 1. Synthesis of the polymer sensor.
The fluorescence wavelength
of the polymer sensor appeared
precursor 2 and 3 were obtained via a typical Pd-catalyzed
Sonogashira reaction. Monomer M-1 was synthesized by 2
and 3 via Sonogashira reaction. 1,4-diazidobenzene (M-2)
could be synthesized from para-phenylenediamine by a 2-
step reaction according to the literature.[11] M-1 and M-2
were served as the monomers for the synthesis of the target
polymer sensor. In this paper, the typical click reaction con-
dition was applied to the synthesis of the polymer sensor.[12]
The polymerization could be easily proceeded by M-1 and
M-2 in THF/H2O solution under the catalysis of sodium as-
corbate (10% mol) and CuSO4·5H2O (5% mol) for 2 d with
a high yield of 90%. The number-average molecular weight
(Mn), the weight-average molecular weight (Mw), and poly-
dispersity index (PDI) of the polymer sensor were deter-
mined by gel permeation chromatography using polystyrene
standards in THF. The values of Mn, Mw and PDI were 7690,
10410 and 1.35, which showed the polymer sensor have
moderate molecular weight. The resulting polymer sensor
could be dissolved in THF, DMF, CHCl3 and the mixed sol-
vents of methanol and water (1:1, v/v) which was attributed
to six hydrophilic tri (ethylene glycol) groups as side chains.
The polymer sensor was a kind of air stable powder with
yellow color. In addition, the ethynyl linker could reduce
steric hindrance between phenyl groups and also had a ben-
eficial influence on the stability of the resulting polymer
sensor. The polymer sensor emitted bright blue light under
UV lamp (l=365 nm) or sunlight even in low concentra-
tions (1ꢁ10À5 molLÀ1 corresponding to triazole unit). The
triazole unit as the metal chelating ligand could orient in a
at 425 nm with a shoulder peak at 450 nm under the excita-
tion at 381 nm in MeOH/H2O (1:1, v/v) solution. The photo-
luminescence efficiency (FPL) of polymer sensor was 0.22 in
THF solution with reference to quinine sulfate in 0.5 mol/L
H2SO4 solution. As is evident from Figure 2, obvious fluo-
rescence quenching response could be observed upon the in-
creasing addition of Hg2+ which led to about 86% fluores-
cence quenching by polymer sensor (1.0ꢁ10À5 molLÀ1) at a
concentration of 4:1 molar ratio. More importantly, the fluo-
rescence color of the polymer sensor gradually disappeared
Figure 1. TGA curve of the polymer sensor.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2725 – 2729

Polymer-based Fluorescence Sensor for Hg2+ Ions
fluorescence intensity of the polymer sensor are not obvi-
ously affected by the addition of Co2+, Ni2+, Na+, Ag+, K+,
Cr3+, Mg2+, Al3+, Ca2+, Pb2+, Fe3+, Cd2+, Cu2+ and Zn2+
even at high concentrations of metal ions (Figure 3). But
Ag+ could lead to 34% fluorescence quench, which can be
attributed to Hg2+ and Ag+ have the similar coordination
feature (Seen supporting information Figure S9). The results
indicate that triazole unit in the main chain backbone of the
polymer sensor can act as a ligand to only coordinate with
Hg2+. Moreover, the excellent fluorescence sensor with
highly sensitive and selective feature need meet one of the
essential requirements: minor or no disturbance from other
metal ions. In this paper, we further investigated the utility
of the polymer sensor as an ion-selective fluorescence
sensor for Hg2+. Herein, we subjected the polymer sensor
(1.0ꢁ10À5 molLÀ1) to a mixture of Hg2+ (4.0ꢁ10À5 molLÀ1)
and other metals (4.0ꢁ10À5 molLÀ1). The deviations from
other metalsꢂ interference were less than 6% in the coexist-
ed metal experiment, which shows the polymer sensor can
exhibit the most pronounced fluorescence quenching re-
sponse to Hg2+ in aqueous solution without much interfer-
ence with other metal ions (Figure 4). Therefore, the poly-
mer sensor can exhibit the pronounced fluorescence re-
sponse toward only Hg2+ with high selectivity and sensitivi-
ty, which might be attributed to several factors, such as the
structural rigidity of the triazole unit, the larger radius of
the Hg2+ and coordination diversification of Hg2+, its soft
acid property, the binding ability of triazole unit and strong
Hg-N binding.[15] Because of its 5d106 s2 electronic configura-
tion and lack of ligand field stabilization energy, Hg2+ can
accommodate a range of coordination numbers and geome-
tries and two-coordinate linear and four coordinate tetrahe-
dral species are common. And also, Hg2+ is a soft acid and
the use of soft donor atoms, including nitrogen, sulfur or
Figure 2. Fluorescence spectra of the polymer sensor (1.0ꢁ10À5 molLÀ1
)
in MeOH/H2O (1:1, v/v) solution with increasing amounts of Hg2+ ions
in H2O. (0, 0.1, 0.2, 0.4, 0.6, 1.0, 2.0, 3.0, 4.0, 5.0ꢁ10À5 molLÀ1; lex
=
381 nm); Inset (Top): The fluorescence image of a solution of the sensors
(10 mm) plus 4.0 equiv of a metal ion excited by a commercially available
UVlamp (lex =365 nm); Inset (Bottom): Plot of the concentration of
Hg2+ ions vs. I/I0, where I is the fluorescence intensity of the polymer
(1.0ꢁ10À5 molLÀ1) with addition of Hg2+ ions at lem =425 nm and I0 is
the fluorescence intensity of the polymer without Hg2+ ions at lem
=
425 nm.
after addition of Hg2+, which could be easily detected by
naked eyes under UV lamp (l=365 nm) (Figure 2, Inset 1).
The results suggested that nitrogen atoms of the triazole
unit in the polymer main chain backbone coordinate with
Hg2+, which led to fluorescence quenching of the polymer
sensor. The quenching mechanism could be attributed to a
reverse PET (photoinduced electron transfer) as well as
heavy metal ion effects.[13] That is, the M-1 unit acts as a
PET donor and the Hg2+ binds triazole unit as an electron
acceptor. The quenching of excimer emission resulted from
the conformational change caused by the two outward-
facing triazole units that turned inward upon binding with
phosphorus, in a chelating unit will generally increase its af-
[6 m]
finity and selectivity for Hg2+
.
Further research work is
still in progress in our laboratory.
Hg2+ [13f,13 h] (Figure 2)
.
The quenching efficiency of polymer sensor is related to
the Stern-volmer constant KSV, and which was determined
by monitoring measurable changes in the fluorescence spec-
tra via the Stern-volmer equation: I0/I=1+KSV [Q], herein,
[Q] is the quencher concentration.[14] The average KSV value
of the polymer sensor was as high as 1.58ꢁ105 mÀ1 which in-
dicated the polymer sensor had highly sensitive to Hg2+. Ac-
cording to the titration result, the lowest limit for Hg2+ de-
tection reached as high as 4.69ꢁ10À7 mollÀ1 in the ppb
range (Seen supporting information Figure S11). The lowest
detection limit was sufficiently low to detect the submillimo-
lar concentrations of Hg2+ found in many chemical and bio-
logical systems.
Figure 3. The selectivity of the polymer toward Hg2+ and other metal
ions. In these experiments, the fluorescence measurement was taken at
lex =381 nm using 10 mm of the polymer in a MeOH/H2O (1:1, v/v) solu-
tion at room temperature and in the absence and presence of 4.0 equiv of
a metal ion. The fluorescence intensity at lem =425 nm was used for plot-
ting versus the analyte.
The above studies prompted us to select the polymer
sensor for further evaluation aimed at determining its selec-
tivity. The fluorescence response behaviors of polymer
sensor on other metal ions were also examined under the
same conditions as Hg2+ determination. We found that the
Chem. Asian J. 2011, 6, 2725 – 2729
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www.chemasianj.org
2727

Y. Cheng et al.
COMMUNICATION
(300 MHz, CDCl3): d=8.99 (br, 2H), 8.12–8.07 (br, 6H), 7.16–7.11 (m,
4H), 4.41–4.22 (m, 12H), 4.00–3.97 (m, 12H), 3.86–3.80 (m, 12H), 3.59–
3.54 (m, 12H), 3.39 (s, 6H), 3.37 (s, 6H), 3.30 ppm (s, 6H). FT-IR (KBr):
n˜ =3489, 2875, 2094, 1510, 1423, 1244, 1215, 1107, 1056 cmÀ1. Anal. calcd
for C62H80N6O18: C 62.19, H 6.73, N 7.02. Found: C 62.14, H 6.67, N 6.98.
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (No.20832001, 21074054), the National Basic Research Program of
China (2010CB923303), and the Bureau of Science and Technology of
Wenzhou (No. S20100007).
Keywords: click chemistry
polymers · sensors
· fluorescence · mercury ·
Figure 4. Metal specificity: the concentration of the polymer is 1.0ꢁ
10À5 molLÀ1, Hg2+ and other metal ion are used at a concentration of
4.0ꢁ10À5 molLÀ1, respectively. Mix=a mixture of Co2+, Ni2+, Na+, K+,
Cr3+, Mg2+, Al3+, Ca2+, Pb2+, Fe3+, Cd2+, Cu2+, and Zn2+ ions in
MeOH/H2O (1:1, v/v; lex =381 nm).
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In summary, a highly selective and sensitive polymer-
based fluorescence sensor incorporating triazole unit for
Hg2+ detection was synthesized by click reaction. Compared
with other cations, such as Co2+, Ni2+, Na+, Ag+, K+, Cr3+
,
Mg2+, Al3+, Ca2+, Pb2+, Fe3+, Cd2+, Cu2+ and Zn2+, only
Hg2+ could exhibit the most pronounced fluorescence re-
sponse to the polymer sensor due to triazole unit as the
metal binding ligand only to coordinate with Hg2+. Com-
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for Hg2+ detection, the polymer sensor in this contribution
can not only work in aqueous solutions, but also has low de-
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fluorescent color of the polymer disappeared after addition
of Hg2+, which could be easily detected by naked eyes
under UV lamp (l=365 nm). The results indicate that the
polymer sensor can be potentially used as an excellent fluo-
rescence chemosensor for rapid Hg2+ detection in aqueous
solution with high selectivity and sensitivity.
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Experimental Section
Preparation of the Polymer Sensor
A mixture of M-1 (110.0 mg, 0.10 mmol), M-2 (16.0 mg, 0.10 mmol),
10 mol% sodium ascorbate (1.98 mg, 0.010 mmol) and 5 mol%
CuSO4·5H2O (1.24 mg, 0.005 mmol) was dissolved in the mixed solvents
of 10 mL THF and 10 mL H2O. The solution was stirred at 30–368C for
4 days under N2. The solvents were removed under reduced pressure and
the residue was extracted with CHCl3 (2ꢁ50 mL). The organic layer was
washed with an aqueous NH4OH solution, water and then dried over an-
hydrous Na2SO4. After the solution was removed, the resulting polymer
sensor was precipitated into methanol, and then filtered and washed with
methanol several times. Further purification could be conducted by dis-
solving the polymer sensor in CHCl3 to precipitate in methanol again.
The polymer sensor was dried in vacuum to give 114.0 mg as a dark
yellow solid in 90% yield. Mw =10410, Mn =7690, PDI=1.35. 1H NMR
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Polymer-based Fluorescence Sensor for Hg2+ Ions
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Received: June 14, 2011
Published online: August 17, 2011
Chem. Asian J. 2011, 6, 2725 – 2729
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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