Synthesis and characterization of a fluorescent polymer containing 2,6-bis(2-thienyl)pyridine moieties as a highly efficient sensor for Pd 2+ detection

Article abstract of DOI:10.1039/c0cc03819a

A novel conjugated polymer chemosensor 1 containing 2,6-bis(2-thienyl) pyridine has been synthesized and exhibits a high sensitivity and selectivity for palladium ion detection based on the fluorescent quenching effect.

Full text of DOI:10.1039/c0cc03819a

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COMMUNICATION
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Synthesis and characterization of a fluorescent polymer containing
2,6-bis(2-thienyl)pyridine moieties as a highly efficient sensor for
Pd2+ detectionw
Bin Liu, Yinyin Bao, Fanfan Du, Hu Wang, Jiao Tian and Ruke Bai*
Received 13th September 2010, Accepted 4th November 2010
DOI: 10.1039/c0cc03819a
A novel conjugated polymer chemosensor 1 containing 2,6-bis
(2-thienyl)pyridine has been synthesized and exhibits a high
sensitivity and selectivity for palladium ion detection based on
the fluorescent quenching effect.
quencher, Pd2+ could be detected by fluorescent sensors
through fluorescence quenching. Unfortunately, most of the
sensors reported in the literature, such as N-9-anthrylmethyl-
N-methyl-N0-benzoylthiourea,6a bis(naphthalenemethyleneoxy)
tetrathia-16-crown-4,6b bathophenanthroline,6c and meso-
tetrakis(4-(carboxymethyleneoxy)phenyl)porphyrin,6d exhibit
poor selectivities for Pd2+ among other transition metallic
cations. There are only three ligands, including 1,2-dithioethene
derivatives,6g thiophenemethyl-amino derivatives,6i and diallyl-
amino derivatives,6m that proved to be highly selective sensors
for Pd2+ ions.
Today, palladium as a transition metal has a wide spectrum of
applications, such as in the electrical and electronic industries,
dental appliances, fuel cells, jewellery, and catalysts.1 Especially
with the rapid growth of the number of cars, significant amounts
of palladium are emitted due to automobile catalytic converter
attrition.2 In the natural environment, palladium exists in three
states: Pd0(metallic), Pd2+, and Pd4+. Its metallic form shows
no or little in vitro cytotoxicity, however, Pd ions are capable of
eliciting a series of cytotoxic effects which may cause severe
primary skin and eye irritations. Epidemiological studies
demonstrated that Pd ions, especially PdCl2, are among the
most frequent reacting sensitizers within metals (second rank
behind nickel). Besides, palladium ions are capable of inhibiting
most major cellular functions, as seen in vivo and in vitro.3c
Owing to the ability of Pd2+ ions to form complexes, some
biomacromolecules such as proteins, DNA, and RNA seem to
be the most sensitive targets.3 Government restrictions on the
levels of residual heavy metals in end products are very strict,
the threshold for palladium in drugs is 5–10 ppm.4 As a result of
the importance of Pd2+ for the environment and human health,
investigation into methodologies for highly sensitive and
selective detection of Pd2+ have attracted more and more
attention.
Over the past decades, conjugated polymers (CPs) have
received considerable attention as sensing probes due to their
sensing ability in response to interactions with analytes such as
metal ions, toxic chemicals and biomolecules.7c Swager
and coworkers have demonstrated 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 excitation.7
Compared with small molecule chemosensors, the CPs display
more advantages such as amplified sensitivity, excellent
film-forming properties, and systematic modification by
different functional groups. We noted that a number of CPs
sensors for various metals or their ions have been investigated,
but there is only one reported for Pd2+ detection,6f and the
mechanism was ascribed to the interpolymer interaction by
Pd2+–pyridyl binding. However, the experimental results
revealed that the interference from platinum ions was obvious.
In this communication, we present a novel fluorescent
polymer 1 as a highly efficient sensor for Pd2+ detection. The
polymer comprises two functional parts, one for the specific
binding of metal ions and the other for signalling the response.
We chose a poly(p-phenylene ethynylene) unit as signal
domain because of its unique photophysical and conductive
properties.7a On the other hand, thiophene moieties not only
play an important role in nonlinear optical materials and
conductivity sensory devices,8 but are also often used for
binding heavy or transition metal cations. Since a sulfur
atom is considered as a ‘‘soft’’ donor atom in a chelating
agent, it generally increases the sensor’s affinity and selectivity
for ‘‘soft’’ metals such as Hg2+ and Pd2+. Herein, we
Conventional methods for palladium detection, including
atomic absorption spectrometry, plasma emission spectro-
scopy, solid phase microextraction-high performance liquid
chromatography, and X-ray fluorescence have the character-
istics of fast measurement and good sensitivity, but require
expensive facilities, experienced operators, and complicated
sample-pretreatment procedures.5 Colorimetric and fluori-
metric methods for Pd2+ detection would be more desirable
because the measurements are comparatively easy and cheap.6
The mechanisms for detection methods to date are mostly
based on the chemical reactions between Pd2+ and detector, or
complexation of Pd2+ with ligands. As a strong fluorescence
introduce
phenylpyridine (TPT) by combination of a pyridine group
and thiophene group, into the conjugated polymer,
expecting to improve sensitivity and selectivity for Pd2+
a
binding building block, 2,6-dithienyl-4-
CAS Key Laboratory of Soft Matter Chemistry, Department
of Polymer Science and Engineering, University of Science and
Technology of China, Hefei, 230026, P. R. China.
E-mail: bairk@ustc.edu.cn; Fax: 0086-551-3631760;
Tel: 0086-551-3600722
w Electronic supplementary information (ESI) available: Synthetic
procedures, experimental details, additional spectroscopic data. See
DOI: 10.1039/c0cc03819a/
a
.
The synthetic approach for the conjugated polymer 1 is
shown in Scheme 1. First of all, 2,6-dithienyl-4-phenylpyridine
3 was synthesized by reaction of cycloimmonium ylides 6 with
c
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Metal ion responsive properties of 1 were examined by
UV-Vis absorbance and fluorescence emission spectroscopy
in THF solution at a polymer repeat unit concentration of
4 ꢀ 10ꢁ6 M. The result indicates that the polymer sensor
displays
a selective chromogenic behavior towards the
palladium ion with a color change from yellow green
to brown, which can be easily observed by the naked eye
(Fig. S1w). It was found that the absorption centered at
426 nm is enhanced about 2-fold with the addition of
palladium ions to the solution of 1, meanwhile, a new peak
of absorption appeared at 330 nm (Fig. 2a). The absorbance at
426 nm is linearly proportional to the amount of Pd2+ in the
range of 1–100 mM (Fig. 2b). After adding Pd2+ ions, the
absorption spectra of 1 show no obvious red-shift, which might
prove that the mode of linkage between the metal and the
ligand is mostly intrachain linkage instead of interchain
linkage.6f Fig. 3a displays the changes to the emission spectra
of 1 with Pd2+ concentrations and it can be seen that with the
increase of Pd2+ concentration, the fluorescence intensity of
the emission maximum at 472 nm decreased dramatically with
the QY changing from 0.24 to 0.1. Fig. 3b is the plot of
fluorescence intensity I472 vs. Pd2+ concentration and it
reveals that the detection limit of Pd2+ is less than 1 ppm.
Scheme 1 Synthesis of the polymer chemosensor.
chalcone 5, and then brominated with NBS to produce a
monomer 2 in a yield of 74%. Finally, the polymer was
obtained by polymerization of the monomer 2 and 2,5-
dicyclohexyloxy-1,4-benzenediyne 4 in the presence of a
catalytic amount of Pd(PPh3)4 and CuI with Et3N under Ar.
The polymer showed good solubility in common organic
solvents, such as THF, DMF and dichloromethane. The
structures of these compounds were fully characterized (see
ESIw).
UV-Vis absorption and fluorescence spectra of 1, measured
in dilute THF solution at room temperature, are shown in
Fig. 1. It can be seen that a strong and broad absorption
appears at the region from 375 to 450 nm, which is attributed
to the effective p–p* transition of the conjugated structure of
the polymers. Moreover, the fluorescence spectrum shows that
1 can emit blue (472 nm) light due to the extended p-electronic
structure in the main chain backbone. The fluorescence
quantum yield (QY) of 1 was calculated to be 0.24, with
quinine bisulfate in 0.1 M H2SO4 as the standard.9
The Stern–Volmer data (KSV = 2.46 ꢀ 104 L molꢁ1
)
(Fig. S2aw) further demonstrates that polymer 1 possesses
the desired high sensitivity.
The quenching of fluorescence could be attributed to the
binding of Pd2+ with TPT units in polymer 1. TPT–Pd2+
complex could be prepared via C–H activation reaction with a
high concentration of Pd2+ ions at high temperature.10
However, when we examined the palladium ion responsive
property of TPT at the same conditions as 1 in THF solution at
a concentration of 4 ꢀ 10ꢁ6 M, the fluorescence spectra
showed a very small change when adding Pd2+. It may
be attributed to the fact that the Stern–Volmer constant of 3
(KSV = 8.81 ꢀ 102 L molꢁ1) (Fig. S2cw) is much lower than
polymer 1. For further confirmation, the palladium ion
responsive property of Br-substituted TPT (compound 2)
was also examined and the Stern–Volmer data of 2 (KSV
=
6.63 ꢀ 102 L molꢁ1) (Fig. S2bw) with palladium ions shows a
similar result to compound 3. These results demonstrate that
the largely conjugated structure of the polymer plays a
particularly important role in the sensitivity of the sensor.
An important feature of the sensor is its high selectivity
towards the target analyte over other competitive species.
Fig. 4a shows the response of the polymer 1 on addition of
Fig. 2 (a) UV-Vis absorption spectra and (b) the plot of the
absorbance at 426 nm of 1 by various concentrations of palladium in
THF ([1] = 4 ꢀ 10ꢁ6 M in repeat units, [Pd2+] = 0, 4, 12, 16, 20, 40,
60, 80, 120 ꢀ 10ꢁ6 M).
Fig. 1 Normalized UV-Vis absorption and emission spectra of polymer
1 in THF. The excitation wavelength is 440 nm. The dashed line is the
absorption spectrum and the solid line is the emission spectrum.
c
1732 Chem. Commun., 2011, 47, 1731–1733
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Pd2+ detection, which not only has potential applications in
human health and environmental protection, but also expands
the methods for palladium analysis in industrial production.
Financial support from the Ministry of Science and Technology
of China (NO. 2007CB936401) is gratefully acknowledged.
Notes and references
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These results indicate that the sensor possesses an excellent
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In conclusion, a novel conjugated polymer 1 has been
successfully synthesized by polymerization of 2,6-bis
(2-thienyl)pyridine with 1,4-bis(ethynyl)-2,5-bis(hexyloxy)benzene
via the Sonogashira coupling reaction. The UV-Vis absorbance
and fluorescence spectra have been investigated in THF. A
selective chromogenic behavior towards Pd2+ can be observed
by the naked eye, and Pd2+ ions can effectively quench the
fluorescence of the polymer with an excellent selectivity. All the
results demonstrate the polymer 1 can be used as an efficient
sensor with a high sensitivity and an excellent selectivity for
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1731–1733 1733