Conjugated polymers containing a 2,2′-biimidazole moiety - A novel fluorescent sensing platform

Article abstract of DOI:10.1039/c1cc15787f

Two novel conjugated polymers containing a 2,2′-biimidazole moiety have been designed, synthesized, and demonstrated to be used as an effective fluorescent sensing platform for detection of Ag⁺ and cysteine. This is the first example utilizing a fluorescent conjugated polymer-Ag⁺ complex to selectively detect Cys with a nanomolar range detection limit.

Full text of DOI:10.1039/c1cc15787f

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Conjugated polymers containing a 2,20-biimidazole moiety—a novel
fluorescent sensing platformw
Yinyin Bao, Qianbiao Li, Bin Liu, Fanfan Du, Jiao Tian, Hu Wang, Yanxue Wang and
Ruke Bai*
Received 19th September 2011, Accepted 21st October 2011
DOI: 10.1039/c1cc15787f
Two novel conjugated polymers containing a 2,20-biimidazole
moiety have been designed, synthesized, and demonstrated to be
used as an effective fluorescent sensing platform for detection of
Ag+ and cysteine. This is the first example utilizing a fluorescent
conjugated polymer–Ag+ complex to selectively detect Cys with
a nanomolar range detection limit.
However, only very few papers have been devoted to the study
of the conjugated polymers containing a biimidazole moiety.
Yamamoto et al. prepared three 2,20-biimidazole homopolymers
by the dehalogenative polycondensation using a zerovalent
nickel catalyst.10a Then MacLean et al. reported conjugated
polymers of 2,20-biimidazole obtained through electrochemical
polymerization.10b The authors did not investigate the fluorescence
properties of the polymers, they focused on the structure and
chemical properties.
Fluorescent conjugated polymers have been widely utilized as
highly sensitive chemosensors for detection of many analytes
in recent years due to the signal amplification effect through
the ‘‘molecular wire’’ of the conjugated backbone.1 Among
these polymers, several N-heterocyclic aromatic structures,
such as pyridine,2 bipyridyl,3 terpyridyl,4 and phenanthroline,5
have been introduced into the backbone as coordination sites
for metal ions. When the interaction occurs between the
coordination sites and metal ions, the conjugated polymers
exhibit fluorescence quenching or emission red/blue-shift, this
is generally attributed to electron density variations on the
main chains or conjugation enhancement along the polymer
backbone.3a,b However, the selectivity of polymer sensors
based on N-heterocycles, especially for the bipyridyl analogue,
remains unsatisfactory compared with that of the small molecule
sensors.6 In order to improve the properties of the polymer
sensors, the design and synthesis of the conjugated polymer
sensors with a new aromatic N-heterocycle as a receptor are
still a quite important and intriguing theme.
In order to explore and design new conjugated polymer
sensors with excellent properties, we designed and prepared
2,20-biimidazole based conjugated polymers, and studied the
fluorescence sensing property of the polymers for the detection
of metal ions and amino acids. Two polymers P1 and P2 were
synthesized through Suzuki condensation and the synthetic
routes are illustrated in Scheme 1. 2,20-Biimidazole was first
prepared according to the published procedure,11a and then
was reacted with n-butyl acrylate and P1 was conveniently
prepared by the Suzuki coupling reaction between M1 and
M2, and P2 was obtained in a similar approach. P1 and P2
were purified by precipitation from hexane and collected as
slightly gray and red solids, respectively. The purified polymers
were characterized by 1H NMR and FTIR (see ESIw), providing
conclusive evidence for the well-controlled structure as we
expected by the incorporation of the monomers into the
polymer backbone. Both polymers readily dissolve in common
organic solvents, such as toluene, CH2Cl2, THF, and dioxane.
The molecular weights (Mn) and polydispersity index (PDI) of
the polymers were determined by gel permeation chromato-
graphy (GPC) with polystyrene as the reference standard
(Table S1, ESIw).
Imidazole is an important aromatic N-heterocycle because
of its significant role in biosystems and attractive chemical
properties.7 Most recently, the conjugated polymers with
imidazole or imidazolium as the side groups exhibit outstanding
sensing properties to metal ions, anions, nitric oxide, and
amino acids.8 2,20-Biimidazole, the dimeric analogue of imidazole,
is one of the most important derivatives of imidazole and plays
a particular role in crystal engineering because of the excellent
coordination ability and diverse coordination modes.9
CAS Key Laboratory of Soft Matter Chemistry, Department of
Polymer Science and Engineering, University of Science and
Technology of China, Hefei, P. R. China 230026.
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/c1cc15787f
Scheme 1 Synthesis of monomers and polymers.
c
118 Chem. Commun., 2012, 48, 118–120
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imidazole moieties in their side chains, which may be due to
the large M–N (metal–nitrogen) bond lengths12 and different
molecular rigidities of the polymer chains. It is presumed that
the distinguishing responsive characteristic of the polymers to
Ag+ ions is attributed to both electron density variations on
the backbone and chain aggregation induced by coordination.
The initial fluorescence quenching of the polymers may be
attributed to the electron transfer interactions between the
polymer backbone and Ag+ ions. Then the later quenching
can be induced by the chain aggregation,13,14 which was
confirmed by the experiment of water induced aggregation
(Fig. S17, ESIw). This means that the FRET effect was
enhanced because of the Ag+ induced aggregation. The inter-
esting phenomenon was not observed in the previous reports
about the conjugated polymer sensor for Ag+ ions.15 The possible
mechanism is illustrated in Scheme S1, ESI.w The Stern–Volmer
quenching constants Ksv of P1–Ag+ and P2–Ag+ systems were
determined to be 2.2 ꢀ 105 Mꢁ1 and 4.6 ꢀ 105 Mꢁ1, respectively.
Since cysteine (Cys) can bind Ag+ through the thiol–Ag+
interaction,16 this provides an approach to detect Cys by
utilizing the polymer–Ag+ complex as a novel sensor. There
are some papers published on the study of Cys fluorescent
sensors based on the Ag+ complexes of the different ligand
species, such as calix[4]arene,17a amino acids,17b and DNA.17c–e
Herein, we prepared the Ag+ complexes of the conjugated
polymers containing 2,20-biimidazole moieties in situ by mixing
P1 and P2 with Ag+ salt, respectively, and studied their properties
as Cys fluorescent sensors. We found that the emission spectra
of the complexes exhibit remarkable changes upon interaction
with Cys (Fig. 2 and Fig. S7 (ESIw)). As shown in Fig. 2a, the
emission peak of the P1–Ag+ complex gradually blue-shifts
from 456 nm to 416 nm, accompanied by an increase in the
fluorescence intensity upon increasing the concentration of Cys.
Moreover, we noticed that the ratio of emission intensities
(F416nm/F456nm) varies from 0.52 to 2.05 and it exhibits a good
linear response to a Cys concentration change from 0 to 8.0 mM
(Fig. 2b). The detection limit of Cys was determined to be as
low as 90 nM by a fluorimetric assay (Fig. S8, ESIw). Similarly,
the intensity ratio (F416nm/F560nm) of P2–Ag+ also varies
linearly with the concentration of Cys and the detection limit
was calculated to be about 150 nM by the titration experiment
(Fig. S9, ESIw). These results indicate that the chemosensing
complexes are excellent ratiometric fluorescent sensors for Cys
Fig. 1 UV-vis absorption and fluorescence spectra of P1 (blue line)
and P2 (red line) in dioxane. [P1] = [P2] = 8.0 mM. lex = 333 nm.
The absorption and emission spectra of the two polymers
were measured in dioxane at room temperature (Fig. 1). P1
exhibits the absorption maximum at 312 nm and its photo-
luminescence spectrum peaked at 416 nm. In comparison with
P1, P2 displays a main absorption peak at 310 nm and the
other weak peak at 450 nm. Similarly in the PL spectrum, P2
shows two peaks centred at 416 and 560 nm, which are due to
the emission of carbazole–biimidazole and carbazole–benzo-
thiazole units, respectively. Simultaneously, a Forster resonance
¨
energy transfer (FRET) process exists in P2, because the weak
absorption peak of the carbazole–benzothiazole unit was
found to overlap with the emission peak of the carbazole–
biimidazole unit (Fig. 1). Both the polymers are emissive in the
visible region under UV irradiation (365 nm), P1 shows a blue-
violet color whereas P2 is a reddish orange color. The fluorescence
quantum yields of P1 and P2 were determined to be 0.2 and
0.17, respectively.
The ion responsive properties of the polymers were studied
by fluorescence emission spectroscopy in dioxane at a concen-
tration of 8.0 mM (Fig. S1–S4, ESIw). It was found that both of
the polymers have no response to the addition of alkali as well
as alkaline earth metal ions. However, upon the addition of
Ag+, the emission peaks at 416 nm exhibit a red shift of 40 nm
with a decreased intensity for both of the polymers, which
causes obvious changes of the fluorescence color. Besides the
Ag+ ion, Zn2+ induces a smaller red shift, while Fe3+, Ni2+
,
Cu2+, and Co2+ ions can cause only fluorescence quenching
in various degrees with no wavelength changes. The other
metal ions examined lead to negligible changes of the fluorescence.
Therefore, Ag+ can be readily distinguished from the other
metal ions by the emission shift. As shown in Fig. S5 (ESIw),
upon interaction with Ag+, the emission peak of P1 gradually
red-shifts to the wavelength of 456 nm along with the concen-
tration of Ag+, and the fluorescence color of the solution changes
from blue-violet to pale blue (Fig. S16a, ESIw). However, com-
pared with P1, P2 exhibits a notable difference during the titration
experiment because of the FRET process. It was noticed that the
emission peak of P2 at 560 nm shows a weaker fluorescence
quenching than that at 416 nm upon the addition of Ag+. As a
result, the intensity ratio F560/F416 gradually increases with Ag+
concentration (Fig. S6, ESIw). Simultaneously, the fluorescence
color of P2 changes from reddish orange to yellow orange
(Fig. S16b, ESIw). The coordination reaction has been confirmed
by a UV-Vis titration experiment (Fig. S23 and S24, ESIw).
It is obvious that the ion responsive properties of P1 and P2
are quite different from those of the conjugated polymers with
the oligopyridyl moieties in their backbones and also with the
Fig. 2 Fluorescence spectra of (a) P1–Ag+ and (c) P2–Ag+ upon the
titration of Cys. (b) Fluorescence intensity of P1–Ag+ and (d)
P2–Ag+ as a function of Cys concentration. lex = 333 nm.
c
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Chem. Commun., 2012, 48, 118–120 119

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with a high sensitivity. In addition, the fluorescence color
changes under a UV lamp can be distinctly observed by the
naked eye (Fig. S16, ESIw).
To determine the selectivity of the Cys-sensing systems, the
fluorescence responses of P1–Ag+ and P2–Ag+ complexes
were further examined with 15 different amino acids. The
experiment results show that the other amino acids exhibit
only a weak binding affinity to the Ag+ ion and lead to
negligible changes in the emission properties of the complexes.
Therefore, Cys can be easily differentiated from other amino
acids either by the fluorescence intensity or by the intensity
ratio (Fig. 3 and Fig. S12–S15 (ESIw)). For comparison, the
interactions of glutathione (GSH) with P1–Ag+ and P2–Ag+
complexes were also studied, which exhibit a similar effect
compared to Cys (Fig. S18, ESIw). This phenomenon has been
also observed in the previous reports.18
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The reversibility of the conjugated polymer sensing systems
was also examined and the results are shown, respectively, in
Fig. S10b and S11b (ESIw). It can be seen that the sensing
processes were reversible, and the emission intensity can be well
recovered at the cyclic addition of Ag+ and Cys. The attenuation
of emission intensity after several cycles may be a result of the
adsorption of the Ag+–Cys complex to the cuvette surface. It
should be pointed out that all of the fluorescence responses do
not exhibit any appreciable time-dependent effects, which makes
the detection of Cys more rapid and efficient.
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In summary, we have successfully designed and synthesized
two novel conjugated polymers containing a 2,20-biimidazole
moiety through the Suzuki coupling reaction, which was
demonstrated to be an effective fluorescent sensing platform
for ratiometric detection of Ag+ and cysteine. This is the first
example utilizing a fluorescent conjugated polymer–Ag+
complex to selectively detect Cys with a nanomolar range
detection limit and this work provides a new strategy for the
development of Cys sensors.
The financial support from Ministry of Science and Technology
of China (NO. 2007CB936401) is gratefully acknowledged.
Notes and references
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120 Chem. Commun., 2012, 48, 118–120
This journal is The Royal Society of Chemistry 2012