2,2′-biimidazole-based conjugated polymers as a novel fluorescent sensing platform for pyrophosphate anion

Article abstract of DOI:10.1021/ma300361t

Three novel conjugated polymers based on 2,2′-biimidazole have been successfully designed and synthesized through the Suzuki coupling reactions, and their fluorescence sensing ability to metal ions and anions was investigated. The emission of the two polymers with hydrophilic side chains can be efficiently quenched by Cu²⁺ through a photoinduced electron transfer process. Moreover, the polymer-Cu²⁺ complexes exhibit excellent turn on sensing properties for detection of pyrophosphate (PPi) anion. These complex sensors possess high selectivity avoiding the interference from other anions, very fast response (less than 3 min) to PPi, and the detection limit of about 0.17 ppm. In addition, the linear detection range of PPi can be tuned conveniently by changing the amount of Cu²⁺ ions. Thus, the conjugated polymers can be used as a novel fluorescent sensing platform, and this work provides a new strategy for the development of PPi sensors.

Full text of DOI:10.1021/ma300361t

Article
pubs.acs.org/Macromolecules
2,2′-Biimidazole-Based Conjugated Polymers as a Novel Fluorescent
Sensing Platform for Pyrophosphate Anion
Yinyin Bao,† Hu Wang,† Qianbiao Li,† Bin Liu,† Qing Li,† Wei Bai,‡ Bangkun Jin,† and Ruke Bai*,†
†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
‡Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: Three novel conjugated polymers based on
2,2′-biimidazole have been successfully designed and synthe-
sized through the Suzuki coupling reactions, and their
fluorescence sensing ability to metal ions and anions was
investigated. The emission of the two polymers with
hydrophilic side chains can be efficiently quenched by Cu2+
through a photoinduced electron transfer process. Moreover,
the polymer−Cu2+ complexes exhibit excellent “turn on”
sensing properties for detection of pyrophosphate (PPi) anion.
These complex sensors possess high selectivity avoiding the
interference from other anions, very fast response (less than 3
min) to PPi, and the detection limit of about 0.17 ppm. In
addition, the linear detection range of PPi can be tuned
conveniently by changing the amount of Cu2+ ions. Thus, the conjugated polymers can be used as a novel fluorescent sensing
platform, and this work provides a new strategy for the development of PPi sensors.
imidazolium as the side groups have been synthesized and
utilized as excellent fluorescent sensors for detection of metal
ions, anions, nitric oxide, and amino acids.8 2,2′-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 In addition, 2,2′-biimidazole can
be easily functionalized through various ways under mild
conditions.10 However, only very few papers have been devoted
to the study of the conjugated polymers containing biimidazole
moiety. Yamamoto et al. prepared three 2,2′-biimidazole
homopolymers by the dehalogenative polycondensation using
a zerovalent nickel catalyst.11a Then MacLean et al. reported
conjugated polymers of 2,2′-biimidazole obtained through
electrochemical polymerization.11b The authors did not
investigate the fluorescence properties of the polymers; they
focused on the structure and chemical properties. From the
literature,12 we know that 2,2′-biimidazole has large M−N
(metal−nitrogen) bond length of 4.2 Å, while 2,2′-bipyridine’s
M−N bond length is only 2.51 Å, which may endow 2,2′-
biimidazole containing conjugated polymers with some
distinguishing sensing properties.
INTRODUCTION
■
In recent years, conjugated polymer-based fluorescent sensors
have been developed as a highly effective tool for detection of a
broad range of environmental and biological analyses.1 With
“molecular wire” effect of the conjugated polymers, the
detection sensitivity of the polymer sensors can be greatly
enhanced because of the fast energy migration along the
conjugated backbone.2 Thus, conjugated polymers have more
advantages over small molecules for sensing applications.
Among these polymers, several N-heterocyclic aromatic units
have been introduced into the backbone as molecular
recognition sites for metal ions, such as bipyridyl,3 terpyridyl,4
and phenanthroline.5 After coordination with different metal
ions, the polymers exhibit different fluorescence changes, such
as quenching or emission red/blue shift, which can be
attributed to electron density variations on the main chains,
aggregation of polymer chains, or conjugation enhancement
along the polymer backbone.3−5 However, the selectivity of
polymer sensors based on N-heterocycle, 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 new aromatic N-
heterocycle as a receptor are still a quite important and
intriguing theme.
We have recently incorporated 2,2′-biimidazole into the
conjugated polymer and used this kind of polymer as a new
As an important aromatic N-heterocycle, imidazole plays a
significant role in biosystems and attractive chemical proper-
ties.7 Most recently, the conjugated polymers with imidazole or
Received: February 21, 2012
Revised: March 30, 2012
Published: April 9, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Monomers and Polymers
platform for design of different fluorescent sensors.13 The
polymer exhibits distinctive fluorescence response to Ag+,
which is quite different from the features of the conjugated
polymers with the oligopyridyl moieties in their backbones and
as well as with the imidazole moieties in their side chains. In
addition, the polymer−Ag+ complexes can be used to highly
selective detection of cysteine with nanomolar detection limit.
In order to further explore new sensors based on the sensing
properties of the N-heterocyclic conjugated polymers, we have
also designed and prepared three new conjugated polymers
based on 2,2′-biimidazole (P1, P2, and P3, Scheme 1) and
found that the polymers exhibit high quantum yields in
solutions. Moreover, the fluorescence of P1 and P2 can be
efficiently quenched by Cu2+ in DMF−H2O mixed solvents.
Therefore, a new sensing platform utilizing the polymer−Cu2+
complexes for detection of pyrophosphate (PPi) anion has
been achieved. The complex sensors exhibit high selectivity and
fast response to PPi with the detection limit of about 0.17 ppm,
and the linear detection range of PPi can be tuned conveniently
by changing the amount of Cu2+ ions. Herein we report the
synthesis, characterization, and sensing properties of the
polymers.
Bruker AVANCEII spectrometer with TMS as an internal standard
and CDCl3 as solvent. Molecular weights and molecular weight
distributions were determined by GPC equipped with a Waters 1515
pump, a Waters 2414 RI detector, and Waters UV/RI detectors (set at
30 °C) using a series of three linear Styragel columns HR3, HR4, and
HR6 at an oven temperature of 45 °C. Mass spectra were performed
on a ProteomeX-LTQ spectrometer. FT-IR spectra were recorded on
a Bruker EQUINOX55 spectrometer. All spectra were measured at
room temperature (temperature controlled at 25 3 °C).
Synthesis of M0. Solid 2,2′-biimidazole14 (0.67 g, 5.0 mmol) was
dispersed in DMF (20 mL), and 0.5 N NaOH(aq) (1.0 mL) was
added. The suspension was stirred and heated to 50 °C. A solution
composed of methoxy poly(ethylene glycol) monoacrylate (5.0 g, 10.3
mmol) solubilized in DMF (10 mL) was added dropwise. After 4 h,
the solution was neutralized with aqueous hydrochloric acid and the
heating discontinued. The solvent was removed by rotary evaporation,
and the product was chromatographed on silica gel (CH2Cl2:methanol
1
= 20:1, v/v) to give M0 (5.5 g) as a pinkish liquid; yield: 98.7%. H
NMR (400 MHz, CDCl3): δ = 7.07 (s, 2H), 7.03 (s, 2H), 4.74 (t, 4H),
4.21 (t, 4H), 3.62 (m, 68H), 3.36 (s, 6H), 2.93 (t, 4H). 13C NMR
(100 MHz, CDCl3): δ = 171.3, 137.7, 127.8, 122.3, 71.9, 70.5, 68.9,
63.8, 59.0, 43.3, 35.5. MS: m/z = 1098.61.
Synthesis of M1. N-Bromosuccinimide (0.38 g, 2.13 mmol) was
added to a chloroform solution (15 mL) of M0 (1.17 g, 1.06 mmol)
under vigorous stirring under air. After 2 h, the reaction mixture was
neutralized with NaOH(aq), and the product was extracted with
chloroform. After being washed with water, the combined organic
layers were dried (Na2SO4) and chromatographed on silica gel
(CH2Cl2:methanol = 20:1, v/v) to give M1 (2.6 g) as light yellow
EXPERIMENTAL SECTION
■
Materials. Metal salts such as NaCl, KCl, MgCl2, CaCl2, FeCl3,
ZnCl2, CdCl2, NiCl2, CuCl2, Mn(OAc)2, Pb(OAc)2, and AgNO3 were
purchased from Shanghai Chemical Co. and used without further
purification. Et3N and 1,4-dioxane were freshly distilled over
appropriate drying agents. The reactions that required oxygen-free
conditions were carried out under an argon atmosphere using standard
Schlenk techniques. All the other reagents were purchased and used
without further purification.
1
liquid; yield: 65.4%. H NMR (400 MHz, CDCl3): δ = 7.12 (s, 2H),
4.70 (t, 4H), 4.07 (t, 4H), 4.21 (t, 4H), 3.63 (m, 68H), 3.37 (s, 6H),
2.91 (t, 4H). 13C NMR (100 MHz, CDCl3): δ = 170.6, 138.4, 128.9,
105.8, 71.9, 70.5, 68.9, 63.9, 59.0, 42.1, 34.3. MS: m/z = 1256.43.
Synthesis of M2. 1,4-Diiodo-2,5-dioctyloxybenzene15 (1.67 g, 3.0
mmol), pinacolborane (1.32 mL, 9.0 mmol), Et3N (2.5 mL), and
PdCl2(dppf) (0.05 g, 0.06 mmol) were dissolved in anhydrous 1,4-
dioxane (12 mL) under an Ar atmosphere, and the solution was stirred
overnight at 90 °C. After cooling to room temperature, the mixture
was poured into water (50 mL) and extracted with Et2O (3 × 20 mL).
Measurements. Fluorescence spectra measurements were per-
formed on a Shimadzu RF-5301PC spectrofluorophotometer.
Absorption spectra were determined on a Pgeneral UV−vis TU-
1
1901 spetrophotometer. H and 13C NMR spectra were taken on a
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The combined organic layers were dried (Na2SO4) and chromato-
graphed on silica gel (petroleum ether:ethyl acetate = 20:1, v/v) to
give M2 (1.2 g) as a brown solid; yield: 72%. H NMR (400 MHz,
RESULTS AND DISCUSSION
Design and Synthesis. The polymers P1, P2, and P3 were
■
1
synthesized by Suzuki condensation, and the synthetic routes
are illustrated in Scheme 1. 2,2′-Biimidazole was prepared
according to the published procedure14 and then was reacted
with methoxy poly(ethylene glycol) monoacrylate through
Michael addition reaction to form M0. Subsequently, M0 was
brominated with N-bromosuccinimide (NBS) at room temper-
ature, resulting 5,5′-dibromo-N,N′-di(methoxy poly(ethylene
glycol) monoacrylate propionato)-2,2′-biimidazole (M1) as a
hydrophilic monomer. A hydrophobic monomer 5,5′-dibromo-
N,N′-di(n-butylpropionato)-2,2′-biimidazole (M4) was synthe-
sized by a similar method.13 Monomer M2 (1,4-bis(4,4,5,5,-
tetramethyl-1,3,2-dioxaborolan-2-yl)-2,5-dioctyloxyphenylene)
was synthesized from 1,4-diiodo-2,5-dioctyloxybenzene15
through borylation reaction catalyzed by PdCl2(dppf),16 and
monomer M3 (4,7-dibromo-2,1,3-benzothiadiazole) was pre-
pared according to the literature procedures.17 P1 was
conveniently formed via Suzuki coupling reaction between
M1 and M2 catalyzed by the Pd(PPh4)3, and P3 was obtained
in a similar approach with M4 and M2. P2 was synthesized
utilizing 1.0 equiv of M2, 0.9 equiv of M1, and 0.1 equiv of M3.
P1 and P2 were purified by precipitation from ether and
collected as brown solids, while P3 was obtained by
precipitation from methanol as a slightly gray solid. It is
worthy to point out that 2,2′-biimidazole-based monomers were
efficiently synthesized under mild conditions, which makes the
2,2′-biimidazole containing polymers more accessible compared
with the other N-hetercycle-based main chain conjugated
polymers. Utilizing the 2,2′-biimidazole-based monomers,
different kinds of conjugated polymers can be prepared
conveniently, resulting a new series of fluorescence sensing
molecules.
CDCl3): δ = 7.08 (s, 2H), 3.93 (t, 2H), 1.74 (m, 2H), 1.47 (m, 2H),
1.34 (s, 12H), 1.30−1.24 (m, 8H), 0.88 (t, 6H). 13C NMR (100 MHz,
CDCl3): δ = 157.7, 119.9, 83.4, 69.7, 31.9, 29.6, 29.5, 29.3, 26.1, 24.8,
22.7, 14.1. MS: m/z = 586.46.
Synthesis of P1. M2 (0.23 g, 0.4 mmol), M1 (0.5 g, 0.4 mmol),
Pd(PPh3)4 (0.05 g, 0.04 mmol), and Na2CO3 (0.85 g, 8.0 mmol) were
added to a flask under an Ar atmosphere. A solvent mixture of toluene
(8 mL), water (4 mL), and ethanol (2 mL) was degassed and added to
the reaction mixture. The resulting mixture was refluxed with vigorous
stirring for 48 h. After cooling to room temperature, the mixture was
poured into water (30 mL) and extracted with CHCl3 (3 × 30 mL).
The collected organic layers were dried (Na2SO4) and purified by
precipitated from ether to give P1 (0.11 g) as a brown solid; yield:
1
19.6%. Mw = 8900, Mw/Mn = 1.27 (GPC, polystyrene calibration). H
NMR (400 MHz, CDCl3): δ = 7.12 (br, ArH), 6.94 (br, ArH), 4.74
(br, −CH2−O−), 4.13−3.89 (br, −CH2−N− and −CH2−O−), 3.63−
3.54 (br, −O−CH2CH2−O−), 3.36 (−O−CH3), 2.76 (br, −CH2−
COO−), 1.69 (br, −CH2−), 1.24 (br, −CH2−), 0.86 (br, −CH3).
UV−vis (DMF, 8.0 × 10−6 mol/L): λmax = 335 nm. IR (thin film), υ
(cm−1): 2928, 2860 (CH2 stretching), 1737 (CO stretching), 1102
(C−O stretching).
Synthesis of P2. P2 was prepared by reaction of M2 (0.17 g, 0.3
mmol), M1 (0.34 g, 0.27 mmol), and 4,7-dibromo-2,1,3-benzothia-
diazole (0.009 g, 0.03 mmol) using the same procedure as P1. The
yield was 23% of P2 (0.09 g) as a brown solid. Mw = 5200, Mw/Mn =
1.28 (GPC, polystyrene calibration). 1H NMR (400 MHz, CDCl3): δ=
7.47−6.93 (br, ArH), 4.75 (br, −CH2−O−), 4.15−3.93 (br, −CH2−
N− and −CH2−O−), 3.63−3.54 (br, −O−CH2CH2−O−), 3.37
(−O−CH3), 2.75 (br, −CH2−COO−), 1.81−1.45 (br, −CH2−), 1.23
(br, −CH2−), 0.86 (br, −CH3). UV−vis (DMF, 8.0 × 10−6 mol/L):
λmax = 326 nm. IR (thin film), υ (cm−1): 2926, 2855 (CH2 stretching),
1735 (CO stretching), 1100 (C−O stretching).
Synthesis of P3. P3 was prepared by reaction of M2 (0.17 g, 0.3
mmol) and M413 (0.16 g, 0.3 mmol) using the same procedure as P1,
except that the polymer was purified by precipitated from methanol to
give a slightly gray solid (0.09 g); yield: 45%. Mw = 12 000, Mw/Mn =
Characterization of Polymers. The purified polymers
1
were characterized by H NMR, FT-IR, UV−vis, and GPC
1
analysis (see Experimental Section and Supporting Information
1.40 (GPC, polystyrene calibration). H NMR (400 MHz, CDCl3): δ
1
= 7.14 (ArH), 6.95 (ArH), 4.78 (br, −CH2−N−), 4.11−3.93 (br,
−CH2−O−), 2.76 (br, −CH2−COO−), 1.71−1.53 (br, −CH2−),
1.34−1.25 (br, −CH2−), 0.91−0.85 (br, −OCHH3). UV−vis (DMF,
8.0 × 10−6 mol/L): λmax = 336 nm. IR (thin film), υ (cm−1): 2965,
2904 (CH2 stretching), 1735 (CO stretching), 1100 (C−O
stretching).
for detailed analysis data). In the H NMR spectra of the
polymers P1, P2, and P3 (Figure 1 and Figures S34 and S35),
Fluorometric Analysis. The stock solutions of P1, P2, and P3
were prepared by dissolving the polymers in DMF (1.0 × 10−3 M) and
were diluted as required before use. The cation stock solutions were
prepared by dissolving the metal salts in deionized water with a
concentration of 0.01 M. The stock solutions of F−, Cl−, Br−, I−,
−
−
−
−
NO3 , HSO4 , AcO−, HCO3 , CO32−, H2PO4 , HPO42−, PO43−, and
P2O72− were prepared in deionized water with a concentration of 0.01
M. A solution of polymer (2.0 mL) was placed in a quartz cell (10.0
mm width), and the fluorescence spectrum was recorded. The sum
volume of each ion solution introduced to the test solution was no
more than 50 μL, and the changes of the fluorescence intensity were
recorded at room temperature (λex = 338 nm). The quantum yields of
the polymers were determined according to the equation
1
Figure 1. H NMR spectra of P1 in chloroform-d. The solvent peak
2
F Asnu
u
was marked with asterisks.
Φ = Φ
u
s FAuns2
s
the chemical shifts are consistent with the proposed structure of
the polymers as demonstrated in Scheme 1. Take P1 for
example; as shown in Figure 1, the two peaks at about 7.12 and
6.94 ppm in the downfield can be ascribed to the aromatic
protons of biimidazole unit and benzene unit, respectively,
indicating M1 and M2 have been successfully copolymerized
where Φ is quantum yield, F is integrated area under the corrected
emission spectra, A is absorbance at the excitation wavelength, n is the
refractive index of the solution, and the subscripts u and s refer to the
unknown and the standard, respectively. Quinine bisulfate in 0.05 M
H2SO4 solution was used as the standard, which has a quantum yield
of 0.55.
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Figure 2. UV−vis absorption (blue line) and fluorescence spectra (red line) of P1 (left), P2 (middle), and P3 (right) in DMF. [P1] = [P2] = [P3] =
8.0 × 10−6 M. λex = 338 nm.
through Suzuki condensation. And the existence of the ether
side chains and octyl side chains can be confirmed by the peaks
at 3.36 and 0.86, which can also be supported by the bands at
2965 and 2904 cm−1 of C−H vibration stretching, 1737 cm−1
of CO stretching, and 1102 cm−1 of C−O stretching in the
FT-IR spectrum of P1 (Figure S36).
decreases along with increasing the water content; this can be
attributed to the chain aggregation-induced fluorescence
quenching. Note that P3 shows much faster quenching rate
than the other two polymers (Figure S7). This phenomenon
indicates that both P1 and P2 with hydrophilic side chains can
be better dispersed in DMF−H2O mixed solvents. Thus, P1
and P2 are more suitable for fluorescence sensing application in
aqueous solution compared with P3. Moreover, as shown in
Figure S5, the fluorescence intensity ratios F550 nm/F423 nm of P2
increases along with raising the water content, and after the
water content reaches 90% the emission peak at 423 nm
disappeared, resulting a single peak at 550 nm (Figure S4). This
result can be largely ascribed to the fact that the FRET effect
existing in P2 system is distinctly enhanced because of water-
induced chain aggregation.13
Since the polymers P1 and P2 show high fluorescence
quantum yield (Table S1) and good dispersion in DMF−H2O
(4:1, v/v), we chose the mixed solvent as the detection media,
and HEPES was used as the buffer agent. The ion responsive
properties of P1 were studied by fluorescence spectroscopy in
DMF−HEPES (pH = 7.4, v/v = 4:1) at a concentration of 8.0
× 10−6 M. The experimental result displays that the
fluorescence of P1 can be efficiently quenched by Cu2+ ions.
As shown in Figure 3, upon the addition of Cu2+, the emission
The three polymers readily dissolve in common organic
solvents, such as toluene, CH2Cl2, THF, and DMF, because of
the flexible alkyl or ether side chains. The molecular weights
(Mn) and polydispersity index (PDI) of the polymers were
determined by gel permeation chromatography (GPC) with
polystyrene as the reference standard (Table S1). The
molecular weights of the polymers are relatively low, which
may be because the polymerization reactivity of the 2,2′-
biimidazole-based monomers is not very high. Even so, the
molecular weights are high enough for the polymers to have
good sensing properties, and a similar phenomenon can be seen
in the previous reports3−5 on polycondensation of N-
heterocycle-based monomers. The UV−vis absorption spectra
of polymers are shown in Figure 2, P1, P2, and P3 have similar
maximum absorption wavelengths around 330 nm in DMF
because of the similar conjugated backbone. But P2 has
stronger absorption from 400 to 550 nm, which can be
attributed to the alkoxybenzene−benzothiadiazole units. All the
evidence indicates that the designed polymers have been
successfully prepared.
Fluorescence Properties and Ion Recognition. The
emission spectra of P1 and P2 were measured in various
organic solvents at room temperature (Figures S1 and S2). It
can be seen that the two polymers exhibit highest emission
intensity in DMF, so it was chosen as the main solvent to
investigate the fluorescence properties of the polymers. The
results show that all the three polymers exhibit a strong blue
fluorescence with a maximum around 425 nm in DMF because
of the conjugated polymer backbone. However, in the presence
of alkoxybenzene−benzothiadiazole units, P2 shows a little
Figure 3. Fluorescence spectra of P1 upon the titration of Cu2+ in
DMF−HEPES (pH = 7.4, v/v = 4:1). Inset: fluorescence intensity of
P1 as a function of Cu2+ concentration. λex = 338 nm. [P1] = 8.0 ×
10−6 M.
broader emission peak. Simultaneously, a Forster resonance
̈
energy transfer (FRET) process may exist in P2 because the
weak absorption peak of the alkoxybenzene−benzothiadiazole
unit can be found to overlap with the emission peak of the
alkoxybenzene−biimidazole unit. The orange emission peak
around 550 nm can be observed more obviously in the solution
of THF, dioxane, and toluene than in DMF (Figure S2), and a
similar phenomenon has also been observed in our previous
report.13 The fluorescence quantum yields of P1, P2, and P3 in
DMF were determined to be 0.22, 0.10, and 0.41, respectively.
In order to examine the properties of these polymers in
aqueous solution, the fluorescence spectra of the three
polymers in DMF−H2O mixed solvents were studied. As
shown in Figures S3−S6, the emission intensity of the polymers
peaks at 423 nm gradually decrease along with increasing the
Cu2+ concentration. The intensity exhibits a good linear
response to a Cu2+ concentration change from 0 to 1.0 equiv,
and after adding 2.5 equiv of Cu2+, 90% of the fluorescence has
been quenched. The quenching efficiency can be described by
the Stern−Volmer equation, I0/I = Ksv[A] + 1, which related to
the fluorescence intensity, I, at different concentrations of
analyte quencher, [A], where I0 is the intensity at [A] = 0 and
Ksv is the Stern−Volmer constant. According to the
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fluorescence titration of P1 in DMF aqueous solution with
Cu2+, Ksv of the system was determined to be 3.3 × 105 M−1
(Figure S8). When the quantum yield of P1 changed from 0.13
to 0.02 upon interaction with Cu2+, the obvious fluorescence
changes could be observed by the naked eye under a UV lamp.
For comparison’s sake, the Cu2+ response properties of P3 were
also studied (Figures S13−S15). However, the Stern−Volmer
quenching constant Ksv of P3−Cu2+ was calculated to be only
2.8 × 103 M−1 (Figure S15), which is much lower than that of
P1−Cu2+. And after addition of 20.0 equiv of Cu2+, only 30% of
the fluorescence can be quenched. This result indicates that P1
has much stronger coordination ability with Cu2+ than P3 in
DMF−H2O mixed solvent, which implies that the hydrophilic
side chains can promote the interaction between the polymer
and Cu2+ ions in aqueous solution. Thus, the reasonable design
of side chains is important to construct conjugated polymer-
based fluorescent sensors.
papers published on the study of PPi fluorescent sensors based
on the metal ion complexes20 and hydrogen-bond or ionic
bond interaction.21 However, only very few PPi sensors based
on conjugated polymers have been reported.22 Herein we
prepared the Cu2+ complexes of the conjugated polymers
containing 2,2′-biimidazole moieties in situ by mixing P1 and
P2 with Cu2+ salt in aqueous DMF, respectively, and studied
their sensing properties for PPi. The results indicate that the
emission spectra of the complexes exhibit remarkable changes
upon interaction with PPi. As shown in Figure 4, the emission
In general, there are two possibilities that metal ions cause an
obvious fluorescence quenching phenomenon of conjugated
polymer: one is electron transfer interactions between the
polymer backbone and metal ions, and the other is
interpolymer aggregation induced by coordination. To further
clarify the mechanism, we investigated the fluorescence
response properties of P2 to Cu2+ in aqueous DMF solution.
If the fluorescence quenching is induced by chain aggregation,
after adding enough Cu2+ ions the FRET enhancement similar
to the phenomenon caused by water (Figure S4) should be
Figure 4. Fluorescence spectra of P1−Cu2+ complex upon the titration
of PPi in DMF−HEPES (pH = 7.4, v/v = 4:1). Inset: fluorescence
intensity of P1−Cu2+ complex as a function of PPi concentration. λex
338 nm. [P1] = 8.0 × 10−6 M. [Cu2+] = 2.0 × 10−5 M.
=
peak of P1−Cu2+ ([P1] = 8.0 × 10−6 M, [Cu2+] = 2.0 × 10−5
M) gradually increases upon addition of different amount of
PPi anions, and the intensity at 423 nm exhibits a good linear
change with a PPi concentration increasing from 1.0× 10−5 to
4.8 × 10−5 M. Moreover, when utilizing 1.0 × 10−5 M Cu2+ and
8.0 × 10−6 M P1 as the sensing ensemble, the emission
intensity changes linearly with the concentration of PPi from
5.0 × 10−6 to 1.6 × 10−5 M (Figure 5). Thus, the linear change
observed, leading to a large change of intensity ratios F550 nm
/
F423 nm. However, upon interaction with Cu2+, the intensity
ratios F550 nm/F423 nm exhibits only a slight increment, and no
obvious emission peak at 550 nm was actually observed in the
P2 system (Figures S9 and S10), which means aggregation of
polymer chains is not the principal contributor to the
quenching phenomenon in the system. Thus, we may
reasonably conclude that the emission quenching of the
polymers is mainly due to electron transfer interactions
between the polymer backbone and Cu2+ ions, which was
also reported in the previous paper.8c In addition, this work
provides an efficient method to investigate the sensing
mechanism of conjugated polymers to metal ions, which was
not reported in the literature of other N-heterocycle-based
polymer sensors.3−5,8 At the same time, the Stern−Volmer
constant Ksv of P2−Cu2+ was calculated to be 2.6 × 105 M−1
(Figure S12), which is of the same order of magnitude as, but a
little lower than, that of the P1−Cu2+ system. This result shows
that the fluorescence of P2 can also be efficiently quenched by
Cu2+, and the lower Stern−Volmer constant compared to that
of P1−Cu2+ system may be attributed to the lower molecular
weight, resulting in a slightly weaker “molecular wire” effect.
Thus, the electron transfer interactions between the polymers
and Cu2+ can be tuned by changing the molecular weight of the
polymer. Therefore, the reasonable design of the backbone is
critical to develop efficient conjugated polymer-based fluo-
rescent sensors. In addition, the coordination reaction of P1
and P2 with Cu2+ has been confirmed by the UV−vis titration
experiment (Figures S16 and S17).
Figure 5. Fluorescence spectra of P1−Cu2+ complex upon the titration
of PPi in DMF−HEPES (pH = 7.4, v/v = 4:1). Inset: fluorescence
intensity of P1−Cu2+ complex as a function of PPi concentration. λex
338 nm. [P1] = 8.0 × 10−6 M. [Cu2+] = 1.0 × 10−5 M.
=
range of PPi can be tuned by varying the amount of Cu2+ ions.
After adding 5.0 equiv of PPi, a 9-fold emission enhancement
can be obtained, and the fluorescence changes under a UV
lamp can be distinctly observed by the naked eye (Chart 1).
From the fluorescence titration experiment, the detection limit
of PPi was estimated to be less than 1.0 × 10−6 M, about 0.17
ppm. Similar to P1, P2 also shows good turn-on sensing
properties for PPi in the presence of Cu2+ ions (Figures S22−
S25). As shown in Figures S23 and S25, the linear range 1.0×
10−5−3.6 × 10−5 M can be changed into 5.0 × 10−6−2.4 × 10−5
M by varying the concentration of Cu2+ from 2.0 × 10−5 to 1.0
As shown in Figures S18−S21, compared with other metal
ions, Cu2+ ion shows the strongest quenching ability to the
polymers. It is well-known that pyrophosphate (PPi) anion has
strong interaction with Cu2+;18 thus, we attempted to use these
polymer−Cu2+ complexes as new sensors for detection of PPi.
PPi is a biologically important target because it is the product of
ATP hydrolysis under cellular conditions.19 There are many
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Chart 1. Schematic Representation of PPi Sensors Based on the Fluorescence “On−Off−On” of 2,2′-Biimidazole Containing
Conjugated Polymers
× 10−5 M. The sensing process is consistent with the
displacement approach reported in the previous report18,20
utilizing metal ion complexes, the fluorescence of the polymers
can be recovered once the Cu2+ ions are removed by PPi, and
the possible sensing mechanism is illustrated in Chart 1. These
sensors are based on the interaction of PPi with Cu2+ ions
coordinated to the biimidazole group on polymer backbone,
while the other polymer PPi sensors22 rely on the interaction of
PPi with Cu2+ on the polymer side chains or directly with the
polymers.
To determine the selectivity of the PPi-sensing systems, the
fluorescence responses of P1−Cu2+ and P2−Cu2+ complexes
were further examined with 13 different anions including
monovalent anions (F−, Cl−, Br−, I−, NO3 , HSO4 , AcO−,
−
−
−
−
HCO3 , H2PO4 ), divalent anions (CO32−, HPO42−), and
trivalent ion PO43−. As shown in Figure 7 and Figures S26 and
The response time is very important for a sensor; therefore,
the effect of the interaction time on the fluorescence emission
of the system has been examined, and the results are shown in
Figure 6. It can be seen that the emission intensity of P1−Cu2+
Figure 6. Effect of reaction time on the fluorescence intensity of P1−
Cu2+ and P2−Cu2+ by PPi ions in DMF−HEPES (pH = 7.4, v/v =
4:1). λex = 338 nm. [P1] = [P2] = 8.0 × 10−6 M. [Cu2+] = 2.0 × 10−5
M. [PPi] = 4.0 × 10−5 M.
Figure 7. (a) Fluorescence spectra of P1−Cu2+ complex and (b)
intensity ratios F/F0 of P1−Cu2+ complex in the presence of 4.0 ×
10−5 M PPi and 8.0 × 10−5 M various anions in DMF−HEPES (pH =
7.4, v/v = 4:1). λex = 338 nm. [P1] = 8.0 × 10−6 M. [Cu2+] = 2.0 ×
−
−
10−5 M. 1, PPi; 2, F−; 3, Cl−;4, Br−; 5, I−; 6, NO3 ; 7, HSO4 ; 8,
reaches to its saturation value in just 3 min due to the strong
interaction of Cu2+ ions with the PPi anions. On the other
hand, the response time of P2−Cu2+ to PPi is only 1 min, much
shorter than that of P1−Cu2+. The reason is that P2 has weaker
binding ability to Cu2+ than P1 due to the lower molecular
weight, which can be deduced from the Stern−Volmer
constants. Since P2−Cu2+ complex possesses a faster response,
it can be used as an excellent sensor for rapid detection of PPi.
We can conclude that the fluorescence sensing properties of
conjugated polymer based fluorescent sensors can be efficiently
optimized through the reasonable structure modification.
−
−
−
ClO4 ; 9, AcO−; 10, HCO3 ; 11, CO32−, 12, H2PO4 ; 13, HPO42−; 14,
PO4
3−
.
S27, the experiment results indicate that all of the other anions
exhibit weak binding affinities to Cu2+ ion and lead to negligible
changes in the fluorescence properties of the complexes.
Therefore, PPi can be easily differentiated from the other
anions, especially from PO43−, by the emission intensity. This
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CONCLUSIONS
■
In summary, we have successfully designed and synthesized
three novel conjugated polymers based on 2,2′-biimidazole
through the Suzuki coupling reaction and also investigated their
ability to sense metal ions and anions. The fluorescence of the
two polymers with hydrophilic side chains can be efficiently
quenched by Cu2+ ions through a photoinduced electron
transfer process. Based on the unique interaction of Cu2+ with
the polymers, the two conjugated polymer−Cu2+ complexes
have been demonstrated to be potential “turn on” fluorescent
sensors for detection of pyrophosphate anion. The sensors
possess high sensitivity to PPi with the detection limit of about
0.17 ppm and excellent selectivity avoiding the interference
from other anions. And the linear detection range of PPi can be
tuned conveniently by changing the amount of Cu2+ ions. Since
the sensors display very fast response (less than 3 min), it
provides a rapid and efficient approach for the detection of PPi.
The experiment results show that the fluorescence sensing
properties of the polymers can be efficiently optimized through
the reasonable structure modification. In addition, the facile
synthesis of the 2,2′-biimidazole-based conjugated polymers
under mild conditions makes the sensors more accessible
compared with the other N-heterocycle-based conjugated
polymers. To the best of our knowledge, this is the first
example utilizing the 2,2′-biimidazole-based conjugated poly-
mers to detect PPi, and this work provides not only a new
strategy for the development of PPi sensors but also a novel
platform to further design different fluorescent sensors for
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ASSOCIATED CONTENT
■
S
* Supporting Information
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1H NMR of monomers; 13C NMR of monomers; H NMR of
1
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organic solvents; optical data of polymers P1−P3 in aquous
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polymers P1 and P2 in the presence of various metal ions in
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AUTHOR INFORMATION
Corresponding Author
*Tel 86-551-3600722; Fax 86-551-3631760; e-mail bairk@ustc.
edu.cn.
■
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The financial support from National Natural Science
Foundation of China (No. 20974104 and No. 21074120) and
Ministry of Science and Technology of China (No.
2007CB936401) is gratefully acknowledged.
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