non-ionic water-soluble crown-ether-substituted polyfluorene as fluorescent probe for lead ion...
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Communication
Non-Ionic Water-SolubleCrown-Ether-Substituted Polyfluorene asFluorescent Probe for Lead Ion Assays
Minghui Yu, Fang He, Yanli Tang, Shu Wang,* Yuliang Li, Daoben Zhu
A new non-ionic water-soluble polyfluorene that contains two benzo-18-crown-6 side chainsper repeat unit (PFDC) is synthesized and characterized. Upon addition of Pb2þ ions to asolution of PFDC, the PFDC fluorescence is strongly quenched. The minor interferencefrom other metal ions, especial from Cu2þ
and Hg2þions, clearly shows that the PFDCcan be used to detect Pb2þ ions with highselectivity. The fluorescence quenching ofPFDC in solution originates from the multi-valent coordination of the crown-ether moi-eties to Pb2þ ions followed by precipitation. Incomparison to ionic conjugated polymers, thepH of the medium shows less effect on thebinding of non-ionic PFDC to Pb2þ ions.
Introduction
Lead is a common contaminant and has inherent toxic
effects on human health and the environment. The release
of Pb2þ ions into the environment originates from
numerous natural and man-made sources, such as fossil
fuel combustion and electronics industries.[1,2] The Pb2þ
ion contamination can affect a wide variety of diseases
such as anemia, irritability, memory loss, and muscle
paralysis.[3,4] For these reasons, there is an exigent need to
develop some effectivemonitoringmethods to detect trace
amounts of Pb2þ ions in the environment, industrial
wastewater, and drinking water. Traditional methods
M. Yu, F. He, Y. Tang, S. Wang, Y. Li, D. ZhuBeijing National Laboratory for Molecular Sciences, Key Labora-tory of Organic Solids, Institute of Chemistry, Chinese Academy ofSciences, Beijing, 100080, ChinaFax: þ86 10 6263 6680; E-mail: [email protected]
Macromol. Rapid Commun. 2007, 28, 1333–1338
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
for Pb2þ ion assays such as atomic absorption spectro-
metry[5] and anodic stripping voltammetry[6] often require
sophisticated instruments and skilled professionals. As a
way to circumvent these limitations, some fluorescent
sensors based on small fluorescent dyes have been
developed,[7–9] however, the lack of high selectivity against
other interference ions and the non-aqueous assay
requirements for most of these systems prevent their
practical application. Although progress has been made to
design highly selective Pb2þ ion detection in aqueous
solution using DNA,[10] peptides, or proteins,[11] these
biological macromolecules are relatively unstable and
expensive. The development of convenient and
sensitive Pb2þ analytical approaches in aqueous solution
with minor or no interference from other metal ions is still
needed.
In comparison to small molecule counterparts, the
electronic structure of a conjugated polymer (CP) coordi-
nates the action of a large number of absorbing units. The
DOI: 10.1002/marc.200700187 1333
M. Yu, F. He, Y. Tang, S. Wang, Y. Li, D. Zhu
1334
excitation energy along the whole backbone of a
conjugated polymer transferring to the energy/electron
acceptor results in the amplification of fluorescent signals.
Therefore, conjugated polymers can be used as optical
platforms in highly sensitive chemical and biological
sensors.[12–19] Although many chemical sensors for alkali
and alkali-earth ions based on conjugated polymers have
been reported,[20] only a few recent examples concern
heavy metal ions, such as mercury (II)[21] and Pb2þ ions.[22]
The group of Bunz reported sugar-substituted poly
(p-phenylene ethynylene) (PPE) as a sensory material for
the amplified fluorescent sensing of Pb2þ and Hg2þ ions
in the non-aqueous solvent N,N-dimethylformamide
(DMF).[22b] Recently they developed a carboxylate-
substituted PPE as a selective and sensitive material to
detect Pb2þ ions in aqueous solution.[22a] The pH of the
aqueous solution decides the ionization state of the
carboxylate-containing ionic conjugated polymer, which
affects the binding of the polymer to metal ions. Thus,
non-ionic water-soluble conjugated polymers capable of
good recognition of Pb2þ ions provide alternative modes of
function to ionic conjugated polymers. It is known that
crown ether derivatives can form sandwich complexes
with Pb2þ ions, which possess a higher complexation
constant than those with other metal ions.[8b,23] In this
contribution we design and synthesize a new non-ionic
water-soluble polyfluorene (PFDC) that contains two
Scheme 1. The chemical structure and synthesis of PFDC.
Macromol. Rapid Commun. 2007, 28, 1333–1338
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
benzo-18-crown-6 side chains per repeat unit (RU) (see
Scheme 1 for the chemical structure) as a selective and
sensitive probe to detect Pb2þ ions in aqueous solution. The
crown ether moieties enhance the water solubility of the
polymer and are also used to recognize Pb2þ ions.
Experimental Part
Materials and Measurements
The chemicals were purchased from Acros or Alfa Aesar, and used
as received. All solvents were purified using standard procedures.
40-Carboxybenzo-18-crown-6[24] and 9,9-bis(60-aminohexyl)-2, 7-
dibromofluorene (1)[25] were prepared according to procedures in
the literature. The 1H and 13C NMR spectra were recorded on an
AV300 or AV400 spectrometer. The gel permeation chromato-
graphy (GPC) measurements were performed on a Water-410
system against polystyrene standards with tetrahydrofuran (THF)
as eluent. UV-vis spectra were taken on a JASCO V-550U
spectrometer. Fluorescence measurements were obtained in
3 mL quartz cuvettes at room temperature using an Hitachi
F-4500 spectrofluorometer equippedwith a xenon lamp excitation
source.
Synthesis of 2,
5-Dioxopyrrolidin-1-yl-4(-carboxybenzo-18-crown-6
ester (2)
N-Hydroxysuccinimide (HOSu, 1.04 g, 9.1 mmol) was added to a
solution of 40-carboxybenzo-18-crown-6 (2.7 g, 7.56 mmol) in dry
dichloromethane (60 mL) at room temperature. The mixture
was stirred for 15 min, and dicyclohexylcarbodiimide (DCC, 2.18 g,
10.6 mmol) was added at �20 8C, and the resulting mixture was
thenwarmed to room temperature and stirred overnight. After the
precipitate was filtered off, the solvent was removed under
reduced pressure. The residue was purified by silica gel column
chromatography with dichloromethane/ethyl acetate/ethylene
glycol dimethyl ether (10/10/1) as eluent to give a yellow solid
(3.36 g, 98%).1H NMR (400 MHz, CDCl3): d¼ 2.90 (m, 4H), 3.68 (m, 12H), 3.93
(t, 4H), 4.19 (t, 4H), 6.92 (d, 1H), 7.44 (s, 1H), 7.78 (d, 1H).13C NMR (100 MHz, CDCl3): d¼28.3, 70.4, 70.8, 114.7, 115.6,
122.2, 129.3, 146.5, 150.6, 168.3, 180.6.
MS (EI): m/z¼ 438.1 (Mþ).
C21H27O9N: Calcd. C 57.68, H 6.22; Found C 57.96, H 6.37.
Synthesis of Monomer 3
A solution of compound 1 (1.25 g, 1.68 mmol) and triethylamine
(0.68 g, 6.72 mmol) in methanol was added dropwise to the
solution of activated ester 2 (1.54 g, 3.53 mmol) in dichloro-
methane over 20 min at �30 8C. The resulting mixture was
warmed to room temperature and stirred overnight. The mixture
was washed with NaHCO3 and brine, and the organic layer was
dried with anhydrous magnesium sulfate, and the solvent was
then removed under reduced pressure. The residue was subjected
DOI: 10.1002/marc.200700187
Non-Ionic Water-Soluble Crown-Ether-Substituted Polyfluorene . . .
to silica gel column chromatography with dichloromethane/
methanol (20/1) as eluent to give a white solid (1.10 g, 52%).1H NMR (300MHz, CDCl3): d¼0.58 (m, 4H) 1.12 (m, 4H), 1.26 (m,
4H), 1.66 (m, 4H), 1.91 (m, 4H), 3.29 (m, 4H), 3.69 (m, 24H), 3.95
(t, 8H), 4.21 (t, 8H), 6.84 (d, 2H), 7.43 (s, 4H), 7.53 (s, 2H) 7.70 (s, 2H),
7.81 (d, 2H).13C NMR (100 MHz, CDCl3): d¼24.5, 26.8, 30.1, 32.6, 40.9, 43.5,
43.9, 70.2, 70.4, 112.4, 115.3, 122.3, 126.4, 129.7, 130.6, 133.5, 146.8,
150.1, 168.7.
MS MALDI-TOF: m/z¼1 237.9 (MþK).
C59H78O14N2Br2: Calcd. C 59.10, H 6.56, N 2.34; Found C 59.45, H
6.71, N 2.38.
Synthetic Procedure for PFDC
A mixture of monomer 3 (0.4 mmol) and 5,5-dimethyl-2-
[4-(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)phenyl]-1,3,2-dioxaboro-
nane (0.120 g, 0.4 mmol) in 10 mL of toluene and 4 mL of
2.0 M K2CO3 was degassed, and Pd(dppf)Cl2 was added under
nitrogen atmosphere. The resulting mixture was stirred at 95 8Cfor 48 h under nitrogen. After cooling to room temperature,
100 mL of distilled water was added and the mixture was
extracted with chloroform. After the solvent was removed, the
residue was precipitated in acetone. The crude polymers were
purified by precipitation from chloroform into acetone again and
dried under vacuum to give yellow solids (0.187 g, 43%).1HNMR (300MHz, CDCl3): d¼ 0.58 (m, 4H), 1.12 (m, 4H), 1.26 (m,
4H), 1.66 (m, 4H), 1.91 (m, 4H), 3.29 (m, 4H), 3.69 (m, 24H), 3.95 (t,
8H), 4.21 (t, 8H), 6.84 (d, 2H), 6.99 (d, 1H), 7.13 (d, 3H), 7.20 (d, 1H),
7.37 (m, 5H), 7.41 (m, 3H), 7.53 (s, 1H).13C NMR (100 MHz, CDCl3): d¼24.5, 26.8, 30.1, 32.6, 40.9, 43.5,
43.9, 70.2, 70.4, 112.4, 115.3, 122.3, 126.4, 128.7, 129.3, 129.6, 136.4,
137.2, 137.3, 142.3, 142.9, 146.8, 150.1, 167.6.
Mn ¼18 800, Mw ¼33270, PDI¼1.78.
UV: lmax¼365 nm, e¼ 12200 M�1 � cm�1. FL: lmax¼ 418 nm.
Fluorescence Quenching of PFDC by Pb2R Ions
The quenching experimentwas performed by successive additions
of Pb2þ ions ([Pb2þ]¼ 0–50�10�6M) to the solution of PFDC
([PFDC]¼ 5�10�6M) in aqueous solution at room temperature,
and the fluorescence spectra were measured immediately.
Figure 1. a) Fluorescence spectra and plot of the fluorescenceintensity at 418 nm (insert) b) Ksv plot of PFDC upon addition ofvariable concentrations of Pb2þ ions in water. [PFDC]¼ 5� 10�6 M
in RU, [Pb2þ]¼0–50� 10�6 M. c) Fluorescence quenching effi-ciencies of PFDC by the Pb2þ ion at different pH. [PFDC]¼ 5�10�6 M in RU, [Pb2þ]¼ 50� 10�6 M. The excitation wavelength is365 nm.
Results and Discussion
The conjugated polymer PFDC was synthesized by the
route depicted in Scheme 1. The monomer 3 was
synthesized by coupling activated 5-dioxopyrrolidin-1-
yl-40-carboxybenzo-18-crown-6 (2) with amine-terminated
fluorene 1 in 52% yield. The PFDC was obtained by
Suzuki-coupling[26] between one equivalent of monomer 3
and 1,4-phenyldiboronic ester in the presence of 2.0 M
aqueous Na2CO3 and Pd(dppf)Cl2 in toluene. All the
polymer and intermediates were characterized by1H NMR, 13C NMR, and/or mass spectroscopy. The PFDC
Macromol. Rapid Commun. 2007, 28, 1333–1338
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is readily dissolved in common organic solvents such as
chloroform and THF, and is also soluble in polar solvents,
such as MeOH and water. The molecular weights of PFDC
were determined by GPC using polystyrene as a standard
with THF as eluent. The number-averagemolecular weight
(Mn) of PFDC is 18 800 with a polydispersity index
(PDI¼Mw=Mn) of 1.78. The absorption and fluorescence
spectra of PFDC were measured in water. The PFDC
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M. Yu, F. He, Y. Tang, S. Wang, Y. Li, D. Zhu
Figure 2. a) The fluorescence response of PFDC to various metalions in aqueous solution. [PFDC]¼ 5� 10�6 M in RU, [metalion]¼ 50� 10�6 M. b) Fluorescence spectra of solutions contain-ing PFDC, amixture ofmetal ions (mix: Fe2þ, Ni2þ, Cu2þ, and Hg2þ,each 50� 10�6 M) and Pb2þ ions in water. [PFDC]¼ 5� 10�6 M inRU, [Pb2þ]¼ 50� 10�6 M. The excitation wavelength is 365 nm.
1336
exhibits an absorption maximum at 365 nm, which
corresponds to the p-p� transition of the conjugated units.
The PFDC emits bright blue fluorescence in water and
shows emission spectra with a lmax at 418 nm and two
shoulders at 445 and 476 nm, which is characteristic of
polyfluorenes.[27]
Figure 1a shows the emission spectra of PFDC in water
with a constant concentration ([PFDC]¼ 5� 10�6M in RU)
upon successive addition of the Pb2þ ion ([Pb2þ]¼(10–50)� 10�6
M) with an excitation wavelength of
365 nm. The addition of the Pb2þ ion leads to a significant
quenching of the PFDC emission. The quenching of PFDC
shows an upward non-linear curvature in the Stern-
Volmer plot (I0/I vs [Pb2þ])28 at higher Pb2þ ion concentra-
tion (� 50� 10�6M) (Figure 1b). At a lower concentration
of Pb2þ ions (0–40� 10�6M), a linear Stern-Volmer plot is
obtained with a Stern-Volmer constant (Ksv) of 2.9�104 M
�1. When the concentration of Pb2þ ions reaches
50� 10�6M, the fluorescence intensity of PFDC is quenched
by 75% calculated from the equation: 1� I/I0, where I and
I0 are the fluorescence intensity measured with and
without the addition of Pb2þ ions.[28] The plot of the
relative fluorescence intensity of the PFDC versus the
concentration of the Pb2þ ion (Figure 1a insert) shows that
the Pb2þ ion can be detected in the range from
(5–50)� 10�6M. The pH value is an important factor that
affects the selectivity and sensitivity of the sensor in
aqueous solution.[29] To check the working pH range for
the Pb2þ ion assay using PFDC, the effect of medium pH
values on the quenching efficiency of the PFDC emission
by the Pb2þ ion was investigated. The PFDC is quenched by
approximately 75% in the pH range from 4.0 to 7.0, and by
60% at pH 3 and 9, that is, a 15% difference for the
quenching efficiency is exhibited over a wide range of pH
values (3.0–9.0). These results indicate that the non-ionic
conjugated polymer PFDC can still detect Pb2þ ions even if
the external pH value is changed.
To examine the selectivity of PFDC for the Pb2þ ion, we
also studied the response of PFDC to other metal ions, such
as Liþ, Naþ, Kþ, Ca2þ, Mg2þ, Ba2þ, Fe2þ, Co2þ, Cu2þ, Mn2þ,
Zn2þ, and Hg2þ ions. Figure 2a shows the influence of
different metal ions ([metal ion]¼ 50� 10�6M) on the
relative intensity change of PFDC fluorescence. The PFDC is
highly selective for Pb2þ ions with a fluorescence decrease
of 4.2-fold. The fluorescence of PFDC is not influenced at all
by the addition of Liþ, Naþ, Kþ, Ca2þ, Mg2þ, Zn2þ, or Mn2þ
ions. Although the fluorescence quenching was detected
upon addition of Fe2þ, Co2þ, Cu2þ, Hg2þ, or Ba2þ ions, the
fluorescence decrease for the Pb2þ ion was 2.5–4 times
higher than those for Fe2þ, Co2þ, Cu2þ, Hg2þ, and Ba2þ ions.
These results indicate that the PFDC has specific recogni-
tion ability for the Pb2þ ion. For Pb2þ ion detection, one of
the essential requirements is minor or no interference
from other metal ions, especially from the most common
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environmentally significant interfering Cu2þ and Hg2þ
ions. It is necessary to test the selectivity of the PFDC with
mixed competing ions. As shown in Figure 2b, for a
solution of PFDC in water ([PFDC]¼ 5� 10�6M in RU) with
a mixture metal ions (Fe2þ, Ni2þ, Cu2þ, and Hg2þ, each
50� 10�6M) added, the fluorescence of PFDC was only
quenched by 25%. When Pb2þ ions ([Pb2þ]¼ 50� 10�6M)
were added to the mixture solution, the fluorescence
intensity of PFDC significantly decreased and a further
quenching efficiency of 50% was obtained whereupon it
wasmore likely that the response of PFDC to themetal ions
reached saturation. The minor interference from other
metal ions clearly shows that PFDC can be used as a Pb2þ
ion probe with good selectivity.
The major advantage of PFDC for Pb2þ ion assay is the
good selectivity. It eliminates the effect of other
heavy metal ions, such as Cu2þ and Hg2þ. To gain insight
into the selectivitymechanism of the PFDC, we studied the
absorption spectra of PFDC in the presence of Pb2þ, Cu2þ,
or Hg2þ ions. As shown in Figure 3a, the absorbance
DOI: 10.1002/marc.200700187
Non-Ionic Water-Soluble Crown-Ether-Substituted Polyfluorene . . .
Figure 3. a) The absorption spectra of PFDC with additions ofvariable concentrations of Pb2þ ions in water. b) The plot of theabsorbance intensity of PFDC at 365 nmwith additions of variableconcentrations of Pb2þ, Cu2þ, or Hg2þ ions in water. [PFDC]¼ 5�10�6 M, [Pb2þ, Cu2þ, or Hg2þ]¼0–50� 10�6 M.
intensity of PFDC decreases gradually along with the
addition of Pb2þ ions, while the spectrum shape doesn’t
change. This indicates that the PFDC/Pb2þ complex
precipitated from the assay solution. In these experiments,
the Pb2þ ion was added into the solution of PFDC and the
absorption spectra were measured after the sample was
incubated for one minute. Almost the same absorption
spectra were obtained even when the PFDC was incubated
for 10 min after the Pb2þ ions were added. This result
indicates that the PFDC precipitates from the assay
solution very quickly within oneminute after the addition
of Pb2þ ions. If the concentrations of both PFDC and Pb2þ
ions were increased, the sedimentation could be seen to
the naked eye. As shown in Figure 3b, under identical
conditions to the Pb2þ ion, the absorbance of the PFDC
declinesmuch less upon adding Cu2þ andHg2þ ions, which
indicates that no precipitation of PFDC with Cu2þ or Hg2þ
ions occurs. These observations are consistent with the
fluorescence experiments, where the emission intensity of
PFDC decreases with the addition of Pb2þ but not Cu2þ
and Hg2þ ions. In fact, Figure 1b shows a non-linear
Macromol. Rapid Commun. 2007, 28, 1333–1338
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
decrease in fluorescence intensity with the addition
of Pb2þ ions. This would suggest that there is more than
one quenching mechanism at work.[22] Considering the
multivalent binding property of the Pb2þ ion, two or three
benzo-18-crown-6 moieties in PFDC can coordinate
one Pb2þ ion to form intra-chain or inter-chain structures
(p-stacking aggregation),[8b,20d,23] which results in the
precipitation of the PFDC/Pb2þ complex from the assay
solution followed by the fluorescence quenching of PFDC.
A similar result has been reported for the peculiarly
multivalent coordination interactions of Pb2þ with ionic
carboxylate-substituted PPE to cause the precipitation of a
PPE/Pb2þ complex from the assay solution.[22a]
In conclusion, a new non-ionic water-soluble polyfluor-
enewith two benzo-18-crown-6moieties in side chains per
repeat unit (PFDC) is synthesized by a Pd-catalyzed
Suzuki-coupling reaction. The crown ether moieties
possess two functions: they enhance the water solubility
of the polymer and recognize Pb2þ ions. The fluorescence-
quenching behavior of PFDC by Pb2þ ions was studied.
Multivalent coordinations of benzo-18-crown-6 moieties
to one Pb2þ ion could form intra/inter-chain p-stacking
aggregates followed by precipitation, which dominates
the quenching behavior of PFDC in the presence of Pb2þ
ions. Thus, the PFDC can be used as a highly selective Pb2þ
ion probe with minor interference from other metal ions,
especial from Cu2þ and Hg2þ ions. The pH change of the
aqueous solution shows less effect on the quenching
efficiency of non-ionic PFDC by Pb2þ ions.
Acknowledgements: The authors are grateful for the financialsupport from the ‘100 Talents’ program of Chinese Academy ofSciences, the National Natural Science Foundation of China(20601027, 20574073 and 20421101), the National High-TechR&D Program (No.2006AA02Z130), the National Basic ResearchProgram of China (No.2006CB806200), and the National MajorResearch Plan of China (No. 2006CB932100).
Received: March 8, 2007; Revised: April 19, 2007; Accepted: April20, 2007; DOI: 10.1002/marc.200700187
Keywords: fluorescence quenching; lead ions; non-ionic conju-gated polymer; selectivity; sensors
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