Non-Ionic Water-Soluble Crown-Ether-Substituted Polyfluorene as Fluorescent Probe for Lead Ion...

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.



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



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

www.mrc-journal.de 1335


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



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



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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DOI: 10.1002/marc.200700187

Non-Ionic Water-Soluble Crown-Ether-Substituted Polyfluorene as Fluorescent Probe for Lead Ion...

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