Analytica Chimica Acta 556 (2006) 313–318

Selective determination of bisphenol A (BPA) in water by a reversiblefluorescence sensor using pyrene/dimethyl�-cyclodextrin complex

Xu Wang, Hulie Zeng, Lixia Zhao, Jin-Ming Lin∗State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences,

Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China

Received 19 July 2005; received in revised form 22 September 2005; accepted 22 September 2005Available online 14 November 2005

Abstract

A bifurcated optical fiber chemical sensor for continuous monitoring of bisphenol A (BPA) has been proposed based on the fluorescence quenching(λex/λem= 286/390 nm) of pyrene/dimethyl�-cyclodextrin (HDM-�-CD) supramolecular complex immobilized in a plasticized poly(vinyl chloride)(PVC) membrane, in which pyrene served as a sensitive fluorescence indicator probe. The decrease of the fluorescence intensity of pyrene/HDM-�-CD complex upon the addition of BPA was attributed to the displacement of pyrene by BPA, which has been utilized as the basis of the fabricationo mic detectionr ity,s te.©

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rinsfehtthftccsh

r-ipole,ond-

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f a BPA-sensitive fluorescence sensor. The response mechanism of the sensor was discussed in detail. The sensor exhibited a dynaange from 7.90× 10−8 to 1.66× 10−5 mol L−1 with a detection limit of 7.00× 10−8 mol L−1, and showed excellent reproducibility, reversibilelectivity, and lifetime. The proposed sensor was successfully used for the determination of BPA in water samples and landfill leacha2005 Elsevier B.V. All rights reserved.

eywords: Chemical sensor; Dimethyl�-cyclodextrin; Pyrene; Bisphenol A; Competitive complexation

. Introduction

The development of optical methods for highly selectiveecognition and detection of environmentally toxic compoundss a challenging work, because it requires the specific recog-ition of particular element in the presence of closely relatedpecies. The host–guest chemistry is a promising approachor chemical sensing[1,2] and environmental monitoring[3],specially the application of multicomponent assemblies ofost molecules has been proved to be one of the approaches

o generate new sensing materials with high specificity forhe sensing process[4,5]. These supramolecular assembliesave exhibited novel functions which are different from those

ound in simple molecules[6] and have opened a new field inhe chemical sensors to promote the application of host–guesthemistry in environmental analysis. Among various hosts,yclodextrins (CDs) are attractive for the construction ofupramolecular recognition system since they possess nanosizeydrophobic cavities which enable them to incorporate various

guest molecules in aqueous solution[7]. Several weak intemolecular forces between host and guest, such as dipole–dhydrophobic, Van der Waals, electrostatic, and hydrogen bing interaction, cooperatively contribute to the molecrecognition process[8]. Formation of a competitive systemwhich two different guest molecules are added simultaneoto a CDs solution is one of the study hotspots of CDs chem[9–11]. This competitive system has been used to deterdrugs with high sensitivity and selectivity[9], construcphotoresponsive hydrogel system[10], and achieve chiral searation in liquid chromatography and capillary electrophor[11].

In the recent years, the ever-increasing number of orgcompounds being detected in waters has risen concernthe contamination of water resources[12,13]. Among thevarious pollutants, bisphenol A (BPA, a chemical intermedwidely used in the synthesis of polycarbonate and epoxy reunsaturated polyester-styrene resins and flame-retardants[14])is mainly released into the environment in wastewater fplastics-producing industrial plants and landfill sites. BPAreported to show potential detrimental reproductive eff

∗ Corresponding author. Tel.: +86 10 62841953; fax: +86 10 62841953.E-mail address: jmlin@mail.rcees.ac.cn (J.-M. Lin).

on wildlife and humans through altering endocrine functionand may disrupt growth and development by interfering

003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2005.09.060

314 X. Wang et al. / Analytica Chimica Acta 556 (2006) 313–318

with the production, release, transport, metabolism, binding,and regulation of development processes[15–19]. Since itsubiquitous nature and its endocrine disrupting potential, BPAhas been included in the environmental water monitoring ordetermining study by several techniques[20–24]. However,these methods require collection and processing of the sampleprior to analysis, steps that often require more time and laborthan the analysis itself. For the purpose of determining analytecontinuously and rapidly, it requires that no separation processsuch as chromatography or electrophoresis take place. So theremust be a recognition molecule or transducer that interactswith the analyte to produce a signal that can be related to itspresence or concentration. The membrane based fiber-opticchemical sensors have been proved to be good candidates forthe continuous recognition process because of their advantagesin real time analysis of drug[25] or water contents[26].

Based on the investigation on the inclusion phenomenaof CDs with pyrene[27–30], a reversible optical fiber basedchemical sensor for the fluorimetric determination of BPAwas fabricated. The sensing compound was the inclusion com-plex formed between pyrene and lipophilic heptakis-(2,6-di-O-methyl)-�-cyclodextrin (HDM-�-CD), which was plasticizedin a poly(vinyl chloride) (PVC) membrane. It showed thatwhen the membrane contacted with solution containing BPA,a remarkable decrease in the monomer emission of pyrenearound 390 nm was observed, which was thought to be asso-c e oft le.S -C nds sen-s A inr owl-e ensorf

2

2

asu of as at wasc enw mete( seta nd ab andlT untei berw em-b dingC d thec riedt thee wa

Fig. 1. Schematic diagram of the flow cell arrangement. (1) Excitation light, (2)signal light, (3) bifurcated optical fibre, (4) screw cap, (5) cell body, (6) quartzglass slide, (7) O silicon ring, (8) sensing membrane, (9) sample in-let, (10)sample out-let.

performed on a Pentium III computer with software of Sigmaplot. For pH measurements, an Orion Model 828 pH meter (MA,USA) was used. A spin-on device[31] was used to prepare themembrane.

2.2. Materials and reagents

Analytical reagent grade chemicals were used unless indi-cated otherwise. High molecular weight PVC was pur-chased from Sigma (St. Louis, USA). BPA was from TokyoKasei Kogyo Co. Ltd. (Tokyo, Japan) and its stock solution(1.0× 10−3 mol L−1) was prepared by dissolving the appropri-ate amount of BPA in ethanol. Working solutions were preparedby successive dilution of the stock solutions with water. Pyrenewas from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). HDM-�-CD was obtained from Sigma (St. Louis, USA). Tetrahydrofuran(THF) and diisooctyl sebacate (DOS) were obtained from Bei-jing Chemical Factory (Beijing, China). The ultra high puritydeionized water used in the experiment was obtained fromEasypure water purification system with a 0.2�m fiber filter(Barnstead, USA).

2.3. Preparation of sensing membrane

The sensing membrane solution was prepared by dissolvinga fD F.A thes singa o thec raneo pro-fi ationo ut0 ation( encei 9 and2 anep

iated with the exclusion of pyrene from inside to outsidhe HDM-�-CD cavity by accommodating a BPA molecuince the specific host–guest interaction between HDM�-D and BPA, a significant improvement in sensitivity aelectivity was achieved using the proposed sensor. Theor was successfully used for the determination of BPeal water samples and landfill leachate. To our best kndge, this is the first continuous fluorescence quenching s

or BPA.

. Experimental

.1. Apparatus

A Dektak 8 surface profiler (Digital Instruments, USA) wsed to determine the film thickness by scanning the edgemall scratch that was applied to the sensing membrane thoated to a circular quartz plate. All fluorescence measuremere carried out on a Hitachi F-2500 fluorescence spectro

Hitachi, Tokyo, Japan) with excitation and emission slitst 5 and 5 nm, respectively. A homemade PTFE flow cell aifurcated optical fiber (50 + 50 quartz fibers, diameter 6 mm

ength 1.2 m) were used for the sensing measurements (Fig. 1).he quartz plate with the sensing membrane on it was mo

n the flow cell. The membrane side is facing the cell chamith the circulating sample solution sweeping over the mrane driven by a peristaltic pump (Lange Instruments, Baohina). The opposite side of the quartz plate tightly matcheommon end of the optical fiber. The excitation light was caro the cell through one arm of the bifurcated optical fiber andmission light collected through the other. Data processing

tsr

d

,

s

mixture of 25 mg of HDM-�-CD, 60 mg of PVC, 120 mg oOS, and 0.2 mg of pyrene in 2.0 mL of freshly distilled THcircular quartz plate of 25 mm diameter was mounted on

pin-on device and then rotated at a frequency of 800 rpm. Usyringe, 0.2 mL of the membrane solution was sprayed t

enter of the plate. After a spinning time of 4 s, a membf 5�m thickness, which was determined by the surfaceler, was then coated on the quartz plate. The concentrf pyrene and HDM-�-CD in the PVC membrane was abo.49 and 9.4 mM, respectively. The relative standard derivR.S.D.) for the membrane thickness and the blank fluorescntensity of 10 newly prepared sensing membranes were 1..0%, respectively, reflecting good reliability of the membrreparation technique.

X. Wang et al. / Analytica Chimica Acta 556 (2006) 313–318 315

Fig. 2. Effect of the amount of HDM-�-CD on fluorescence intensity of 0.49 mMpyrene in membrane: (1) 0 mM HDM-�-CD, (2) 2.45 mM HDM-�-CD, (3)4.9 mM HDM-�-CD, (4) 7.35 mM HDM-�-CD and (5) 9.4 mM HDM-�-CD.The inset shows the dependence of the I/III intensity ratio for pyrene on the sameconcentration of HDM-�-CD in membrane.

2.4. Measurement procedure

The fluorescence measurements were carried out at the max-imum excitation wavelength of 286 nm and the maximum emis-sion wavelength of 390 nm. The sample solution was fed throughthe detecting chamber of the flow cell by the peristaltic pump ata rate of 2.0 mL min−1. After each measurement, the flow cellwas washed with water until the fluorescence intensity of thesensor reached the original blank value.

3. Results and discussion

3.1. The supramolecular interaction between pyrene andHDM-β-CD

The interaction between pyrene and CDs has been extensively studied and complex with 1:1, 1:2, and 2:2 (pyrene:CD)stoichiometries has been reported[27–30].Fig. 2shows the fluo-rescence emission spectra of pyrene in the presence of differeconcentrations of HDM-�-CD in the sensing membrane. Theemission band from 370 to 400 nm is attributed to the pyrene

monomer, while the broad band extending from 410 to 550 nmis ascribed to the excimer emission[32]. Noticeable fluores-cence enhancement of pyrene was observed in the presenceof HDM-�-CD, indicating the inclusion of pyrene within therelatively nonpolar cyclodextrin cavity. The substitution of thehydroxyl groups on�-CD by alkyl groups reduces to some extentthe polarity of�-CD, facilitating the complexation process andcan decrease the degree of freedom in the motion of pyrenemolecule entrapped in CD’s cavity, which provides more pro-tection for pyrene. Since the host–guest recognition involved in aplasticized polymer membrane was often a microheterogeneousreaction[33,34], more persuasive evidence of the formationof HDM-�-CD/pyrene inclusion complex can be provided bythe pyrene’s monomer emission ratio of the I (373 nm) to III(383 nm) bands, which is sensitive to the microenvironmentalpolarity around pyrene[35,36]. The inset ofFig. 2shows the I/IIIemission-intensity ratio. As can be seen, with increasing HDM-�-CD concentrations, the I/III ratio was gradually decreasedtill a constant value obtained, revealing the maximum inclusionequilibrium between pyrene and HDM-�-CD occurred in themembrane. Thus, a hydrophobic and protective microenviron-ment for pyrene was formulated shielding the excited state ofpyrene from quenching by water molecules and dissolved oxy-gen. The Benesi–Hildbrand equation[37] was used to obtaina 1:1 complex ratio between pyrene and HDM-�-CD with anapparent formation constant of 1.23× 103 L mol−1.

3

endss ranec OS,a -� th aD n oft nflu-e itivity.F em-b em-b ationc asingo

Table 1Analysis characteristics of the sensing membranes of different compositions

Membrane no. Pyrene HDM-�-CD Fluore

1 42.02 47.03 55.54 55.65 41.06 45.07 48.0

nse p

mg mM mg mM

0.20 0.49 13.0 4.90.20 0.49 19.7 7.40.20 0.49 25.0 9.40.20 0.49 32.7 12.30.40 0.98 13.0 4.90.40 0.98 25.0 9.40.40 0.98 32.7 12.3

a Mean value of three determination.b The detection range was defined as 0.05 <α < 0.95, whereα was the respoc Method detection limit using 3σ method[38].

-

nt

.2. Optimization of membrane compositions

The response behavior for a given optical sensor depignificantly on the membrane composition. Several membompositions were compared using varying ratio of PVC, Dnd pyrene/HDM-�-CD complex. At each pyrene and HDM-CD concentration, maximum sensitivity was obtained wiOS/PVC weight ratio of 2:1. Moreover, the concentratio

he sensing material incorporated within the membrane inced the magnitude of the sensor response and its sensrom Table 1, the fluorescence intensity of the sensing mrane increased with an increasing amount of pyrene in the mrane up to about 0.1 wt.%. But too high pyrene concentrould cause inner filtering effect and fluorescence decref the membrane was observed. Since the pyrene/HDM-�-CD

scence intensitya Detection range (mol L−1)b MDL (mol L−1)c

8.20× 10−7 to 2.56× 10−6 6.80× 10−7

1.50× 10−7 to 2.43× 10−6 1.00× 10−7

7.90× 10−8 to 1.66× 10−5 7.00× 10−8

1.20× 10−7 to 9.80× 10−6 8.50× 10−8

1.05× 10−6 to 8.80× 10−6 7.20× 10−7

8.30× 10−7 to 5.50× 10−6 5.40× 10−7

4.80× 10−7 to 4.50× 10−6 2.50× 10−7

arameter[41].

316 X. Wang et al. / Analytica Chimica Acta 556 (2006) 313–318

Fig. 3. Fluorescence spectra of the sensing membrane in the presence ofdifferent concentrations of BPA: (1) blank solution, (2) 1.0× 10−7 mol L−1,(3) 5.0× 10−7 mol L−1, (4) 8.0× 10−7 mol L−1, (5) 3.0× 10−6 mol L−1, (6)6.0× 10−6 mol L−1, (7) 8.0× 10−6 mol L−1 and (8) 1.0× 10−5 mol L−1.

inclusion complex was used as the probe, the amount ofHDM-�-CD was very crucial. Membrane 3 (pyrene 0.2 mg,HDM-�-CD 25 mg) showed the best performance in terms ofdetection range and method detection limit (MDL)[38] for theassay of BPA (Table 1). The compositions of pyrene and HDM-�-CD in membrane 3 were used in subsequent experiments,corresponding to 0.49 and 9.4 mM in the membrane-preparingcocktail.

3.3. Response of the sensor to BPA

Fig. 3 shows the fluorescence spectra of the sensing membrane exposed to a solution containing different concentrationof BPA. As can be seen, the presence of BPA caused a remarable decrease in the fluorescence intensity of pyrene/HDM�-CD complex, which can be explained by excluding pyrenefrom the cavity associated with BPA accommodation. Sincethis guest-induced fluorescence variation is linked with thecavity-competition reaction, it was suggested that this sys-tem can be used as a sensing system for detecting BPin water.

3.4. Effect of solution pH

The effect of solution pH on the sensor was investigatedo e of8 -i atp emb A ina aset ucedi -o eana sens onsr

3.5. Determination of the complex ratio and theequilibrium constant

Based on an operation principle reported[40], the relation-ship between response parameter,α, and BPA concentration[G(aq)] can be represented as:

αn

1 − α= 1

nKCn−1H [G(aq)]m

(1)

where m and n are the complexing number of BPA andpyrene/HDM-�-CD inclusion complex, respectively.CH andK refer to the total amount of pyrene/HDM-�-CD inclusioncomplex in membrane and the overall equilibrium constant,respectively. Eq.(1) is the basis of the quantitative determinationof BPA in aqueous solution by using the proposed sensor. Theexperimental data were fitted to Eq.(1) by changing the ratio ofn tom and adjusting the overall equilibrium constant,K. It showsthat the curve referring to the 1:1 complex ratio andK = 7.05 ×105 is the best one fitted to the experimental data (Fig. 4), whichcan serve as the calibration curve for the determination of BPA.

3.6. Response characteristics of the sensor

The response of the sensor toward BPA is associated with theanalyte diffusion in polymer entity and the reversible interactionb , thes1 c-t hes n,u oingf r9 in,w ge of1 culesc v-i theo quite

Ff

ver a wide pH range from 2.0 to 12.0 in the presenc.0× 10−7 mol L−1 BPA. Because the pKa of BPA is approx

mate 9.28[39], the dissociation of BPA was inhibitedH < 9.50 and the fluorescence intensity of the sensing mrane remained a constant; while at relatively higher pH, BPqueous solution inclined to dissociate, which in turn decre

he extraction efficiency of BPA into the membrane and redts supramolecular inclusion with HDM-�-CD. Thus, the flurescence intensity of the membrane increased. This mdjustment of solution pH was unnecessary since theor’s response already fell within the most sensitive respegion.

-sk--

A

-

d

s-

e

etween host and the analyte. With the optimum conditionsenor exhibits a dynamic detection range from 7.90× 10−8 to.66× 10−5 mol L−1 (0.05 <α < 0.95) [41] and method dete

ion limit of 7.00× 10−8 mol L−1. The response behavior of tensor to BPA was shown inFig. 5 (curve 1). As can be seender the optimum conditions, the forward response time (g

rom lower to higher BPA concentrations),t95 (time needed fo5% of the total signal change to occur) was within 1.3 mhereas the time for the reverse response was in the ran.8 min over the entire concentration range. The BPA moleomplexed with HDM-�-CD could be eluted out of the CD caty quickly and completely by pyrene solution, whereuponriginal fluorescence was restored. Thus, the sensor was

ig. 4. The best fitting result with 1:1 of the response parameter valuesα as aunction of the logarithm of BPA concentration according to Eq.(1).

X. Wang et al. / Analytica Chimica Acta 556 (2006) 313–318 317

Fig. 5. The fluorescence intensity response (λex = 286 nm, λem= 390 nm)of the sensor vs. time after several repeated BPA concentrations changesbetween (a) 5.0× 10−6 mol L−1 pyrene solution, (b) 1.0× 10−7 mol L−1, (c)8.0× 10−7 mol L−1 and (d) 6.0× 10−6 mol L−1 BPA solution. Curve 1 andcurve 2 refer to the membrane with or without HDM-�-CD, respectively.

reversible and no noticeable hysteresis effect was observed,which indicated that the migration rate of the analyte in thedynamic process was fast[42]. Without HDM-�-CD in the mem-brane, the response behavior of the sensor was also recorded andshowed inFig. 5(curve 2). It was apparent that when no HDM-�-CD coexisted in the membrane, the fluorescence changes upoBPA addition can hardly be observed. The presence of HDM-�-CD turns out to be essential to observe the time responsereflecting that the competitive reaction could not occur unlessan inclusion complex was formed in the membrane.

3.7. Short-time stability

The short-time stability of the sensor was tested by recording the fluorescence intensity of 2.0× 10−7 mol L−1 BPA over aperiod of 8 h for 48 measurements. Between each measurementhe sensor was regenerated with pyrene. A relative standard devation of the fluorescence reading of 2.0% was recorded. Thfluorescence intensity of the membrane decreased 3.5% aftercontinuous 200 measurement. The sensing membrane exhibiteneither bleaching of the fluorophore, chemical decompositionnor peeling-off of the polymer layer from the glass support for aperiod of longer than 3 months when stored in the dark at roomtemperature.

3

ationo ito eatet t to2NNfC

PO43−; 300 for o-cresol,m-cresol,p-cresol; 200 for salicylic

acid, formic acid, 1-naphthol, 2-naphthol, 2,6-xylenol; 180 forpyrocatechol, resorcinol, hydroquinone,p-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and 50 for phenol, respec-tively.

3.9. The response mechanism of the sensor

BPA has been reported to be included into the CD’s cavity[43,44]. When BPA was introduced to the host–guest systemof pyrene/HDM-�-CD as a third component, it would com-pete for the HDM-�-CD cavity with pyrene. The partial pyrenemolecules would be expelled from the HDM-�-CD cavities.As the photochemical and photophysical properties of pyrenestrongly depended on its local microenvironment, the exclusionwould make it lose the protection of HDM-�-CD cavity, whichresulted in the decrease of its fluorescence intensity.

3.10. Method evaluation

Since the present work was to develop a simple and con-venient method for the determination of BPA in water, it wasevaluated with reference to the USEPA methods[38]. Resultsshowed that the mean recovery and the R.S.D. was in the rangeof 95.3–97.5 and 2.55–3.85%, respectively.

3

n ofB pringa ment.T etate

TD

S )

T

R

S

R

.8. Selectivity of the sensor

The effects of some foreign species on the determinf 2.0× 10−7 mol L−1 BPA were studied. The tolerable limf a foreign species was taken as a relative error not gr

han±5%. The tolerable concentration ratios with respec.0× 10−7 mol L−1 BPA were determined as 5000 for Na+, K+,H4

+, Mg2+, Zn2+, Cl−, NO3−, Br−, CO3

2−; 3000 for F−,O2

−; 2000 for Ca2+, Ni2+, Cu2+, Cd2+, SO42−, SO3

2−; 1500or Fe2+, Fe3+, Al3+; 1000 for Pb2+, Co2+, Mn2+, Cr3+, HCOO−,H3COO−, MnO4

−, SiO32−, C2O4

2−, MoO42−, Cr2O7

2−,

n

,

-

t,i-

ead

,

r

.11. Analytical application

The proposed sensor was applied to the determinatioPA in tap, river, spring, and rain water samples. The tap, snd rain water samples were analyzed without any pretreathe river water samples were filtered through a cellulose ac

able 2etermination of BPA in tap, river, and spring water samples

ample BPA spiked(mol L−1)

BPA found (mol L−1),meana ± S.D.b

Recovery (%

ap water1 9.00× 10−8 (9.45± 0.13)× 10−8 1052 2.00× 10−7 (1.93± 0.23)× 10−7 963 1.00× 10−6 (9.73± 0.19)× 10−7 97

iver waterc

1 9.50× 10−8 (8.93± 0.21)× 10−8 942 3.00× 10−7 (3.18± 0.20)× 10−7 1063 2.50× 10−6 (2.38± 0.17)× 10−6 95

pring waterd

1 9.80× 10−8 (1.03± 0.25)× 10−7 1052 2.50× 10−7 (2.43± 0.35)× 10−7 973 2.00× 10−6 (2.01± 0.12)× 10−6 101

ain water1 1.40× 10−7 (1.46± 0.18)× 10−7 1042 3.50× 10−7 (3.71± 0.23)× 10−7 1063 8.50× 10−8 (8.30± 0.30)× 10−7 98

a Mean of five determinations.b S.D.: standard deviation.c From Qinghe River, Beijing.d From Yunmeng Mountain, Beijing.

318 X. Wang et al. / Analytica Chimica Acta 556 (2006) 313–318

Table 3Determination of BPA in landfill leachate (n = 5)

Samplea BPA determined (mol L−1) BPA added (mol L−1) BPA found (mol L−1) Recovery (%)

This method Ref.[45]

1 8.18× 10−6 7.85× 10−6 8.00× 10−6 1.66× 10−5 1052 9.39× 10−6 9.02× 10−6 4.50× 10−6 1.41× 10−5 105

a From Liulitun Landfill Factory.

filter with a pore size of 0.45�m before analysis. All the watersamples were spiked with BPA at different concentration levelsand then analyzed. The results were shown inTable 2. Further-more, the results obtained using the proposed method in thedetermination of BPA in landfill leachate were also comparedwith those obtained by the standard GC–MS method[45]. Thelandfill leachate was first centrifugated at 5000 rpm and thesupernate was filtered through a 0.45�m cellulose acetate filter,then diluted to 1:100 with deionized water. As can be seen fromTable 3, there was good agreement between the two methods.

4. Conclusions

A simple, sensitive, selective and fast fiber-optic chemicalsensor for the determination of BPA in water has been developedbased on the fluorescence quenching of pyrene/HDM-�-CDsupramolecular complex, which was thought to be associatedwith the exclusion of pyrene from inside to outside of theHDM-�-CD cavity by accommodating a BPA molecule. Thefluorescence signal variation was very reversible and was pro-portional to BPA concentration. The study results enabled abetter understanding of host–guest chemistry and showed thatthe supramolecular assembly can be used as a powerful tool fordeveloping a high-quality chemical sensor. The proposed sensorwas used to analyze BPA in real water samples with satisfactoryresults.

A

f theN s ofC n oC gramo

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124

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un.

l. 3,

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[ 03)

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cknowledgements

The authors gratefully acknowledge financial support oational Science Fund for Distinguished Young Scholarhina (No. 20125514), National Natural Science Foundatiohina (No. 20437020, 50273046) and Major Research Prof Chinese Academy of Sciences (KZCX3-SW-432).

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