AnalyticalMethods

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A novel molecula

aCollege of Environment and Resources, Fu

E-mail: lixiaojuan@fzu.edu.cn; Tel: +86-591bCollege of Life Sciences, Fujian Agriculture

ChinacAQSIQ Key Laboratory of Drug Detection,

Center, Fujian Entry-Exit Inspection and

350001, ChinadFujian Radiation Environmental SupervisioeState Key Laboratory of Photocatalysis o

Chemistry, Fuzhou University, Fuzhou 3500

Cite this: Anal. Methods, 2016, 8, 7445

Received 22nd July 2016Accepted 13th September 2016

DOI: 10.1039/c6ay02087a

www.rsc.org/methods

This journal is © The Royal Society of C

rly imprintedelectrochemiluminescence sensor based ona Ru(bpy)3

2+/MWCNTs/nano-TiO2-Nafionelectrode for the detection of bisphenol A

Yulin Lin,a Jie Cao,bc Xiaojuan Li,*a Xinying Zhang,a Jianhua Zhangd and Zhenyu Line

A novel molecularly imprinted electrochemiluminescence sensor based on a Ru(bpy)32+/multi-walled

carbon nanotubes (MWCNTs)/nano-titanium dioxide (nano-TiO2)-Nafion electrode was established for

the ultrasensitive and ultraselective detection of bisphenol A (BPA). The introduction of MWCNTs/nano-

TiO2 nanohybrids into the Nafion film could provide an excellent basis for effective immobilization of

Ru(bpy)32+ and improve the electrochemical and ECL properties of Ru(bpy)3

2+. The MIP-ECL sensor

fabricated by the combination of the MIT with the Ru(bpy)32+ ECL system could provide specific binding

sites for BPA and the selectivity of the sensor was significantly improved. Under the best optimum

conditions, the proposed MIP-ECL sensor not only exhibited high sensitivity and selectivity for BPA, but

also displayed acceptable reproducibility and stability. A linear range from 0.01 mg L�1 to 2 mg L�1 for the

detection of BPA was observed with a detection limit of 0.0041 mg L�1. Moreover, the fabricated sensor

was successfully used to detect BPA in ambient water samples and relatively satisfactory recoveries were

obtained.

1. Introduction

Bisphenol A (BPA) is an important industrial chemical materialand is widely used for the production of epoxy resins and pol-ycarbonate plastics.1 It is reported that BPA can act as a typicalendocrine disruptor and has negative impacts on reproduction,neurobehavioral development, and metabolism, leading to e.g.,obesity, diabetes, heart disease, and thyroid and liver dysfunc-tion.2 The European Union (EU) has set food migration limitsfor BPA as 0.6 mg kg�1. And low levels of BPA will do great harmto human health and aquatic life.3 Therefore, the developmentof sensitive and selective methods for BPA detection hasbecome one of the most attractive subjects in analyticalchemistry.

Recently, various analytical methods have been reportedfor the determination of BPA, including gas chromatographymass spectrometry (GC-MS),4 liquid chromatography-mass

zhou University, Fuzhou, 350116, China.

-22866086

and Forestry University, Fuzhou, 350002,

Fujian International Travel Healthcare

Quarantine Bureau of P.R.C., Fuzhou,

n Station, Fuzhou, 350013, China

n Energy and Environment, College of

02, China

hemistry 2016

spectrometry (LC-MS),5 and uorescence spectrometry (FL).6

These methods are effective, but they are usually quite expen-sive, time-consuming, have poor detection limits or requirecomplex pretreatments, which could not meet the requirementsof on-site and rapid detection. Hence, it is still a great challengeto develop a novel method for the determination of BPA withsimple operation, high sensitivity and good selectivity. Andsome new analytical methods such as enzyme-linked immu-nosorbent assay (ELISA),7 electrochemistry8 and electro-chemiluminescence (ECL)9,10 have been developed for theselective determination of BPA.

Among the methods mentioned above, ECL methods, alsowell known as electrogenerated CL, combining the advantagesof electrochemistry and CL, have received great attention due totheir high sensitivity, wide linear range, simple operation andcontrollability.10 Ru(bpy)3

2+ ECL, a typical and most used ECLsystem, can be used to detect analytes at low concentrations.11

However, the inherent water-solubility and low selectivity to co-reactants of Ru(bpy)3

2+ are bottlenecks of ECL sensor applica-tions.12 Thus, the immobilization of Ru(bpy)3

2+ onto a solidelectrode surface and improving the selectivity of these sensorsare important for Ru(bpy)3

2+ ECL applications.Naon is a sulfonated ion-exchange polymer which exhibits

high chemical, electrochemical and thermal stabilities andgood biocompatibility.13 Naon is oen employed for theimmobilization of Ru(bpy)3

2+ due to its high ion-exchangeselectivity coefficient towards Ru(bpy)3

2+. But the rate of charge

Anal. Methods, 2016, 8, 7445–7452 | 7445

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transfer of Ru(bpy)32+ in the pure Naon lm is quite low. Also,

it has been reported that the immobilized Ru(bpy)32+ can

migrate into the electrochemically inactive hydrophobic regionof Naon over the course of time and consequently its electro-chemical signal is not stable.13 Thus, there is a great interest toovercome the mentioned problems by introducing newapproaches. A variety of new nanomaterials with unique char-acteristics, such as multi-walled carbon nanotubes (MWCNTs),14

graphene,15 nano-silica (nano-SiO2)16 and nano-titanium dioxide(nano-TiO2),17 have been introduced into the Naon lm in orderto improve the electrochemical and ECL properties of Ru(bpy)3

2+

and also to increase the amount of the immobilized Ru(bpy)32+

and to prevent luminophore leaching. Moreover, to furtherimprove the performance of the Ru(bpy)3

2+-Naon based ECLsystem, nanohybrids with the integrated properties of twonanocomponents, such as nano-ZnO/MWCNTs,13 nano-SiO2/MWCNTs18 and Au NPs/MWCNTs,19 have been used for thefabrication of Ru(bpy)3

2+-Naon based ECL sensors, and theresults revealed that the nanohybrids had signicant enhancingeffects on the electrochemical and ECL properties of luminol.However, to the best of our knowledge, the MWCNTs/nano-TiO2

doped Ru(bpy)32+-Naon ECL sensor has rarely been studied.

Molecular imprinting technology (MIT) is an approach tosynthesize molecularly imprinted polymer (MIP) matrices withmolecular recognition sites, which can selectively recognize andbind to target molecules (template molecules).20 Recently, MIPshave been used in the form of free-standing membranes forsensor technology. However, because of the slow diffusion ofanalytes to recognition sites and inefficient communicationbetween binding sites and transducers, MIP sensors oen showrelatively low sensitivity.21 The fabrication of a MIP-ECL sensorby the combination of the MIT with the Ru(bpy)3

2+ ECL systemwould be able to solve the problem. Currently, lots of MIP-ECLsensors have been utilized to detect different organiccompounds and biomacromolecules, and the investigationsrevealed that MIP-ECL is a detection method with high sensi-tivity and desirable selectivity.22–25

In this study, a novel molecularly imprinted electro-chemiluminescence sensor based on a Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrode was successfully designed forultrasensitive and ultraselective detection of BPA. Naon wasemployed for the immobilization of Ru(bpy)3

2+. The MWCNTsand nano-TiO2 were together introduced into the Naon lm inorder to improve the electrochemical and ECL properties ofRu(bpy)3

2+. Finally, the BPA-imprinted sol was scribbled ontothe Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrode surfaceand the novel MIP-ECL sensor was obtained. The above-mentioned MIP-ECL sensor exhibited excellent performance forthe analysis of BPA, paving the way for a novel approach for thedetermination of other small molecules even macromolecules.

2. Experimental2.1. Materials

Bisphenol A (BPA), (3-aminopropyl)triethoxysilane (APTEOS),methoxytrimethylsilane (MTMOS), phenyl methoxysilane(PTMOS), potassium ferricyanide (K3Fe(CN)6), multi-walled

7446 | Anal. Methods, 2016, 8, 7445–7452

carbon nanotubes (MWCNTs), dimethylsulfoxide (DMSO),bipyridine ruthenium (Ru(bpy)3Cl2$6H2O), and Naon(5 wt%)were obtained from Sigma Chemical Co. (St. Louis, MO, USA).Butyl titanate (TPOT), isopropanol, dipotassium phosphate(NaH2PO4), potassium dihydrogen phosphate (Na2HPO4) andethanol were purchased from Sinopharm Chemical ReagentCo., Ltd., China. The phosphate buffer saline solution (PBS) wasprepared by mixing the solutions of NaH2PO4 and Na2HPO4. Allchemicals and solvents used were of analytical grade and wereused as received without further purication. Ultrapure water(18.2 MU cm) used was processed with a Milli-Q water puri-cation system (Millipore Co., USA) and used throughout allexperiments.

2.2. Apparatus

Cyclic voltammetry (CV) was conducted with a CHI 660E elec-trochemistry workstation (Shanghai CH Instruments Co.,China). The ECL measurements were performed on a BPCLweak luminescence measuring instrument (Beijing ChineseAcademy of Sciences, China). The classical three-electrodesystem consisted of a gold electrode or a molecularly imprintedpolymer-Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrode asthe working electrode, a silver–silver chloride (Ag/AgCl) as thereference electrode, and a platinum wire electrode as thecounter electrode. The morphologies of the nanomaterials wereobserved by using a eld emission scanning electron micro-scope (FE-SEM; Nova Nano SEM 230, FEI Co., USA). The struc-ture of MIPs was characterized by using an atomic forcemicroscope (AFM; 5500, Agilent, USA).

2.3. Preparation of the Ru(bpy)32+/MWCNTs/nano-TiO2-

Naon electrode

5 mL ultrapure water and 7 mL 0.1 mol L�1 HCl solution wereadded to 1 mL 0.15 mol L�1 TPOT solution with stirring for 1 hat 40 �C and a nano-TiO2 sol was obtained.26,27 A nano-TiO2-Naon solution was prepared by mixing the nano-TiO2 sol withNaon (5 wt%) under ultrasonication for 1 h.

The carboxylated-MWCNTs were prepared according to themethod reported previously.28 The commercially availableMWCNTs were dispersed in a 1 : 3 (v/v) mixture of concentratedHNO3 and H2SO4 under stirring for 12 h at room temperature.Then the carboxylated-MWCNTs were washed with water untilneutral, ltered through a microporous membrane, and driedat 80 �C. Subsequently, 1.0 mg obtained carboxylated-MWCNTswere immersed in 1mL dimethyl sulfoxide and sonicated for 1 hto form a stable suspension.

The carboxylated-MWCNT suspension was then mixed withthe nano-TiO2-Naon solution to form a MWCNTs/nano-TiO2-Naon solution under ultrasonication for 1 h.

Prior to modication, the gold electrode (2 mm diameter)was polished to a mirror-like surface with 0.3 and 0.05 mmalumina slurry followed by successive sonication in ethanol andultrapure water for 5 min and dried in air.

The fabrication procedure of the Ru(bpy)32+/MWCNTs/nano-

TiO2-Naon electrode is displayed in Scheme 1. Initially, 7.5 mLMWCNTs/nano-TiO2-Naon solution was dropped onto the bare

This journal is © The Royal Society of Chemistry 2016

Scheme 1 Schematic illustrations of the fabrication procedure of the MIP-ECL sensor.

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gold electrode and evaporated in air to form a homogeneous lmto prepare a MWCNTs/nano-TiO2-Naon electrode. Then, themodied electrode was soaked in 10 mL 1 mM Ru(bpy)3

2+ solu-tion for 30 min. Finally, the electrode was thoroughly rinsed withultrapure water and dried at room temperature. For comparison,a MWCNTs-Naon electrode and a Ru(bpy)3

2+/MWCNTs-Naonelectrode were also synthesized using the same procedure butwithout the addition of the nano-TiO2 sol.

2.4. Preparation of the MIP-ECL sensor

The template molecular BPA was embedded to mark the lmusing the sol–gel process. The non-imprinted polymer (NIP) solwas made of 200 mL APTEOS, 60 mL MTMOS, 60 mL PTMOS, 200mL anhydrous ethanol, 10 mL HCl (0.1 M) and 200 mL ultrapurewater.24 Then the molecularly imprinted polymer (MIP) sol wasobtained bymixing 40 mL 100mg L�1 BPA solution with the NIPs.

As schematically shown in Scheme 1, the MIP-ECL sensorwas prepared as follows: 10 mL of MIPs was dropped onto thesurface of the Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrodeand dried at room temperature. Then it was washed thoroughlywith ultrapure water for 40 min at 40 �C in order to remove thetemplate molecule. Ultimately, the prepared sensor wasstored at 4 �C when not in use. For comparison, a non-imprinted electrochemiluminescence sensor based on the

Fig. 1 FE-SEM images of (A) carboxylated-MWCNTs, and (B) TiO2-Nafio

This journal is © The Royal Society of Chemistry 2016

Ru(bpy)32+/MWCNTs/nano-TiO2-Naon electrode (NIP-ECL)

was also prepared using the same procedure.

2.5. Electrochemical and ECL measurements

The production of the electrochemical signal is due to theoxidation or reduction of [Fe(CN)6]

3�/4� on the electrode.Hence, [Fe(CN)6]

3�/4� was used as an electrochemically activeprobe to study the response of the electrodes.29

Ru(bpy)32+, which can produce an ECL signal, was used to

study the response of the MIP-ECL sensor. The MIP-ECL sensorwas incubated in different concentration BPA solutions for20 min and then the ECL behavior was nally monitored overa scan range of �0.2–1.4 V in PBS solution (pH ¼ 7.4) witha scan rate of 100 mV s�1.

All electrochemical and ECL experiments were carried out atroom temperature. Estrone (E1), 17b-estradiol (E2) and non-ylphenol (NP) were chosen as interfering molecules to evaluatethe selectivity of the MIP-ECL sensor.

3. Results and discussion3.1. Preparation of the MIP-ECL sensor

The purpose of this study was to prepare a MIP-ECL sensor forBPA detection with both high sensitivity and selectivity. A two-

n and (C) MWCNTs/TiO2-Nafion nanomaterials.

Anal. Methods, 2016, 8, 7445–7452 | 7447

Fig. 2 (A) Cyclic voltammograms of the bare gold electrode (curve a), MWCNTs-Nafion electrode (curve b) and MWCNTs/nano-TiO2-Nafionelectrode (curve c) in 5mMK3Fe(CN)6 solution (scan rate: 100mV s�1); (B) ECL intensity of the Ru(bpy)3

2+/MWCNTs/nano-TiO2-Nafion electrode(curve a) and Ru(bpy)3

2+/MWCNTs-Nafion electrode (curve b) in 0.1 M PBS solution (pH ¼ 7.4).

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step fabrication procedure, as illustrated in Scheme 1, wasproposed. The rst step was to coat a light-emitting layer ona gold electrode and the second step involved the covering ofa MIP layer on the light-emitting layer.

The BPA-imprinted lm was fabricated by an organosiloxanesol–gel technology and involved hydrolysis/condensation reac-tions of PTMOS, MTMOS and APTEOS in the presence of BPA.PTMOS and MTMOS played the role of functional monomers.The phenyl group of PTMOS can form p–p stacking (or hydro-phobic) interaction with BPA.30 The hydroxy group of PTMOSand MTMOS could form hydrogen bonds with the hydroxygroup existing in BPA. As a cross-linker, APTEOS could forma polymeric network highly rigid around the BPA templatemolecule through Si–O bonds via hydrolysis, wherefore thesiloxane lm presented amino groups and hydroxy groups,31,32

which could interact with the template BPA through hydrogen

Fig. 3 AFM images of (A) the non-eluted NIP film, (B) eluted NIP film, (C

7448 | Anal. Methods, 2016, 8, 7445–7452

bonds. In addition, the aminopropyl groups in an aqueoussolution establish an equilibrium with the (CH2)3–NH3

+ group,which can interact with the anionic species of the imprintedmolecules.33 All these interactions were reversible and were thedriving force for the recognition process of BPA in the testsolution.

3.2. Characterization of the nanomaterials

FE-SEM images of carboxylated-MWCNTs, nano-TiO2-Naon andMWCNTs/nano-TiO2-Naon nanomaterials are shown in Fig. 1.Clearly, there was a great difference in the morphologies betweencarboxylated-MWCNTs, nano-TiO2-Naon and MWCNTs/nano-TiO2-Naon. Fig. 1A shows the ber-like morphologies of theMWCNTs and it can be seen that the MWCNTs were welldispersed. Mixing the nano-TiO2 sol with Naon formed irregularnano-TiO2 particles with a certain degree of agglomeration and

) non-eluted MIP film and (D) eluted MIP film.

This journal is © The Royal Society of Chemistry 2016

Fig. 4 (A) Cyclic voltammograms of the MIP-ECL sensor before (curvea) and after (curve b) elution in 5mMK3Fe(CN)6 solution (scan rate: 100mV s�1); (B) cyclic voltammograms of the MIP-ECL sensor after (curvea) and before (curve b) re-adsorption of BPA and the NIP-ECL sensorafter adsorption of BPA (curve c) in the 0.1 M PBS solution (scan rate:100 mV s�1); (C) ECL responses of the MIP-ECL sensor after (curve a)and before (curve b) re-adsorption of BPA and the NIP-ECL sensorafter adsorption of BPA (curve c), and the inset shows the ECLresponses of the MIP-ECL sensor before re-adsorption of BPA (curveb) and the NIP-ECL sensor after adsorption of BPA (curve c) in 0.1 MPBS solution (pH ¼ 7.4).

Fig. 5 Effect of (A) pH and (B) accumulation time (in the presence ofa 100 mg L�1 BPA solution).

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the diameter was approximately 100 nm (Fig. 1B). Fig. 1C showsthat the MWCNTs were distributed densely and uniformly in thenano-TiO2-Naon, so that the block was formed integrally, whichwas benecial to the sensor performance.19,34 The wrinkled,rough and multilayer structure provided a larger surface area forthe adsorption of Ru(bpy)3

2+ and template molecules.12

This journal is © The Royal Society of Chemistry 2016

3.3. Electrochemical and ECL characterization of theRu(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrode

The electrochemical properties of the MWCNTs/nano-TiO2-Naon electrode, MWCNTs-Naon electrode and bare goldelectrode were studied by cyclic voltammetry. Fig. 2A depicts thecyclic voltammograms of the bare gold electrode and themodied electrodes in 5 mM K3Fe(CN)6 solution. It can be seenthat there was a pair of reduction–oxidation peaks at 0.3 V and0.5 V on the bare gold electrode (Fig. 2A, curve a). Aer beingmodied with MWCNTs-Naon and MWCNTs/nano-TiO2-Naon, the peak currents all increased. In addition, comparedto the MWCNTs-Naon electrode (Fig. 2A, curve b), the reduc-tion–oxidation peak current dramatically increased on theMWCNTs/nano-TiO2-Naon electrode (Fig. 2A, curve c). Theresult demonstrated that the addition of nano-TiO2, which hasa high electrical conductivity and large surface area, couldsignicantly improve the electrochemical capability of theelectrode. Similar enhancements were also found in previousstudies.35–37

The ECL behaviors of the Ru(bpy)32+/MWCNTs/nano-TiO2-

Naon electrode and Ru(bpy)32+/MWCNTs-Naon electrode in

0.1 M PBS solution were then investigated. As shown in Fig. 2B,the ECL intensity of the Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naonelectrode was enhanced with the existence of nano-TiO2.This trend can be rationalized as follows:38 more Ru(bpy)3

2+

molecules were immobilized on the MWCNTs/nano-TiO2-Naon electrode surface, charge transfer resistance at the

Anal. Methods, 2016, 8, 7445–7452 | 7449

Fig. 6 (A) The ECL responses of the MIP-ECL sensor to BPA, E1, E2, and NP, and the inset shows the ECL responses of the MIP-ECL sensor to E1,E2, and NP; (B) ECL emission from the MIP-ECL sensor before (curve a) and after (curve b) 30 days in 0.1 M PBS solution (pH¼ 7.4) containing 50mg L�1 BPA under continuous cyclic voltammetry for ten cycles (scan rate: 100 mV s�1).

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Ru(bpy)32+/MWCNTs/nano-TiO2-Naon electrode was smaller,

and the immobilization of Ru(bpy)32+ on the MWCNTs/nano-

TiO2-Naon electrode via ion exchange and electrostatic inter-action was relatively stable.

3.4. AFM images of the NIPs and MIPs

The morphologies of NIP and MIP lms were investigated byAFM. As shown in Fig. 3, AFM images of the non-eluted NIP lm(Fig. 3A), eluted NIP lm (Fig. 3B) and non-eluted MIP lm(Fig. 3C) were without cavities, surface roughness, and a tightrigid structure, while the eluted MIP lm (Fig. 3D) hada uniform distribution of cavities. The results indicated that thetemplate molecule (BPA) might be successfully eluted fromMIPs at 40 �C with ultrapure water and the cavities appeared inthe MIPs aer eluting.

Fig. 7 Linear relationship between the concentration of BPA and ECLintensity, BPA concentrations: 0.01 mg L�1 to 2 mg L�1, and the insetshows the calibration curve of BPA.

3.5. Electrochemical and ECL characterization of the MIP-ECL senor

The electrochemical behaviors of the MIP-ECL senor before andaer elution were measured by cyclic voltammetry in 5 mMK3Fe(CN)6 solution and the results are shown in Fig. 4A. Noreduction peak appeared in the cyclic voltammogram of theMIP-ECL sensor before elution (Fig. 4A, curve a) while a signif-icantly high reduction peak was found in the cyclic voltammo-gram of the MIP-ECL sensor aer elution (Fig. 4A, curve b).Removing the template molecule (BPA) from the molecularlyimprinted membrane by eluting would leave cavities on thepolymer lm and the cavities could serve as the tunnels for thetransfer of the [Fe(CN)6]

3�/4� probe, resulting in a correspond-ing reduction peak in 5 mM K3Fe(CN)6 solution. The electro-chemical characteristics of the MIP-ECL sensor before/aer re-adsorption of BPA and the NIP-ECL sensor aer adsorption ofBPA were also determined in 0.1 M PBS solution (Fig. 4B). Aerre-adsorption of BPA onto the MIP-ECL sensor, the reduction–oxidation peak current dropped. This result could verify that thecavities could be occupied again by the BPA and the tunnels ofelectron transfer would be blocked. The cyclic voltammogram ofthe NIP-ECL sensor aer adsorption of BPA was almost identical

7450 | Anal. Methods, 2016, 8, 7445–7452

to that of the MIP-ECL sensor aer re-adsorption of BPA, sincethere were no cavities on the surface of both sensors.

The ECL characterization of the MIP-ECL sensor before/aerre-adsorption of BPA and NIP-ECL sensors aer adsorption ofBPA in 0.1 M PBS solution was performed and the results areshown in Fig. 4C. The ECL intensity of MIP-ECL sensors aer re-adsorption of BPA (Fig. 4C, curve a) was signicantly higherthan that of MIP-ECL sensors without adsorption of BPA(Fig. 4C, curve b) and also higher than that of the NIP-ECLsensor aer adsorption of BPA (Fig. 4C, curve c). It had beendemonstrated that BPA could be oxidized on the gold electrode,giving rise to BPA+c cation radicals. Then, the short-livedcationic radical BPA+ccould lose a proton to form the stronglyreducing intermediate BPAc. Finally, BPAc was able to reduceRu(bpy)3

3+ to Ru(bpy)32+*, which produced ECL emission.10,12

When the sensor was immersed into the BPA solution, the BPAmolecules could selectively adsorb on the recognition sites inthe imprinted cavities. Then the ECL process occurred and theadsorbed BPA could cross the cavities, come into contact withthe light-emitting layer, and lead to the oxidation–reductionreaction, resulting in a highest ECL signal intensity. If there was

This journal is © The Royal Society of Chemistry 2016

Table 1 Comparison of detection results of BPA by this method and literature methods

Methods Linear range (mg L�1) Detection limit (mg L�1) Recovery (%) Ref.

GC-MS 0.05–2.50 0.01 93.9–104.3 39LC-MS 0.2–50.0 0.01 85.7–118.6 40ELISA 1.3–292.2 0.50 86.7–109.2 7Electrochemistry 11.4–91.3 0.68 96.0–101.2 41MIP-ECL sensor 0.01–2.0 0.0041 80.9–101.9 This study

Table 2 Detection and analysis of the ambient water samples spikedwith BPA

SampleFound(mg L�1)

Add(mg L�1)

Total found(mg L�1)

RSD(%)

Recovery(%)

A 0.040 0.1 0.127 5.7 86.61 1.015 4.7 97.5

B 0.229 0.1 0.309 5.0 80.91 1.247 13.0 101.9

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no BPA on the MIP-ECL sensor, a few free ions in the PBSsolution such as hydroxide ions might come into contact withthe light-emitting layer, resulting in a certain degree of the ECLintensity signal. However, this signal was signicantly weakerthan that of the MIP-ECL sensor aer re-adsorption of BPA.There were no recognition sites for BPA in the dense non-imprinted lm of the NIP-ECL sensor and the BPA moleculescould not selectively come into contact with the light-emittinglayer, resulting in the weakest ECL intensity. The results showedthat the MIP-ECL sensor could provide specic binding sites forBPA.

3.6. Optimization of ECL detection conditions

In order to obtain the maximum ECL intensity, the effects of pHand accumulation time on the ECL intensity of the MIP-ECLsensing system were investigated. As shown in Fig. 5, theoptimal conditions were as follows: 7.4 pH PBS and 20 minaccumulation time.

3.7. Selectivity, reproducibility and stability of the sensor

Selectivity, reproducibility and stability are important factorsfor evaluating the performance of the sensor. To illustrate theselectivity of the MIP-ECL sensor for BPA, various interferenceendocrine disruptors (100 mg L�1 E1, 100 mg L�1 E2 and 100 mgL�1 NP) that co-exist with BPA in water samples were chosen asthe sources of interference. The determination was carried outunder the optimum conditions. As shown in Fig. 6A, there wasalmost no signal from these interfering components (E1, E2, andNP), which revealed that the MIP-ECL sensor could be appliedto detect BPA molecules with great specic selectivity. Thereproducibility and stability of the MIP-ECL sensor were inves-tigated by detecting 50 mg L�1 BPA. The sensor was required toelute before each use. A MIP-ECL sensor was investigated inparallel 10 times in the BPA solution, and the relative standarddeviation (RSD) was 7.58%. Ten different MIP-ECL sensors weremeasured and resulted in a RSD of 7.92%, showing a goodreproducibility. In addition, the ECL response of the sensoraer 30 days remained up to 76.1% of its initial value (Fig. 6B).These results indicated that the proposed MIP-ECL sensor hadgood selectivity, reproducibility and stability.

3.8. Quantication detection of BPA

The ECL intensity of BPA was recorded under optimal experi-mental conditions. As shown in Fig. 7, the ECL intensity peakcurrent increased with the increase of BPA concentration. Thecalibration curve is also shown in Fig. 7, the regression equation

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is DY ¼ 1493.004 + 237.942 ln(C � 0.002) (DY is the differencebetween the ECL intensity of standard solution and that of theblank solution, and C is the BPA concentration (mg L�1)), thelinear response range is 0.01 mg L�1 to 2 mg L�1 with thecorrelation coefficient R2 ¼ 0.9960 and the detection limit is0.0041 mg L�1.

Table 1 lists different methods for the detection of BPA.Compared with other methods, the MIP-ECL sensor in thisstudy has a lower linear range and detection limit, which weresuitable for the rapid detection of low level concentrations ofBPA. It was clear that the MIP-ECL sensor exhibited excellentperformance for the analysis of BPA.

3.9. Application

To examine the practical applicability of the developedMIP-ECLsensor, two river water samples from Fuzhou City (Fujian,China) were studied. Sample A is pond water near the schoolliving area, and sample B is river water near the farmers’market. The samples were analyzed by the proposed methodaer a simple ltration and pH adjustment process. As shownin Table 2, the recoveries and RSD of samples added with 0.1 mgL�1 BPA were 86.6%, 80.9% and 5.7%, 5.0%, while those ofsamples added with 1 mg L�1 BPA were 97.5%, 101.9% and4.7%, 13%. The ECL response of the sensor would be affected bythe impurities in actual water samples when the added stan-dard matter amount was at low levels, which led to relatively lowrecoveries of samples with low BPA concentrations, but it is stillwithin an acceptable limit. This indicated that the MIP-ECLsensor has a promising potential application in ambient watersamples.

4. Conclusions

In summary, we have developed a novel molecularly imprintedelectrochemiluminescence sensor based on a Ru(bpy)3

2+/MWCNTs/nano-TiO2-Naon electrode for ultrasensitive and

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ultraselective monitoring of BPA. The introduction of MWCNTs/nano-TiO2 nanohybrids into the Naon lm could provide anexcellent basis for effective immobilization of Ru(bpy)3

2+ andimprove the electrochemical and ECL properties of Ru(bpy)3

2+.The MIP-ECL sensor fabricated by the combination of the MITwith the Ru(bpy)3

2+ ECL system could provide the specicbinding sites for BPA and the selectivity of the sensor wassignicantly improved. Under the best optimum conditions, theproposed ECL sensor exhibited high sensitivity, a low detectionlimit, acceptable reproducibility and stability and desirablespecicity for BPA and can be applied to the detection of BPA inambient water samples. This study paves the way for a novelapproach to constructing molecularly imprinted electro-chemiluminescence sensors to detect other small moleculeseven macromolecules.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (no. 21407026), the Independent ResearchProject of State Key Laboratory of Photocatalysis on Energy andEnvironment (no. 2014B02 and no. 2014C05), the NationalNatural Science Foundation of Fujian Province of China (no.2014J01181) and the Science and Technology Program of FujianEntry-Exit Inspection and Quarantine Bureau of P.R.C of China(no. FK2013-29 and FK2014-01).

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