A label-free photoelectrochemical aptasensor for bisphenol A based on surface plasmon resonance of gold nanoparticle-sensitized ZnO nanopencils

Author’s Accepted Manuscript A label-free photoelectrochemical aptasensor for bisphenol A based on surface plasmon resonance of gold nanoparticle-sensitized ZnO nanopencils Yunfei Qiao, Jing Li, Hongbo Li, Hailin Fang, Dahe Fan, Wei Wang www.elsevier.com/locate/bios

PII: DOI: Reference:

S0956-5663(16)30602-9 http://dx.doi.org/10.1016/j.bios.2016.06.062 BIOS8854

To appear in: Biosensors and Bioelectronic Received date: 7 May 2016 Revised date: 18 June 2016 Accepted date: 21 June 2016 Cite this article as: Yunfei Qiao, Jing Li, Hongbo Li, Hailin Fang, Dahe Fan and Wei Wang, A label-free photoelectrochemical aptasensor for bisphenol A based on surface plasmon resonance of gold nanoparticle-sensitized ZnO nanopencils, Biosensors and Bioelectronic, http://dx.doi.org/10.1016/j.bios.2016.06.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A label-free photoelectrochemical aptasensor for bisphenol A based on surface plasmon resonance of gold nanoparticle-sensitized ZnO nanopencils

Yunfei Qiaoa,b, Jing Lia, Hongbo Lia*, Hailin Fanga, Dahe Fana, Wei Wanga* a

School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, 211 East

Jianjun Road, Yancheng 224051, PR China b

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR

China

[email protected] [email protected]

*

Tel. / fax: +86 515 88298735.

Abstract A simple and novel photoelectrochemical (PEC) aptasensor for selective detection of bisphenol A (BPA) was developed using surface plasmon resonance of Au nanoparticles activated ZnO nanopencils. With the irradiation of simulated light, the increased photocurrent of nano-Au/ZnO than that of pure ZnO nanopencil is induced by the hot electrons from excited Au nanoparticles. The perfect selectivity is attributed to the specific binding of BPA to its aptamer. With the addition of BPA, the conformation of aptamer changed to a G-quadruplex structure, which resulted in the blockages of photogenerated electron-transfer channels. Based on the above mechanisms and the optimized conditions, the assembled PEC aptasensor was linear 1

with the concentration of BPA in the range of 1-1000 nmol L-1 with a detection limit of 0.5 nmol L-1. The presence of the same concentration and similar structure of other organics did not interfere in the detection of BPA and the recovery was between 96.2% and 108.4%. It has been successfully applied to the detection of BPA in drinking water and liquid milk samples. This PEC aptasensor has good performances in novelty, selectivity, sensitivity and low cost, and it provides an alternative approach to the detection of BPA.

Keywords: Photoelectrochemistry, Aptasensor, Surface plasmon resonance, Au/ZnO nanopencil, Bisphenol A

1. Introduction Photoelectrochemical (PEC) detection represents a novel and dynamically developing analytical technique which has attracted substantial research scrutiny for its satisfactory analytical performances (Zang et al. 2014; Zhao et al. 2015). PEC detection possesses potential merits than traditional electrochemical methods because of its total separation and different energy form of the excitation source (light) and detection signal (photocurrent) (Dai et al. 2014; Li et al. 2014a; Zhao et al. 2014). Different from electrochemical analysis, PEC detection system requires a photoactive working electrode to produce photocurrent signal under photoirradiation. Various semiconductors such as ZnO, TiO2, CdS, CdTe, and CdSe possessing high photocatalytic activity have been employed to prepare photoelectric beacons (Li et al. 2

2016). Recently, the noble metal nanoparticles modified ZnO nanorod arrays have roused great interest because the noble metals can effectively promote the charge separation and excite surface plasmon resonance (SPR) in the visible and even infrared wavelength range (He et al. 2014; Wu et al. 2014b). Among these noble metals, Gold is an excellent plasmonic metal and has a tendency to be a better choice due to their properties of being relatively stable such as high electron transfer ability, good biocompatibility and favorable microenvironment and strongly interact with the visible light (Bian et al. 2014; Wang et al. 2015). SPR can be described as the resonant photon-induced collective oscillation of valence electrons; the unique capacity of plasmonic nanostructures to scatter electromagnetic radiation, concentrate electromagnetic fields, or convert the energy of photons into heat makes them suitable for various applications (Ghosh et al. 2015; Wei et al. 2013). Therefore, it is a tendency to apply the favorable SPR of Au nanostructures into PEC sensing. As high specific and affinity molecular recognition elements, aptamers have been successfully used to the detection of various target molecules (Cui et al. 2016; Ping et al. 2015). Comparing to conventional chemical and immunological recognition molecules, aptamers possess a variety of advantages, including better stability, smaller size, easier artificial synthesis and modification, and higher specificity (Feng et al. 2014). Therefore, the good performances of aptamers are advantages to construct PEC aptasensors. In this work, bisphenol A (BPA) as a model molecule because it is an organic compound widely used in the production of polycarbonate products, such as feeding 3

bottles, water bottles, microwave ovenware and reusable food containers (Wang et al. 2015; Zhang et al. 2014). However, BPA is one of the endocrine disrupting compounds, which has a wide variety of adverse health effects to human beings (Hou and Cronin 2013; Wang et al. 2014). It is also found to be an environmental pollutant in ecosystems by affecting development and reproduction of aquatic organisms (Ragavan et al. 2013; Rochester and Bolden 2015), which primarily comes from plastic-producing industrial. Due to its serious threat to human health and the environment, it is very necessary to develop an efficient, high selectivity and sensitivity analytical method for BPA detection. Traditional analytical methods for BPA mainly include high-performance liquid chromatography (HPLC) (Braunrath et al. 2005), liquid chromatography (LC) (Kang et al. 2007), gas chromatography (GC) (Vandenberg et al. 2007), gas chromatography-mass spectrometry (GC-MS) (Chang et al. 2005). These conventional methods have some disadvantages such as time-consuming of sample pretreatments, high cost of the instruments and require trained operators. As a consequence, there remains a need for developing a more simple and sensitive method for effective and fast determination of BPA. In this work, a label-free PEC aptasensor for BPA was developed by immobilizing aptamer on the surface of Au/ZnO nanopencil photoanode (see Scheme 1). Except as the immobilization matrix for BPA-aptamer, herein gold nanoparticles could also inject hot electrons into the conduction band of ZnO nanopencils under visible light irradiation due to the SPR effect and thus enhanced the photoelectric conversion efficiency and the analytical performances. The analytical properties of the prepared 4

aptamer-based PEC sensor were investigated systematically.

2. Experimental 2.1 Reagents and apparatus All reagents used were analytical grade and were used directly without purification. Bisphenol A was obtained from Sigma-Aldrich (St. Louis, MO). Zinc nitrate

hexahydrate

(Zn(NO3)2∙6H2O),

potassium

hydroxide

(KOH),

6-mercapto-1-hexanol (MCH), tris (hydroxymethyl) aminomethane (Tris-HCl), chloroauric acid hydrate (HAuCl4∙4H2O) and methyl alcohol (CH3OH) were purchased from sinopharm chemical reagent Co., Ltd. (Shanghai, PR China). Aptamer was chosen according to the prior reported literature (Mei et al. 2013). The oligonucleotides used were purchased from Sangon Biotech Co., Ltd. (Shanghai, PR China) with the sequences: 5'-(SH)-(CH2)6-CCGGTGGGTGGTCAGGTGGGATAGC GTTCCGCGTATGGCCCAGCGCATCACGGGTTCGCACCA-3'. The aptamer was dissolved in Tris-EDTA buffer (TE, pH 8.0) and kept frozen. In this work, 0.1 mol L−1 phosphate buffer solution (PBS) of pH 7.0 was always employed as the supporting electrolyte for photoelectrochemical detection. The ultrapure water with 18.2 MΩ cm−1 was used throughout the whole experiments. PEC measurements were performed with a PEC system equipped with a 150 W Xe lamp as the irradiation source (simulated sunlight irradiation) (Zolix, Beijing, China). The photocurrent was measured by the current–time curve experimental technique using a CHI660E electrochemical workstation (CH Instruments, Shanghai, 5

China). All experiments were carried out at room temperature using a conventional three-electrode system: a modified ITO (φ= 5 mm, resistivity 10 Ω/sq, Zhuhai Kaivo Electronic Components Co. Ltd., China) as the working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode as the reference electrode. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 scanning electron microscope (Hitachi, Tokyo, Japan). Transmission electron micrographs (TEM) were performed using a Tecnai 12 TEM (Philips, Netherlands). X-ray diffraction (XRD) patterns of ZnO and Au/ZnO were measured in the range of 2θ = 10–80° by step scanning on the Bruker D8 Advance (superspeed) diffractometer (Bruker-AEX, Germany) with Cu Kα radiation (κ = 0.15406 nm) operated at 40 kV and 100 mA. UV-vis diffuse reflection spectra were recorded at room temperature with a Cary 5000 ultraviolet and visible spectrophotometer (Varian, USA). X-ray photoelectron spectroscopy (XPS) measurements were performed with an Ultra Axis spectrophotometer equipped with a monochromatic Al Kα source operated at 150W (V.G. Scientific. Ltd, England).

2.2 Preparation of Au/ZnO nanopencils-based PEC sensing The synthesis of Au/ZnO nanopencils was according to the prior reported literature with little modification (Wang et al. 2012). Briefly, 10 mL of Zn(NO3)2∙6H2O (0.5 mol L-1) aqueous solution was added to 10 mL of KOH (4.0 mol L-1) aqueous solution drop by drop under stirring, then the solution was transferred to a 100 mL borosilicate glass vial and maintained at 30 ◦C for 12 h. After the reaction 6

completed, the products were washed with ultrapure water and absolute ethanol for several times, and then the pure ZnO nanopencils were obtained after drying at 70 ◦C. A suitable amount of HAuCl4∙4H2O aqueous solution (0.01 g mL-1) and 1 mL of methanol were added in 20 mL of ultrapure water, and then 30 mg of as-prepared ZnO was added in the above mixed solution. After slow stirring for about 1 h, the complex solution was transferred to a 50 mL Teflon-lined stainless steel autoclave, and maintained at 120 ◦C for 1 h and then cooled down to room temperature naturally, the samples were collected by centrifugation and washed with deionized water and absolute ethanol for several times, and then dried at 70 ◦C in air. Before modification, the ITO electrode (1 cm×4 cm) was cleaned with the mixed solution containing hydrogen peroxide, ammonia and water with volume ratio of 1 : 1 : 50, and it was finally washed by ultrapure water and dried in air. Afterward, 20 μL of 1.0 mg mL-1 Au/ZnO nanopencils suspension was dropped onto the ITO electrode with the geometric area was 5 mm in diameter. After drying in air, the film was sintered at 450 ◦

C for 1 h in air to enhance the adhesion strength between the film and the substrate.

Then, it was cooled to room temperature naturally. After that, the Au/ZnO/ITO electrode was immersed in aptamer solution (5 μmol L-1) at 4 ◦C overnight in order to assemble the aptamer on the electrode surface, and it was rinsed with Tris–HCl buffer to remove the unlinked aptamers. Then, the assembled aptamer/Au/ZnO/ITO electrode was covered with 10 μL of 1 mmol L-1 MCH and kept at room temperature for 30 min to block nonspecific sites, and then was thoroughly rinsed with ethanol and ultrapure water, respectively. The MCH/aptamer/Au/ZnO/ITO photoanode was thus 7

successfully developed.

3. Results and discussion 3.1 Characterization of synthesized nanomaterials The morphology and microstructure of the ZnO and Au/ZnO nanopencils were observed by SEM and TEM, the images were shown in Fig. S-1. As shown in Fig. S-1(A) and (B), the mean length of ZnO nanopencils was about 1-2 μm and the tip diameter was about 50 nm; the average length and the tip diameter of Au/ZnO nanopencils in Fig. S-1 (C) and (D) were similar to those of pure ZnO nanopencils. The size of Au nanoparticles was about 10–20 nm. From the close view of TEM images, Au nanoparticles that deposited on the smooth surface of ZnO nanopencils exhibited a dark contrast. The ZnO nanopencils were substantially coarsened after attachment of Au nanoparticles, which was possible that the reagent of HAuCl4∙4H2O caused an acid corrosion on the surface of the ZnO nanopencils (Wang et al. 2012). The powder X-ray diffraction (XRD) patterns of ZnO nanopencils and Au/ZnO nanopencils were shown in Fig. 1(A). All the diffraction peaks of ZnO nanopencils could be indexed to the wurtzite (hexagonal) ZnO (JPCDS 36-1451) and no peaks for any other phase or impurities could be detected. By comparison, it can be observed that the main diffraction peaks of Au/ZnO nanopencils were similar to that of pure ZnO nanopencils, indicating that the formation of Au nanoparticles in the hydrothermal reaction process had little influence on crystal structure of ZnO nanopencils (Zhang et al. 2012). The four diffraction peaks at 38.3°, 44.5°, 64.6° and 8

77.5° could be assigned to the (111), (200), (220) and (311) planes of face-centered cubic (fcc) Au (JCPDS 01-1172) in comparison to pure ZnO nanopencils.

The optical absorption spectra of pure ZnO nanopencils and Au/ZnO nanopencils were recorded in the range of 200-800 nm. As shown in Fig. 1(B), it could be clearly seen that the pure ZnO nanopencils showed a strong absorption at the wavelength under 400 nm, indicating that the pure ZnO nanopencils were transparent in the visible region. Compared with pure ZnO nanopencils, the extra absorption band of Au/ZnO was observed in the visible wavelength of 500–800 nm. The enhanced absorption was attributed to the surface plasmon resonance (SPR) of Au nanoparticles. The utilization of SPR effect of Au NPs helped extending the photoresponse range of ZnO nanopencils. Thus, ZnO nanopencils modified with Au nanoparticles was beneficial to improve the sensitivity of PEC sensing. XPS is a very useful technique to analyze the surface composition of materials (Wang et al. 2014) and has been carried out to check the chemical composition of the Au/ZnO nanopencils. As shown in Fig. 2(A), all of the peaks were assigned to Zn, Au, C, and O elements, further elucidating that the Au/ZnO nanopencils was successfully obtained without any existing impurity. The high resolution XPS spectrum of Zn 2p shown in Fig. 2(B) clearly indicated that the two peaks positioned at 1021.5 eV and 1044.6 eV corresponded to Zn 2p3/2 and Zn 2p1/2, respectively. And the energy difference between these two peaks was 23.1 eV, clearly demonstrating that zinc species was in the formal Zn2+ valence state (Ahmad et al. 2011). As shown in Fig. 9

2(C), the binding energies at 83.4 eV and 87.3 eV corresponding to Au 4f7/2 and Au 4f5/2, respectively. It exhibited that binding energy of Au 4f7/2 showed a negative shift of 0.6 eV in comparison to 84.0 eV of bulk Au (Li et al. 2014b; Ranasingha et al. 2015). It was commonly believed that this slight shift was caused by electron transfer from Au to ZnO due to the strong electronic interaction between Au nanoparticles and ZnO (Yang et al. 2013; Yin et al. 2011). Furthermore, the other peaks centered at 88.4 eV and 91.5 eV corresponded to Zn 3p3/2 and Zn 3p1/2, respectively. The carbon 1s spectrum with four binding energy positions located at 283.6 eV, 285.3 eV, 286.9 eV and 288.7 eV was shown in Fig. 2(D), these carbon materials were mainly derived from the atmosphere. The binding energies of O 1s were presented in Fig. 2(E), which had three different binding energy positions. The peak located at about 530.4 eV was ascribed to the lattice oxygen O2− of ZnO; the other two peaks located at 532.1 and 532.8 eV were attributed to OH− or O− of surface-adsorbed oxygen and chemisorbed oxygen O22− caused by the surface hydroxyl groups (Yang et al. 2013), respectively.

3.2 PEC sensing response and analytical performance 3.2.1 Effect of different percent of Au NPs Control experiments were carried out to investigate the relationship between different percent of Au nanoparticles and photocurrents. As shown in Fig. S-2, the photocurrent of pure ZnO nanopencils modified ITO was about 0.39 μA (curve a), the weight ratio of x% Au : ZnO (x = 1, 3, 5, 6, 7, 8, 9, 10) nanopencils modified ITO were about 0.49 μA (curve b), 0.66 μA (curve c), 0.83 μA (curve d), 0.95 μA (curve 10

e), 1.22 μA (curve f), 1.01 μA (curve g), 0.86 μA (curve h) and 0.69 μA (curve i), respectively. As could be seen from Fig. S-2, the photocurrent reached a maximum level at the weight ratio of 7% Au : ZnO. As the amount of Au increased, the photocurrent of nano Au/ZnO could be enhanced, confirming the positive effects of Au nanoparticles. However, when the weight ratio was above 7%, the photocurrent responses slowed down. This could be attributed to the transient accumulation of mass photoexcited charges. Therefore, 7 wt% Au : ZnO was chosen for further study.

3.2.2 PEC mechanism The pure ZnO nanopencils modified ITO showed a photocurrent of 0.39 μA at a bias of 0.2 V in 0.1 mol L-1 PBS (pH 7.0) upon photoexcitation with the simulated sunlight (Fig. 4A, curve a), while the Au/ZnO nanopencils modified ITO showed a photocurrent of 1.22 μA (Fig. 4A, curve b), which was 2.13 times increment of that observed at the ZnO/ITO. This obvious enhancement could be explained that upon the visible light irradiation hot electrons in the Au nanoparticles could be generated and injected into the conduction band of ZnO nanopencil owing to SPR effect. Additionally, the rapid electron transfer from Au nanoparticles to ZnO nanopencil reduced the recombination of photoexcited electron-hole pairs (Hou and Cronin 2013; Zhang et al. 2013). While BPA aptamer was immobilized onto the Au/ZnO photoanode, the photocurrent obviously decreased to about 0.91 μA (Fig. 4A, curve c). On one hand, this could be explained that the immobilization of BPA aptamer on Au nanoparticles blocked the incident light arriving at Au/ZnO nanopencils surfaces and 11

inhibited the SPR effects therein; on the other hand, BPA aptamer coated on the photoanode inhibited electron transfer (Fan et al. 2013). In the presence of 60 nmol L-1 BPA, the photocurrent decreased to around 0.54 μA (Fig. 4A, curve d) due to the conformation change of aptamer to G-quadruplex structure. The steric G-quadruplex structure subsequently closed the entrance of long passageways, resulting in the blockage of electron transfer. The more concentration of BPA will bring about more formation of closed aptamer gate, which leads to the more decreased photocurrent. Therefore, the concentration of BPA could be quantified by the decreased photocurrent.

3.3 Analytical performance The photocurrent–time curves of the assembled PEC aptasensor clearly indicated the rapid response in different concentrations of BPA at a bias voltage of 0.2 V under the irradiation of a Xe lamp. As shown in Fig. 3(A), the photocurrent gradually decreased with increased concentration of BPA. Fig. 3(B) clearly showed that there was a linear relationship between log C and (I0 – I) / I0. Here, log C was the logarithm of the concentrations of BPA, while the I0 and I represented the photocurrent intensity of the modified ITOs in the absence and presence of BPA, respectively. The liner range was from 1.0 nmol L-1 to 1000 nmol L-1 with a detection limit of 0.5 nmol L-1, which was lower than most previous reports (as shown in Table S-1). The good performances are attributed to the SPR of gold nanoparticles, which effectively promote the charges separation of ZnO nanopencils and amplify their photocurrents 12

with irradiation. In this case, the increased photocurrent of 2.13 times was favorable for the construction of "signal-off" PEC sensing model.

Stability is an important parameter for the performance of the PEC aptasensor. Fig. 4 (A) illustrated the photoexcited process repeated 5 times over 110 s, which indicated the photocurrent responses of the assembled PEC aptasensor at 0.1 mol L-1 PBS (pH 7.0) in the absence (curve c) and presence (curve d) of 60 nmol L-1 BPA at a bias voltage of 0.2 V under the irradiation of a 150 W Xe lamp. There was almost no notable change found in the photoexcitation process, indicating that this PEC aptasensor possessed stable performance. The selectivity of the prepared PEC aptasensor for BPA was investigated by comparison with other common species, such as 4, 4'-bisphenol, 6F bisphenol A and bisphenol B at the same concentration of 60 nmol L-1. As could be seen in Fig. 4 (B), the influences of the above potential interferences on the determination of bisphenol A were no more than 6%. It could be attributed to the specificity of BPA to its aptamer, which indicating that the prepared PEC aptasensor possessed high selectivity.

To determinate the feasibility of the PEC sensor applied to the real samples, the drinking water and liquid milk were chosen as real samples (see Table 1). The drinking water and liquid milk were purchased from Wenfeng supermarket (Yancheng, China). The drinking water samples were pretreated according to the prior reported 13

literature (Xue et al. 2012) with little modification. Briefly, the spiked drinking water samples were detected without any further pretreatments by just mixing with 0.2 mol L-1 PBS (pH 7.0) at a ratio of 1:1 to increase the ionic strength of the water samples. The liquid milk samples were pretreated according to the references (Yin et al. 2011) with the same modification. Briefly, 20 mL milk was mixed with 40 mL anhydrous alcohol. After 15 min sonication and 10 min shaking, the mixture was centrifuged for 10 min, and then the supernatant was filtrated. The filtrate was collected and added into 100 mL volumetric flask, diluted with ultrapure water to the marked line. Then, the above solution mixed with 0.2 mol L-1 PBS (pH 7.0) at a ratio of 1:1.

4. Conclusions In summary, a label-free and novel PEC aptasensor for selective detection of BPA was successfully fabricated based on Au nanoparticle modified ZnO as the photoelectric beacon. The pure ZnO nanopencils and Au/ZnO nanopencils were characterized by SEM, TEM, XRD, XPS, and UV-Vis DRS. Meanwhile, the PEC aptasensor exhibited high stability, good selectivity, and excellent sensitivity. Moreover, the PEC aptasensor had been applied to detection of BPA in drinking water and liquid milk samples. The PEC platform based on the combination of Au/ZnO nanopencils and aptamer has great potential for application in food analysis.

Acknowledgements We gratefully acknowledge the financial support from the Natural Science 14

Foundation of China (21305123, 21505117, 21575123), the Natural Science Foundation

of

Jiangsu

Province

(BK2012247,

BK20131218),

the

Industry-University-Research Cooperative Innovation Foundation of Jiangsu Province (BY2014108-08, BY2014108-19), the Foundation of International Cooperation of Jiangsu Province (BZ2010053), the Foundation of Jiangsu Key Laboratory of Environmental Material and Environmental Engineering (K13064), the Foundation of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (AE201162, AE201017), the research fund of Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments

(GX2015103).

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Ghosh, A., Guha, P., Samantara, A.K., Jena, B.K., Bar, R., Ray, S., Satyam, P.V., 2015. ACS Appl. Mater. Interfaces 7, 9486-9496. He, W., Kim, H.K., Wamer, W.G., Melka, D., Callahan, J.H., Yin, J.J., 2014. J. Am. Chem. Soc. 136, 750-757. Hou, W., Cronin, S.B., 2013. Adv. Funct. Mater. 23, 1612-1619. Kang, J.H., Asai, D., Katayama, Y., 2007. Crit. Rev. Toxicol. 37, 607-625. Li, H., Qiao, Y., Li, J., Fang, H., Fan, D., Wang, W., 2016. Biosens. Bioelectron. 77, 378-384. Li, H., Xue, Y., Wang, W., 2014a. Biosens. Bioelectron. 54, 317-322. Li, X., Zhou, X., Guo, H., Wang, C., Liu, J., Sun, P., Liu, F., Lu, G., 2014b. ACS Appl. Mater. Interfaces 6, 18661-18667. Mei, Z., Chu, H., Chen, W., Xue, F., Liu, J., Xu, H., Zhang, R., Zheng, L., 2013. Biosens. Bioelectron. 39, 26-30. Ping, J., Zhou, Y., Wu, Y., Papper, V., Boujday, S., Marks, R.S., Steele, T.W., 2015. Biosens. Bioelectron. 64, 373-385. Ragavan, K.V., Rastogi, N.K., Thakur, M.S., 2013. TrAC, Trends Anal. Chem. 52, 248-260. Ranasingha, O.K., Wang, C., Ohodnicki, P.R., Lekse, J.W., Lewis, J.P., Matranga, C., 2015. J. Mater. Chem. A 3, 15141-15147. Rochester, J.R., Bolden, A.L., 2015. Environ. Health Persp. 123, 643-650. Vandenberg, L.N., Hauser, R., Marcus, M., Olea, N., Welshons, W.V., 2007. Reprod. Toxicol. 24, 139-177. Wang, T., Jin, B., Jiao, Z., Lu, G., Ye, J., Bi, Y., 2014. J. Mater. Chem. A 2, 15553-15559. Wang, T., Lv, R., Zhang, P., Li, C., Gong, J., 2015. Nanoscale 7, 77-81. 16

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Figure and table captions Scheme 1. Schematic illustration of the prepared PEC aptasensor based on Au/ZnO nanopencils modified with BPA aptamers. Fig. 1. (A) X-ray diffraction patterns and (B) UV-vis diffuse reflection of ZnO nanopencils (a) and 7 wt% Au : ZnO nanopencils (b). Fig. 2. XPS spectrum of Au/ZnO nanopencils. (A) XPS survey; (B), (C), (D) and (E) correspond 17

to the high-resolution XPS at Zn 2p, Au 4f, C 1s and O 1s, respectively. Fig. 3. (A) Photocurrent responses of the prepared aptamer-based Au/ZnO nanopencils photoanode in 0.1 mol·L-1 PBS (pH 7.0) containing 0, 1, 5, 10, 30, 60, 100, 200, 400, 600, 800 and 1000 nmol·L-1 (from a to l) BPA at a bias potential of 0.2 V upon the simulated sunlight irradiation and (B) the linear calibration curve. Fig. 4. (A) Photocurrent response of (a) ZnO/ITO; (b) Au/ZnO/ITO; Aptamer-based Au/ZnO/ITO in the absence (c) and presence (d) of 60 nmol L-1 BPA at a bias voltage of 0.2 V in 0.1 mol L-1 PBS (pH 7.0) under the irradiation of a 150 W Xe lamp. (B) Selectivity of the prepared PEC aptasensor for BPA detection, the concentration of other common species was the same (60 nmol L-1) as that of BPA.

Scheme 1

Figure 1

18

Figure 2

Figure 3

19

Figure 4

Table 1 Analytical results of a BPA in drinking water and liquid milk samples using the proposed method (n=3).

Sample Drinking water

Liquid milk

Added (nmol L-1)

Found (nmol L-1)

Recovery (%)

RSD (%)

0 50 100 200 0 50 100 200

0 54.2 97.3 209.8 0 48.1 105.7 207.6

– 108.4 97.3 104.9 – 96.2 105.7 103.8

1.7 2.6 3.9 3.5 2.3 4.8 3.2 4.1

n is the repetitive measurements number.

20

Highlights 

A novel and label-free photoelectrochemical aptasensor for bisphenol A.



Enhanced photocurrent responses were triggered by the hot electrons from SPR of nano golds.



Aptamer G-quadruplex structures accordingly blocked photogenerated electron transfer.



The PEC aptasensor exhibited good performances for bisphenol A detection.

21