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A Cathodic “Signal-off” Photoelectrochemical Aptasensor for Ultrasensitive and Selective Detection of Oxytetracycline Kai Yan, Yong Liu, Yaohua Yang, and Jingdong Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03139 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 11, 2015

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A Cathodic “Signal-off” Photoelectrochemical Aptasensor for Ultrasensitive and Selective Detection of Oxytetracycline Kai Yan, Yong Liu, Yaohua Yang, Jingdong Zhang* Key Laboratory for Large-Format Battery Materials and System (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, P.R. China ABSTRACT: A novel cathodic “signal-off” strategy was proposed for photoelectrochemical (PEC) aptasensing of oxytetracycline (OTC). The PEC sensor was constructed by employing a p-type semiconductor BiOI doped with graphene (G) as photoactive species and OTC-binding aptamer as a recognition element. The morphological structure and crystalline phases of obtained BiOI-G nanocomposites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The UV-visible absorption spectroscopic analysis indicated that doping of BiOI with graphene improved the absorption of materials in the visible light region. Moreover, graphene could facilitate the electron transfer of BiOI modified electrode. As a result, the cathodic photocurrent response of BiOI under visible light irradiation was significantly promoted when a suitable amount of graphene was doped. When amine-functionalized OTC-binding aptamer was immobilized on the BiOI-G modified electrode, a cathodic PEC aptasensor was fabricated, which exhibited a declined photocurrent response to OTC. Under the optimized conditions, the photocurrent response of aptamer/BiOIG/FTO was linearly proportional to the concentration of OTC ranging from 4.0 nM to 150 nM, with a detection limit (3S/N) of 0.9 nM. This novel PEC sensing strategy demonstrated an ultrasensitive method for OTC detection with high selectivity and good stability.

Due to the advantages of simple instrumentation, low cost, rapid analysis and high sensitivity, photoelectrochemical (PEC) sensors have attracted increasing research interest in recent years.1-3 Typically in a PEC sensing system, a photoelectrode is employed to convert photoirradiation to electrical signal which is proportional to the concentration of analyte.4 Thus, photoelectrode plays an important role in PEC detection. Various n-type semiconductors such as TiO2, ZnO, CdS and CdSe that can efficiently convert UV or visible light irradiation to anodic current have been extensively explored to prepare photoelectrodes (photoanodes).5-8 On the other hand, in order to achieve a specific photoelectrical response to analyte, DNA,9, 10 molecularly imprinted polymers,11 antibodies,12, 13 enzymes,14 and aptamers15 have been coupled with photoelectrodes as recognition elements in PEC sensors. Among these, aptamers have the advantages of high specific binding ability, wide target range, and acceptable stability. Based on photoelectrodes and immobilized aptamers, PEC aptasensing has been widely applied to the determination of various analytes including organic compounds, 16, 17 inorganic ions,18 and proteins. 19 Actually, p-type semiconductors are also very useful to prepare photoelectrodes (photocathodes) that can generate cathodic current under photoirradiation.20, 21 Unlike commonly used n-type semiconductors-based photoanodes, photocathodes can avoid the intrinsic hole oxidation reactions occurring at the photoanode/electrolyte interface and broaden the scope of photoelectrochemistry in biosensing.22 Owing to the

strong absorption in the visible light region and small band gap (1.8 eV), BiOI has been one of the most intensively studied p-type semiconductors employed in photocatalysis23, 24 and solar cell.25 Recently, BiOI has been successfully demonstrated as useful photoactive materials for the fabrication of PEC sensors. Zhou et al. have developed a PEC biosensor for the detection of DNA methyltransferase activity based on BiOI nanoflakes.26 Gong et al. have fabricated a sensitive and selective biosensor for the PEC detection of organophosphate pesticides using crossed BiOI nanoflake arrays. 27 Oxytetracycline (OTC) is one of the most commonly used tetracycline antibiotics to treat livestock infections due to its effective antimicrobial properties, low cost and low side effects.28 However, excessive residual of OTC in the environment can cause antibiotic resistance.29 OTC has been listed as pharmaceutical and personal care products (PPCPs) by the U.S. environmental protection agency (EPA).30 Moreover, OTC may accumulate in food products.31 It is therefore necessary to develop highly sensitive and selective methods to monitor OTC in the environment and determine OTC residues in food. Various analytical techniques including electrochemiluminescence (ECL),32 colorimetry,33 electrochemistry,34, 35 high performance liquid chromatography (HPLC),36 and fluorescence37 have been established for the determination of OTC. Since the aptamer of OTC was selected by Niazi et al. in 2008,38 several OTC aptasensors have been developed in combination with distinct detection strategies such as

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Scheme 1. Schematic illustration of cathodic “signal-off” PEC aptasensor based on BiOI-G nanocomposites.

electrochemistry,39, 40 colorimetry,41 fluorescence,42 and cantilever array.43 In the present work, we fabricated a cathodic PEC aptasensor for OTC detection based on BiOI-graphene (G) nanocomposites. In this sensor, p-type semiconductor BiOI was employed as photoactive species to generate a cathodic photocurrent signal while OTC-binding aptamer was introduced as the specific recognition element. Thephotocurrent response of photocathode was amplified by doping BiOI with a suitable amount of graphene. When OTC was present in solution, a decrease of photocurrent was recorded on the sensor owing to the specific capture of OTC by aptamer, as illustrated in Scheme 1. The sensor exhibited a linear response to OTC in the concentration range from 4.0 nM to 150 nM, and the detection limit (3S/N) was down to 0.9 nM. Thus, a novel strategy of cathodic “signal-off” PEC aptasensor for ultrasensitive and selective detection of OTC was developed. EXPERIMENTAL SECTION Materials and Reagents. Oxytetracycline, aureomycin, kanamycin, vibramycin, ofloxacin, and poly(diallyldi-methylammonium chloride) (PDDA, Mw 100 000-200 000, 20% wt. in water) were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Chitosan (CHIT) (degree of deacetylation>90.0%) was provided by Shanghai Ruji Biological Technology Co., Ltd. (Shanghai, China). Amino-functionalized OTC aptamer with sequences of 5’-NH2-(CH2)6-GGA ATT CGC TAG CAC GTT GAC GCT GGT GCC CGG TTG TGG TGC GAG TGT TGT GTG GAT CCG AGC TCC ACG TG-3’ was synthesized by Shanghai Sangon Biotech Co., Ltd. (Shanghai, China). Commercial oxytetracycline tablets were purchased from Yichang Humanwell Pharmaceutical Co., Ltd. (Yichang, China). Other reagents of analytical grade were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 0.1 M phosphate buffer solution (PBS, pH 8.0) was used for the preparation of aptamer solution. Doubly distilled water was used throughout the investigation. Apparatus. A Quanta 200 field emission scanning electron (FEI, Netherlands) was employed for the morphology characterization. X-ray diffraction (XRD) pat-

terns were obtained using a Bruker D8 Advance X-ray diffractometer (Bruker Instruments, Darmstadt, Germany) with Cu Kα radiation. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The UV-visible absorption spectra were measured with a TU1900 UV-visible spectrometer (Beijing Purkinje General Instrument Company, China). All electrochemical and PEC measurements were performed on a CHI660A electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) in a conventional three-electrode system. A fabricated FTO electrode, a platinum wire, and a saturated calomel electrode (SCE) were employed as the working, auxiliary and reference electrodes, respectively. A PLS-SXE300 xenon lamp (Perfect Co., Beijing, China) with an optical filter (λ>400 nm) was employed as the irradiation source, and the distance between the light source and electrode surface was 10 cm. The high-performance liquid chromatograph (HPLC) measurements were performed on an Agilent (USA) 1100 module system with C18 column (150 mm×4.6 mm). The mobile phase was a 35:65 (v/v) mixture of acetonitrile: 0.01 M NaH2PO4 (in water and the pH was adjusted to 2.5 with 30% HNO3) with flow rate of 1 mL/min. Detection wavelength was set at 355 nm and the column temperature was 303 K. Synthesis of Graphene and BiOI Nanoplates. Graphene (G) was prepared according to our previous report.44 A 1 mg—mL-1 graphene suspension was prepared by dispersing suitable amount of graphene in water with the assistance of ultrasonic agitation. BiOI nanoplates were synthesized via a solvothermal method45 with a little modification. Briefly, 30 mL of 0.10 mol—L-1 Bi(NO3)3—5H2O dissolved in glycol was added drop-wisely into 30 mL of 0.10 mol—L-1 KI dissolved in glycol under vigorous stirring, leading to the formation of an orange-red solution. The mixture was transferred into a 100 mL Teflon-lined autoclave. After stirring at room temperature for 1 h, the autoclave was sealed in a stainless steel tank, and the temperature was maintained at 160 oC for 12 h. The product was washed three times with deionized water and ethanol respectively, and dried at 60 oC for 4 h. A 1 mg—mL-1 BiOI suspension was pre-

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pared by dispersing suitable amount of BiOI in 0.035% CHIT aqueous solution containing 0.5% acetic acid with the aid of 2 h ultrasonic agitation. The BiOI-G nanocomposites were prepared by directly mixing BiOI (1 mg—mL-1) and graphene (1 mg—mL-1) suspensions under vigorous stirring. Different weight contents of graphene in BiOI-G composites were obtained by varying the mixture ratio of two suspensions; and the asprepared composites were marked as BiOI-x%G, where x represented the weight percentage of graphene to BiOI. Generally, BiOI-5%G was used except where otherwise indicated. Fabrication of PEC Aptasensor. Prior to modification, the FTO glass (0.8 cm × 1.2 cm) was cleaned by successive sonication in acetone, mixed solution of ethanol and 2 M NaOH (v/v, 1:1), and pure water for 20 min. After being dried with nitrogen gas, the FTO glass with an exposed geometric area of 0.096 cm2 was coated with 15 µL of BiOI-G suspension and dried at 60 oC in an oven, followed by rinsing with water to remove loosely adsorbed materials. The obtained modified electrode was marked as BiOI-G/FTO. Then, amine-functionalized OTC aptamer was immobilized on the BiOI-G/FTO electrode surface using glutaraldehyde (GA) as crosslinking.19 Briefly, a 10 µL of GA aqueous solution (0.25%, v/w) was dropped onto the BiOI-G/FTO surface, and kept in dark for 1 h at room temperature, followed by rinsing with water. After being dried under nitrogen gas, the BiOI-G/FTO surface was coated with 8 µL of aptamer solution and incubated at 4 oC for 12 h. The obtained aptamer/BiOI-G/FTO electrode was rinsed thoroughly with water to remove any unbounded aptamers. Detection of OTC. The fabricated sensor (aptamer/BiOI-G/FTO) was incubated with 10 µL of 0.1 M PBS containing different concentrations of OTC at 60 oC for 1 h, followed by thoroughly rinsing with deionized water. The photocurrent responses of the fabricated sensor before and after incubation with OTC were recorded. RESULTS AND DISCUSSION Materials Characterization. The obtained graphene and BiOI were characterized by SEM. It was observed that graphene showed the typical flake-like shape with a wrinkled structure (Figure 1A); whereas BiOI was composed of a large quantity of nanoplates with a thickness of 40 to 60 nm (Figure 1B). When BiOI-G composites were utilized to modify FTO electrode, a rough but uniform film structure was exhibited (Figure 1C), and many BiOI nanoplates were embedded into the sheets of graphene. Meanwhile, the crystalline nature of various materials was analyzed by XRD. As shown in Figure S1 in the Supporting Information, graphene showed a broad peak around 23◦, assigned to the hexagonal graphite structures of C (002). For the sample of BiOI, all the diffraction peaks were readily indexed to the tetragonal phase of BiOI. In the XRD pattern of BiOI-G composites, all the diffraction peaks of BiOI were clearly exhibited but no diffraction of graphene was found, which might be due to the strong intensities of diffraction peaks from crystalline BiOI.46

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Figure 2. Photocurrent responses of different BiOI-G composites modified FTO electrodes recorded in 0.1 M Na2SO4 at a bias potential of -0.1 V: (a) BiOI-0%G/FTO, (b) BiOI-1%G/FTO, (c) BiOI-3%G/FTO, (d) BiOI5%G/FTO, and (e) BiOI-7%G/FTO. On the other hand, the absorption behavior of various materials modified electrodes was studied by UV-visible absorption spectroscopy. As shown in Figure S2 in the Supporting Information, both graphene and BiOI showed strong adsorption in visible region. When BiOI was doped with graphene, the absorption was significantly increased. This result demonstrates that the introduction of graphene into BiOI is favorable to promote the visible light harvesting property of materials. PEC Activity of BiOI-G Composites. The photocurrents on BiOI-G composite modified electrodes with different contents of graphene were recorded in 0.1 M Na2SO4. As shown in Figure 2, all BiOI-G/FTO electrodes responded sensitively to the irradiation of visible light; and cathodic photocurrents were generated under a bias potential of -0.1 V, which was induced by the reduction of oxygen on the working electrode surface.47 The cathodic photocurrent of BiOI-G/FTO increased significantly with increasing the weight ratio of graphene from 0% to 5%. The increment of cathodic photocurrent could be attributed to the higher optical absorption and better

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fore, the proposed aptamer/BiOI-G/FTO electrode can be utilized as the visible light PEC sensor for OTC. 800

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electrical conductivity after graphene was doped in the semiconducting materials.48 While the weight ratio of graphene in the composites reached 7%, the light absorption of BiOI nanoplates was limited,10 which hindered the photocurrent intensity. Accordingly, BiOI-5%G/FTO was selected for further investigation in this work except where otherwise indicated. Electrochemical Impedance Analysis and PEC Response of Sensor. Considering that different materials were used to modify the electrode surface during the sensor fabrication, the interfacial properties of various modified electrodes were investigated using electrochemical impedance spectroscopic (EIS) technique (Figure 3A). Based on the semicircle diameters of the Nyquist plots, the electron-transfer resistance (Ret) values of different modified electrodes were evaluated and compared. For bare FTO electrode, a Ret value of 608 Ω was obtained (curve a in Figure 3A). While FTO was coated with BiOI, the Ret value was obviously decreased to 154 Ω(curve b in Figure 3A), demonstrating that the presence of BiOI nanoplates facilitated the electron transfer on the electrode surface. When BiOI-G composites were used to modify FTO, the Ret value was further decreased to 126 Ω (curve c in Figure 3A), which was lower than that of BiOI/FTO, confirming that graphene with high conductivity could facilitate the electron transfer. However, the Ret value dramatically increased to 1049 Ω after aptamer was immobilized on the electrode (curve d in Figure 3A). The increase in Ret can be ascribed to the negative charges of aptamer molecules, which repel the negatively charged [Fe(CN)6]3-/4- species and inhibit the electron transfer.49 After the sensor was incubated in the solution of OTC, the Ret value further increased to 1406 Ω (curve e in Figure 3A), confirming the capture of OTC by aptamer immobilized on the electrode surface. In order to understand the PEC sensing mechanism of the proposed sensor, the photocurrent responses of various modified electrodes were recorded in 0.1 M Na2SO4 under visible light illumination by applying a bias potential of -0.1 V. As shown in curve a in Figure 3B, BiOI-G composites possessed a good photoelectrical property and exhibited a cathodic photocurrent of 0.27 µA in airsaturated electrolyte. When the solution was purged with nitrogen, the photocurrent response of BiOI-G/FTO decreased significantly (curve b in Figure 3B), confirming that oxygen acted as electron acceptor and participated in the generation of cathodic photocurrent.22 On the other hand, as compared with BiOI-G/FTO, the photocurrent of aptamer/BiOI-G/FTO was decreased by ca. 20% when aptamer was immobilized on the BiOI-G/FTO electrode (curve c in Figure 3B). The decreased photocurrent can be attributed to the presence of aptamer molecules which enhance the steric hindrance of the electrode interface and block the electron transfer between the electrode and electron acceptor as indicated by EIS analysis. While the analyte OTC was captured by aptamer immobilized on the electrode surface, the cathodic photocurrent response was drastically decreased (curve d in Figure 3B). Obviously, the formation of aptamer-analyte complexes leads to the increment in steric hindrance for diffusion of electron acceptor to electrode surface. There-

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Figure 3. (A) Nyquist plots of different electrodes in 0.1 M KCl containing 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1): (a) bare FTO substrate, (b) BiOI/FTO, (c) BiOI-G/FTO, (d) aptamer/BiOI-G/FTO and (e) aptamer/BiOI-G/FTO incubated with 100 nM OTC. The frequency range for EIS measurements was from 0.01 to 100 000 Hz at an applied potential of 0.2 V. (B) Photocurrent responses of different modified FTO electrodes recorded in 0.1 M Na2SO4 at a bias potential of -0.1 V: (a) BiOI-G/FTO, (b) BiOI-G/FTO in deaerated electrolyte, (c) aptamer/BiOI-G/FTO, (d) aptamer/BiOI-G/FTO incubated with 100 nM OTC.

Optimization of PEC Sensor. The influences of BiOI-G suspension amount, aptamer concentration and applied potential on the photocurrent response of aptamer/BiOI-G/FTO to 100 nM OTC were investigated in 0.1 M Na2SO4. To quantitatively compare the response of the sensor to OTC, the photocurrent difference (∆PI) before and after incubation with OTC was evaluated. Figure S3 in the Supporting Information illustrates the influence of BiOI-G suspension amount on the photocurrent response of the proposed sensor to 100 nM OTC. It was seen that the response of the sensor increased when the volume of BiOI-G suspension dropped on the FTO surface was increased from 9 to 15 µL. However, with further increasing the volume of BiOI-G suspension to 17 µL, the response declined slightly. This is probably due to the increased thickness of BiOI-G film, which may lead to the decrease of electron transfer rate and en-

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hancement of undesirable recombination of photogenerated electrons/holes. Therefore, the optimized amount of BiOI-G suspension solution was chosen as 15 µL. Figure S4 in the Supporting Information compares the photocurrent responses of several PEC sensors prepared with different aptamer concentrations. The response increased with increasing the aptamer concentration from 0.5 µM to 1.5 µM, indicating that more OTC molecules were captured on the electrode surface. However, when the aptamer concentration further increased to 2.5 µM, the response levelled off, meaning that the amount of aptamer molecules immobilized on electrode surface reached saturation. Thus, the optimized aptamer concentration used for sensor fabrication was 1.5 µM. Moreover, the applied potential also plays an important role in photoelectrochemical sensing since the bias potential affects the recombination probability of the photogenerated electrons/holes.50 As shown in Figure S5 in the Supporting Information, the response was found to be increased with changing the bias potentials from 0.00 V to -0.10 V, indicating that the electron acceptor molecules were efficiently reduced under negative potential. Then the response became saturated as the potential was more negative than -0.1 V. So, -0.1 V was considered as the optimized potential applied on the sensor. PEC Sensing of OTC. Based on the above optimum conditions, the developed aptamer/BiOI-G/FTO electrode was applied to the quantitative determination of OTC. As shown in Figure 4, the cathodic photocurrent response was found to be linearly decreased with increasing the concentration of OTC from 4.0 nM to 150 nM. The linear regression equation was expressed as ∆PI/nA=1.04c(OTC)/nM+44.9, with a correlation coefficient (R2) of 0.997. The detection limit (3S/N) was estimated to be 0.9 nM. Compared with previously reported methods for OTC determination (Table S1 in the Supporting Information), the proposed PEC aptamer sensor showed much lower detection limit. 0.00

get analyte OTC. To evaluate the detection selectivity, interference studies were carried out in 100 nM OTC solution under the optimum conditions by adding 100 nM of some other antibiotics such as aureomycin, kanamycin, vibramycin and ofloxacin. As shown in Figure 5, all these antibiotics did not show obvious interference in the determination of OTC, confirming the specific interaction of aptamer and target OTC molecules. The result reveals that the proposed sensor has a high selectivity toward OTC against other antibiotics. Moreover, the reproducibility of the developed aptasensor was evaluated by checking the responses of six independently prepared aptamer/BiOI-G/FTO electrodes toward 100 nM OTC. A relative standard deviation value of 4.5% was obtained, showing a good reproducibility. In addition, the stability of the developed aptasensor was also investigated. The fabricated aptamer/BiOI-G/FTO electrode retained 95.1% of its original photocurrent response toward 100 nM OTC after the electrode was stored at 4 oC in a refrigerator for two weeks, indicating a good stability. The feasibility of the proposed aptasensor for possible application in real samples was evaluated by assaying OTC in a commercial tablet. The PEC sensing result was 245.0 ± 9.5 mg per tablet, showing a good agreement with the result obtained by HPLC (254.5 ± 2.3 mg per tablet). The result indicates that the proposed aptasensor could be employed as a reliable method for OTC detection in real samples with an acceptable accuracy.

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Time / s Figure 4. Photocurrent response of the fabricated sensor incubated with different concentrations of OTC: (from a to i) 0, 4, 10, 20, 50, 80, 100, 125, 150 nM. Inset: Calibration curve for OTC on the PEC sensor. Error bars were derived from the standard deviation of four measurements. Since aptamer possesses the specific recognition ability, the proposed aptasensor should only capture the tar-

CONCLUSIONS An ultrasensitive and selective PEC detection of OTC was achieved by employing BiOI-G composites as photoactive materials and aptamer as recognition element. The cathodic photocurrent response of BiOI was promoted by doped graphene. After OTC-binding aptamer was introduced in BiOI-G modified electrode, a cathodic PEC sensor for OTC was developed. The fabricated sensor exhibited high sensitivity and selectivity, good reproducibility, and long-term stability. Such a cathodic “signal-off” sensor demonstrates a novel PEC aptasensing strategy.

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ASSOCIATED CONTENT Supporting Information Additional information as noted in text: five figures showing XRD patterns, UV-visible absorption spectra and condition optimization results; one table showing comparison of different methods for OTC determination. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +86-27-87543032. Fax: +86-27-87543632. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No.61172005). We also thank the Analytical and Testing Center of HUST for help in the characterization of synthesized materials.

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