Engineering of Heterojunction-Mediated Biointerface for

May 12, 2017 - Finally, the detection of MC-LR was realized by monitoring the change of the PEC ... The CdTe-Bi2S3 heterojunction formed here was base...
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Engineering of Heterojunction-Mediated Biointerface for Photoelectrochemical Aptasensing: Case of Direct Z‑Scheme CdTeBi2S3 Heterojunction with Improved Visible-Light-Driven Photoelectrical Conversion Efficiency Qian Liu,† Juan Huan,‡ Nan Hao,‡ Jing Qian,‡ Hanping Mao,*,† and Kun Wang*,‡ †

Key Laboratory of Modern Agriculture Equipment and Technology, School of Agricultural Equipment Engineering, Jiangsu University, Zhenjiang 212013, P.R. China ‡ School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P.R. China S Supporting Information *

ABSTRACT: This work presents a heterojunction-mediated photoelectrochemical (PEC) biointerface for selective detection of microcystin-LR (MCLR) by introducing a direct Z-scheme heterojunction as efficient visible-lightdriven photoactive species. Specifically, the Z-scheme type CdTe-Bi2S3 heterojunction was designed and synthesized as an ideal photoactive material, which exhibited higher PEC activity as compared with either CdTe quantum dots or Bi2S3 nanorods due to the improved photogenerated charges separation efficiency of heterojunction. Then the MC-LR aptamer was employed for selective recognition of MC-LR target, which was immobilized on the CdTe-Bi2S3 film by the formation of phosphor-amidate bonds between the phosphate group of aptamer and amino group of the chitosan film on the electrode. The proposed aptasensor showed a photocurrent signal due to the photoactive CdTe-Bi2S3 heterojunction, while the presence of MC-LR resulted in a dose-responsive decrease in PEC response, which allowed the quantification analysis of MC-LR by measuring the photocurrent signal of the fabricated aptasensor. Under optimal conditions, the resulted PEC aptasensor showed wide linear range (0.01−100 pM) and low detection limit (0.005 pM) for MC-LR determination with high selectivity and acceptable reproducibility. Finally, the proposed aptasensing method was successfully applied in MC-LR detection in real water samples. KEYWORDS: Z-scheme heterojunction, CdTe quantum dot, Bi2S3 nanorod, photoelectrochemical aptasensor, microcystin-LR detection

1. INTRODUCTION

chemical applications as it is higher utilization efficiency of the solar light.8,9 Besides, Bi2S3 possesses the high absorption coefficient (in the order of 104 ∼ 105 cm−1) and reasonable photon-electron conversion efficiency (∼5%),10 which make it an excellent candidate as a light-harvesting substrate for PEC biosensing and photocatalytic applications.11,12 By virtue of these prominent advantages above, Bi2S3 was considered as a potential candidate in the fabrication of PEC sensors.13,14 For example, by employing Bi2S3 nanorods (NRs) to fabricate the photosensitive layer, Ai’s group fabricated a PEC immunosensor for Avian leukosis viruses detection. Nevertheless, research is still continuing in searching for efficient carrier transportation to improve their photoelectric conversion efficiency.15,16 In the ongoing pursuits for this purpose, the formation of heterostructure by coupling of two different semiconductors with appropriate band positions has emerged

Photoelectrochemical (PEC) aptasensing combines the merits of PEC analysis and inherent properties of aptamer and has become a promising approach and provided a new exciting technique for bioanalysis in recent years.1−3 Typically, a general PEC aptasensor necessitates two essential ingredients: the PEC active species which convert photoirradiation to electrical signal and the biological recognition aptamer which is in intimate contact with the transducer.4 As known to all, the selectivity of PEC aptasensor is guaranteed by the employment of the targetdependent aptamer with high specificity,5,6 and the sensitivity of a PEC aptasensor, another important factor for bioanalysis, are more closely related to the PEC active species-based photoelectrode.7 Thus, it has great significance for screening efficient PEC active species with superior photoelectric conversion efficiency in the fabrication of PEC aptasensing platform with high-performance. Bismuth sulfide (Bi2S3), as a highly photoconductive semiconductor with a direct band gap (Eg) of ca. 1.3 eV, is demonstrated to be an ideal candidate for various photo© 2017 American Chemical Society

Received: March 27, 2017 Accepted: May 12, 2017 Published: May 12, 2017 18369

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

Research Article

ACS Applied Materials & Interfaces

photoelectron spectrometer (ESCALAB MKII). The transient timeresolved PL decay measurements were recorded on a FS5 fluorescence spectrometer (Edinburgh Instruments, UK), and the solutions of all the samples were excited by a 470 nm laser. The zeta potential of the samples were taken with a Zetasizer Nano-ZS (Malvern, UK). Electrochemical impedance spectroscopy (EIS) was measured by a ZENNIUM electrochemical workstation (Zahner Instruments, Germany) in 0.1 M KCl with 5.0 mM Fe(CN)63−/4−. The PEC detection was taken with a ZENNIUM electrochemical workstation with a controlled intensity modulated photo spectrometer, and a BLR01 LED light with wavelength at 470 nm was used as the accessory light source. A conventional three-electrode system was used in the electrochemical experiments where a modified ITO electrode, a platinum wire, and a saturated calomel electrode (SCE) were used as the working, auxiliary, and reference electrode, respectively. Preparation of Bi2S3 NRs and CdTe-Bi2S3 Heterojunction. Bi2S3 NRs were prepared by the modified method from previous literature.24 In a typical synthesis, 3.75 mmol Bi(NO3)3·5H2O was added into 25 mL EG and dissolved, which was then deaerated by nitrogen gas bubbling for 20 min to obtain solution A. Meanwhile, 17.3 mmol Na2S was dissolved in a mixture which contained 10 mL EG and 20 mL deionized water and was stirred for 15 min, thus obtaining solution B. Then under magnetic stirring, the solution B was added dropwise to solution A, along with plenty of black suspended matter. Subsequently, 32.0 mmol carbamide and 20 mL deionized water were added to the mixture above and stirred for another 30 min. Then the resulting solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 160 °C for 20 h. After naturally cooling to room temperature, the precipitate was collected by vacuum filtration and washed successively by ethanol and water three times. Finally, the obtained products were dried under vacuum at 60 °C to obtain the Bi2S3 NRs. To prepare CdTe-Bi2S3 heterojunction, 3 mL of Bi2S3 NRs suspension (2 mg mL−1) was mixed directly with 0.5 mL CdTe QDs by magnetic stirring for 3 h to obtain a CdTe-Bi2S3 heterojunction suspension. The CdTe-Bi2S3 heterojunction formed here was based on the electrostatic adsorption interaction between CdTe QDs and Bi2S3 NRs, which was further confirmed by comparing the zeta potential of CdTe QDs, Bi2S3 NRs, and the CdTe-Bi2S3 heterojunction (shown in Table S1). Construction of PEC Aptasensing Interface. Prior to modification, the ITO electrode was cleaned with NaOH solution (1.0 M) and sequentially sonicated in ethanol and ultrapure water for about 10 min. The preparation process of the PEC aptasensor was shown in Scheme 1. Twenty microliters of CdTe-Bi2S3 suspension was

as a promising technique, since it can extraordinarily facilitate the photoexcited electron−hole pairs separation and thus improve the efficiency of charge transfer.17,18 In this work, Bi2S3 NRs with improved photoelectric conversion efficiency was selected as a photoactive species, and then the photocurrent signal amplification was obtained by the formation of the direct Z-scheme type CdTe-Bi2S 3 heterostructure, which was beneficial to improve the sensitivity of the fabricated PEC bioassays. Microcystins are a family of stable heptapeptides released mainly by cyanbacteria during eutrophication.19 MicrocystinsLR (MC-LR), considered as the most common and also the most toxic congener in various microcystin variants, may cause the potential threat of cancer.20 Since 1998, the World Health Organization has set a maximum permitted level (1 μg/L) for MC-LR in drinking water to protect water quality and human health,21 therefore, it is of great urgency for researchers to quantify MC-LR in water. Herein, a PEC-aptasensing biointerface with high sensitivity and selectivity for MC-LR detection was proposed on the basis of the incorporation of the sensitivity of PEC technique, along with the selectivity of aptamer. In the aptasensor, a desired initial PEC signal was obtained by the formaion of CdTe-Bi2S3 heterostructure to significantly enhance the PEC response for improving the sensitivity of the sensing biointerface. Whereafter, the MC-LR aptamers for MC-LR recognition were anchored on the electrode surface via the phosphor-amidate bonds. Finally, the detection of MC-LR was realized by monitoring the change of the PEC signal of the photoelectrode induced by the presence of MC-LR. The proposed PEC biointerface exerts satisfactory reliability for MC-LR determination, which revealed a promising way for accurate monitoring of other kinds of contaminants.

2. EXPERIMENTAL DETAILS Materials and Reagents. Sodium sulfide nonahydrate (Na2S· 9H2O), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), carbamide (CON2H4), and ethylene glycol (EG, C2H6O2) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). The CdTe QDs with emission peak at ca. 710 nm were prepared through the method in our previous work,22 and the concentration of the CdTe QDs solution was estimated to be ∼15 μM, according to the UV−vis spectrum of the diluted solution in Figure S1.23 Tris(hydroxymethyl) aminomethane (Tris), ethylenediaminetetraacetic acid disodium salt (EDTA), chitosan, 1-ethyl-3-(3-(dimethylamino)-propyl) carbodiimide (EDC), MC-LR, and other microcystins (MC-LA and MC-YR) were obtained from Shanghai J&K Scientific Ltd. The MC-LR aptamers (5′-GGC GCC AAA CAG GAC CAC CAT GAC AAT TAC CCA TAC CAC CTC ATT ATG CCC CAT CTC CGC-3′) were obtained from Shanghai Sangon Biotech Co., Ltd. and purified by an HPLC method. Standard microcystins solutions with various concentrations were prepared by dissolving microcystins samples in 50 mM Tris-HCl buffer (pH 7.4), which contained 200 mM KCl, 100 mM NaCl, 5 mM MgCl2, and 1 mM EDTA to obtain various concentrations of microcystins. The aptamer solution was received by dissolving aptamer into the Tris-HCl buffer. The buffer of phosphate-buffered saline (0.1 M, PBS) was used in the process of detection, and ultrapure water (18.4 MΩ) purified by a Milli-QTM system (Millipore) was used for preparing all aqueous solutions in this work. Apparatus. X-ray diffraction (XRD) and transmission electron microscopy (TEM) images were performed on a Bruker D8 diffractometer and a JEOL 2100 TEM (JEOL, Japan), respectively. The UV−vis absorption and the photoluminescence (PL) spectra were taken with a UV-2450 spectrophotometer (Shimadzu, Japan) and a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan). X-ray photoelectron spectrometry (XPS) was recorded by an X-ray

Scheme 1. Schematic Representation for the Construction of PEC Aptasensor and Mechanism for MC-LR Detection

coated on the surface of the ITO electrode to form a CdTe-Bi2S3 heterojunction-modified electrode (denoted as ITO/CdTe-Bi2S3); then, 25 μL of 0.3% chitosan solution was casted onto the ITO electrode. After standing at room temperature for 2 h, an amount of 10 μL aptamer solution (4 μM) containing 50 mM EDC was decorated on the ITO/CdTe-Bi2S3 surface and remained for another 1.5 h, and then the electrode above was washed with the binding buffer to remove the excess aptamer; the aptasensing interface was thus 18370

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

Research Article

ACS Applied Materials & Interfaces

Figure 1. (A) XRD patterns and (B) UV−vis diffuse reflectance spectrum of pure Bi2S3 NRs; TEM images of (C) pure Bi2S3 NRs and (D) CdTeBi2S3 heterojunction.

Figure 2. (A) XPS survey scan of CdTe-Bi2S3 heterojunction; (B) Bi 4f and S 2p, (C) Cd 3d, and (D) Te 3d spectrum of CdTe-Bi2S3 heterojunction. established (denoted as ITO/CdTe-Bi2S3/Apt). It would be specially mentioned that the aptamer was modified on the aptasensing interface by the phosphoramidate bonds between the phosphate group of aptamer and the amino group of chitosan.25,26 To investigate the aptasensor performance, 20 μL of MC-LR solution with various concentrations was casted onto the ITO/CdTe-Bi2S3/Apt and incubated for 40 min at room temperature. Finally, the PEC response of the ITO/CdTe-Bi2S3/Apt/MC-LR was used for MC-LR monitoring.

3. RESULTS AND DISCUSSION Characterization of CdTe-Bi2S3 Heterojunction. XRD analysis was employed to characterize the phase of the asprepared Bi2S3 NRs. As shown in Figure 1A, several main diffraction peaks located at 25.09, 28.73, 31.88, 35.66, 46.60, and 52.76° were observed for the Bi2S3 NRs, which were readily indexed to the (130), (211), (301), (240), (431), and (351) 18371

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

Research Article

ACS Applied Materials & Interfaces planes of orthorhombic Bi2S3 (JCPDS 17-0320).27 In addition, no other diffraction peaks other than those of Bi2S3 could be observed for the sample, suggesting the high-purity of obtained samples. The absorption spectrum of Bi2S3 NRs was evaluated by the diffuse reflectance spectrum in Figure 1B, demonstrating a strong absorption of the synthesized Bi2S3 NRs in the whole range of visible light until 850 nm. The morphologies of the Bi2S3 and CdTe-Bi2S3 products were studied by TEM. As displayed in Figure 1C, the prepared Bi2S3 NRs were of uniform size with a length of up to ca. 200 nm and a diameter of ca. 45 nm, and the edge of the Bi2S3 NRs was smooth and clear to be seen. Figure 1D shows the TEM image of the CdTeBi2S3 heterojunction; it can be seen obviously that the CdTe QDs was decorated on the Bi2S3 NRs surface, and the edge of the Bi2S3 NRs became rough due to the formation of the heterojunction. In order to elucidate the composition of the synthetic CdTeBi2S3 sample, XPS analysis was performed. Figure 2A presents the low-resolution XPS spectrum of the CdTe-Bi2S3, which revealed the Cd, Te, Bi, and S elements in the product. The additional signals of C and O elements were attributed to the absorbed oxygen and reference sample. The high-resolution XPS spectra of Bi 4f and S 2p shown in Figure 2B revealed three peaks present at 158.46, 163.78, and 161.18 eV, respectively, which corresponded to Bi 4f7/2, Bi 4f5/2, and S 2p spin states.28 Meanwhile, Figure 2C displays the highresolution spectrum of Cd 3d, and the sharp peaks which were located at 404.93 and 411.66 eV were assigned to the 3d3/2 and 3d5/2 peaks of Cd(II).29 In addition, the high-resolution spectrum of Te 3d (Figure 2D) presented two doublets of Te 3d5/2 and Te 3d3/2 (I and II) located at 572.14, 575.86, 582.53, and 586.16 eV, respectively.30 All of these results above confirmed the successful formation of the CdTe-Bi 2 S 3 heterojunction. Amplified PEC Behavior of CdTe-Bi2S3 Heterojunction. The PEC behaviors of pure CdTe QDs, Bi2S3 NRs, and CdTeBi2S3 heterojunction were investigated under the irradiation with visible light at 470 nm in Figure 3. In detail, the

ITO electrode effectively due to the lower combination efficiency of the photoexicited electron−hole pairs in the CdTe-Bi2S3 composite. Moreover, the PL and time-resolved PL decay spectra of pure CdTe QDs and CdTe-Bi2S3 samples were displayed in Figure 4. It can be clearly seen from Figure 4A that the fluorescence intensity of CdTe-Bi2S3 was decreased compared with pure CdTe QDs, and correspondingly, the photographs under visible-light and UV illumination showed in the inset of Figure 4A vividly demonstrated the fluorescence of pure CdTe QDs was quenched by the introduction of Bi2S3 NRs. Generally speaking, the lower PL signal signifies the higher separation efficiency of electron−hole pairs,31 and in other words, the PL results here demonstrated the improved electron−hole pairs separation efficiency of the CdTe-Bi2S3 heterostructure. To better understand the charge transfer process between CdTe QDs and Bi2S3 NRs, the time-resolved PL spectra of CdTe QDs and CdTe-Bi2S3 were also investigated. As shown in Figure 4B, an increase in average lifetime was observed in the case of CdTe-Bi2S3 (65.59 ns), which was longer than that of pure CdTe QDs (58.97 ns). Such slower PL decay kinetics of CdTe-Bi2S3 implied that the photoexcited electrons transfer from the CB of Bi2S3 NRs to the CB of CdTe QDs.32,36 It is important to know the energy levels of CdTe QDs and Bi2S3 NRs, which play an important role in determining the flowchart of photoexcited electrons transfer in the CdTe-Bi2S3 heterojunction. The conduction band (CB) and valence band (VB) levels (vs vacuum) of Bi2S3 NRs were calculated according to the formula as following:33 ECB(eV) = −χ + 0.5Eg

(1)

E VB(eV) = −χ − 0.5Eg

(2)

where Eg and χ are the band gap and the electronegativity of Bi2S3. The Eg values of Bi2S3 could be calculated from the UV− vis diffuse reflectance spectrum of Bi2S3 NRs, which was calculated as 1.34 eV, according to the Tauc’s plots shown in Figure S2.34 The χ values for Bi2S3 are calculated to be 5.95 eV from previous literature.35 Therefore, the CB and VB levels of Bi2S3 NRs was calculated as −5.28 and −6.62 eV, according to the above eqs 1 and 2, respectively. Also, the energy levels of the CdTe QDs were investigated by the electrochemical method.36,37 As shown in Figure S3, the peak A1 from the first positive sweep and peak C1 from the first negative sweep are located at ca. 0.99 and −0.52 V (vs NHE), respectively. From the results above, the VB and CB levels of CdTe QDs were estimated as −5.49 and −3.98 eV, respectively, and the electrochemical band gap of CdTe QDs was 1.51 eV. Compared with previous reports,36,37 the electrochemical band gap of CdTe QDs was much smaller, which was attributed to the larger particle size of QDs in this work. Thus, it is apparent that the calculated EVB of Bi2S3 was lower than that of the normal oxidation potential of the H2O/O2 (−5.73 eV vs vacuum), and the ECB of CdTe QDs was higher than the normal reduction potential of O2/•O2− (−4.45 eV vs vacuum).38,39 In accordance with the description above, a direct Z-schemetype charge transfer in CdTe-Bi2S3 is presented. As displayed in Figure 5, both CdTe QDs and Bi2S3 NRs are easily excited under visible-light irradiation and, correspondingly, photoexcited electrons and holes are generated in CB and VB, respectively. The photoinduced electrons on the CB of Bi2S3 are moved to the VB of CdTe QDs, and then the electrons can

Figure 3. Photocurrent responses of (a) pure CdTe QDs, (b) Bi2S3 NRs, and (c) CdTe-Bi2S3 heterojunction modified ITO electrodes.

photocurrent signals of pure CdTe QDs (curve a) and Bi2S3 NRs (curve b) were ca. 70 and 450 nA, respectively. After the engineering of CdTe QDs on Bi2S3 NRs, the heterojunction exhibited ca. 2 and 13 times higher than the photocurrent signals of the Bi2S3 NRs and CdTe QDs. This fact suggested that the formation of the CdTe-Bi2S3 heterojunction could enhance the photocurrent signal of the CdTe-Bi2S3-modified 18372

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Figure 4. (A) The PL spectra of (a) pure CdTe QDs and (b) CdTe-Bi2S3 heterojunction, inset: the photographs of (a) pure CdTe QDs and (b) CdTe-Bi2S3 heterojunction solution under (I) visible light and (II) UV light (λex = 365 nm), respectively. (B) Time-resolved PL decay traces of (a) CdTe QDs and (b) CdTe-Bi2S3 (λex = 470 nm).

Figure 5. Schematic diagram of enhanced photocurrent generation mechanism for direct Z-scheme CdTe/Bi2S3 heterojunction.

be further excited to the CB of CdTe QDs, resulting in leaving behind the holes in the VB of Bi2S3, which can react with H2O to generate O2.40 In such a way, the photoexcited electrons and holes in the CdTe-Bi2S3 composite can be separated efficiently. Electrochemical Impedance Analysis and PEC Investigation of the Aptasensing Interface. EIS was performed to monitor each step of the sensing interface modification in Figure 6A, and the values of semicircle diameters in the impedance spectra reflect the electron-transfer resistance (Ret) values of the modified electrodes. Prior to the modification of the CdTe/Bi2S3 on the ITO electrode, the Ret of the bare ITO electrode was quite small (ca. 130 Ω) due to the fast electrons transfer of the sensing interface (curve a), while the immobilization of the CdTe/Bi2S3 seminconductor on ITO resulted in the increase of the Ret to ca. 470 Ω (curve b). However, the Ret value dramatically increased to ca. 1180 Ω after the MC-LR targeting aptamer was anchored on the electrode surface, which was ascribed to the aptamer with negative charges hindering the negatively charged Fe(CN)63−/4− and thus inhibits the electron transfer from Fe(CN)63−/4− to the electrode.6 When the aptasensing interface was incubated with MC-LR, the Ret decreased due to the conformation change of the MC-LR targeting aptamer, which was induced by the MC-LR immobilized on the interface.41 These EIS results above confirmed the effective modification of the aptasensor and the specific binding of target MC-LR on the aptasensor. By virtue of the amplified photocurrent response of CdTe/ Bi2S3 heterojunction stated above, a protocol for aptasensing application was then constructed. Figure 6B displays the fabrication process of the aptasensor recorded by transient photocurrent responses. As shown in curve a, the photo-

Figure 6. (A) Nyquist plots of (a) bare ITO, (b) ITO/CdTe-Bi2S3, (c) ITO/CdTe-Bi2S3/Apt, and (d) ITO/CdTe-Bi2S3/Apt/MC-LR; (B) photocurrent responses of (a) ITO/CdTe-Bi2S3, (b) ITO/CdTeBi2S3/Apt, and (c) ITO/CdTe-Bi2S3/Apt/MC-LR.

electrode of CdTe/Bi2S3/ITO showed a photocurrent of ca. 880 nA, and after anchoring the aptamer on the CdTe/Bi2S3/ ITO photoelectrode, an obvious decrease of photocurrent was observed (600 nA, curve b), which was attributed to the modification of aptamer suppressing the interfacial electron transfer.41 This result also demonstrated the successful fabrication of the aptasensing interface. After MC-LR was captured on the aptasensor, the photocurrent was decreased obviously; such finding indicated that the photocurrent signal of the aptasensor was inhibited by the capture of MC-LR on the biointerface, and thus the quantitative detection of MC-LR can be realized. However, the phenomenon of such inhibited photocurrent of aptasensor induced by MC-LR was not consistent with our recent report, which has enhanced PEC 18373

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

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Figure 7. (A) Photocurrent responses of the aptasensor at different concentrations of target MC-LR; (B) the corresponding calibration curve for MC-LR detection; (C) selectivity of the MC-LR PEC aptasensor; and (D) stability test of MC-LR PEC aptasensor for MC-LR at the concentration of 1 pM.

binding time prolonged from 5 to 40 min. This was ascribed to the fact that the amount of target MC-LR molecules captured by the anchored aptamer would be increased as the binding time rose. Nevertheless, when the binding time exceeded 40 min, the photocurrent did not show any further increase, which indicated the saturation of the captured MC-LR on the aptasensor. Thus, 40 min binding time was used for the PEC aptasensing. PEC Detection of MC-LR. Since the photocurrent signal is related to the target-based reaction directly, an effective PEC aptasensor toward MC-LR could be fabricated by tracking the variation of photocurrent responses. Figure 7A records the corresponding photocurrent responses of the aptasensor toward the target MC-LR with variable concentrations under the optimum conditions, and it can be observed that the photocurrent decreased gradually with the increasing MC-LR concentration. As presented in Figure 7B, a calibration curve reporting the photocurrent intensity as a function of the MCLR concentration from 0.01 to 100 pM was derived. The detection limit of the as-prepared PEC aptasensor was calculated to be 0.005 pM (S/N = 3); this value was much lower than previous aptamer-based methods (Table S2) for MC-LR determination.41,43−46 Selectivity, Reproducibility, and Real Sample Analysis. The specificity of the proposed aptasensor was studied by selecting some representative interfering agents involving two similar structure molecules (MC-LA and MC-YR) and common pesticide molecules (acetamiprid, carbaryl, and paraquat) for the interference test. In this process, the aptasensor was evaluated by monitoring the photocurrent signals of 50 pM interfering agents with 5 pM MC-LR under the same experimental conditions. As shown in Figure 7C, obvious photocurrent changes of the aptasensor toward 5 pM MC-LR were observed, while no remarkable decrease was found in the photocurrent when the aptasensor incubated with

responses upon the capture of MC-LR on the BiOBr nanoflakes/N-doped graphene photoelectrode,41 and the distinct results of MC-LR target on photoelectrode might be attributed to the different energy levels of the photoactive species for photoelectrodes.42 Optimization of Experimental Conditions. In order to obtain superior performance in the PEC detection of MC-LR, experimental parameters including the wavelength of accessory light source, the aptamer concentration, and the binding time of MC-LR with the aptamer were optimized. The wavelength of accessory light source on the photocurrent of the semiconductor photoelectrode was first investigated in Figure S4A. As shown, with the variation of wavelength from 430 to 630 nm, significant changes of the photocurrent response of ITO/CdTe-Bi2S3 were observed, and the maximum photocurrent was found at the wavelength of 470 nm. The result also indicated the superior visible-light driven photocurrent response of the ITO/CdTe-Bi2S3 photoelectrode. Therefore, an accessory light source with the wavelength of 470 nm was selected for the PEC tests. Figure S4B shows the effect of aptamer concentration on the photocurrent intensity of the aptasensor. It is observed that the photocurrent intensity decreased with the increasing of the aptamer concentration from 1 to 4 μ M, which was due to the fact that higher concentration of aptamer immobilized on the electrode could suppress the interfacial electron transfer more effectively. As the aptamer concentration further increased beyond 4 μM, the response kept constant, which illustrated that the aptamer immobilized on the surface of the photoelectrode reached a certain saturation point. Consequently, 4 μM aptamer was chosen as the optimized aptamer concentration for the aptasensor fabrication. Moreover, the influence of binding time between the immobilized aptamer and target MC-LR on the response of the aptasensor was studied. It can be observed in Figure S4C that the photocurrent response reduced gradually as the 18374

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

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ACS Applied Materials & Interfaces Notes

10-fold higher concentration of nontarget molecules, indicating the good specificity of the as-prepared aptasensor. The photocurrent response was recorded under eight on/off irradiation cycles in Figure 7D and no noticeable variation occurred, indicating the stable readout for signal collection. Moreover, the reproducibility of the resulting aptasensor was assessed by an interassay relative standard deviation (RSD) through testing 5 pM MC-LR with four independent electrodes, which were modified at the same experimental condition, and RSD of 7.3% was calculated, suggesting the acceptable reproducibility of the resulting aptasensor. The feasibility of the proposed PEC aptasensor was studied in tap water and environmental water samples collected from Yangtze River in Zhenjiang city by the proposed method using the standard addition method. As shown in Table S3, different concentrations (0.2, 2, and 20 pM) of standard MC-LR solution were added into the real water samples. The average recoveries of the aptasensor were in the range of 95−110% and the RSD was 5.3−6.8%. The satisfactory results for real water sample analysis implyed the reliability and potential applicability of the resulting aptasensor for trace MC-LR monitoring in real environmental samples.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by the National Natural Science Foundation of China (Grants 21375050, 21405063, 21505055, and 61601204), Provincial Natural Science Foundation of Jiangsu (Grant BK20160542), Project Funded by Qing Lan, China Postdoctoral Science Foundation (Grants 2015M581745 and 2015T80517), and Jiangsu Province Postdoctoral Science Foundation (Grant 1601204C).

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4. CONCLUSIONS In summary, the present work focuses on the fabrication of a selective and sensitive PEC aptasensing biointerface by exploiting the superior selectivity of aptamer as well as the improved photoactivity of the Z-scheme CdTe-Bi2S3 heterojunction. Also, the mechanism for the improved PEC activity of CdTe-Bi2S3 heterojunction was investigated. Due to the advantages of aptamer and CdTe-Bi2S3 photoactive materials, the developed PEC aptasensor demonstrated great advantages in selectivity and sensitivity, and further, a relatively wide linear range and low detection limit for MC-LR determination was obtained. Moreover, satisfactory recoveries were obtained when the resulting aptasensor was employed to monitor MC-LR in water samples. The design of such sensitized Z-scheme heterojunction opened a new way for the fabrication of a universal biointerface for PEC bioassays and also photovoltaic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04310. UV−vis spectrum of CdTe QDs, descriptions for calculating the size and concentration of CdTe QDs, Tauc’s plots for Bi2S3 NRs, cyclic voltammograms for calculating the energy levels of CdTe QDs, effects of experimental parameters on the photocurrent response of aptasensor, and tables for zeta potential results, analytical performance of aptasensor, and analysis of real water samples (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Tel: +86 511 88791800. Fax: +86 511 88791708. E-mail: [email protected]. ORCID

Kun Wang: 0000-0001-6764-8686 18375

DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b04310 ACS Appl. Mater. Interfaces 2017, 9, 18369−18376