Graphitic Carbon Nitride Sensitized with CdS Quantum Dots for Visible-Light-Driven Photoelectrochemical Aptasensing of Tetracycline.

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Graphitic Carbon Nitride Sensitized with CdS Quantum Dots for Visible-Light-Driven Photoelectrochemical Aptasensing of Tetracycline Yong Liu, Kai Yan, and 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 S Supporting Information *

ABSTRACT: Graphitic carbon nitride (g-C3N4) is a new type of metal-free semiconducting material with promising applications in photocatalytic and photoelectrochemical (PEC) devices. In the present work, g-C3N4 coupled with CdS quantum dots (QDs) was synthesized and served as highly efficient photoactive species in a PEC sensor. The surface morphological analysis showed that CdS QDs with a size of ca. 4 nm were grafted on the surface of g-C3N4 with closely contacted interfaces. The UV−visible diffuse reflection spectra (DRS) indicated that the absorption of g-C3N4 in the visible region was enhanced by CdS QDs. As a result, g-C3N4− CdS nanocomposites demonstrated higher PEC activity as compared with either pristine g-C3N4 or CdS QDs. When gC3N4−CdS nanocomposites were utilized as transducer and tetracycline (TET)-binding aptamer was immobilized as biorecognition element, a visible light-driven PEC aptasensing platform for TET determination was readily fabricated. The sensor showed a linear PEC response to TET in the concentration range from 10 to 250 nM with a detection limit (3S/N) of 5.3 nM. Thus, g-C3N4 sensitized with CdS QDs was successfully demonstrated as useful photoactive nanomaterials for developing a highly sensitive and selective PEC aptasensor. KEYWORDS: graphitic carbon nitride, photoelectrochemical aptasensor, CdS quantum dots, aptamer, tetracycline

1. INTRODUCTION Graphitic carbon nitride (g-C3N4), a new type of metal-free layered π-conjugated semiconducting material with medium band gap, has recently attracted much research interest. Because of its nontoxic and inexpensive characteristics, high thermal and chemical stability, and good electrical and optical properties, g-C3N4 has been regarded as a promising photocatalyst for splitting water and degrading organic pollutants under sunlight irradiation.1,2 However, the phototo-current conversion efficiency of pristine g-C3N4 is still limited because of the relatively wider optical band gap (ca. 2.7 eV) and high recombination rate of photogenerated electron−hole pairs.3 To improve the photocatalytic activity of g-C3N4, many approaches such as regulating shapes,4 doping with metal or nonmetal elements,5,6 and constructing heterojunction structures between g-C3N4 and other semiconductors7 have been carried out. CdS quantum dots (QDs), an important II−IV semiconductor with a narrow band gap, have been intensively studied as popular visible light-active materials for PEC sensing.8 However, pure CdS QDs also suffer from the high recombination rate of photogenerated electrons and holes.9 This problem can be overcome by coupling CdS QDs with gC3N4. In g-C3N4−CdS composites, the photogenerated charge © XXXX American Chemical Society

carriers can be effectively separated and transferred by a strong interfacial electric field, owing to the well-matched overlapping band-structures.10 So far, g-C3N4−CdS composites have been explored as efficient visible-light photocatalysts for pollutant degradation10,11 and hydrogen production.12 Photoelectrochemical (PEC) detection is a newly developed technique which can be performed on an ordinary electrochemical instrument in cooperation with a light source. In such a detection system, the light source is utilized to excite the photoactive species to produce a photocurrent signal which can be recorded by the electrochemical instrument. The PEC detection is considered to offer the advantages of both optical analysis and electrochemical analysis.13 Moreover, because of the complete separation of light excitation source and photocurrent detection signal, the PEC strategy exhibits lower background but higher sensitivity than common electrochemical analysis.14 Different from electrochemical Special Issue: Electrochemical Applications of Carbon Nanomaterials and Interfaces Received: September 4, 2015 Accepted: November 2, 2015

A

DOI: 10.1021/acsami.5b08275 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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CTTGG GTTGG TCCCA CTGCG CGTGG ATCCG AGCTC CACGT G-3′ according to reported literature.33 The stock solution of oligonucleotides was prepared in phosphate buffer solution (PBS) (0.01 M, pH 7.4). The F-doped SnO2 conducting glass (FTO) was purchased from Dalian Heptachroma SolarTech Co., Ltd. (Dalian, China). Ultrpure water with a specific resistivity >18.2 MΩ cm was used throughout the investigation. 2.2. Synthesis of g-C3N4 and CdS QDs. Water-dispersible gC3N4 was prepared according to our previous work.30 First, bulk C3N4 (b-C3N4) was synthesized by annealing 5 g of melamine in a semiclosed system at 550 °C for 4 h. Then, 1 g of as-prepared b-C3N4 powder was refluxed in 100 mL of 5 M HNO3 at 125 °C for 12 h and kept overnight. The formed precipitate was centrifuged, washed by deionized water until neutral, and dried at 60 °C. Thus, waterdispersible g-C3N4 was obtained for further use. CdS QDs were synthesized via a hydrothermal method with a little modification.34 Briefly, 1.5 mL of mercaptoacetic acid was added into 100 mL of 0.2 M Cd(ClO4)2 solution, and then the pH of the solution was adjusted to 10 with 2.0 M NaOH. The resulting mixture was transferred into a 250 mL round-bottomed flask and heated at boiling point under constant passage of high purity nitrogen gas. After 30 min, 100 mL of 0.2 M Na2S solution was added into the mixture, and the reaction was continued for 4 h. The product was collected by centrifugation, washed several times with ethanol, and dried at 60 °C. 2.3. Fabrication of Aptasensor. The g-C3N4−CdS nanocomposites were prepared by directly mixing the aqueous solutions of g-C3N4 (3 g L−1) and CdS QDs (3 g L−1) by ultrasonic agitation for at least 0.5 h. Prior to electrode modification, the FTO slices were cleaned by sonication in acetone and ethanol for 30 min sequentially, and followed by dipping in saturated NaOH solution for 30 min. After being thoroughly rinsed with deionized water and dried with high purity nitrogen gas, the FTO substrate surface (exposed geometry area of 0.096 cm2) was coated with 8 μL of g-C3N4−CdS mixed solution (containing 1.0 g L−1 of g-C3N4 and 0.5 g L−1 of CdS). After being dried at 60 °C, the obtained g-C3N4−CdS/FTO electrode was coated with 8 μL of aptamer at a desired concentration (diluted with 0.01 M PBS), and then dried at 60 °C for 2 h to ensure the effective immobilization of aptamer on g-C3N4. The aptamer/g-C3N4−CdS/ FTO electrode was thoroughly rinsed with 0.01 M PBS and dried naturally at room temperature for further use. 2.4. Apparatus. The crystalline phases of prepared materials were analyzed by X-ray diffraction (XRD, Bruker Instruments, Germany) with Cu Kα radiation. The surface morphology was characterized by a Quanta 200 field emission scanning electron microscope (FE-SEM) (FEI, The Netherlands) and a Tecnai G220 transmission electron microscope (TEM) (FEI, The Netherlands). The X-ray photoelectron spectra (XPS) were measured on a 5300 ESCA instrument (PerkinElmer PHI Co., USA) using an Al Kα X-ray source at a power of 250 W. The Fourier transform infrared (FT-IR) spectra were recorded with an Equinox 55 FTIR spectrophotometer (Bruker Co., Germany). The UV−visible diffuse reflectance spectra (DRS) were collected on a UV-2550 spectrophotometer (Shimadzu, Japan). The photoluminescence (PL) spectra were measured with a RF-5310PC fluorometer (Shimadzu, Japan) at an excitation wavelength of 300 nm. Electrochemical impedance spectroscopic (EIS) analysis was carried out on a CHI 660A electrochemical workstation (Chenhua Instrument Co., Shanghai, China). 2.5. PEC Sensing Procedure. The as-prepared aptamer/g-C3N4− CdS/FTO electrode was incubated with 0.01 M PBS containing desired amount of TET at 60 °C for 2.0 h. After being thoroughly rinsed with PBS, the aptamer/g-C3N4−CdS/FTO electrode was put into a three-electrode cell containing 0.1 M Na2SO4 solution to serve as the working electrode. A platinum wire and a saturated calomel electrode (SCE) were employed as the auxiliary and reference electrodes, respectively. Then, the PEC detection was carried out on a CHI 830C electrochemical working station (Shanghai Chenhua Instrument Co., China). A PLS-SXE 300 xenon lamp (Perfect Co., China) (λ > 420 nm) was utilized as the irradiation source, and the distance between the light source and working electrode is 10 cm.

analysis, PEC detection system requires a photoactive working electrode to produce photocurrent signal under photoirradiation. Various semiconductors possessing high photocatalytic activity such as TiO2, ZnO, CdS, CdSe, and CdTe have been employed to prepare photoactive electrodes.15−18 Recently, composite semiconducting materials with enhanced photocatalytic activity have been used instead of single semiconductor, which has been considered as a promising strategy for amplifying the PEC response signal.17−20 Therefore, it is of great significance to explore new composite photoactive materials for developing high-performance PEC sensors. As one of the primarily broad spectrum antibiotics groups, tetracycline (TET) possesses a wide range of antimicrobial activity against Gram-positive and Gram-negative microorganisms. Owing to the high antimicrobial activity and costeffectiveness, TET has been widely used for the treatment of bacterial infections in human therapy or animal husbandry. In particular, it has been intensively used as a feed additive to promote the growth of livestock in agriculture sector.21 Consequently, the abuse of TET has led to its accumulation in human directly from drugs or indirectly from animal products consumed by human, which can provoke some serious risks such as increasing drug resistance of microbial strains, causing allergic or toxic reactions among some hypersensitive individuals, and inhibiting bone growth. 22,23 This has tremendously spurred the development of various instrumental methods for rapid and accurate detection of TET, such as high performance liquid chromatography (HPLC),24 liquid chromatography−mass spectrometry (LC-MS),25 fluorescence,26 chemiluminescence,27 capillary electrophoresis,28 and immunoassays.29 However, these methods usually require expensive equipment or time-consuming operation procedures. It is necessary to develop rapid, simple, low-cost, and specific methods for sensitive detection of TET. In the present work, g-C3N4−CdS composites were explored for constructing a PEC sensing platform for the first time. In such a PEC sensor, g-C3N4 sensitized with CdS QDs served as highly efficient photoactive materials, whereas low-cost labelfree TET-binding aptamer was used as a biorecognition element. The formation of g-C3N4−CdS composites was found to dramatically promote the photocurrent response of g-C3N4. Owing to the large specific surface area and πconjugated structure of g-C3N4, the aptamer could be facilely anchored onto g-C 3 N 4 surface through π−π stacking interaction between the nucleobases of DNA and gC3N4.30−32 During the process of PEC sensing, the aptamer would specifically capture TET present in the solution, producing an enhanced photocurrent signal through the reaction between the captured TET and photogenerated holes. Thus, a new PEC aptasensor based on g-C3N4−CdS nanocomposites exhibiting a highly sensitive and selective response to TET was successfully constructed.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Cd(ClO4)2·6H2O was purchased from Alfa Aesar Chemical Co. Ltd. (Tianjin, China). Tetracycline, chlortetracycline, neomycin sulfate, kanamycin, ciprofloxacin, doxycycline and chloramphenicol were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). All other reagents of analytical grade were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The DNA aptamer for TET was synthesized by Sangon Biotech Co., Ltd. (Shanghai, China) with the following sequence: 5′CGTAC GGAAT TCGCT AGCCC CCCGG CAGGC CACGG B


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contacted nanocomposites.10 As a result, the charge transfer between the two semiconductors would be spatially smooth, which is beneficial to the promotion of photoelectrical conversion efficiency. The chemical composition of g-C3N4−CdS and chemical state of constituent elements were investigated by XPS. The XPS survey spectrum (Figure S1 in the Supporting Information) indicates that the elements of C, N, Cd, S and small amount of O are present in the sample. The highresolution XPS spectrum of C 1s can be fitted with five component peaks (Figure 3A). The peak at 284.9 eV is identified as graphitic carbon (C−C, CC),38 which should be ascribed to the surface adventitious carbon. Three peaks at 285.5, 288.2, and 288.8 eV can be respectively assigned to sp2 C bonded to N,39 C bonded with three N atoms40 and sp2 C bonded to N-containing aromatic structure (N−CN).6 The fifth peak centered at 289.5 eV is derived from C-OOH bonds,41 which would be very beneficial for the dispersibility of the synthesized composites in water. Furthermore, three peaks centered at 399.0, 400.0, and 401.3 eV can be identified from N 1s spectrum (Figure 3B). Two peaks at 400.0 and 401.3 eV should be attributed to tertiary nitrogen (N−(C)3) and amino functional groups with a hydrogen atom (C−N−H).11 The main peak at 399.0 eV is typically derived from the sp2-bonded N involved in the triazine rings (C−NC),7 confirming the presence of sp2-bonded g-C3N4. On the other hand, the photoelectron peaks for Cd 3d (Figure 3C) appear at 405.1 and 411.8 eV, which are assigned to the Cd 3d5/2 and Cd 3d3/2 for Cd2+ in CdS QDs.7 The S 2p peaks at 161.5 eV (S 2p3/2) and 162.5 eV (S 2p1/2) (Figure 3D) are ascribed to be S2− in CdS QDs.42 Thus, these XPS results further confirm the existence of graphite-like sp2-bonded structure in g-C3N4 and the successful formation of g-C3N4−CdS composites. Figure 4A shows the FT-IR spectra of g-C3N4, CdS QDs, and g-C3N4−CdS composites. The pristine g-C3N4 presents three characteristic absorption regions at 3174.7, 1200−1700, and 810.3 cm−1. The broad band at 3174.7 cm−1 can be attributed to the stretching mode of N−H bond.2 Several strong characteristic absorption bands observed at 1200−1700 cm−1 should be ascribed to the typical stretching modes of CN heterocycles in g-C3N4.2,10 The representative absorption peaks for − COO− at 1386.0 and 1576.4 cm−1 are found,30 further confirming the good water-dispersibility of the prepared gC3N4. Additionally, the band at 810.3 cm−1 corresponds to the characteristic breathing mode of triazine units of g-C3N4.43 For CdS QDs, a broad band centered at 3430.2 cm−1 and an absorption peak at 1629.7 cm−1 should be attributed to the surface-adsorbed water molecules.44 The characteristic absorption bands at 1385.2 and 1128.3 cm−1 are from the vibrations of Cd−S bond.11 The peak at 1568.4 cm−1 is due to the stretching vibration of COO− for mercaptoacetic acid during the synthesis process of CdS QDs,45 which ensures the excellent watersolubility of CdS QDs. In the FT-IR spectrum of g-C3N4−CdS composites, all of the IR characteristic bands of g-C3N4 are present, whereas the peaks of CdS QDs are not observable. The result might be attributed to two facts. First, the relative higher percentage of g-C3N4 present in the composites leads to the stronger IR absorption intensity of g-C3N4 than that of CdS QDs. Second, unlike organic compounds, CdS itself only exhibits weak absorption in the IR region. Figure 4B compares the UV−visible DRS of g-C3N4, CdS QDs, and g-C3N4−CdS composites. As can be seen, the pristine g-C3N4 exhibits a fundamental absorption edge at 460 nm,

3. RESULTS AND DISCUSSION 3.1. Characterization of g-C3N4−CdS Composites. The crystalline phases of the synthesized materials were analyzed by XRD (Figure 1). As can be seen, the XRD pattern of g-C3N4

Figure 1. XRD patterns of (a) g-C3N4, (b) CdS QDs, and(c) g-C3N4− CdS.

exhibits two distinct diffraction peaks at 13.0 and 27.4°. These peaks can be indexed as the (100) and (002) peaks for graphitic materials, corresponding to the in-plane structure of tri-striazine units and interlayer stacking of the conjugated aromatic groups.35 For CdS QDs, there appear three pronounced diffraction peaks at 26.4, 43.6, and 51.8° assigned respectively to the (111), (220), and (311) crystal planes of CdS, which match well with those of cadmium yellow CdS phase (JCPDS 41−1049).36 When g-C3N4 and CdS form composites, almost all diffraction peaks for g-C3N4 and CdS QDs are clearly observed in the XRD pattern except the weak (100) diffraction of g-C3N4. The morphological structures of g-C3N4, CdS QDs, and gC3N4 −CdS composites modified FTO electrodes were observed by SEM (Figure 2). It can be seen from Figure 2A that g-C3N4 on electrode surface displays a typical slate-like structure. The TEM observation reveals the sheet structure of g-C3N4 (Figure 2B), consistent with the previous reports.4,10 Accordingly, g-C3N4 with slatelike morphology on FTO surface is due to the aggregation of g-C3N4 nanosheets. For CdS QDmodified electrode, a great number of CdS nanoparticles are distributed on FTO surface presenting a uniform and granular structure with well-defined grain boundaries (Figure 2C). The TEM analysis shows that the size of CdS QDs is ca. 4 nm (Figure 2D). For g-C3N4−CdS composites modified electrode, it is clear that CdS nanoparticles are homogeneously and densely dispersed onto the surfaces of g-C3N4 with closely contacted interfaces (Figure 2E). This should be ascribed to the fact that the large surface area of g-C3N4 can provide effective anchor sites for loading of CdS nanoparticles.37 Moreover, the TEM image of g-C3N4−CdS composites (Figure 2F) further intuitively demonstrates that g-C3N4 are decorated with CdS QDs. The high-resolution TEM (HR-TEM) image (inset of Figure 2F) shows the lattice spacing of 0.206, 0.336, and 0.325 nm, which are respectively assigned to the (220) and (111) crystal planes of CdS (JCPDS 89−0440) and (002) crystal plane of g-C3N4 (JCPDS 87−1526). Additionally, this HRTEM image also clearly reveals a very close interface between gC3N4 and CdS QDs in the composites, demonstrating that CdS nanoparticles spread onto the g-C3N4 and form closely C


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Figure 2. SEM images of (A) g-C3N4, (C) CdS QDs, and (E) g-C3N4−CdS modified FTO electrodes. TEM images of (B) g-C3N4, (D) CdS QDs, and (F) g-C3N4−CdS. Inset of F: HR-TEM image of g-C3N4−CdS.

consistent with its band gap of 2.70 eV.46 The pure CdS QDs has strong absorption in a wide visible region, and the absorption edge is at 574 nm. When g-C3N4 is coupled with CdS QDs, the absorption in the visible region is obviously broader than that of g-C3N4 but slightly narrower than that of CdS QDs. Thus, g-C3N4 is successfully sensitized by CdS QDs, and the composites are able to efficiently utilize the solar spectrum. 3.2. PEC Response of g-C3N4−CdS/FTO Electrode. To evaluate the PEC performance of the synthesized materials, the photocurrent responses of g-C3N4-, CdS-, and g-C3N4−CdSmodified FTO electrodes were recorded in 0.1 M Na2SO4 at a bias potential of +0.4 V (vs SCE) under visible light irradiation (Figure 5A). As can be seen, g-C3N4/FTO electrode exhibits a low photocurrent response to light illumination (curve a in Figure 5A). When FTO electrode is modified by CdS QDs, the photocurrent is obviously increased owing to the narrow band gap of CdS QDs (curve b in Figure 5A).47 After g-C3N4 is

coupled with CdS QDs, the dramatic promotion of photocurrent is observed on the g-C3N4−CdS/FTO electrode, confirming the synergy between the PEC activities of g-C3N4 and CdS QDs (curve c in Figure 5A). This should be attributed to the formation of nanocomposites, which results in high separation and transfer of photogenerated electron−hole pairs at the interfaces of composites.7,10 Such a charge transfer phenomenon could be clarified by analyzing the PL spectra of g-C3N4 before and after coupling with CdS (Figure S2). The pristine g-C3N4 exhibits a strong PL emission peak around 462 nm, attributed to the recombination of electron−hole pairs. After g-C3N4 is coupled with CdS, the PL intensity significantly decreases. This indicates that the recombination of photogenerated electrons and holes on g-C3N4−CdS composites is much lower than that on g-C3N4, due to the charge transfer at the interfaces of g-C3N4 and CdS QDs.11 As illustrated in Scheme 1, the photogenerated holes and electrons would be facilely transferred between CdS and g-C3N4 owing to the wellD


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Figure 3. High-resolution XPS spectra for (A) C 1s, (B) N 1s, (C) Cd 5d, and (D) S 2p of g-C3N4−CdS composite.

matched overlapping band-structures of the two materials.10 This can efficiently impede the recombination of holes and electrons, leading to enhanced photocurrent. The photocurrent responses of modified electrodes prepared with g-C3N4−CdS composite solutions containing different amounts of CdS QDs were studied. In this study, the concentration of CdS QDs was varied from 0.25 to 1.5 g L−1. To ensure enough amount of g-C3N4 for subsequent aptamer immobilization, the concentration of g-C3N4 was increased correspondingly, whereas the ratio of g-C3N4 to CdS QDs was always kept at 2:1 in the composite solutions. As can be seen in Figure 5B, the photocurrent of g-C3N4−CdS/FTO electrode significantly increases with increasing the concentration of CdS QDs from 0.25 to 0.75 g L−1, illustrating that more g-C3N4− CdS nanocomposites could provide more photogenerated holes and electrons to participate in the PEC process. When the concentration of CdS QDs exceeds 0.75 g L−1, the photocurrent decreases. This result may be due to the fact that excessive amount of g-C3N4 and CdS QDs would block the transfer of photogenerated electron. Thus, 0.75 g L−1 CdS QDs was used for fabricating the PEC sensor. In addition, the evolution of the PEC response of the gC3N4−CdS/FTO electrode with time was examined in 0.1 M Na2SO4 at a bias potential of +0.4 V. To minimize the influence of charging current on the PEC response, the photocurrent was recorded after 250 s. As illustrated in Figure S3, the photocurrent is decayed with pronging of time until 1000 s. Considering that shorter responsive time and higher photocurrent are preferred for PEC sensing, we adopt the photocurrent measurement at the initial period of 250−270 s to represent the PEC response of the g-C3N4−CdS/FTO electrode despite the fact that the current in this period did not reach the steady state. To avoid the problem of decayed current during the period of 250−270 s, the transient photocurrent

Figure 4. (A) FT-IR spectra and (B) UV−visible diffuse reflectance spectra (DRS) of the as-prepared samples.

E


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troscopy (EIS) using [Fe(CN)6]3‑/4‑ as a redox probe (Figure S4). Before electrode modification, the EIS of bare FTO electrode was measured. According to the semicircle diameter appearing in the Nyquist plot of EIS, the interfacial chargetransfer resistance (Rct) value of bare FTO electrode is 242 Ω. After FTO is coated with g-C3N4−CdS composites, Rct value is increased to 627 Ω, indicating that the low conductivity of gC3N4−CdS composites blocks the electron transfer between [Fe(CN)6]3−/4− reaction species and electrode. Furthermore, the EIS of g-C3N4−CdS/FTO electrode after aptamer immobilization was studied. The Rct value of the aptamer/gC3N4−CdS/FTO is found to be drastically increased to 1978 Ω. This result could be attributed to the fact that aptamer molecules carrying negative charges induce electrostatic repulsion between electrode surface and negatively charged redox species of [Fe(CN)6]3−/4−.30,32 The considerable Rct value of aptamer modified electrode also confirms the effective immobilization of aptamer on the surface of g-C3N4−CdS/ FTO electrode. 3.4. Optimizing Response of Aptasensor to TET. To study the PEC performance of the aptasensor, we recorded the photocurrent response of the as-prepared aptamer/g-C3N4− CdS/FTO electrode toward TET in 0.1 M Na2SO4 by applying an anodic bias potential of +0.4 V. The result showed that the photocurrent of aptamer/g-C3N4−CdS/FTO increased when TET was present in the solution. Thus, the aptamer/g-C3N4− CdS/FTO electrode could be used as a PEC sensor for TET detection. The increased photocurrent response in this sensor can be analyzed by the PEC reactions of TET molecules captured by aptamer on the electrode surface,48 as illustrated in Scheme 2. According to previously reported photocatalytic oxidation reactions for TET, photogenerated holes on electrode surface could directly oxidize adsorbed TET molecules, or react with OH− or H2O to form OH• radicals, which then oxidize adsorbed TET molecules.49

Figure 5. (A) Photocurrent responses of (a) g-C3N4/FTO, (b) CdS/ FTO, and (c) g-C3N4−CdS/FTO electrodes. (B) Influence of CdS concentration on the photocurrent response of g-C3N4−CdS modified electrodes. The PEC measurements were recorded in 0.1 M Na2SO4 at a bias potential of +0.4 V.

Scheme 1. Schematic Illustration of Separation Mechanism of Photogenerated Electron−Hole Pairs between g-C3N4 and CdS

TET + h+ → oxidation products

(1)

OH− + h+ → OH•

(2)

H 2O + h+ → OH• + H+

(3)

TET + OH• → degradation products

(4)

Because some holes are scavenged directly or indirectly by adsorbed TET according to these reactions, the recombination of photogenerated electron−hole pairs is inhibited. The left photogenerated electrons would be driven to the counter electrode by the bias potential through the external circuit, and hence facilitate the generation of a high photocurrent.48 On the basis of the photocurrent response of TET, the aptamer/g-C3N4−CdS/FTO electrode was used as a PEC sensor for TET detection. To achieve the best sensing performance, the influence of aptamer concentration on the response of the sensor toward TET was investigated, and the PEC response was quantitatively evaluated by the photocurrent difference (ΔPI) before and after incubation with TET. Figure S5 displays the effect of aptamer concentration on the photocurrent response. It is observed that the PEC response toward TET remarkably increases with increasing the aptamer concentration from 0.5 to 1 μM. This result is consistent with the fact that higher concentration of aptamer immobilized on the electrode could capture more TET molecules. However, the PEC response decreases when the aptamer concentration

value at the middle moment of this period is determined for quantitative analysis in the developed PEC sensor. 3.3. Electrochemical Impedance Spectroscopic Analysis. The interfacial behavior of g-C3N4−CdS modified electrode was analyzed by electrochemical impedance specF


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ACS Applied Materials & Interfaces Scheme 2. Schematic Illustration of PEC Aptasensing of TET Based on g-C3N4−CdS Composites

exceeds 1 μM. It is likely that steric hindrance created by excess aptamer results in the blockage of electrons transfer.16 Therefore, 1 μM aptamer was chosen for fabricating the sensor. The bias potential also showed an obvious effect on the PEC response of the sensor. As shown in Figure S6, the photocurrent response of TET increases with increasing the bias potential from +0.2 to +0.4 V, demonstrating that larger anodic bias potential could drive more photogenerated electrons to the counter electrode and suppress the recombination of hole−electron pairs more effectively.16,34 However, the response toward TET does not show further enhancement when the applied potential is greater than +0.4 V, which may result from the saturation of photogenerated holes consumed by TET. Thus, +0.4 V was the optimum potential applied for PEC sensing. Moreover, the influence of incubation time on the response of the sensor was studied (Figure S7). It can be observed that the photocurrent response toward TET increases with prolonging the incubation time from 0.5 to 2.0 h, which should be ascribed to the fact that long incubation time can increase the amount of TET molecules captured by aptamer. Nevertheless, the response does not show further enhancement when the incubation time exceeds 2.0 h, demonstrating the saturation of captured TET molecules on the sensor. Consequently, 2.0 h was used as the optimum incubation time for PEC sensing. 3.5. PEC Aptasensing of TET. The developed PEC aptamer sensor was applied to the quantitative determination of TET. The PEC responses of the proposed aptamer/gC3N4−CdS/FTO electrode toward different concentrations of TET were recorded in 0.1 M Na2SO4 solution (Figure 6). As can be seen, the photocurrent response increases with increasing the concentrations of TET, meaning that more TET molecules are captured and participate in the PEC process. Furthermore, the PEC response is found to be linearly related to the concentration of TET in the range from 10 nM to 250 nM. The linear regression equation is expressed as ΔPI/ μA = 0.00472C/nM+0.05271 with a correlation coefficient of 0.998. According to the method of signal-to-noise ratio of 3 (S/ N = 3), the detection limit is estimated. In this method, the standard deviation (S.D.) of PEC responses of five replicated measurements of an aptamer/g-C3N4−CdS/FTO electrode in 0.1 M Na2SO4 solution is first determined; and then 3S.D./k is calculated as the value of detection limit, where k is the slope of

Figure 6. Photocurrent responses of aptamer/g-C3N4−CdS/FTO toward TET at different concentrations: (a) 0, (b) 10, (c) 30, (d) 60, (e) 100, (f) 150, (g) 200, (h) 250 nM. The PEC measurements were recorded in 0.1 M Na2SO4 at a bias potential of +0.4 V. Inset: calibration curve. The error bars were derived from the standard deviation of three measurements.

the linear regression equation of calibration curve. Thus, the detection limit (3S/N) is estimated to be 5.3 nM. Compared with many previously reported methods for the determination of TET (Table 1), this PEC aptasensor shows lower detection limit. To study the selectivity of this PEC aptamer sensor, we recorded the responses of the aptamer/g-C3N4−CdS/FTO electrode in 0.1 M Na2SO4 solution after incubation with 0.01 Table 1. Comparison of Various Methods for the Determination of TET method fluorescent sensor chemiluminescence amperometric detection electrochemical sensor colorimetric aptasensor PEC aptasensor

G

minear range (M)

detection limit (M)

1.0 × 10−7−2.0 × 10−5 2.0 × 10−7−5.0 × 10−6 1.0 × 10−6 − 5.0 × 10−4

6.0 × 10−8 6.0 × 10−8 9.6 × 10−7

26 50 51

3.0 × 10−7−5.2 × 10−5

1.0 × 10−7

23

4.2 × 10−7−4.2 × 10−6

8.1 × 10−8

33

1.0 × 10−8−2.5 × 10−7

5.3 × 10−9

This work

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4. CONCLUSIONS In summary, g-C3N4 sensitized with CdS QDs was prepared to fabricate a novel visible-light PEC aptasensor for TET. The nanocomposites remarkably improved the PEC performance compared with single photoactive material. The aptamer was effectively anchored on the surface of composites via the π−π stacking interaction between g-C3N4 and aptamer. Thus, a high performance PEC aptamer sensor with high sensitivity and selectivity for TET detection was successfully constructed. The proposed PEC aptamer sensor was successfully applied to TET detection in environmental water samples with desirable precision and accuracy, demonstrating the potential applications of g-C3N4 and CdS QDs nanocomposites in PEC devices.

M PBS solution containing 100 nM of various antibiotics including tetracycline, chlortetracycline, neomycin sulfate, kanamycin, ciprofloxacin, doxycycline, and chloramphenicol at 60 °C for 2.0 h. Different from TET, all these antibiotics do not exhibit obvious responses on the aptamer/g-C3N4−CdS/FTO electrode (Figure 7), revealing the proposed sensor has a high

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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.5b08275. XPS and PL spectra of composite materials; evolution of PEC response of g-C3N4−CdS/FTO with time; EIS of modified electrodes; influences of aptamer concentration, bias potential, and incubation time on PEC response of sensor (PDF)

Figure 7. PEC responses of aptamer/g-C3N4−CdS/FTO toward various antibiotics at 100 nM. The PEC measurements were recorded in 0.1 M Na2SO4 at a bias potential of +0.4 V. The error bars were derived from the standard deviation of three measurements.

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AUTHOR INFORMATION

Corresponding Author

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

selectivity due to the specific recognition between aptamer and target TET molecules. Moreover, the reproducibility of the PEC aptamer sensor was also evaluated. The relative standard deviation (RSD) of the responses for detection of 100 nM TET using five independently aptamer/g-C3N4−CdS/FTO electrodes is 2.7%, suggesting the good reproducibility. Furthermore, no obvious change was observed for the response of the aptamer/g-C3N4−CdS/FTO electrode stored at 4 °C for 2 weeks, indicating the high stability of the sensor. The applicability of the PEC aptasensor was studied in environmental water samples collected from three lakes in Wuhan City by the proposed method using the standard addition method. Because no TET was detected in original water samples, different amounts of standard TET were spiked into these samples. As can be seen (Table 2), the recoveries of the proposed method for these water samples are in the range from 98.2 to 101.1%. The result demonstrates the feasibility of the proposed sensor for TET determination in environmental water samples.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 61172005). The authors thank the Analytical and Testing Center of HUST for help in the characterization of materials.

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Table 2. Determination of TET in Environmental Water Samples Employing the PEC Aptamer Sensor (n = 3) sample

added (nM)

found (nM)

1

0 100 150 0 100 150 0 100 150

0 101.1 151.7 0 99.1 150.9 0 98.2 149.2

2

3

RSD (%)

recovery (%)

1.3 1.6

101.1 101.1

1.4 1.6

99.1 100.6

1.7 1.3

98.2 99.5

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