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BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin
Accepted Manuscript Graphitic carbon nitride/BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin Li Xu, Henan Li, Pengcheng Yan, Jiexiang Xia, Jingxia Qiu, Qian Xu, Shanqing Zhang, Huaming Li, Shouqi Yuan PII: DOI: Reference:
S0021-9797(16)30570-7 http://dx.doi.org/10.1016/j.jcis.2016.08.015 YJCIS 21483
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
7 June 2016 24 July 2016 6 August 2016
Please cite this article as: L. Xu, H. Li, P. Yan, J. Xia, J. Qiu, Q. Xu, S. Zhang, H. Li, S. Yuan, Graphitic carbon nitride/BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.08.015
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Graphitic carbon nitride/BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin Li Xua,c, Henan Li c,d, Pengcheng Yanb, Jiexiang Xiab, Jingxia Qiua , Qian Xua , Shanqing Zhang c, Huaming Li*a,b, Shouqi Yuan* a
a Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China c Centre for Clean Environment and Energy, Environmental Futures Research Institute and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia d School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia *Corresponding authors. E-mail addresses: [email protected]; [email protected] Tel: +86-511-88799500.
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Abstract Ciprofloxacin, as a second generation of fluoroquinolone antibiotics, has been proved to cause environmental harm and exhibits toxic effects on the wastewater and surface water even at low concentrations due to their continuous input and persistence. Despite tremendous efforts, developing ciprofloxacin detection method with accuracy and sensitivity at low-cost remains a great challenge. Herein, graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) has been designed for a facile and sensitive photoelectrochemical (PEC) monitoring platform of ciprofloxacin at first time. BiOCl can be modified with the g-CN nanosheets which are obtained via solvothermal process at low-temperature conditions. The use of g-CN is shown to strongly enhance the PEC response of BiOCl due to the formation of heterojunctions. The photocurrent generated at the g-CN/BiOCl-modified ITO (with 13 wt% g-CN content) is much higher and more
stable than that
of a
BiOCl-modified ITO.
Based on these
findings,
the
g-CN/BiOCl-modified ITO was used to design a PEC assay for the antibiotic ciprofloxacin. Furthermore, the limit of detection of the ciprofloxacin PEC sensor has been significantly lowered to 0.2 ng⋅mL-1. In addition, the PEC sensor can detect ciprofloxacin in the wide range of 0.5-1840 ng mL-1. Key words: Bismuth oxychloride; graphitic carbon nitride; ciprofloxacin; photoelectrochemical detection
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1. Introduction Photoelectrochemical (PEC) detection exerts a tremendous fascination on analytical field. In the case of the PEC detection process, the electrons excited from semiconductor materials by light are transferred and producing photocurrent which is utilized as detection signal[1]. Quite a few background signals can be reduced greatly benefiting from the efficient separation of the excitation source (light) and detection signal (photocurrent)[2]. PEC detection demonstrates the superiorities of both optical and electrochemical methods such as low cost, easy-to-operate, remarkable sensitivity, rapid response, etc., which have been widely captured for the interest of researchers[3,4]. Many efforts have been devoted to develop photo-electrochemical detection method towards different analysts, such as p-phenylenediamine[5], catechol[6], bisphenol A[7], chlorpyrifos[8], and so on. With development of photoelectrochemistry, a multitude of materials with PEC response are appeared, including quantum dots, semiconducting nanoparticles, metal complex and so on.[9] Among them, bismuth oxyhalides (BiOCl, BiOBr, and BiOI) have drawn much attention for their unique and excellent optical and electrical properties[10-12]. The crystal structure of bismuth oxyhalides is a layered structure that is built by interlacing [Bi2O2] slabs with double halogen slabs[13]. In particular, the layered structure endows bismuth oxyhalides with the self-built internal static electric fields which can promote the effective separation of the photoinduced hole-electron pairs[14]. Owing to small band gap (-1.8 eV) and excellent absorption capacity in the visible-light region, BiOI has been applied in PEC detection, such as organophosphate pesticide[15]and perfluorooctanic acid[2]. However, BiOCl have not yet been applied in the PEC detection field. BiOCl belongs to the wide-band-gap (3.4 eV) p-type semiconductor[16] which results in low utilization ratio of light. A multitude of strategies have been proposed to enhance the properties of photoelectric materials[17]. A method, coupling two semiconductors to form heterojunction, is a shared approach to improve the PEC response of the semiconductors. Recently, the graphitic carbon nitride (g-CN), an organic semiconductor, has stimulated intensive interest in photoelectron-chemical field[18]. Due to its visible light response ability, high thermal and chemical stability, unique electronic band structure (2.7 eV), and layer structure like graphene, g-CN frequently couples with other semiconductors to enhance the PEC response[19].
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Some pioneering studies have proved that energy levels of g-CN and BiOCl are well-matched and can overlap in band structure[20,21]. Thus, the combination of g-CN and BiOCl can construct heterojunction catalysts, which can enhance charge separation and improve the PEC response[22]. Inspired by this, the excellent PEC performance can be achieved by constructing composite of g-CN and BiOCl. Ciprofloxacin, as a second generation of fluoroquinolone antibiotics, is active against a broad spectrum of Gram-negative and Gram-positive bacteria, and has been widely used for humans and veterinary purposes[23]. Although benefits of the application of ciprofloxacin in medicine are obvious, ciprofloxacin has been proved to cause environmental harm and exhibits toxic effects on the wastewater and surface water even at low concentrations due to their continuous input and persistence[24]. Therefore, it is crucial to develop sensitive and effective methods for the careful monitoring of ciprofloxacin concentrations. Recently, Gayen and Chaplin have developed a sensitive, selective, and fast electrochemical method for the determination of ciprofloxacin in water by modifying a boron-doped diamond electrode with porous nafion/multiwalled carbon nanotube composite film[25]. Herein, we have been fabricated graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) via a facile a one-pot ethylene glycol-assisted solvothermal process in the presence of ionic liquid 1-hexadecyl-3-methylimidazolium chloride ([C16mim]Cl). The g-CN obtained by solvothermal process at low-temperature conditions possesses high light-harvesting capability and decreased band gap[18]. It can benefit to enhance the PEC performance of BiOCl. In addition, the g-CN nanosheets can cover the surface of BiOCl microspheres, which can form the heterojunction structures. Therefore, the g-CN/BiOCl composites possessed a high light response and exhibited much more enhanced PEC response than that of pure BiOCl. The PEC sensor for ciprofloxacin can be fabricated by g-CN/BiOCl composites at first time, which can exhibit high sensitivity and stability. 2. Materials and methods 2.1 Materials The ionic liquid [C16mim]Cl (99%) was purchased from Shanghai Chengjie Chemical Co. Ltd. 2,4,6-trichloro-1,3,5-triazine, melamine and Bi(NO3)3⋅5H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All the reagents were of analytical purity and were used as 4
received. Phosphate buffer solution (0.1 M, pH 7.0) was prepared by Na2HPO4·12H2O and NaH2PO4·2H2O, and adjusted the pH with 0.1 M NaOH or H3PO4. 2.2 Synthesis of g-CN/BiOCl composites According to the literature[18,19], the g-CN was synthesized by solvothermal process. 2,4,6-trichloro-1,3,5-triazine (5 mmol) and melamine (2.5 mmol) was added into acetonitrile (20 mL). The mixture was stirred for 30 min and then transferred into 25 mL Teflon-lined autoclave. The autoclave was then heated at 160 ºC for 48 h and cooled down to room temperature. Then, the samples were centrifuged by high-speed centrifuge, washed by deionized water and dried at 60 ºC. A typical synthesis of g-CN/BiOCl composites (with 13 wt% g-CN content) is shown in the following way: firstly, g-CN (40 mg) was added into an ethylene glycol solution (20 mL) containing [C16mim]Cl (1 mmol), and then ultrasonicated for 30 min. Secondly, Bi(NO3)3⋅5H2O (1 mmol) was dissolved into above solution, the mixture was stirred for 30 min and then transferred into 25 mL Teflon-lined autoclave. The autoclave was then heated at 140 ºC for 24 h and cooled down to room temperature. The obtained samples were centrifuged and washed with distilled water and ethanol three times. Finally, the samples were dried in a vacuum oven at 50 ºC for 24 h before further characterization. For comparison, the BiOCl samples were also prepared without g-CN under the same condition. 2.3 Apparatus X-Ray powder diffraction (XRD) analysis was carried out on a Bruker D8 diffractometer with high-intensity Cu-Kα (λ=1.54 Å). The structural information for samples was measured by Fourier transform spectrophotometer (FT-IR, Nexus 470, Thermo Electron Corporation) using the standard KBr disk method. X-ray photoemission spectroscopy (XPS) was recorded on a VG MultiLab 2000 system with a monochromatic Mg-Kα source operated at 20 kV. The field-emission scanning electron microscopy (SEM) measurements were carried out with a field-emission
scanning
electron
microscope
(JEOL
JSM-7001F)
equipped
with
an
energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. The photoluminescence (PL) spectra were detected using a Varian Cary Eclipse spectrometer. The diffuse reflectance spectra (DRS) were measured using a UV-vis spectrophotometer (Shimadzu UV-2450, Japan) in the range of 200 to 800 nm. BaSO4 was used as the reflectance standard
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material. PEC performance measurements were performed on a CHI-660B electrochemical workstation (CH Instruments, Inc.), which was connected to a conventional three electrodes cell using indium tin oxide (ITO) as a working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a platinum wire as a counter electrode. A 150 W xenon lamp was used as the light source. Electrochemical impedance spectroscopy (EIS) was performed in a 0.1 M potassium chloride solution containing 5 mM ferricyanide/ferrocyanide with a frequency range from 0.01 Hz to 10 kHz at 0.24 V, and the amplitude of the applied sine wave potential in each case was 5 mV. 2.4 Fabrication of the modified electrodes Prior to the modification of ITO electrode, the g-CN/BiOCl composites were dissolved in ethanol to obtain a 2 mg mL-1 suspension. The ITO electrodes were placed in boiling water containing 0.1 M sodium hydroxide for 10 min, and were then cleaned ultrasonically in water followed by alcohol for 10 min each. Afterwards, 20 µL of the g-CN/BiOCl composites suspension was dropped onto the surface of the ITO electrode with a fixed area of 0.5 cm2 and then dried at room temperature to obtain the g-CN/BiOCl composites modified ITO. For comparison, pristine BiOCl modified ITO was also prepared in the same way. 3. Results and discussion 3.1 Characterization of the g-CN/BiOCl composite The XRD patterns of the g-CN/BiOCl composites and BiOCl were shown in Fig. 1 (a). All the diffraction peaks of the g-CN/BiOCl composites and BiOCl corresponded to the structure of the tetragonal phase of BiOCl (JCPDS card No. 06-0249), with the lattice constants a = 3.890 Å, c = 7.890 Å. No other characteristic peaks were found, which implied the high purity of the samples. Meanwhile, it can be seen from the XRD diffraction peaks that the samples had a good crystallinity. However, there were no characteristic peaks of the g-CN at about 13.3° and 27.4° appeared, which might due to the low g-CN content and the high dispersion of g-CN in the g-CN/BiOCl composites[26]. FT-IR spectra analysis had been applied to determine the presence of g-CN in g-CN/BiOCl composites. It was found by contrasted Fig.1 (b) that the g-CN/BiOCl composites possessed characteristic absorption peaks of g-CN and BiOCl. For the g-CN/BiOCl composites, a strong absorption band in the range of 1000-1800 cm-1, with the peaks at about 1210, 6
1338, 1447, 1538, 1617, 1664, and 1731 cm-1, were associated with typical skeletal stretching vibrations of the s-triazine or tri-s-triazine,[18,21] which indicated g-CN basic structure was maintained after hybridization. The peaks at 1210 and 1617 cm-1 were related attributable to the C-N and C=N stretching vibration modes[27], respectively. The peak at 807 cm-1 was attributable to the s-triazine ring modes[28]. The absorption peak at about 530 cm-1 was attributed to the stretching vibrations of the Bi-O in BiOCl[29].Thus, the results showed that the g-CN had been successfully introduced to the BiOCl material. In order to specify the chemical states of surface elements and the chemical compositions of g-CN/BiOCl composite, XPS analysis was further carried out. In the Fig. 2, the survey scan XPS spectra provided C 1s, N 1s, Bi 4f, O 1s and Cl 2p peaks for g-CN/BiOCl composite. To further determine the valence state of various elements for g-CN/BiOCl composite, high resolution XPS analysis were conducted. In the high-resolution Bi 4f XPS spectra (Fig.2b), the binding energy of 164.5 eV and 159.2 eV corresponded to the Bi 4f5/2 and Bi 4f7/2 of Bi3+ in BiOCl[30], respectively. No photoelectron signals associated with Bi2+, Bi4+ or Bi5+ states in bismuth oxides appear, suggesting the high purity of the samples. As can be seen from Fig. 2(c), the Cl 2p XPS spectra consisted of Cl 2p3/2 and Cl 2p1/2 photolines that were fitted well with the peak at 197.9 eV and 199.2 eV, respectively, consistent with the values for Cl- [17]. It can be also found in Fig. 2(d) that the peak at 530.2 eV was attributed to O 1s, which came from the Bi-O bond in BiOCl. Fig. 2(e and f) showed the high-resolution C 1s and N 1s XPS spectra of the samples. C 1s peaks at 284.8 eV and 288.0 eV were assigned to carbon and defect-containing sp2-bonded carbon (C=N)[31]. The N 1s peak was located at 399.1 eV, which was ascribed to sp2-hybridized nitrogen (C=N-C)[18]. The results of FT-IR and XPS analysis indicated the coexistence of g-CN and BiOCl in the g-CN/BiOCl composites. The morphology of g-CN/BiOCl composites was visualized by scanning electron microscopy (SEM). As shown in Fig. 3(a,b), the g-CN/BiOCl composites had a three-dimensional sphere-like structure. As can be shown from the SEM image, the g-CN nanosheets covered the surface of BiOCl microspheres. The g-CN and BiOCl were in close contact with each other, indicating an interaction between g-CN and BiOCl exists. The morphology of BiOCl was also a similar three-dimensional sphere-like structure (Fig. 3(c,d)), indicating that the morphologies of BiOCl
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were not obviously changed in the g-CN/BiOCl composites after introducing g-CN. The elemental mapping of g-CN/BiOCl composites was conducted using SEM with an energy dispersive X-ray. The images in Fig.3(b) represented elemental mapping of Bi, Cl, O, C and N elements, respectively. The C and N elements were homogeneously distributed, besides the Bi, Cl and O elements, which provided the direct evidence for the good dispersion and attachment of g-CN on the surfaces of BiOCl microspheres. It further proved that the samples were g-CN/BiOCl composites and the result of SEM was consistent with the results of FT-IR and XPS analysis. To investigate information for the photoexcited charge carriers transfer and recombination processes, PL spectra of the g-CN/BiOCl composites and BiOCl were recorded. Fig. 4 showed the PL spectra of the g-CN/BiOCl microspheres and BiOCl excited at 360 nm. The emission peak of pure BiOCl appeared at about 468 nm. For g-CN/BiOCl composites, the position of the emission peak in the PL spectrum was similar to that of the pure BiOCl, but the emission intensity was decreasing. This result demonstrated that the g-CN/ BiOCl composites had a much lower recombination rate of photoinduced electrons and holes than that of pure BiOCl, which was attributed to that the recombination of photoinduced electrons and holes was greatly inhibited by the introduction of g-CN. This result also indicated that the g-CN/BiOCl composites were expected to exhibit improved PEC properties. UV-vis diffuse reflectance spectroscopy (DRS) was used to examine the optical absorption of the g-CN/BiOCl composites. Fig. 5 showed DRS spectra of the g-CN/BiOCl composites, BiOCl and g-CN. Optical absorption of g-CN was from approximately 500 to 680 nm, which was in accordance with the work by Wang[18]. As shown in Fig. 5, it can be clearly observed that the DRS of the g-CN/BiOCl composites and BiOCl present a similar absorption edge between about 300 to 400 nm, indicating that the g-CN/BiOCl composites and BiOCl mainly absorb UV light. Noticeably, the g-CN/BiOCl composites had a new extra consecutive visible light absorption region from 400 nm to 570 nm. These indicated that the absorption region had been broadened after introducing g-CN, which was contributed to absorb more light for generating more electrons and holes, and improve the PEC performance.
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3.2 Electrochemical and PEC performance of the g-CN/BiOCl composite To gain further insight into the electron-transport property of the g-CN/BiOCl composites and BiOCl, EIS was performed with the g-CN/BiOCl composites modified ITO electrode (g-CN/BiOCl-modified ITO) electrode and the BiOCl modified ITO electrode (BiOCl-modified ITO) using [Fe(CN)6]3-/4- as a redox probe (Fig. 6a). EIS of the modified electrodes showed the similar Nyquist plots with a semicircle in the high-frequency region and an inclined straight line at low frequency. The radius of the g-CN/BiOCl-modified ITO was smaller than that of the BiOCl-modified ITO. It can be deduced that the g-CN/BiOCl composites improved the conductivity of the electrode. That is, the g-CN/BiOCl composites possessed excellent conductivity, which can obviously decrease the interface resistance between the electrochemical probe and the electrode[32]. The PEC behaviors of the different modified electrodes were investigated in 0.1 M phosphate buffered solution (pH 7.0) (Fig. 6b). At an applied potential of 0 V (versus SCE), the g-CN/BiOCl-modified ITO and the BiOCl-modified ITO showed a photocurrent response under light illumination with a light on/off circle of 20 s. The photocurrent of the g-CN/BiOCl-modified ITO did not show obvious changes, that is, the photocurrent responses of the modified electrodes were very stable. The photocurrent of the g-CN/BiOCl-modified ITO was almost twice higher than that of the BiOCl-modified ITO. Meanwhile, an optimized special density of g-CN nanosheets combined with BiOCl is necessary to achieve excellent photocurrent response. It is
found that about 13 wt% g-CN content is optimum to get the highest photocurrent response (Fig. S1). Compared with the BiOCl-modified ITO, the g-CN/BiOCl-modified ITO (with 13 wt% g-CN content) was more suitable for constructing PEC detection system of ciprofloxacin due to the high and stable of the photocurrent responses. Fig. 7 summarized the proposed mechanism for the photoelectron-chemical behaviors of the g-CN/BiOCl-modified ITO. When the modified electrodes were irradiated by light, the g-CN/BiOCl composites absorbed photons and excited electrons and holes. The g-CN possessed an excellent photoresponse which was easily excited and engendered the corresponding photoinduced electron-hole pairs under light irradiation[33]. The theoretical calculated values of the conduction band (CB) and valence band (VB) potentials of the n-type semiconductor g-CN material were -1.12 and 1.57 eV[34]. According to previous
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report[20], the conduction band edge potential of g-CN was more negative than that of BiOCl, and the valence band of BiOCl was more positive than that of g-CN. The g-CN and BiOCl were easily form heterojunction structure. When the g-CN and BiOCl were excited by light, photoinduced electrons were injected with high efficiency from conduction band of g-CN to conduction band of BiOCl. Simultaneously, the holes on the valence band of BiOCl migrated to that of g-CN. In such a way, the charge transfer would suppress the photoinduced electron-hole pair recombination and prolong the lifetime of photoinduced electron-hole, which contributed to enhance PEC properties of the g-CN/BiOCl composite. The addition of ciprofloxacin hindered the electron transfer from g-CN/BiOCl composites towards the ITO electrode and resulted in the recombination of photoinduced electron-hole pair[2,8,35-37]. The ultimate result was a decrease in the photocurrent intensity. It indicated that the quantitative detection of ciprofloxacin was able to achieve by monitoring the photocurrent intensity decrease of the g-CN/BiOCl-modified ITO after the adding of ciprofloxacin[36]. 3.3 Construction of the PEC sensor for ciprofloxacin detection The properties of PEC sensor based on the g-CN/BiOCl composites were subsequently tested by monitoring ciprofloxacin. As shown in Fig. 8(a), the photocurrent of the g-CN/BiOCl-modified ITO gradually decreased when ciprofloxacin was injected in phosphate buffered solution. The photocurrent intensity of the g-CN/BiOCl-modified ITO decreased with the increase of ciprofloxacin concentration. As shown in Fig. S2, the Ret value of the g-CN/BiOCl-modified
ITO actually increased in the presence of ciprofloxacin. It indicated that ciprofloxacin inhibited electron transfer of the g-CN/BiOCl-modified ITO, and consequently leading to a decrease in photocurrent[35]. FT-IR spectra analysis of the g-CN/BiOCl composites after detecting ciprofloxacin showed that absorption band of g-CN/BiOCl composites was shifted to low wavenumber compared with before detecting ciprofloxacin (Fig.S3). It indicated that the ciprofloxacin adsorbed onto the surface of g-CN/BiOCl composites. The ciprofloxacin was able to introduce a strong steric hindrance effects toward the diffusion of electron[2]. On this basis, the possible mechanism of ciprofloxacin detection can be proposed: When the g-CN/BiOCl-modified ITO detected ciprofloxacin, ciprofloxacin on the surface of g-CN/BiOCl composites gave an increase in steric hindrance which trailed off the electron 10
transfer of g-CN/BiOCl composites towards the ITO electrode surface, and consequently resulted in a decrease in photocurrent[8]. As shown in Fig. 8(b), it can be found that the relative change of the photocurrent ((i0-i)/i0 , where i0 and i were the photocurrent before and after incubation in ciprofloxacin) was two linear segments with different slopes to the ciprofloxacin concentrations in the range of 0.5-1840.0 ng mL-1. The corresponding linearization equation was (I) (i0-i)/i0= 0.1359+ 6.3690* 10-4 c/ng mL-1 (R2=0.994, cciprofloxacin: 0.5-400 ng mL-1); (II) (i0-i)/i0= 0.2533+2.8204*10-4 c/ng mL-1 (R2=0.998, cciprofloxacin: 400-1840 ng mL-1), respectively. The detection limit (S/N=3) was estimated to be 0.2 ng mL-1 based on the response of three times the standard deviation of zero-dose response[2]. Compared with other monitoring methods of ciprofloxacin (Table S1), the PEC sensor, based on the g-CN/BiOCl composites, was demonstrated to be superior to that of other monitoring methods, which had lower detection limit and wider linear range.
The interference study of the ciprofloxacin photoelectrochemical sensor has been investigated by measuring the sensor responses to metal ion (Fe3+, Zn2+, Cd2+, Pb2+), 4-hydroxybenzoic acid, 4-aminobenzoic acid, Rhodamine B, methyl orange and phenol at a concentration of 4000 ng mL-1 that is 10-fold to ciprofloxacin are shown in Fig.S4. The photocurrent responses to the metal ion were very close to the blank test, with minimal deviations below 5%. There were more effects on the blank signal with organic species, especially 4-hydroxybenzoic acid, 4-aminobenzoic acid and phenol, which caused about 10% deviations. There results indicated that the ciprofloxacin photoelectrochemical sensor had an acceptable anti-interference capacity for ciprofloxacin detection. A recovery study has been performed in the detection of ciprofloxacin in Yangtze River water samples with different standard concentrations (Table S2). The PEC sensor can detect ciprofloxacin in real water samples with good recoveries varied in the range of 90.5-112.6%. It indicated that this PEC sensor had satisfactory accuracy and reliability for detection of ciprofloxacin in real water samples. 4. Conclusions In this study, the g-CN/BiOCl composites have been fabricated via a facile a one-pot ethylene glycol-assisted solvothermal process in the presence of ionic liquid [C16mim]Cl. SEM 11
analysis indicated g-CN covered the surface of sphere-like BiOCl and formed heterojunctions, which contributed to enhance the PEC response of the g-CN/BiOCl composites. The photocurrent response of the g-CN/BiOCl-modified ITO was high and stable, and indicated that the g-CN/BiOCl-modified ITO was suitable as a PEC sensor. PEC sensor of ciprofloxacin was designed based on the g-CN/BiOCl composites for the first time. In addition, the PEC sensor detected ciprofloxacin in the wide range of 0.5-1840 ng mL-1 with a low detection limit of 0.2 ng mL-1. The above results demonstrated that the g-CN/BiOCl composites can be applied for detection of ciprofloxacin. Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 21476098, 21576123, 21506081, 21506077), the Jiangsu Province Postdoctoral Science Foundation (1501026B), University Natural Science Research of Jiangsu (15KJB530004), Jiangsu University Scientific Research Funding (15JDG048), Chinese Postdoctoral Foundation (2016M590420), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] D.Q. Jin, Q. Xu, Y.J. Wang, X.Y. Hu, Talanta, 127(2014) 169-174. [2] J.M. Gong, T. Fang, D.H. Peng, A.M. Li, L.Z. Zhang, Biosens. Bioelectron., 73(2015) 256-263. [3] G.M. Wen, H.X. Ju, Talanta, 134(2015) 496-500. [4] R.X. Li, J. Gao, P.C. Gao, S. Zhang, Y.X. Liu, B. Du, Q. Wei, New J. Chem., 39(2015) 731-738. [5] Y.H. Zhu, K. Yan, Y. Liu, J.D. Zhang, Anal. Chim. Acta, 884(2015) 29-36. [6] Y. Liu, R. Wang, Y.H. Zhu, R.Z. Li, J.D. Zhang, Sens. Actuators B, 210(2015) 355-361. [7] B.T. Zhang, L.L. Lu, F. Huang, Z. Lin, Anal. Chim. Acta, 887(2015) 59-66. [8] J. Qian, Z.T. Yang, C.Q. Wang, K. Wang, Q. Liu, D. Jiang, Y.T. Yan, K. Wang, J. Mater. Chem. A, 3(2015) 13671-13678.
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[27] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Energy Environ. Sci., 4(2011) 2922-2929. [28] Q.J. Xiang, J.G. Yu, M. Jaroniec, J. Phys. Chem. C, 115(2011) 7355-7363. [29] J.X. Xia, L. Xu, J. Zhang, S. Yin, H.M. Li, H. Xu, J. Di, CrystEngComm, 15(2013) 10132-10141. [30] X. Zhang, X.B. Wang, L.W. Wang, W.K. Wang, L.L. Long, W.W. Li, H.Q. Yu, ACS Appl. Mater. Interfaces, 6(2014) 7766-7772. [31] S.B. Yang, Y.J. Gong, J.S. Zhang, L. Zhan, L.L. Ma, Z.Y. Fang, R. Vajtai, X.C. Wang, P.M. Ajayan, Adv. Mater., 25(2013) 2452-2456. [32] Z.T. Yang, J.Qian, X.W. Yang, D.Jiang, X.J. Du, K. Wang, H.P. Mao, K. Wang, Biosens. Bioelectron., 65(2015) 39-46. [33] C.S. Pan, J. Xu, Y.J. Wang, D. Li, Y.F. Zhu, Adv. Funct. Mater., 22(2012) 1518-1524. [34] N.N. Wang, Y. Zhou, C.H. Chen, L.Y. Cheng, H.M. Ding, Catal. Commun., 73(2016) 74-79. [35] Y. Zhang, A. Shoaib, J.J. Li, M.W. Ji, J.J. Liu, M. Xu, B. Tong, J.T. Zhang, Q. Wei, Biosens. Bioelectron., 79(2016) 866-873. [36] D.W. Fan, C.J. Guo, H.M. Ma, D. Zhao, Y.N. Li, D. Wu, Q. Wei, Biosens. Bioelectron., 75(2016) 116-122. [37] D. Jiang, X.J. Du, D.Y. Chen, Y.Q. Li, N. Hao, J. Qian, H. Zhong, T.Y. You, K. Wang, Carbon, 102(2016) 10-17.
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Figure Captions Fig.1 (a) XRD pattern of the g-CN/BiOCl composites and pure BiOCl; (b) FT-IR analysis for g-CN/BiOCl composites (I), g-CN (II) and pure BiOCl (III). Fig.2 XPS spectra of the g-CN/BiOCl composites: (a) survey, (b) Bi 4f, (c) Cl 2p, (d) O 1s, (e) C 1s, (f) N 1s. Fig.3 SEM images of the g-CN/BiOCl composites (a,b) and pure BiOCl (c,d); (b) Element mapping of Bi, Cl, O, C and N of the g-CN/BiOCl composites. Fig.4 PL spectra of the g-CN/BiOCl composites and pure BiOCl excited at 360 nm. Fig.5 UV-vis diffuse reflectance spectra of the pure BiOCl (a), g-CN/BiOCl composites (b) and g-CN (c). Fig.6 (a) EIS of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO by applying an AC voltage of 5 mV in the 0.1 Hz to 100 kHz frequency range; (b) Time-based photocurrent response of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO in 0.1 M phosphate buffered solution (pH = 7). Fig.7 Schematic illustration of the exciton trapping mechanism of the g-CN/BiOCl composites. Fig.8 (a) Photocurrent responses of the g-CN/BiOCl-modified ITO toward ciprofloxacin at increasing concentrations in the supporting electrolyte of 0.1 M phosphate buffered solution (pH 7.0). (B) Linear calibration curve. Error bars indicate the standard deviation of five repeated measurements. Fig. S1. Photocurrent response of the g-CN/BiOCl-modified ITO electrodes with different content of g-CN: (a) 15 wt%, (b) 13 wt%, (c) 10 wt%, (d) 5 wt%, (e) pure BiOCl. Fig.S2 EIS of the g-CN/BiOCl-modified ITO before (a) and after (b) the addition of ciprofloxacin (1000 ng mL-1) Fig.S3 (a) FT-IR analysis for g-CN/BiOCl composites before and after detection; (b) FT-IR analysis for ciprofloxacin. Fig.S4 Influence of particular possible interfering substances on the responses of the photoelectrochemical sensor for 400 ng mL-1 ciprofloxacin in 0.1 M phosphate buffered 15
solution (pH 7.0): 1. ciprofloxacin; 2. Fe3+; 3. Zn2+; 4. Cd2+; 5. Pb2+; 6. 4-hydroxybenzoic acid; 7. 4-aminobenzoic acid; 8. Rhodamine B; 9. methyl orange; 10. phenol. Table S1 Comparison results of the determination of ciprofloxacin by other monitoring methods. Table S2 Photoelectrochemical detection of ciprofloxacin in Real Water Samples by the photoelectrochemical sensor
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(110)
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Fig.1 (a) XRD pattern of the g-CN/BiOCl composites and pure BiOCl; (b) FT-IR analysis for g-CN/BiOCl composites (I), g-CN (II) and pure BiOCl (III).
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1200
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Bi 4f
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Bi 5d O 2S
C 1s Cl 2s Cl 2p
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Fig.2 XPS spectra of the g-CN/BiOCl composites: (a) survey, (b) Bi 4f, (c) Cl 2p, (d) O 1s, (e) C 1s, (f) N 1s.
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(b) g-CN
Fig.3 SEM images of the g-CN/BiOCl composites (a,b) and pure BiOCl (c,d); (b) Element mapping of Bi, Cl, O, C and N of the g-CN/BiOCl composites.
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5x106 g-CN/BiOCl BiOCl
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Fig.4 PL spectra of the g-CN/BiOCl composites and pure BiOCl excited at 360 nm.
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Fig.5 UV-vis diffuse reflectance spectra of the pure BiOCl(a), g-CN/BiOCl composites(b) and g-CN(c).
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Fig.6 (a) EIS of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO by applying an AC voltage of 5 mV in the 0.1 Hz to 100 kHz frequency range; (b) Time-based photocurrent response of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO in 0.1 M phosphate buffered solution (pH = 7).
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Fig.7 Schematic illustration of the exciton trapping mechanism of the g-CN/BiOCl composites.
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Fig.8 (a) Photocurrent responses of the g-CN/BiOCl-modified ITO toward ciprofloxacin at increasing concentrations in the supporting electrolyte of 0.1 M phosphate buffered solution (pH 7.0). (B) Linear calibration curve. Error bars indicate the standard deviation of five repeated measurements.
24
Graphical abstract
The graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) has been designed for a facile and sensitive photoelectrochemical (PEC) sensing platform of ciprofloxacin at first time. The present study can serve as a foundation to the application of the g-CN/BiOCl composites in photoelectrochemical monitoring antibiotic compounds and be easily extended to photoelectrochemical detection of other organic contaminant sensing.
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S0021-9797(16)30570-7 http://dx.doi.org/10.1016/j.jcis.2016.08.015 YJCIS 21483
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
7 June 2016 24 July 2016 6 August 2016
Please cite this article as: L. Xu, H. Li, P. Yan, J. Xia, J. Qiu, Q. Xu, S. Zhang, H. Li, S. Yuan, Graphitic carbon nitride/BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.08.015
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 proof before it is published in its final 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.
Graphitic carbon nitride/BiOCl composites for sensitive photoelectrochemical detection of ciprofloxacin Li Xua,c, Henan Li c,d, Pengcheng Yanb, Jiexiang Xiab, Jingxia Qiua , Qian Xua , Shanqing Zhang c, Huaming Li*a,b, Shouqi Yuan* a
a Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China c Centre for Clean Environment and Energy, Environmental Futures Research Institute and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia d School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia *Corresponding authors. E-mail addresses: [email protected]; [email protected] Tel: +86-511-88799500.
1
Abstract Ciprofloxacin, as a second generation of fluoroquinolone antibiotics, has been proved to cause environmental harm and exhibits toxic effects on the wastewater and surface water even at low concentrations due to their continuous input and persistence. Despite tremendous efforts, developing ciprofloxacin detection method with accuracy and sensitivity at low-cost remains a great challenge. Herein, graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) has been designed for a facile and sensitive photoelectrochemical (PEC) monitoring platform of ciprofloxacin at first time. BiOCl can be modified with the g-CN nanosheets which are obtained via solvothermal process at low-temperature conditions. The use of g-CN is shown to strongly enhance the PEC response of BiOCl due to the formation of heterojunctions. The photocurrent generated at the g-CN/BiOCl-modified ITO (with 13 wt% g-CN content) is much higher and more
stable than that
of a
BiOCl-modified ITO.
Based on these
findings,
the
g-CN/BiOCl-modified ITO was used to design a PEC assay for the antibiotic ciprofloxacin. Furthermore, the limit of detection of the ciprofloxacin PEC sensor has been significantly lowered to 0.2 ng⋅mL-1. In addition, the PEC sensor can detect ciprofloxacin in the wide range of 0.5-1840 ng mL-1. Key words: Bismuth oxychloride; graphitic carbon nitride; ciprofloxacin; photoelectrochemical detection
2
1. Introduction Photoelectrochemical (PEC) detection exerts a tremendous fascination on analytical field. In the case of the PEC detection process, the electrons excited from semiconductor materials by light are transferred and producing photocurrent which is utilized as detection signal[1]. Quite a few background signals can be reduced greatly benefiting from the efficient separation of the excitation source (light) and detection signal (photocurrent)[2]. PEC detection demonstrates the superiorities of both optical and electrochemical methods such as low cost, easy-to-operate, remarkable sensitivity, rapid response, etc., which have been widely captured for the interest of researchers[3,4]. Many efforts have been devoted to develop photo-electrochemical detection method towards different analysts, such as p-phenylenediamine[5], catechol[6], bisphenol A[7], chlorpyrifos[8], and so on. With development of photoelectrochemistry, a multitude of materials with PEC response are appeared, including quantum dots, semiconducting nanoparticles, metal complex and so on.[9] Among them, bismuth oxyhalides (BiOCl, BiOBr, and BiOI) have drawn much attention for their unique and excellent optical and electrical properties[10-12]. The crystal structure of bismuth oxyhalides is a layered structure that is built by interlacing [Bi2O2] slabs with double halogen slabs[13]. In particular, the layered structure endows bismuth oxyhalides with the self-built internal static electric fields which can promote the effective separation of the photoinduced hole-electron pairs[14]. Owing to small band gap (-1.8 eV) and excellent absorption capacity in the visible-light region, BiOI has been applied in PEC detection, such as organophosphate pesticide[15]and perfluorooctanic acid[2]. However, BiOCl have not yet been applied in the PEC detection field. BiOCl belongs to the wide-band-gap (3.4 eV) p-type semiconductor[16] which results in low utilization ratio of light. A multitude of strategies have been proposed to enhance the properties of photoelectric materials[17]. A method, coupling two semiconductors to form heterojunction, is a shared approach to improve the PEC response of the semiconductors. Recently, the graphitic carbon nitride (g-CN), an organic semiconductor, has stimulated intensive interest in photoelectron-chemical field[18]. Due to its visible light response ability, high thermal and chemical stability, unique electronic band structure (2.7 eV), and layer structure like graphene, g-CN frequently couples with other semiconductors to enhance the PEC response[19].
3
Some pioneering studies have proved that energy levels of g-CN and BiOCl are well-matched and can overlap in band structure[20,21]. Thus, the combination of g-CN and BiOCl can construct heterojunction catalysts, which can enhance charge separation and improve the PEC response[22]. Inspired by this, the excellent PEC performance can be achieved by constructing composite of g-CN and BiOCl. Ciprofloxacin, as a second generation of fluoroquinolone antibiotics, is active against a broad spectrum of Gram-negative and Gram-positive bacteria, and has been widely used for humans and veterinary purposes[23]. Although benefits of the application of ciprofloxacin in medicine are obvious, ciprofloxacin has been proved to cause environmental harm and exhibits toxic effects on the wastewater and surface water even at low concentrations due to their continuous input and persistence[24]. Therefore, it is crucial to develop sensitive and effective methods for the careful monitoring of ciprofloxacin concentrations. Recently, Gayen and Chaplin have developed a sensitive, selective, and fast electrochemical method for the determination of ciprofloxacin in water by modifying a boron-doped diamond electrode with porous nafion/multiwalled carbon nanotube composite film[25]. Herein, we have been fabricated graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) via a facile a one-pot ethylene glycol-assisted solvothermal process in the presence of ionic liquid 1-hexadecyl-3-methylimidazolium chloride ([C16mim]Cl). The g-CN obtained by solvothermal process at low-temperature conditions possesses high light-harvesting capability and decreased band gap[18]. It can benefit to enhance the PEC performance of BiOCl. In addition, the g-CN nanosheets can cover the surface of BiOCl microspheres, which can form the heterojunction structures. Therefore, the g-CN/BiOCl composites possessed a high light response and exhibited much more enhanced PEC response than that of pure BiOCl. The PEC sensor for ciprofloxacin can be fabricated by g-CN/BiOCl composites at first time, which can exhibit high sensitivity and stability. 2. Materials and methods 2.1 Materials The ionic liquid [C16mim]Cl (99%) was purchased from Shanghai Chengjie Chemical Co. Ltd. 2,4,6-trichloro-1,3,5-triazine, melamine and Bi(NO3)3⋅5H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All the reagents were of analytical purity and were used as 4
received. Phosphate buffer solution (0.1 M, pH 7.0) was prepared by Na2HPO4·12H2O and NaH2PO4·2H2O, and adjusted the pH with 0.1 M NaOH or H3PO4. 2.2 Synthesis of g-CN/BiOCl composites According to the literature[18,19], the g-CN was synthesized by solvothermal process. 2,4,6-trichloro-1,3,5-triazine (5 mmol) and melamine (2.5 mmol) was added into acetonitrile (20 mL). The mixture was stirred for 30 min and then transferred into 25 mL Teflon-lined autoclave. The autoclave was then heated at 160 ºC for 48 h and cooled down to room temperature. Then, the samples were centrifuged by high-speed centrifuge, washed by deionized water and dried at 60 ºC. A typical synthesis of g-CN/BiOCl composites (with 13 wt% g-CN content) is shown in the following way: firstly, g-CN (40 mg) was added into an ethylene glycol solution (20 mL) containing [C16mim]Cl (1 mmol), and then ultrasonicated for 30 min. Secondly, Bi(NO3)3⋅5H2O (1 mmol) was dissolved into above solution, the mixture was stirred for 30 min and then transferred into 25 mL Teflon-lined autoclave. The autoclave was then heated at 140 ºC for 24 h and cooled down to room temperature. The obtained samples were centrifuged and washed with distilled water and ethanol three times. Finally, the samples were dried in a vacuum oven at 50 ºC for 24 h before further characterization. For comparison, the BiOCl samples were also prepared without g-CN under the same condition. 2.3 Apparatus X-Ray powder diffraction (XRD) analysis was carried out on a Bruker D8 diffractometer with high-intensity Cu-Kα (λ=1.54 Å). The structural information for samples was measured by Fourier transform spectrophotometer (FT-IR, Nexus 470, Thermo Electron Corporation) using the standard KBr disk method. X-ray photoemission spectroscopy (XPS) was recorded on a VG MultiLab 2000 system with a monochromatic Mg-Kα source operated at 20 kV. The field-emission scanning electron microscopy (SEM) measurements were carried out with a field-emission
scanning
electron
microscope
(JEOL
JSM-7001F)
equipped
with
an
energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. The photoluminescence (PL) spectra were detected using a Varian Cary Eclipse spectrometer. The diffuse reflectance spectra (DRS) were measured using a UV-vis spectrophotometer (Shimadzu UV-2450, Japan) in the range of 200 to 800 nm. BaSO4 was used as the reflectance standard
5
material. PEC performance measurements were performed on a CHI-660B electrochemical workstation (CH Instruments, Inc.), which was connected to a conventional three electrodes cell using indium tin oxide (ITO) as a working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a platinum wire as a counter electrode. A 150 W xenon lamp was used as the light source. Electrochemical impedance spectroscopy (EIS) was performed in a 0.1 M potassium chloride solution containing 5 mM ferricyanide/ferrocyanide with a frequency range from 0.01 Hz to 10 kHz at 0.24 V, and the amplitude of the applied sine wave potential in each case was 5 mV. 2.4 Fabrication of the modified electrodes Prior to the modification of ITO electrode, the g-CN/BiOCl composites were dissolved in ethanol to obtain a 2 mg mL-1 suspension. The ITO electrodes were placed in boiling water containing 0.1 M sodium hydroxide for 10 min, and were then cleaned ultrasonically in water followed by alcohol for 10 min each. Afterwards, 20 µL of the g-CN/BiOCl composites suspension was dropped onto the surface of the ITO electrode with a fixed area of 0.5 cm2 and then dried at room temperature to obtain the g-CN/BiOCl composites modified ITO. For comparison, pristine BiOCl modified ITO was also prepared in the same way. 3. Results and discussion 3.1 Characterization of the g-CN/BiOCl composite The XRD patterns of the g-CN/BiOCl composites and BiOCl were shown in Fig. 1 (a). All the diffraction peaks of the g-CN/BiOCl composites and BiOCl corresponded to the structure of the tetragonal phase of BiOCl (JCPDS card No. 06-0249), with the lattice constants a = 3.890 Å, c = 7.890 Å. No other characteristic peaks were found, which implied the high purity of the samples. Meanwhile, it can be seen from the XRD diffraction peaks that the samples had a good crystallinity. However, there were no characteristic peaks of the g-CN at about 13.3° and 27.4° appeared, which might due to the low g-CN content and the high dispersion of g-CN in the g-CN/BiOCl composites[26]. FT-IR spectra analysis had been applied to determine the presence of g-CN in g-CN/BiOCl composites. It was found by contrasted Fig.1 (b) that the g-CN/BiOCl composites possessed characteristic absorption peaks of g-CN and BiOCl. For the g-CN/BiOCl composites, a strong absorption band in the range of 1000-1800 cm-1, with the peaks at about 1210, 6
1338, 1447, 1538, 1617, 1664, and 1731 cm-1, were associated with typical skeletal stretching vibrations of the s-triazine or tri-s-triazine,[18,21] which indicated g-CN basic structure was maintained after hybridization. The peaks at 1210 and 1617 cm-1 were related attributable to the C-N and C=N stretching vibration modes[27], respectively. The peak at 807 cm-1 was attributable to the s-triazine ring modes[28]. The absorption peak at about 530 cm-1 was attributed to the stretching vibrations of the Bi-O in BiOCl[29].Thus, the results showed that the g-CN had been successfully introduced to the BiOCl material. In order to specify the chemical states of surface elements and the chemical compositions of g-CN/BiOCl composite, XPS analysis was further carried out. In the Fig. 2, the survey scan XPS spectra provided C 1s, N 1s, Bi 4f, O 1s and Cl 2p peaks for g-CN/BiOCl composite. To further determine the valence state of various elements for g-CN/BiOCl composite, high resolution XPS analysis were conducted. In the high-resolution Bi 4f XPS spectra (Fig.2b), the binding energy of 164.5 eV and 159.2 eV corresponded to the Bi 4f5/2 and Bi 4f7/2 of Bi3+ in BiOCl[30], respectively. No photoelectron signals associated with Bi2+, Bi4+ or Bi5+ states in bismuth oxides appear, suggesting the high purity of the samples. As can be seen from Fig. 2(c), the Cl 2p XPS spectra consisted of Cl 2p3/2 and Cl 2p1/2 photolines that were fitted well with the peak at 197.9 eV and 199.2 eV, respectively, consistent with the values for Cl- [17]. It can be also found in Fig. 2(d) that the peak at 530.2 eV was attributed to O 1s, which came from the Bi-O bond in BiOCl. Fig. 2(e and f) showed the high-resolution C 1s and N 1s XPS spectra of the samples. C 1s peaks at 284.8 eV and 288.0 eV were assigned to carbon and defect-containing sp2-bonded carbon (C=N)[31]. The N 1s peak was located at 399.1 eV, which was ascribed to sp2-hybridized nitrogen (C=N-C)[18]. The results of FT-IR and XPS analysis indicated the coexistence of g-CN and BiOCl in the g-CN/BiOCl composites. The morphology of g-CN/BiOCl composites was visualized by scanning electron microscopy (SEM). As shown in Fig. 3(a,b), the g-CN/BiOCl composites had a three-dimensional sphere-like structure. As can be shown from the SEM image, the g-CN nanosheets covered the surface of BiOCl microspheres. The g-CN and BiOCl were in close contact with each other, indicating an interaction between g-CN and BiOCl exists. The morphology of BiOCl was also a similar three-dimensional sphere-like structure (Fig. 3(c,d)), indicating that the morphologies of BiOCl
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were not obviously changed in the g-CN/BiOCl composites after introducing g-CN. The elemental mapping of g-CN/BiOCl composites was conducted using SEM with an energy dispersive X-ray. The images in Fig.3(b) represented elemental mapping of Bi, Cl, O, C and N elements, respectively. The C and N elements were homogeneously distributed, besides the Bi, Cl and O elements, which provided the direct evidence for the good dispersion and attachment of g-CN on the surfaces of BiOCl microspheres. It further proved that the samples were g-CN/BiOCl composites and the result of SEM was consistent with the results of FT-IR and XPS analysis. To investigate information for the photoexcited charge carriers transfer and recombination processes, PL spectra of the g-CN/BiOCl composites and BiOCl were recorded. Fig. 4 showed the PL spectra of the g-CN/BiOCl microspheres and BiOCl excited at 360 nm. The emission peak of pure BiOCl appeared at about 468 nm. For g-CN/BiOCl composites, the position of the emission peak in the PL spectrum was similar to that of the pure BiOCl, but the emission intensity was decreasing. This result demonstrated that the g-CN/ BiOCl composites had a much lower recombination rate of photoinduced electrons and holes than that of pure BiOCl, which was attributed to that the recombination of photoinduced electrons and holes was greatly inhibited by the introduction of g-CN. This result also indicated that the g-CN/BiOCl composites were expected to exhibit improved PEC properties. UV-vis diffuse reflectance spectroscopy (DRS) was used to examine the optical absorption of the g-CN/BiOCl composites. Fig. 5 showed DRS spectra of the g-CN/BiOCl composites, BiOCl and g-CN. Optical absorption of g-CN was from approximately 500 to 680 nm, which was in accordance with the work by Wang[18]. As shown in Fig. 5, it can be clearly observed that the DRS of the g-CN/BiOCl composites and BiOCl present a similar absorption edge between about 300 to 400 nm, indicating that the g-CN/BiOCl composites and BiOCl mainly absorb UV light. Noticeably, the g-CN/BiOCl composites had a new extra consecutive visible light absorption region from 400 nm to 570 nm. These indicated that the absorption region had been broadened after introducing g-CN, which was contributed to absorb more light for generating more electrons and holes, and improve the PEC performance.
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3.2 Electrochemical and PEC performance of the g-CN/BiOCl composite To gain further insight into the electron-transport property of the g-CN/BiOCl composites and BiOCl, EIS was performed with the g-CN/BiOCl composites modified ITO electrode (g-CN/BiOCl-modified ITO) electrode and the BiOCl modified ITO electrode (BiOCl-modified ITO) using [Fe(CN)6]3-/4- as a redox probe (Fig. 6a). EIS of the modified electrodes showed the similar Nyquist plots with a semicircle in the high-frequency region and an inclined straight line at low frequency. The radius of the g-CN/BiOCl-modified ITO was smaller than that of the BiOCl-modified ITO. It can be deduced that the g-CN/BiOCl composites improved the conductivity of the electrode. That is, the g-CN/BiOCl composites possessed excellent conductivity, which can obviously decrease the interface resistance between the electrochemical probe and the electrode[32]. The PEC behaviors of the different modified electrodes were investigated in 0.1 M phosphate buffered solution (pH 7.0) (Fig. 6b). At an applied potential of 0 V (versus SCE), the g-CN/BiOCl-modified ITO and the BiOCl-modified ITO showed a photocurrent response under light illumination with a light on/off circle of 20 s. The photocurrent of the g-CN/BiOCl-modified ITO did not show obvious changes, that is, the photocurrent responses of the modified electrodes were very stable. The photocurrent of the g-CN/BiOCl-modified ITO was almost twice higher than that of the BiOCl-modified ITO. Meanwhile, an optimized special density of g-CN nanosheets combined with BiOCl is necessary to achieve excellent photocurrent response. It is
found that about 13 wt% g-CN content is optimum to get the highest photocurrent response (Fig. S1). Compared with the BiOCl-modified ITO, the g-CN/BiOCl-modified ITO (with 13 wt% g-CN content) was more suitable for constructing PEC detection system of ciprofloxacin due to the high and stable of the photocurrent responses. Fig. 7 summarized the proposed mechanism for the photoelectron-chemical behaviors of the g-CN/BiOCl-modified ITO. When the modified electrodes were irradiated by light, the g-CN/BiOCl composites absorbed photons and excited electrons and holes. The g-CN possessed an excellent photoresponse which was easily excited and engendered the corresponding photoinduced electron-hole pairs under light irradiation[33]. The theoretical calculated values of the conduction band (CB) and valence band (VB) potentials of the n-type semiconductor g-CN material were -1.12 and 1.57 eV[34]. According to previous
9
report[20], the conduction band edge potential of g-CN was more negative than that of BiOCl, and the valence band of BiOCl was more positive than that of g-CN. The g-CN and BiOCl were easily form heterojunction structure. When the g-CN and BiOCl were excited by light, photoinduced electrons were injected with high efficiency from conduction band of g-CN to conduction band of BiOCl. Simultaneously, the holes on the valence band of BiOCl migrated to that of g-CN. In such a way, the charge transfer would suppress the photoinduced electron-hole pair recombination and prolong the lifetime of photoinduced electron-hole, which contributed to enhance PEC properties of the g-CN/BiOCl composite. The addition of ciprofloxacin hindered the electron transfer from g-CN/BiOCl composites towards the ITO electrode and resulted in the recombination of photoinduced electron-hole pair[2,8,35-37]. The ultimate result was a decrease in the photocurrent intensity. It indicated that the quantitative detection of ciprofloxacin was able to achieve by monitoring the photocurrent intensity decrease of the g-CN/BiOCl-modified ITO after the adding of ciprofloxacin[36]. 3.3 Construction of the PEC sensor for ciprofloxacin detection The properties of PEC sensor based on the g-CN/BiOCl composites were subsequently tested by monitoring ciprofloxacin. As shown in Fig. 8(a), the photocurrent of the g-CN/BiOCl-modified ITO gradually decreased when ciprofloxacin was injected in phosphate buffered solution. The photocurrent intensity of the g-CN/BiOCl-modified ITO decreased with the increase of ciprofloxacin concentration. As shown in Fig. S2, the Ret value of the g-CN/BiOCl-modified
ITO actually increased in the presence of ciprofloxacin. It indicated that ciprofloxacin inhibited electron transfer of the g-CN/BiOCl-modified ITO, and consequently leading to a decrease in photocurrent[35]. FT-IR spectra analysis of the g-CN/BiOCl composites after detecting ciprofloxacin showed that absorption band of g-CN/BiOCl composites was shifted to low wavenumber compared with before detecting ciprofloxacin (Fig.S3). It indicated that the ciprofloxacin adsorbed onto the surface of g-CN/BiOCl composites. The ciprofloxacin was able to introduce a strong steric hindrance effects toward the diffusion of electron[2]. On this basis, the possible mechanism of ciprofloxacin detection can be proposed: When the g-CN/BiOCl-modified ITO detected ciprofloxacin, ciprofloxacin on the surface of g-CN/BiOCl composites gave an increase in steric hindrance which trailed off the electron 10
transfer of g-CN/BiOCl composites towards the ITO electrode surface, and consequently resulted in a decrease in photocurrent[8]. As shown in Fig. 8(b), it can be found that the relative change of the photocurrent ((i0-i)/i0 , where i0 and i were the photocurrent before and after incubation in ciprofloxacin) was two linear segments with different slopes to the ciprofloxacin concentrations in the range of 0.5-1840.0 ng mL-1. The corresponding linearization equation was (I) (i0-i)/i0= 0.1359+ 6.3690* 10-4 c/ng mL-1 (R2=0.994, cciprofloxacin: 0.5-400 ng mL-1); (II) (i0-i)/i0= 0.2533+2.8204*10-4 c/ng mL-1 (R2=0.998, cciprofloxacin: 400-1840 ng mL-1), respectively. The detection limit (S/N=3) was estimated to be 0.2 ng mL-1 based on the response of three times the standard deviation of zero-dose response[2]. Compared with other monitoring methods of ciprofloxacin (Table S1), the PEC sensor, based on the g-CN/BiOCl composites, was demonstrated to be superior to that of other monitoring methods, which had lower detection limit and wider linear range.
The interference study of the ciprofloxacin photoelectrochemical sensor has been investigated by measuring the sensor responses to metal ion (Fe3+, Zn2+, Cd2+, Pb2+), 4-hydroxybenzoic acid, 4-aminobenzoic acid, Rhodamine B, methyl orange and phenol at a concentration of 4000 ng mL-1 that is 10-fold to ciprofloxacin are shown in Fig.S4. The photocurrent responses to the metal ion were very close to the blank test, with minimal deviations below 5%. There were more effects on the blank signal with organic species, especially 4-hydroxybenzoic acid, 4-aminobenzoic acid and phenol, which caused about 10% deviations. There results indicated that the ciprofloxacin photoelectrochemical sensor had an acceptable anti-interference capacity for ciprofloxacin detection. A recovery study has been performed in the detection of ciprofloxacin in Yangtze River water samples with different standard concentrations (Table S2). The PEC sensor can detect ciprofloxacin in real water samples with good recoveries varied in the range of 90.5-112.6%. It indicated that this PEC sensor had satisfactory accuracy and reliability for detection of ciprofloxacin in real water samples. 4. Conclusions In this study, the g-CN/BiOCl composites have been fabricated via a facile a one-pot ethylene glycol-assisted solvothermal process in the presence of ionic liquid [C16mim]Cl. SEM 11
analysis indicated g-CN covered the surface of sphere-like BiOCl and formed heterojunctions, which contributed to enhance the PEC response of the g-CN/BiOCl composites. The photocurrent response of the g-CN/BiOCl-modified ITO was high and stable, and indicated that the g-CN/BiOCl-modified ITO was suitable as a PEC sensor. PEC sensor of ciprofloxacin was designed based on the g-CN/BiOCl composites for the first time. In addition, the PEC sensor detected ciprofloxacin in the wide range of 0.5-1840 ng mL-1 with a low detection limit of 0.2 ng mL-1. The above results demonstrated that the g-CN/BiOCl composites can be applied for detection of ciprofloxacin. Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 21476098, 21576123, 21506081, 21506077), the Jiangsu Province Postdoctoral Science Foundation (1501026B), University Natural Science Research of Jiangsu (15KJB530004), Jiangsu University Scientific Research Funding (15JDG048), Chinese Postdoctoral Foundation (2016M590420), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] D.Q. Jin, Q. Xu, Y.J. Wang, X.Y. Hu, Talanta, 127(2014) 169-174. [2] J.M. Gong, T. Fang, D.H. Peng, A.M. Li, L.Z. Zhang, Biosens. Bioelectron., 73(2015) 256-263. [3] G.M. Wen, H.X. Ju, Talanta, 134(2015) 496-500. [4] R.X. Li, J. Gao, P.C. Gao, S. Zhang, Y.X. Liu, B. Du, Q. Wei, New J. Chem., 39(2015) 731-738. [5] Y.H. Zhu, K. Yan, Y. Liu, J.D. Zhang, Anal. Chim. Acta, 884(2015) 29-36. [6] Y. Liu, R. Wang, Y.H. Zhu, R.Z. Li, J.D. Zhang, Sens. Actuators B, 210(2015) 355-361. [7] B.T. Zhang, L.L. Lu, F. Huang, Z. Lin, Anal. Chim. Acta, 887(2015) 59-66. [8] J. Qian, Z.T. Yang, C.Q. Wang, K. Wang, Q. Liu, D. Jiang, Y.T. Yan, K. Wang, J. Mater. Chem. A, 3(2015) 13671-13678.
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Figure Captions Fig.1 (a) XRD pattern of the g-CN/BiOCl composites and pure BiOCl; (b) FT-IR analysis for g-CN/BiOCl composites (I), g-CN (II) and pure BiOCl (III). Fig.2 XPS spectra of the g-CN/BiOCl composites: (a) survey, (b) Bi 4f, (c) Cl 2p, (d) O 1s, (e) C 1s, (f) N 1s. Fig.3 SEM images of the g-CN/BiOCl composites (a,b) and pure BiOCl (c,d); (b) Element mapping of Bi, Cl, O, C and N of the g-CN/BiOCl composites. Fig.4 PL spectra of the g-CN/BiOCl composites and pure BiOCl excited at 360 nm. Fig.5 UV-vis diffuse reflectance spectra of the pure BiOCl (a), g-CN/BiOCl composites (b) and g-CN (c). Fig.6 (a) EIS of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO by applying an AC voltage of 5 mV in the 0.1 Hz to 100 kHz frequency range; (b) Time-based photocurrent response of the g-CN/BiOCl-modified ITO and BiOCl-modified ITO in 0.1 M phosphate buffered solution (pH = 7). Fig.7 Schematic illustration of the exciton trapping mechanism of the g-CN/BiOCl composites. Fig.8 (a) Photocurrent responses of the g-CN/BiOCl-modified ITO toward ciprofloxacin at increasing concentrations in the supporting electrolyte of 0.1 M phosphate buffered solution (pH 7.0). (B) Linear calibration curve. Error bars indicate the standard deviation of five repeated measurements. Fig. S1. Photocurrent response of the g-CN/BiOCl-modified ITO electrodes with different content of g-CN: (a) 15 wt%, (b) 13 wt%, (c) 10 wt%, (d) 5 wt%, (e) pure BiOCl. Fig.S2 EIS of the g-CN/BiOCl-modified ITO before (a) and after (b) the addition of ciprofloxacin (1000 ng mL-1) Fig.S3 (a) FT-IR analysis for g-CN/BiOCl composites before and after detection; (b) FT-IR analysis for ciprofloxacin. Fig.S4 Influence of particular possible interfering substances on the responses of the photoelectrochemical sensor for 400 ng mL-1 ciprofloxacin in 0.1 M phosphate buffered 15
solution (pH 7.0): 1. ciprofloxacin; 2. Fe3+; 3. Zn2+; 4. Cd2+; 5. Pb2+; 6. 4-hydroxybenzoic acid; 7. 4-aminobenzoic acid; 8. Rhodamine B; 9. methyl orange; 10. phenol. Table S1 Comparison results of the determination of ciprofloxacin by other monitoring methods. Table S2 Photoelectrochemical detection of ciprofloxacin in Real Water Samples by the photoelectrochemical sensor
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Fig.1 (a) XRD pattern of the g-CN/BiOCl composites and pure BiOCl; (b) FT-IR analysis for g-CN/BiOCl composites (I), g-CN (II) and pure BiOCl (III).
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(b) g-CN
Fig.3 SEM images of the g-CN/BiOCl composites (a,b) and pure BiOCl (c,d); (b) Element mapping of Bi, Cl, O, C and N of the g-CN/BiOCl composites.
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Fig.5 UV-vis diffuse reflectance spectra of the pure BiOCl(a), g-CN/BiOCl composites(b) and g-CN(c).
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Fig.7 Schematic illustration of the exciton trapping mechanism of the g-CN/BiOCl composites.
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Graphical abstract
The graphitic carbon nitride/BiOCl composite (g-CN/BiOCl) has been designed for a facile and sensitive photoelectrochemical (PEC) sensing platform of ciprofloxacin at first time. The present study can serve as a foundation to the application of the g-CN/BiOCl composites in photoelectrochemical monitoring antibiotic compounds and be easily extended to photoelectrochemical detection of other organic contaminant sensing.
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