Investigation on the Electrocatalytic Determination and Photocatalytic

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Investigation on the Electrocatalytic Determination and Photocatalytic Degradation of Neurotoxicity Drug Clioquinol by Sn(MoO4)2 Nanoplates Raj Karthik, Jeyaraj Vinoth Kumar, Shen-Ming Chen, Kumar Seerangan, Chelladurai Karuppiah, Tse Wei Chen, and Velluchamy Muthuraj ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06851 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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ACS Applied Materials & Interfaces

Investigation on the Electrocatalytic Determination and Photocatalytic Degradation of Neurotoxicity Drug Clioquinol by Sn(MoO4)2 Nanoplates Raj Karthika, Jeyaraj Vinoth Kumarb, Shen-Ming Chena*, Kumar Seeranganc, Chelladurai Karuppiahd, Tse-Wei Chena, Velluchamy Muthurajb a

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and

Biotechnology, National Taipei University of Technology, No.1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan (R.O.C). b

c

Department of Chemistry, VHNSN College, Virudhunagar – 626001, Tamilnadu, India.

Institute of plant and microbial biology (IPMB), Academia Sinica, Taipei, Taiwan R. O.C.

d

Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei

City, 24301, Taiwan, ROC

*Corresponding author. Tel: +886 2270 17147, fax: +886 2270 25238. E-mail address: [email protected] (S-M Chen)

KEYWORDS: Tin molybdate, Sn(MoO4)2 nanoplates, Anti-protozoal, Neurotoxicity, Clioquinol, Electrocatalysis, Photocatalysis

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ABSTRACT: Transition metal molybdates have concerned enormous curiosity as supercapacitors, photocatalysts and electrocatalysts. These materials are best alternative to noble metal based catalysts which are generally show a limited photocatalytic and electrocatalytic activity. As well, the

anti-protozoal

drug

can

usually

pollute

the

environment

through

improper

disposable/incomplete metabolism and it is very dangerous to the human as well as aquatic animals.

Therefore,

here,

we have studied

the electrochemical

determination

and

photodegradation of neurotoxicity clioquinol (CQL) drug by nanoplates-like tin molybdate (Sn(MoO4)2; denoted as SnM), which is played as both electro and photocatalyst. The asprepared catalyst delivered a highly efficient activity towards the detection and degradation of CQL. The proposed nanoplates-like SnM was prepared through a simple wet-chemical route and its physicochemical properties were characterized by various spectroscopic and analytical techniques. As an electrochemical sensor, the SnM electrocatalyst exhibited tremendous activity for the detection of CQL in terms of lower potential and enhanced anodic peak current. Also, it showed high selectivity, wide linear concentration range, lower detection limit and good sensitivity. From the UV-Vis spectroscopy study, the SnM photocatalyst delivered an excellent photocatalytic activity towards the degradation of CQL in terms of increasing contact time and reducing CQL concentration, results the increasing of the degradation efficiency about 98% within 70 min under visible light irradiation and it shows an appreciable stability by observing the reusability of the catalyst.


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1. INTRODUCTION In recent years, mixed binary transition metal oxides have been focused with stimulated research interest in worldwide owing to their environment benignity, stronger electronic conductivity and higher electrochemical properties than single-component metal oxides.1-3 In particular, metal molybdates (MMoxOy = Zn, Mn, Mg, Sr, Ba, Ni, Co, Pb, Ca, Cd, Cu, Sn and so forth) is one of the most significant families of inorganic materials that have been widely used in the various fields including energy storage devices4-12, photocatalysis13, humidity and gas sensors14-16, electrochemical sensors17, heavy metal disposal18, magnetic properties19, and laser applications20. Among them, tin molybdate (SnMoxOy) is an interesting inorganic material due to their excellent peculiar properties such as optical, electronic structure and it can be used in some important applications such as inorganic ion-exchanger membranes.21 Recently, Sakthikumar et al. demonstrated the shape-selective synthesis of Sn(MoO4)2 nanomaterials for catalysis and supercapacitors application.22 Also, Ede et al. reported the microwave assisted synthesis of Sn(MoO4)2 nanoassemblies on DNA scaffold for lithium-ion batteries application.23 However, the photocatalytic and electrocatalytic behavior of Sn(MoO4)2 has not yet been reported elsewhere. Therefore, we made an attempt for the synthesis and fabrication of plate-like Sn(MoO4)2 via simple wet-chemical route and evaluated its photochemical and electrochemical activity. To the best of our knowledge, there is no reports available for the synthesis of Sn(MoO4)2 (SnM) nanoplates and utilized as an electrochemical sensor as well as photocatalyst for the sensitive detection and degradation of antifungal and anti-protozoal (neurotoxic) drug clioquinol for the first time. Clioquinol (5-chloro-7-iodo-8-hydroxyquinoline; CQL) is halogenated derivatives of 8hydroxyquinoline and widely used as anti-protozoal and anti-fungal drug which belongs to the family of anti-infective drug. In 19th century, CQL was introduced and used (oral intestinal amebicide) to treat the intestinal diseases such as traveller’s diarrhea, lambliasis, chronic nonspecific diarrhea and shigellosis. It can also be used to treat the skin infections such as athlete’s foot, eczema and other kind of fungal infection in the form of topical cream.24 Moreover, CQL acts as zinc and copper chelator and it can be successfully used in combination with vitamin B12 for the treatment of Alzheimer’s disease.25-27 In earlier of 1970s, the CQL was banned from the market in the oral consumption due to cause neurotoxicity, a syndrome called Subacute myelooptic neuropathy (SMON).24,28,29 Subacute myelo-optic neuropathy is an iatrogenic disease of the 3 ACS Paragon Plus Environment


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nervous system30 leading to a disabling paralysis, blindness and even death. SMON disease was very common in Japan, where it reportedly more than 10,000 people are affected by this disease. The over dosage of CQL can cause important side effects to human such as neurotoxicity etc. Therefore, CQL was restricted or discontinued in some countries, however, it still being used in some other countries. As a result, it is a big challenge to develop a methodology for the accurate detection of CQL. The literature survey showed only a few reports are available for the detection of CQL by electrochemical, spectrophotometry and HPLC techniques.31,32 However, the development of selective and sensitive electrochemical detection of CQL is very important concern to the human as well as pharmaceutical fields. Here, we have chosen electrochemical methods for the detection of CQL, because, the electrochemical methods hold more advantages such as simplicity, portability, rapid analysis, high sensitivity and low-cost instrumentation compared than other techniques.31,32 Due to these attractive features, the electrochemical methods are representing a promising alternative technique for the detection of CQL. In addition, the removal of CQL from the aquatic and soil environment is another important concern due to its non-biodegradable nature. For degradation, photocatalysis is the best tool; since, it is a green, simple and eco-friendly method.33-35 Hence, the SnM nanoplates were used here for the removal of CQL from the environmental samples.13 In this study, we report the SnM nanoplates tailored via simple wet-chemical technique and evaluated as an electrocatalyst as well as photocatalyst for the detection and degradation of CQL. The physicochemical properties of the as-prepared SnM nanoplates were analyzed by various spectroscopic and analytical techniques such as XRD, Raman, SEM, EDS, TEM, XPS, UV-DRS. Interestingly, we found that the as-prepared SnM nanoplates showed a higher electrocatalytic performance for the determination of CQL. Furthermore, it showed an excellent photocatalytic activity towards the degradation of CQL with 98% degradation efficiency under visible light irradiation.

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus Stannous chloride dihydrate (SnCl2.2H2O), sodium molybdate (Na2MoO4), urea (CH4N2O), Clioquinol (C9H5ClINO), chloramphenicol (C11H12Cl2N2O5), acetaminophen (C8H9NO2), amoxicillin (C16H19N3O5S), metronidazole (C6H9N3O3), dopamine (C8H11NO2), uric 4 ACS Paragon Plus Environment

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acid (C5H4N4O3), ascorbic acid (C6H8O6), glucose (C6H12O6), copper chloride (CuCl2), iron nitrate (Fe(NO3)3), potassium nitrate (KNO3), calcium chloride (CaCl2), sodium hydroxide (NaOH) and sodium nitrate (NaNO3) were purchased from Sigma-Aldrich & Merck companies and used without further purification. The electrolyte solution was prepared by using NaOH. All the other required solutions were prepared by using deionized (DI) water throughout the experiments. Powder X-ray powder diffraction (XRD) was used to examine the crystal structure of the product’s via PANalytical X´Pert diffractometer measured with Cu-Kα radiation (λ = 1.54178 Å) in the 2θ range of 10-80˚. Raman spectra were collected on Horiba HR 800UV confocal Raman spectrophotometer. Scanning electron microscope (SEM) and Energy dispersive X-ray (EDS) spectral studies have analyzed by using Hitachi S-3000 H scanning electron microscope (SEM Tech Solutions, USA) and HORIBA EMAX X-ACT, respectively. Transmission electron microscopy (TEM) images were examined on a Shimadzu JEM-1200 EX with an accelerating voltage of 100 kV. X-ray photoelectron spectroscopy (XPS) results were carried out using Thermo ESCALAB 250 instrument. The specific surface area and pore size distribution were determined using the BET (Micromeritics, ASAP 2020M). UV-Vis absorption spectra of the photodegradation experiments were analyzed by using Shimadzu DUV-3700 spectrophotometer. The total organic carbon (TOC) analysis was performed by using a Shimadzu TOC-L CPN instrument. The electrocatalytic behavior and the detection of CQL were performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using CHI 405a and CHI 900 (CH Instruments, USA). A conventional three-electrode system has been used for the electrocatalytic studies where the modified GCE is the working electrode (0.07 cm2), platinum wire is an auxiliary electrode and Ag/AgCl (in saturated KCl) is used as a reference electrode. All the electrochemical studies were carried out at room temperature. 2.2. Synthesis of Sn(MoO4)2 Nanoplates In a typical procedure, 0.5 M of SnCl2.2H2O and Na2MoO4.2H2O were dissolved in 70 mL of DI water and stirred for 30 min to form a homogeneous suspension. Then, 10 mL (0.5 M) of NaOH was added to the above suspension and aged for 12 h with constant stirring. Subsequently, the obtained precipitates were washed with DI water and acetone to remove any possible impurities and dried at 80 oC for overnight. The dried products were calcined at 600 oC for 4 h in an air atmosphere. 5 ACS Paragon Plus Environment


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2.3. Fabrication of Modified Electrodes Before to modify on the GCE surface, the electrode was polished with 0.05 µm alumina slurry and washed with DI water to remove the alumina from the GCE surface. The as-prepared SnM nanoplates were re-dispersed in DI water and sonicated for 10 min. After that, the optimum level of (8 µL) SnM nanoplates suspension was drop coated on the GCE surface (GCE working area = 0.07 cm2). The drop coated electrode is allowed to dry at room temperature and then gently rinsed by DI water to remove the loosely bounded particles. The modified electrode was used for further electrochemical measurements. 2.4. Photocatalytic Experiments The photocatalytic activity was investigated using the as-synthesized SnM nanoplates for the degradation of CQL under visible light illumination. In this system, 50 mg of the catalyst and 100 mL of CQL solution (20 mg/L) was stirred in the dark condition for 2 h to reach adsorptiondesorption equilibrium between the CQL solution and photocatalyst. Subsequently, the mixture was irradiated under visible light. A 500 W tungsten incandescent lamp was used as the visible light source equipped with a UV cut-off filter in the wavelength range of (λ > 400 nm) and the intensity of the lamp is 150 mW/cm2. For each 10 min, 4 mL of the solution was collected and centrifuged to remove the photocatalyst. The concentration changes of the supernatant were analyzed by UV-Vis spectrophotometer. For the recycling test, the photocatalyst was collected by centrifugation after the first reaction, then washed, dried and used it again for next reactions.

3. RESULTS AND DISCUSSION 3.1. Characterization of Sn(MoO4)2 Nanoplates The crystalline structure and quality of the as-synthesized SnM were investigated by XRD analysis as shown in Fig.1A. The appearance of distinctive diffraction peaks in the 2θ ranges at 12.79˚, 23.52˚, 25.82˚, 26.9˚, 29.7˚, 34.02˚, 35.5˚, 39.01˚, 46.07˚, 52.3˚, 58.08˚, 64.18˚ and 68.3˚ were corresponding to the (011), (112), (210), (211), (202), (032), (222), (114), (015), (402), (026), (424) and (306) planes of Sn(MoO4)2 (JCPDS File No: 36-1240).23 Fig.1B represents the Raman spectrum of the as-prepared SnM which displays the high intense peaks at 904 and 721 cm-1, ascribed to the Mo=O stretching and Mo-O-Mo asymmetric stretching vibrations of Sn(MoO4)2, respectively. The peaks at 559, 261 and 216 cm-1 were corresponding


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to the Mo-O-Mo symmetric stretching, Mo=O bending and distorted deformation Mo-O-Mo vibrations, respectively.36

Fig.1 (A) XRD patterns and (B) Raman spectrum of as-synthesized SnM nanoplates.

The surface topography of the as-prepared SnM were determined by using scanning electron microscopy (SEM) and the results are presented in Fig.2 (A, B). The result obviously shows that the prepared SnM has plates-like structures which are randomly arranged to one another. The thickness of each plate was obtained approximately less than 100 nm. The elemental composition of as-synthesized SnM was identified by EDS analysis, as depicted in Fig.2 (C). The EDS spectrum portrayed the existence of tin (Sn), molybdenum (Mo) and oxygen (O) elements without any other significant impurities. In addition, Fig. 2D shows quantitative results of Sn, Mo and O with the ratio of 1:2:8 this is good accordance to the experimental protocols. The in-depth morphological features of the as-prepared SnM were investigated by TEM analysis, as it can be seen in Fig. 3 (A, B). It was clearly confirms the plates-like structure of SnM with smooth surfaces and the average length is to be 80 nm. The suitable kind of light region for the as-synthesized SnM nanoplates were determined by DRS UV-Vis studies, as shown in Fig. 3C. It observed that the broad absorption peak in the wavelength ranges from 330 to 460 nm, which corresponds to the bathochromic shifts of SnM nanoplates. The energy gap value of SnM nanoplates was calculated by Tauc’s plot equation as follows, 7 ACS Paragon Plus Environment


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(α ) =

A −1 / 2 hν (hν − E g )

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(1)

where, α is the absorption coefficient, h is Planck’s constant, ν is the frequency of light, A is a constant and Eg is the band gap. The calculated energy gap is to be 2.52 eV as represented in Fig. 3D.

Fig.2 (A, B) SEM images of plate-like SnM, (C) corresponding to EDS spectrum of SnM and (D) Quantitative results of SnM. The elemental composition and their oxidation states were accurately investigated by XPS analysis and it can be seen in Fig.4. The wide scan survey displays (Fig.4A) the presence of C, Mo, Sn and O elements in SnM nanoplates which is well agreed to the EDS reports. The presence of C element is due to the presence of hydrocarbon into the XPS instrument itself. From


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Fig.3 TEM images at different magnifications of (A) 50 nm and (B) 100 nm, (C) DRS UV-Vis spectrum and (D) Energy gap spectrum of SnM nanoplates.

the high magnification XPS spectrum of Sn 3d (Fig.4B), the peaks at 487.1 and 495.4 eV corresponds to the Sn 3d5/2 and Sn 3d3/2 spin-orbit of Sn4+, respectively.23 In the case of Mo 3d spectra (Fig.4C), the high intense strong peaks at 232.7 and 235.8 eV can be attributed to the Mo 3d5/2 and Mo 3d3/2 of Mo6+ state, respectively.17 The binding energies in the range of 529-532 eV were ascribed to the O 1s spin-orbit level of O2- oxidation state (Fig.4D). The de-convolution peaks at 529.2 and 530.8 eV were due to the appearance of lattice oxygen, chemisorbed oxygen and adsorbed water molecules, respectively. The peak at 531.9 eV is related to the adsorbed water molecules on the surface of the Sn(MoO4)2.37 The specific surface area and pore-size



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distribution of the as-prepared SnM nanoplates were determined by N2 adsorption-desorption isotherm analysis. Fig.5A portrayed the Brunauer–Emmett–Teller (BET) surface area of the SnM nanoplates was 4.387 m2/g. The Barrett–Joyner–Halenda (BJH) pattern in Fig. 5B demonstrated that the pore-size distribution of SnM nanoplates which shows the mesoporous like structure with the diameter of 13 nm.

Fig.4 (A) Wide-scan XPS survey spectrum of SnM nanoplates; High-magnification XPS spectra of (B) Sn 3d, (C) Mo 3d and (D) O 1s.


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Fig.5 (A) Nitrogen adsorption-desorption isotherms and (B) BJH pore-size distribution plots of SnM nanoplates. 3.2. Electrochemical Behavior of CQL at Different Modified Electrodes The loading (optimization) of SnM nanoplates on the modified GCE is very important and it can be directly affected the sensitivity of the electrode. In order to investigate the optimization effect of loading level, a different amount of SnM nanoplates were modified on GCE and observed the CV response for the detection of CQL drug. The obtained optimization results are present in Fig.S1. It can be clearly seen that a maximum anodic peak current and the higher detection sensitivity of CQL was found at 8 µL of SnM drop coated GCE. Therefore, 8 µL drop coated SnM/GCE’s were used for further electrochemical experiments. The as-prepared SnM nanoplates were used as an excellent electron mediator for the fabrication of CQL biosensor. Fig.6A shows the typical CV curves for the unmodified GCE (a) and SnM nanoplates modified GCE (c) in the presence (b, d) and absence (a, c) of 200 µM CQL containing 0.1 M NaOH at a scan rate of 50 mVs-1. Interestingly, it can be seen that the SnM nanoplates modified GCE and unmodified GCE does not show any anodic peak current response in absence of CQL. However, a sharp and well-defined anodic peak current (in presence of CQL) were obtained for the oxidation of CQL on SnM/GCE at + 0.48 V. Moreover, in reverse scan, there is no reduction peak was observed which indicates that the oxidation of CQL is irreversible process. The obtained irreversible oxidation peak of CQL at 0.48V confirms the direct oxidation of CQL to quinone-like structure with one proton and one electron transfer process.



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Fig.6 (A) The CVs of bare GCE (a), SnM/GCE (c) in presence (b, d) and absence (a, c) of 200 µM CQL containing 0.1 M NaOH solution at a scan rate of 50 mVs−1. (B) CVs of SnM modified GCE in absence and presence of CQL with different concentrations (a-m; 0 -300 µM) at a scan rate of 50 mVs−1. (C) CVs of SnM modified electrode in 200 µM CQL containing 0.1 M NaOH at different scan rates (a–j: 20 – 200 mVs−1). (D) The calibration plot of scan rate vs. Ipa of CQL. The oxidation mechanism of CQL has given in Scheme 1. At the same time, in presence of CQL at unmodified GCE, a little broad anodic peak current response was obtained at + 0.5 V. However, the obtained enhanced anodic peak current of CQL at SnM/GCE is 2.5 folds higher than bare GCE and the CQL oxidized potential is also lower when compared to unmodified GCE. The obtained result suggests that the SnM nanoplates offers the excellent platform and proficient electron mediator for the electrocatalysis of CQL by contributing surplus electroactive 12 ACS Paragon Plus Environment

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sites, thus it enhance the electrocatalytic activity for the detection of CQL. Fig.6B reveals the CV responses at SnM nanoplates modified GCE towards CQL with different concentration ranges from 0 to 300 µM (a-m) in 0.1 M NaOH at the scan rate 50 mVs-1. The CQL oxidation peak current was increased linearly when increasing the concentrations of CQL from 20 to 300 µM, which indicating that the SnM nanoplates modified GCE had excellent electrocatalytic activity towards antifungal and antiprotozoal drug CQL. Furthermore, the electrochemical characteristic such as sensitivity, limit of detection (LOD) and the linear response ranges are discussed and detailed in the section 3.2.2.

Scheme 1 the proposed electro-oxidation mechanism of CQL. 3.2.1. The Effect of Scan Rate The effect of scan rate on CQL oxidation at SnM/GCE was also investigated by CV. Fig.6C shows the CV responses for different scan rates effect on SnM/GCE toward CQL oxidation in 0.1 M NaOH solution containing 200 µM of CQL. It can be seen that the oxidation peak currents of CQL at SnM/GCE increased gradually while increasing the scan rate. The linear relationship of oxidation peak current of CQL with the scan rates is plotted and depicted in Fig.6D. At SnM/GCE, the oxidation peak current was directly proportional to the scan rates in the range between 20-200 mVs-1, which follows the linearity with corresponding equation of Ip (µA) = 0.02 ν (mVs–1) +1.85 and the correlation coefficient of R2 = 0.9966. The above results suggest that the oxidation of CQL is an adsorption controlled and an irreversible electrode process.



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Fig.7 (A) The DPV signals of the SnM modified GCE to successive addition of CQL from 0.05 to 1024 µM in 0.1 M NaOH solution. (B) The calibration plot between concentrations of CQL vs. anodic peak current. (C) The DPV responses of CQL oxidation in presence of interfering compounds 50-fold excess concentrations of common metal ions; Cu2+, Fe2+, K+, Ca2+, Na+, Cl- , Br-, I-; 20-fold excess concentrations of biological interferents; DA, UA, AA, Glu and 20-fold excess concentrations of commonly used drug; CAP, ACP, AMO, Met. (D) Relative error analysis for the interfering samples; x axis shows the tested interfering metal ions, biological molecule and antibiotic drugs and y axis shows the interference effect in percentage scale.


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3.2.2. Electrochemical Determination of CQL To investigate the sensor characteristics such as correlation coefficient, sensitivity and detection limit, we have chosen DPV techniques for the determination of CQL due to the higher sensitivity and better resolution when compared with conventional CVs. Under the optimized CVs experimental conditions, the DPV were carried out by using SnM nanoplates modified GCE. Fig.7A depicts the DPV performance of SnM/GCE for the successive addition of different concentration of CQL (from 0.05 - 1024 µM) in 0.1 M NaOH electrolyte at the potential window of 0 - 0.8 V. A sharp and well-defined anodic current response is observed for each addition (from lower to higher concentrations) of CQL, which is due to the quick electro-oxidation of CQL on SnM/GCE surface. The SnM/GCE can detect the CQL even in very low concentration such as 0.05 µM, which indicating the quick diffusion of CQL on the electrode surface. The proposed sensor exhibits a sensitive current response and it is clearly showed that the increasing concentration of CQL could also be increased the oxidation peak current. This result reveals that the SnM nanoplates modified GC electrode had fast electrocatalytic ability towards the CQL sensing. The oxidation peak current of CQL increases linearly over the concentration range of 0.05 – 234 µM; the linear regression equation can be expressed as Ipa (µA) = 0.02 [CQL]/µM + 0.08 and the correlation coefficient of R2 = 0.9949 (Fig.7B). From the calibration curve, the LOD and the sensitivity can be calculated as 14 nM and 0.25 µAµM-1cm-2, respectively. The superior electroanalytical parameters (i.e., LOD, linear concentration range and sensitivity) of CQL drug were briefly examined using SnM nanoplates modified electrode for the first time in this work. These excellent performances of SnM/GCE toward CQL detection is obtained from the good electrocatalytic activity and the accessibility of large surface area of SnM nanoplates. 3.2.3. Selective Determination of CQL Using SnM/GCE The selectivity study is a significant part for the newly developed electrochemical sensors and biosensors. In order to study the selectivity, the fabricated SnM/GCE was used and detected the CQL by DPV techniques. The influences of some potentially interfering compounds including common cations and anions, biological molecules and some commonly used drugs on the oxidation signal of 50 µM CQL were demonstrated. A number of common metal ions such as 50 fold excess concentrations of Cu2+, Fe2+, K+, Ca2+, Na+, Cl- , Br-, I-; 20 fold excess concentrations of biologically co-interfering substances such as dopamine (DA), uric acid (UA), 15 ACS Paragon Plus Environment


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ascorbic acid (AA), glucose (Glu) and 20 fold excess concentrations of commonly used some known drug such as chloramphenicol (CAP), acetaminophen (ACP), amoxicillin (AMO) and metronidazole (Met) which are selected for the interference studies. From the Fig.7 (C), it can be clearly seen that the oxidation peak current of CQL was not changed in the presence of 50-fold of common metal ions, 20-fold of biological molecules and 20-fold of antibiotic drug samples. Fig.7D signifies the relative error (%) for the detection of CQL in the presence of selected metal ions, biological compounds and antibiotic drugs. Most of biological compounds (∼1%) and inorganic species (-1 to -6%) have an interfering effect on the CQL detection. Also, the response current of CQL changed by ∼2% in the presence of a 20 fold concentrations of antibiotic drugs (CAP, ACP, AMO and Met). As a result, the fabricated SnM nanoplates modified electrode exhibited an excellent selectivity even in the presence of aforementioned interfering compounds for the detection of CQL. Therefore, the proposed SnM nanoplates modified electrode can be applicable either biosensors or pharmaceutical formulations for the selective detection of CQL. 3.2.4. Stability, Reproducibility and Repeatability Measurements The stability of the fabricated (as follows Section 2.3) electrochemical sensor was investigated by DPV. The SnM modified GCE was used to monitor the anodic peak current response of CQL in 50 µM CQL containing 0.1M NaOH solution for everyday in room temperature. After 7 days, there is only a small (4%) current changes was observed from its original current response of CQL, which is suggesting that the good storage stability of the proposed sensor. For the reproducibility studies, we have modified SnM suspensions on 3 independent GCE and their electrochemical responses were tested toward 50 µM of CQL. The anodic peak current responses for 3 different modified electrodes were observed clearly with a relative standard deviation (RSD) of 2.2 %, indicates the SnM/GCE had good reproducibility to CQL determination. Although, to study the repeatability, one GCE was modified with SnM suspension and its’ electrochemical oxidation behavior to CQL was examined with 5 consecutive measurements with RSD of 3.1%, suggesting an acceptable repeatability of the SnM nanoplates modified electrode. The above results exhibited that the proposed CQL sensor had good storage stability, repeatability and good reproducibility.


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3.3. Photocatalytic Activity The photocatalytic activity of as-synthesized SnM nanoplates was evaluated against the photodegradation of antifungal and antiprotozoal drug CQL in an aqueous solution under visible light irradiation. Fig.8A represented that the time-dependent UV-Vis absorption spectrum of CQL photodegradation process over 50 mg of the photocatalyst under visible light illumination. As can be seen that, the major absorption peak of CQL at 267 nm was gradually decreased with increasing the time and almost vanished to zero after 70 min of illumination. There are no other discernible peaks were observed which suggested that the SnM nanoplates do not alter the photodegradation pathway of the CQL solution in the present system. The photocatalytic efficacy of SnM nanoplates was further compared to the commercially available TiO2 and MoO3 and the results are presented in Fig.8B. It was found that the commercial TiO2 and MoO3 were exhibited the photodegradation efficiency of 34 and 41 % of CQL solution under visible light illumination. However, the CQL degradation was not observed while in the absence of catalyst and/or light irradiation. The results demonstrated that the SnM nanoplates possessed excellent photocatalytic activity towards the CQL drug sample solution. Furthermore, the photodegradation performance of SnM nanoplates were established by kinetic studies and the degradation rate was followed pseudo-first order kinetic equation as follows, ln C/C0 = kt

(2)

where, k is the rate constant and t is the reaction time, C0 and C are the CQL initial concentration and at different time t, respectively. The calculated rate constant of SnM nanoplates for the photodegradation of CQL is to be 2.81 min-1 and the plots of ln C/C0 vs. time is shown in Fig.8C. The amount of catalyst dosage is significant parameter on the photodegradation process. In this study, the effect of SnM amount dosages on the photodegradation of CQL were analyzed by varying the amount from 25 to 100 mg/mL and the results are shown in Fig.8D. It obviously shows that the photodegradation efficiency increases with increasing the catalyst dosage from 25 to 50 mg/mL. Beyond 50 mg/mL, the photodegradation efficiency was decreases which might be due to the over dosage can cause the agglomeration of the catalyst (particle-particle interactions), resulting to decrease the availability of active surface area for light absorption. The involvement of major reactive oxidative species such as hydroxyl radical (•OH), superoxide (•O2−), and holes (h+) in the photodegradation of CQL using SnM under visible light 17 ACS Paragon Plus Environment


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irradiation was determined by radical scavenging experiments. In the present study, benzoic acid (BA), acryl amide (AA) and ammonium oxalate (AO) were used as the scavengers for •OH, •O2−, and h+, respectively. The effect of various scavengers on the degradation of CQL was displayed in Fig 9A. The photocatalytic degradation efficacy of CQL was decreased from 99% to 26% and 99% to 32% after the addition of BA and AA, respectively. From these observations, •OH and •O2− are the major reactive oxidative species for the photodegradation of CQL over SnM nanoplates under visible light irradiation. When the addition of AO, there is no remarkable changes observed in the photocatalytic degradation of CQL which obviously revealed that very little involvement of h+ in the photocatalytic process. 3.3.1. Plausible mechanism Based on the above trapping experiments, the mechanism of the photocatalytic degradation of CQL in the presence of Sn(MoO4)2 nanoplates under visible light irradiation was described as follows. Sn(MoO ) + hν → Sn(MoO ) (e + h ) (1)

Sn(MoO ) (h ) + H O → Sn(MoO ) + HO • (2) . Sn(MoO ) (e ) + O → Sn(MoO ) + O (3)

O . + H → • HO (4) • HO • , O.

,• HO , +CQL → CQL + H O (5)

CQL• + HO • , O . → degradation products (6) The photodegradation mechanism is depending on the electron-hole charge separation. During the photodegradation process, the electrons produced from the valence band (VB) which migrate easily to the conduction band (CB) by absorbing light, leaving holes in the valence band (Eq. 1). The holes in the valence band react with water to form OH● and electrons in the conduction band reacts with oxygen to produce into superoxide radical ion (O2●-) which react with H+ to form ●HO2 (Eq. 2-4). As the trapping results suggested that the both OH● and O2●- are actively participated in the photocatalytic degradation reaction. The reactive oxidative species were reacted with the CQL molecule to produce its ion as illustrated in Eq. 5. After all, the formed active species react with CQL on the catalyst surface to form degradation products (Eq. 6). Apart from the photocatalytic activity, mineralization efficiency is another significant parameter for the photocatalyst. Consequently, the mineralization performances of SnM 18 ACS Paragon Plus Environment

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nanoplates towards the photodegradation of CQL were investigated by chemical oxygen demand (COD) and total organic carbon (TOC) analysis under optimized reaction conditions. Fig.S2 (A and B) showed the TOC and COD results which suggested that the removal of organic matters into the CQL solution increases gradually with increasing light exposure and the mineralization efficiency reaches around 77% for COD and 82% for TOC even after 70 min of irradiation.

Fig.8 (A) Time-dependent absorption spectrum of CQL photodegradation in the presence of 50 mg/mL SnM nanoplates under UV-light illumination, (B) Effect of different catalyst dosage on the photodegradation of CQL, (C) Kinetic curve for the photodegradation of CQL by SnM nanoplates and (D) Effect of catalyst amount dosage on the photodegradation of CQL.



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The stability and recycling ability is the primary concern of the catalyst owing to its practical application. Therefore, we evaluated the reusability tests of SnM nanoplates and the results are presented in Fig.9B. As seen, SnM nanoplates showed above 90% of degradation even after 4th cycle and the slight variation was observed after 4th cycle. In general, the photocatalyst is irradiated with the light of equal or higher energy than the band gap energy of the photocatalysts. In the process, the electron excited to the conduction band and the photocatalytic reaction is occurred only at the surface of the photocatalyst. The pollutant/substrate uptake the electron and proceeds the photocatalytic reaction. However, it is possible that the photocatalyst itself reduced rather than the pollutant/substrate.38 In the present study, the metals Sn and Mo are possible to undergo the photocorrosion during the photocatalytic reaction. Hence, the efficiency of the photocatalyst is decreased after the 4th cycle.

Fig.9 (A) Effect of different scavengers on the photodegradation of CQL and (B) Reusability study of plates-like SnM photocatalyst. 4. CONCLUSIONS In conclusions, we reported a novel SnM nanoplates prepared by a simple wet-chemical route and it was characterized by various spectroscopic and analytical techniques. The asdeveloped SnM nanoplates were scrutinized for its electrochemical determination and photocatalytic degradation performances towards neurotoxicity anti-protozoal drug CQL. The electrochemical studies exhibited that the as-prepared SnM nanoplates modified GCE showed a good analytical performance towards the detection of CQL including lower detection limit, wide 20 ACS Paragon Plus Environment

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linear response range, good sensitivity and excellent selectivity even in the existence of common interferents and some known antibiotic as well as antiprotozoal drug. In addition, the as-prepared SnM nanoplates exhibited an excellent photocatalytic activity towards the degradation of CQL drug under visible light illumination with high degradation rate of above 98% within 70 min. The represented results indicates that SnM could be used as an efficient electron mediator (electrode active material) as well as admirable photocatalytic material towards the detection and degradation of CQL with the advantages of unique features, simple fabrication, high sensing performance and proficient catalytic activity.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxx The optimized loading level of SnM on the GCE and TOC and COD analysis AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S-M Chen). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This project was supported by the Ministry of Science and Technology of Taiwan (Republic of China).


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Table of Contents


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Investigation on the Electrocatalytic Determination and Photocatalytic

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