Synthesis of Two-Dimensional Sr-Doped MoSe2 Nanosheets and its

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C: Physical Processes in Nanomaterials and Nanostructures

Synthesis of Two-Dimensional Sr-Doped MoSe2 Nanosheets and its Application for Efficient Electrochemical Reduction of Metronidazole Mani Sakthivel, Ramaraj Sukanya, Shen-Ming Chen, and Bose Dinesh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02188 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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The Journal of Physical Chemistry

Synthesis of Two-Dimensional Sr-Doped MoSe2 Nanosheets and its Application for Efficient Electrochemical Reduction of Metronidazole Mani Sakthivela, Ramaraj Sukanyaa, Shen-Ming Chena* and Bose Dineshb a

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology,

National Taipei University of Technology, Taipei 10608, Taiwan. b

Nano and Bioelectrochemistry Research Laboratory, Department of Chemistry, School of Advanced

Sciences, Vellore Institute of Technology University, Vellore – 632 014, Tamil Nadu, India

ABSTRACT: we have prepared a new type of two-dimensional strontium doped molybdenum diselenide (SrMoSe2) nanosheets by using a simple hydrothermal process and characterized by using various analytical techniques. Especially, the transmission electron microscopy (TEM) and Raman spectroscopy technique are evidently confirming the formation of SrMoSe2 nanosheets. In addition, the electrochemical properties of SrMoSe2 nanosheets modified glassy carbon electrode (SrMoSe2/GCE) was studied by using various electrochemical techniques. As a result, the SrMoSe2/GCE was detected with low electron transfer resistance (16.58 Ω), excellent active surface area (0.104 cm2) and superior charge transfer coefficient (0.89). Moreover, the SrMoSe2/GCE was developed for electrochemical detection of antimicrobial metronidazole (MTZ) drug. As expected, SrMoSe2/GCE exhibited excellent electrocatalytic activity toward the detection of MTZ with the low detection limit (0.001 µM), better sensitivity (1.13 µA µM-1 cm-2) and wide linear range (0.05 to 914.92 µM). Remarkably, the real-time detection of MTZ in human urine sample also stated the substantial practicability of SrMoSe2/GCE. 

INTRODUCTION

The graphene-like materials especially the 2D transition metal dichalcogenides have an enormous interest in recent research due to their interesting physical and electrochemical properties. In general, the bulk transition metal dichalcogenides are formed by combining the IV, V, VI and VII group of transition metal elements with chalcogen atoms (S, Se, Te) via the strong covalent bond. And also, the bulk transition metal dichalcogenides consists weakly bounded multilayers like graphite structure, whereas the multilayers stacked together by weak van der Waals interaction1,2. 1 ACS Paragon Plus Environment

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This type of weak interaction offers the exfoliation of multilayers into the few or single layers. These exfoliated few layer transition metal dichalcogenide is known as 2D transition metal dichalcogenides, which are exhibiting enhanced electronic and mechanical properties than bulk form of transition metal dichalcogenides due to the two-dimensional confinement of charge carriers3. For decades, diverse 2D transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2, NbSe2, Bi2Se3, Bi2Te3, TaS2, TiS2, ZrS2, and Sb2Se3 etc.,) have been synthesized and used as an active electrode material for variety of applications including supercapacitors4, batteries5, 6, solar cells7, 8, sensors9, fuel cells10, hydrogen evolution reaction11, 12, oxygen evolution reaction13, field effect transistors14, photodetectors15,16 and, photocatalyst for heavy metal reduction17 etc. Among these all 2D transition metal dichalcogenides, MoS2 and MoSe2 dichalcogenides have been widely used in these all aforementioned applications due to various oxidation states of Mo atom with the narrow band gap, high surface area, high electronic conductivity, low charge transfer resistance, high specific capacitance, excellent electrocatalytic activity and good chemical stability18,

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Especially, MoSe2 possesses a higher electrocatalytic activity and electronic conductivity owing to the higher metallic property and an electrocatalytically active unsaturated edge of Se in MoSe220. These significant characteristics of MoSe2 are favorable for electrochemical applications, especially for electrochemical sensors. Recently, Xue Liu et al., demonstrated net like molybdenum selenide-acetylene black supported Ni foam for high-performance supercapacitor electrode and hydrogen evolution reaction with excellent specific capacitance21. Ke-Jing Huang et al., synthesized layered molybdenum selenide stacking flowers like nanostructure coupled with guanine-rich DNA sequence for ultrasensitive ochratoxin A aptasensor application with very low detection limit and significant reproducibility20. However, the electrochemical sensor based on MoSe2 is rarely studied compared to MoS2. Recently, the advanced research has been developed 2 ACS Paragon Plus Environment

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to enhance the electrocatalytic activity of MoSe2 chalcogenide such as (i) increased the active sites by cation-doping/substitution, which introduce the defect or strain on the basal planes, and (ii) hybridization of carbonaceous materials, which act as the excellent catalyst support to improve both electrocatalytic activity and electrical conductivity71,72. Inspired by this above studies, later we have prepared Sr doped/substituted MoSe2 nanosheets using the hydrothermal method and applied as an electrode material for electrochemical detection of MTZ. Recently, Sr-doped MoO4, LaFeCoO3, LaMnO3, LaCoO3, PrCuO4, SnO2, V2O6, Fe2Se2, and CdS-ZnS were used as an electrode material in superconductors, batteries, fuel cells, photocatalyst and electrochemical sensors due to the improved higher photocatalytic activity, chemical stability, superconductivity and higher electrocatalytic activity for electrochemical reaction22-30. The doping of Sr2+ effectively increases the surface area and decrease the bandgap of MoO4, whereas the lowest region in the conduction band of SrMoO4 is mainly the d state of Mo and Sr. The d states of the Sr2+ is mostly present in the conduction band region. In addition, the pseudo-gap (width = ̴ 0.4 eV) is present in the middle of the conduction band. Herein, the presence of pseudo-gap accounts for the covalent bonding in the molybdate. Therefore, the doping of Sr2+ slightly decreases the band gap of molybdate31,32. For example, Meul Hw published an article on Sr based molybdenum chalcogenides with enhanced superconductivity64.

Hence, the doping of Sr2+ with MoSe2 is

believed to facilitate the higher electrocatalytic activity towards the electrochemical detection of MTZ. MTZ (2-methyl-5-nitroimidazole-1-ethanol) is known as a derivative of 5-nitroimidazole, which is used as an antimicrobial agent, antiamebic, and, antiprotozoal for the treatment of infections or diseases in human and animal caused by various microorganisms including balantidium coli, giardia lamblia, anaerobic bacterial, escherichia coli, and, salmonella enterica typhi etc33, 3 ACS Paragon Plus Environment

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Moreover, MTZ has been used in animal nutrition to improve the feed efficiency of the agriculture industry. In 1960, WHO listed the MTZ as a safest medicine in a healthy system. Although MTZ has been widely used in pharmaceutical, food and agriculture industry, the dosage level should be limited due to its serious side effects for both human (diarrhea, dizziness, vaginal itching, liver diseases, crohn’s disease, anemia, and nerve disorder etc.,) and animals (cancer, blood in urine, fever, and drooling etc.,)35, 36. Because of these toxic effects of MTZ, which is prohibited in many countries such as Europe, India, and UK. Hence, the trace level detection of MTZ is essential in food and drug industry to avoid these serious side effects. For more than two decades, variety of analytical techniques have been used for detection of MTZ such as High performance liquid chromatography technique (HPLC)37, fluorescence technique38, UV spectrophotometer method39, spectrophotometry method40, ion mobility spectrometry41, gas chromatography and flame ionization detector method42, polarography method43, least square method44, and electrochemical method45. Among these all followed methods, recently electrochemical approach is a promising and more interested due to its low cost, simple fabrication process, fast response, and high sensitive detection even at low concentration of targeted analyte46. In addition, the MTZ contains the nitro group, which is an electrochemically reducible active center. Thus, the electrochemical technique is considered as a more suitable method for electrochemical detection of MTZ. In this work, we have demonstrated the synthesis of 2D SrMoSe2 nanosheets by using hydrothermal technique and characterized by using various analytical techniques. Remarkably, the SEM and TEM analysis evidently confirmed the 2D nanosheets like structure of SrMoSe2. The chemical and electronic state of Sr, Mo, and Se elements in SrMoSe2 nanosheets was strongly demonstrated by using XPS analysis. Moreover, the uniform distribution of Sr, Mo, and Se elements in prepared sample was observed from the elemental mapping studies. In addition, 4 ACS Paragon Plus Environment

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RAMAN and XRD techniques also provide the evidence for the formation of 2D SrMoSe2 nanosheets. Further, the electrochemical activity of SrMoSe2 towards the detection of MTZ were studied using various electrochemical techniques such as CV and DPV techniques. As the result, the active surface area (A) and charge transfer coefficient (α) of SrMoSe2 modified glassy carbon electrode were detected. In addition, the SrMoSe2 modified glassy carbon electrode exhibited higher electrochemical response with low detection limit, higher sensitivity, wide linear range and excellent selectivity of MTZ. 

EXPERIMENT SECTION

Materials and reagents The strontium nitrate (Sr(NO3)2), sodium molybdate dihydrate (Na2MoO4 · 2H2O), selenide powder (Se), and ethylenediamine (NH2CH2CH2NH2) and metronidazole (C8H9N3O3) were purchased from Sigma Aldrich. The hydrazine hydrate solution (N2H4. xH2O) purchased from ACROS chemicals. In the all following electrochemical studies, 0.05 M phosphate buffer solution (PBS) (pH 7) was used as supporting electrolyte, which is prepared by mixing of 0.05 M Na2HPO4 and NaH2PO4. The pH of the PBS was changed by adding of NaOH/H2SO4. All the chemical reagents were of analytical grade and used without any further purification. Hydrothermal synthesis of SrMoSe2 nanosheets The SrMoSe2 nanosheets were prepared by using the simple hydrothermal technique. In this synthesis method, 0.02 M Sr(NO3)2 and 0.02 M Na2MoO4 · 2H2O were added in 12 mL of H2O under vigorous stirring for 15 min. Then, 0.04 M Se powder in 8 mL of ethylenediamine was added to the above mixture solution and followed by continuous stirring for 15 min. Subsequently, 1 mL of N2H4 · xH2O was added drop by drop into the above solution and obtained the black color precipitation. Finally, the resultant black color precipitation was transferred into 50 mL Teflon equipped autoclave and kept at 180 ̊ C for 12 hr in hot air oven. After the hydrothermal process, 5 ACS Paragon Plus Environment


the heated autoclave was allowed to cool at room temperature. Then, collected the precipitation and washed several time by using water/ethanol and dried in air oven at 45 ̊ C for overnight47,48. The formation of SrMoSe2 nanosheets is clearly represented by following equations, Sr(NO3)2 + Na2(MoO4).2H2O → Sr(MoO4) Se (0) → Se2Sr(MoO4) + Se2- + Se → SrMoSe2

Characterization techniques The surface characterization of SrMoSe2 nanosheets was characterized by using scanning electron microscopy (SEM, Hitachi S-3000 H electron microscope) and transmission electron microscopy (TEM, JEOL 2100F). The elemental distribution of Sr, Mo and Se elements in prepared SrMoSe2 nanosheets was quantitatively measured by using energy dispersive X-ray (EDX, HORIBA EMAX X-ACT). The crystalline nature of SrMoSe2 nanosheets was scrutinized by using X-ray diffraction technique (XRD, XPERT-3 diffract meter with Cu Kα radiation (K= 1.54 Å)). Raman spectroscopy was performed to understand the structural fingerprint and formation of SrMoSe2 nanosheets by using WITech CRM200 confocal microscopy Raman system with a 488 nm laser. In addition, the surface electronic state of Sr, Mo, and Se elements in prepared SrMoSe2 nanosheets was recorded by using X-ray photoelectron spectroscopy (XPS, Thermo scientific multilab 2000). Electrochemical impedance spectroscopy (EIS, IM6ex ZAHNER impedance measurement unit) has been performed to understand the interfacial electrochemical property between SrMoSe2/GCE and electrolyte. The electrochemical property of SrMoSe2/GCE has been studied by using CV (CHI611A electrochemical analyzer) and, DPV (CHI900 electrochemical analyzer) techniques. These electrochemical studies were recorded by using the conventional three-electrode system, where glassy carbon electrode (GCE) was used as a working electrode, a saturated Ag/AgCl 6 ACS Paragon Plus Environment

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electrode was used as a reference electrode, and a platinum wire was used as an auxiliary electrode. All measurements were carried out at room temperature. Fabrication of SrMoSe2 modified glassy carbon electrode (SrMoSe2/GCE) In order to study the electrochemical properties of SrMoSe2 nanosheets, which is modified on bare GCE and used as a working electrode towards the electrochemical reduction of MTZ. For this, SrMoSe2 (1 mg) was dispersed in 1 mL of ethanol by using ultrasonication for 15 min. Then, the prepared 6 µL of the SrMoSe2 suspension was drop casted on the surface of GCE and dried in an oven at 45 ̊C. After drying process, the modified GCE was softly rinshed with DD water to remove the weakly bounded materials on GCE surface. Finally, the fabricated SrMoSe2 modified GCE was used as a working electrode in all further electrochemical studies. 

RESULTS AND DISCUSSION

Characterization of 2D SrMoSe2 nanosheets The surface morphology of prepared 2D SrMoSe2 nanosheets was studied by using SEM and TEM analysis. Herein, Figure. 1a shows the SEM image of aggregated SrMoSe2 nanosheets and the corresponding EDX spectra is shown in Figure. 1b with sharp peaks of Sr, Mo, and Se elements. From this EDX spectra, the quantitative result was detected with relative weight percentage of Sr (0.92%), Mo (0.65%), and Se (0.57%). In addition, the elemental mapping studies also were demonstrated in Figure. 1c(i, iii), which is clearly display the uniform distribution of Sr, Mo, and Se elements in the surface of SrMoSe2 nanosheets. It preliminarily confirms the formation of SrMoSe2 nanosheets. Further, the TEM analysis provides the clear evidence for the formation of 2D layered nanosheets structure of SrMoSe2. Figure. 2a-c shows the curling structured thin nanosheets of SrMoSe2. In addition, the high-resolution HR-TEM image (Figure. 2d) displays the perfect lattice fringes for the layered crystal structure of SrMoSe2 nanosheets with the interplanar distance of 0.69 nm, which is corresponding to (002) plane of hexagonal SrMoSe2. The selected 7 ACS Paragon Plus Environment


area electron diffraction (SAED) pattern of SrMoSe2 (inset of Figure. 2d) with diffraction rings formed by bright spots, which are labeled to (200), (006), (103), (105), (004), (100), and (002) planes of SrMoSe2 hexagonal phase. It is good agreement with XRD pattern of SrMoSe2 nanosheets. The crystalline nature of SrMoSe2 nanosheets were characterized by using XRD analysis. Figure. 3a shows the major diffraction peaks at 11.8 ̊, 24.9 ̊, 31.4 ̊, 36.1 ̊, 41.2 ̊, 44 ̊, 47.6 ̊, 57 ̊, 59.8 ̊, and 73.6 ̊ for corresponding crystal planes are (002), (004), (100), (103), (006), (200), (105), (110), (112) and (116) respectively, which are strongly assigned to the hexagonal phase of MoSe2 (JCPDS card no: 29-0914). On the other hand, the diffraction peaks at 23.92 ̊, 25.24 ̊ and 35.1̊ for corresponding crystal planes (010) (002) and (012) which are due to hexagonal phase of strontium (JCPDS card no; 98-007-4722)49, 50. It is suggesting that the successful integration of Sr2+ with MoSe2, which is approximately labeled as SrMoSe2. For further confirmation, Raman spectroscopy was used to identify the formation of the SrMoSe2 nanosheets based on molecular vibrations. Figure. 3b shows the Raman spectrum of SrMoSe2 with the peak observed at 229 cm-1 for Mo-Se out of plane vibration (A1g mode) and the peak at 313.44 cm-1 for Mo-Se in plane 1 vibration (E2g ). The spectrum displays the additional peaks at 797.65, 848.94, and 892.96 cm-1

that are corresponding to the MoSe2 as reported in previous literature20. The peak at 1079.5 cm-1, which is related to the presence of Sr2+ in prepared SrMoSe2 nanosheets50. The surface electronic state and chemical composition of SrMoSe2 nanosheets were successfully studied by using XPS analysis and demonstrated in Figure. 4. The wide scan XPS spectrum (Figure. 4a) was observed with the major peaks of Sr, Mo, and Se elements in the corresponding electronic state. Whereas, the peak of C (as the reference) and O elements were recorded due to the surface adsorption of hydrocarbon contaminants and exposure to air. Thus, the wide scan XPS spectrum strongly confirmed the existence of Sr, Mo, and Se elements in the near-surface range of 8 ACS Paragon Plus Environment

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SrMoSe2 nanosheets. The high-resolution XPS spectrum of Sr 3d (Figure. 4b) was split into two major peaks at 132.97 and 136.62 eV, which are assigned to Sr 3d5/2 and Sr 3d3/2 states respectively51. As shown in Figure. 4c, the core level high-resolution spectrum of Mo 3d exhibits the two major peaks at 230.84 and 231.99 eV for Mo 3d5/2 and Mo 3d3/2 states respectively, which is ascribed to the trivalent state of Mo element. The another peak at 235.05 is affirming the existence of a Mo6+ state, which might be due to the reduction of MoO2− 4 during the hydrothermal reaction, while the peak at 227.42 can assign to zero valent Mo0 state52. The XPS spectrum was presented in the Figure. 4d confirms the Se 3d at a relative binding energy of 54.1 eV53. Finally, the recorded XPS spectra scrutinized the electronic state of Sr, Mo, and Se elements, thus the formation of SrMoSe2 nanosheets was evidently proved. Electrochemical activity of SrMoSe2 modified glassy carbon electrode (SrMoSe2/GCE) Electrochemical Impedance Spectroscopy (EIS) is a promising technique to understand the interfacial electrochemical property between the surface of SrMoSe2/GCE and electrolyte66. Thus, various modified electrodes such as bare GCE, MoSe2/GCE, and SrMoSe2/GCE were treated in 5 mM of [Fe(CN)6]3-/4- contain 0.1 M of KCl, where the frequency range applied from 0.1 Hz to 100 kHz and fixed AC applied potential to be 10 mV. The obtained EIS result was demonstrated in the form of Nyquist plots as shown in Figure. 5a with corresponding Randles circuit. The fitted Randle circuit (inset of Figure. 5a) implies that the interfacial property of electrode is mainly related to charge transfer resistance (Rct), electrolyte resistance (Rs), Warburg impedance (Zw), and double layer capacitance (Cdl). In general, the diameter of the semicircle in EIS curve is directly proportional to Rct value. The low Rct value indicates the fast electron transfer kinetic of the redox probe at the interface of electrode and electrolyte. From the Figure. 5a, Rct values were detected for bare GCE, MoSe2/GCE, and SrMoSe2/GCE of about 211.0, 32.44 and 16.58 Ω respectively. 9 ACS Paragon Plus Environment


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As the result, SrMoSe2/GCE exhibited ultra low Rct value, which is indicating that the incorporation of Sr2+ with MoSe2 is relatively decreased the charge transfer resistance of SrMoSe2. Moreover, the CV technique also was performed to evaluate the electron transfer kinetic at SrMoSe2/GCE based on fundamental electrochemical redox process of [Fe(CN)6]3-/4-. Figure. 5b shows the CV profile of bare GCE, MoSe2/GCE, and SrMoSe2/GCE in 5 mM of [Fe(CN)6]3-/4contains 0.1 M of KCl at 50 mV s-1. Apparently, SrMoSe2/GCE showed the higher reversible redox peak current response (Ipa = -107. 21 µA and Ipc = 106.02 µA ) with lowest peak potential separation (ΔEp = 0.068 V). For comparison, ΔEp value was calculated for GCE and, MoSe2/GCE of 0.147 and 0.077 V respectively. As the result of this experiment, the lowest value of ΔEp and higher redox peak current response are indicating that SrMoSe2/GCE has larger electroactive surface area and faster electron transfer kinetics than that of MoSe2 and bare GCE. In addition, the detailed electrochemical activity of SrMoSe2/GCE was studied in [Fe(CN)6]3-/4- by varying scan rate from 10 to 100 mV s-1 as shown in Figure. 5c, which clearly display the increasing redox peak current response by increasing scan rate. The corresponding linear calibration plot for the square root of the scan rate vs. redox peak current (inset of Figure. 5c), It indicates that the overall kinetic in electrochemical redox process of [Fe(CN)6]3-/4- is a diffusion controlled process. Finally, the active surface area of the modified electrodes was calculated by using Randles-Sevick equation54: Ip = 2.69 × 105 A D1/2n3/2 γ1/2 C

(1)

Where Ip is the peak current, D is the diffusion coefficient of ferricyanide solution (cm2 s-1), n is the number of electron transfer (n = 1), γ is the scan rate (V s-1) and, C is the concentration of ferricyanide in bulk solution. By using the Equation. 1, the active surface area was calculate to be 0.069, 0.085, and 0.104 cm2 for bare GCE, MoSe2, and SrMoSe2 nanosheets respectively. The higher active surface area of SrMoSe2 reveals that the incorporation of Sr with MoSe2 effectively 10 ACS Paragon Plus Environment

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enhanced the active surface area of SrMoSe2 nanosheets. Thus, SrMoSe2/GCE is found as a promising electrode material with fast electron transfer kinetic and higher active surface area, which can facilitate the efficient electrochemical activity towards the electrochemical reduction of MTZ. Electrochemical reduction of MTZ on SrMoSe2/GCE Further, the CV response of SrMoSe2/GCE towards the electrochemical reduction of MTZ (0.47 mM) was recorded and compared with other modified electrodes including bare GCE, and MoSe2/GCE as shown in Figure. 5d, where N2 saturated PBS (pH 7) was used as an electrolyte and the scan rate was fixed to be 50 mV s-1. The electrochemical reduction of MTZ at bare GCE was observed at Epc = -0.745 V with low cathodic peak current (Ipc) of 5.65 µA, which is due to the slow electron transfer kinetic. At the same time, MoSe2/GCE, and SrMoSe2/GCE were observed with a sharp reduction peak current response at Epc = -0.72 and -0.69 V respectively. Especially, the reduction peak current for MTZ at SrMoSe2/GCE is 12.82 and 1.83 fold higher than that of bare GCE and MoSe2/GCE respectively. The resultant reduction peak of MTZ at various modified electrodes is ascribed due to the four electron reduction of the nitro group (-NO2) to corresponding hydroxylamine group (-NHOH). The possible reduction mechanism of MTZ is clearly demonstrated in Scheme 1. Thus, SrMoSe2/GCE is found as an active modified electrode for electrochemical reduction of MTZ. Effect of MTZ concentration on SrMoSe2/GCE Figure. 6a displays the CV profile for varying the concentration of MTZ on SrMoSe2/GCE in N2 saturated 0.05 M PBS (pH 7) at the scan rate of about 50 mV s-1. As the result, SrMoSe2/GCE exhibited linear increment in reduction peak current for linear addition of MTZ from 0.09 to 0.740 mM. The corresponding linear calibration plot for reduction peak current vs. concentration of MTZ 11 ACS Paragon Plus Environment


is shown in Figure. 6b with resultant linear regression equation and correlation coefficient of y = 132.27x + 7.395 (y = Ipc (µA), x = mM) and R2= 0.9926 respectively. It reveals the efficient electrocatalytic activity of SrMoSe2/GCE towards the reduction of MTZ. In addition, Figure. 6c demonstrates the linear calibration plot for the logarithm of reduction peak current vs. logarithm of the concentration of MTZ. The corresponding linear calibration plot was measured as y = 0.9784x + 2.1043 (y = log (Ipc (µA)), x = log (mM)) and R2= 0.9926 respectively, where the slope value was measured to nearly 1. It is indicating that the electrochemical reduction of MTZ on SrMoSe2/GCE is first order kinetic process. Effect of scan rate and pH for MTZ reduction on SrMoSe2/GCE Further electrochemical characterizations were carried out to understand the electrochemical activity of SrMoSe2/GCE towards the reduction of MTZ. In general, the electrochemical behaviors of the modified electrode are depending upon the number of factors including the applied scan rate (rate of voltage changes per second) and pH of the electrolyte. Hence, the fabricated SrMoSe2/GCE were tested under varying scan rate and different pH (3 to 11). Figure. 7a shows the CV curve for varying the scan rate from 10 to 100 mV s-1 on SrMoSe2/GCE with the presence of MTZ (0.47 mM) in N2 saturated 0.05 M PBS (pH 7). The corresponding linear calibration plot (Figure. 7b) for reduction peak current vs. scan rate was measured with calculated linear regression equation and correlation coefficient y = 1.3227x + 8.316 (y = Ipc (µA), x = v (mV s-1)) and R2= 0.9926 respectively. It clearly demonstrated that surface control process participates in the kinetic of overall reduction process. In addition, the linear calibration plot for reduction peak potential (Epc (V)) vs. logarithm of scan rate is shown in Figure. 7c. It indicates that the peak potential for catalytic reduction of MTZ increases by increasing the applied scan rate, which is related to the kinetic limitation during electrochemical reduction (charge transfer kinetic) between 12 ACS Paragon Plus Environment

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SrMoSe2/GCE and MTZ. For the further understanding on rate-determining step, a Tafel plot was drawn by using CV curve obtained at 10 mV s-1 for MTZ (0.47 mM) as shown in Figure. 7d. The kinetic parameter (charge transfer coefficient) was measured by using slope (Δ) value, which can represent as55, 56: Δ = n (1- α) F/2.3RT

(2)

Where, n is the number electron participated in the rate determining step, α denoted as charge transfer coefficient, F represents the Faraday constant (96485 C/mol), R is the gas constant (8.314 J mol-1 K-1), and T is the room temperature (300 K). From the Tafe-plot, the slope value was observed to be 6.9655 (V decade)-1. By using Equation.2, α value was measured to be 0.89 (n = 4) for SrMoSe2/GCE. In order to study the effect of pH on SrMoSe2/GCE, the electrochemical reduction of MTZ (0.47 mM) on SrMoSe2/GCE was studied by varying the pH of 0.05 M PBS from 3 to 11 at a fixed scan rate of 50 mV s-1. The obtained CV curve is shown in Figure. 7e. The corresponding calibration plot was drawn for reduction peak current vs. pH of 0.05 M PBS. In this plot, increased reduction peak current was observed for increasing pH from 3 to 7 , while the peak current value decreased with increasing pH from 7 to 11. It suggesting that the higher (pH > 7) is not favorable for better electrochemical reduction of MTZ. At the time, the maximum electrochemical reduction of MTZ was achieved at pH 7. It is clearly demonstrated in calibration plot (Figure. 7f). Hence, 0.05 M PBS (pH 7) was chosen as optimized electrolyte condition for all further electrochemical experiments. Differential pulse voltammetry study for determination of MTZ on SrMoSe2/GCE The DPV technique was performed to understand the exact electrochemical performance of SrMoSe2/GCE based on essential electrochemical parameters such as limit of detection (LOD), 13 ACS Paragon Plus Environment


linear range and sensitivity of MTZ. Figure. 8a shows the DPV response of SrMoSe2/GCE for the linear addition of MTZ concentration from 0.05 to 914.92 µM under the optimized condition. The obtained sharp DPV response for electrochemical reduction of MTZ on SrMoSe2/GCE is suggesting the effective electrochemical reduction of the nitro group in MTZ via four electron reduction process. Moreover, the corresponding linear calibration plot was drawn for reduction peak current vs. concentration of MTZ is shown in Figure. 8b with calculated linear regression equation with correlation coefficient to be y = 0.0832x + 10.227 (y = (Ipc (µA), x = µM) and R2= 0.991 respectively. The limit of detection (LOD) has been calculated by using the standard equation: LOD = 3σ/S

(3)

where σ is the standard deviation of the blank signal and S is the slope. By following the Eq. 3, LOD and sensitivity of MTZ on SrMoSe2/GCE were calculated to be (0.001 µM) and (1.13 µA µM-1 cm-2) respectively. Eventually, the obtained LOD of MTZ on SrMoSe2/GCE is comparatively lower than previously reported MTZ sensor as shown in comparison Table.1. Ten continuous additions of 50×10-6 M MTZ showed an RSD value of 1.64% Selectivity, Stability and Reproducibility of MTZ The practicability of the proposed SrMoSe2/GCE sensor is relatively depending upon the number factors such as selectivity, stability, and reproducibility in the followed electrochemical reduction of MTZ. Thus, the selectivity of SrMoSe2/GCE towards the detection of MTZ was recorded by using DPV technique under the same experimental condition as followed in Figure. 8a. As a result, Figure. 8c shows the DPV curve for addition of MTZ (100 µM) and interference species such as 2+ 200 fold higher concentration of metal ions (NO3− , PO3− 4 , and Zn ) and 100 fold higher

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concentration of organic compounds (Dopamine (DA), 4-Nitrophenol (NP), Alanine (Ala), Nifedipine (NIF), Cystine (Cys), Citric acid (CA), Ascorbic acid (AA), and Tartaric acid (TA)). The corresponding relative error bar diagram is given in Figure. 8d. It suggests that the presence of interference species produced the negligible amount of signal changes (less than 5%) only. As a result, the proposed SrMoSe2/GCE sensor exhibited the excellent selectivity for electrochemical detection of MTZ. In order to evaluate the stability and reproducibility of SrMoSe2/GCE, which was treated in DPV technique with the presence of MTZ (100 µM) in N2 saturated 0.05 M PBS (pH 7). The corresponding DPV response of MTZ reduction for 1st and 20th consecutive measurement is shown in Figure. 9a. As the result, SrMoSe2/GCE retains 92.63% of its initial (at 1st measurement) reduction peak current response of MTZ after 20th measurement. It implies that the higher stability of SrMoSe2/GCE towards the detection of MTZ. In addition, the reproducibility of the reported sensor was demonstrated by fabricating the five individual SrMoSe2/GCE and recorded their DPV response for reduction of MTZ (0.6 mM). The corresponding bar diagram is shown in Figure. 9b. Herein, the relative standard deviation (RSD) value was measured to be 3.7%, which is suggesting the excellent reproducibility of SrMoSe2/GCE for the detection of MTZ. These obtained substantial selectivity, stability, and reproducibility of SrMoSe2/GCE could be facilitated higher electrochemical response of MTZ in real sample analysis. Real sample analysis of MTZ in human urine sample The real-time application of SrMoSe2/GCE was also analyzed by using DPV techniques. In this experiment, a human urine sample was used as the real sample. Further, the experiment was performed by adding the known concentration of MTZ spiked urine sample in N2 saturated 0.05 M PBS (pH 7) and tested for five times (n = 5). Here, the standard addition method was followed 15 ACS Paragon Plus Environment


to measure the recoveries. The corresponding DPV response is shown in Figure. 9c. The resultant sharp peak current response is demonstrating that the effective estimation of MTZ spiked real samples on SrMoSe2/GCE. The calculated recovery values are ranging from 92 to 112 % and presented in Table 2. The considerable result in real sample analysis suggests that the reported SrMoSe2/GCE is a promising electrode material for electrochemical determination of MTZ in human urine samples also. 

CONCLUSION

In summary, the SrMoSe2 nanosheets were prepared by using a simple hydrothermal technique and the analogous multilayer stacked graphite structure of prepared SrMoSe2 was clearly demonstrated by using TEM and Raman analysis, where TEM shows the curling structured super thin nanosheets of SrMoSe2. The SEM, EDX, XRD, and XPS techniques also evidently proved the formation of SrMoSe2. In addition, the electrochemical property of SrMoSe2 was found using various electrochemical techniques. The estimated electrochemical parameters such as the higher electroactive surface area and charge transfer coefficient are aiding for the higher electrocatalytic activity for SrMoSe2 nanosheets. Therefore, the fabricated SrMoSe2/GCE sensor exhibits higher electrocatalytic activity with a low detection limit of MTZ. The real sample analysis also shows the excellent electrocatalytic activity of SrMoSe2/GCE towards the detection of MTZ in the human urine sample. 

ACKNOWLEDGEMENTS

Dr Bose Dinesh gratefully acknowledges DST-SERB for a National Postdoctoral Fellowship (PDF/2015/000174). Financial supports of this work by the Ministry of Science and Technology, Taiwan (MOST106-2811-M-027-004) is gratefully acknowledged. 

AUTHOR INFORMATION

Corresponding Authors 16 ACS Paragon Plus Environment

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*E-mail: [email protected]. Tel: +886 2270 17147. Fax: +886 2270 25238. Shen-Ming Chen: 0000-0002-8605-643X NOTES The authors declare no competing financial interest

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Figures

Figure. 1 (a) SEM image, (b) EDX spectra (inset: Quantitative result), and (c) SEM image for elemental mapping ((i) Sr, (ii) Mo, and (iii) Se element) of SrMoSe2 nanosheets.



Figure.2 (a-c) Different magnified TEM images, (d) HR-TEM images, and (insert) SAED pattern of SrMoSe2 nanosheets.


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Figure.3 (a) XRD pattern, and (b) Raman spectrum of SrMoSe2 nanosheets.



Figure.4 (a) Wide scan XPS spectra of SrMoSe2 nanosheets, and XPS spectra of (b) Sr 3d, (c) Mo 3d, and (d) Se 3d.


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Figure. 5 (a) EIS curve of (i) bare GCE, (ii) MoSe2/GCE, and (iii) SrMoSe2/GCE in 5 mM of [Fe(CN)6]3/4contain 0.1 M of KCl in the frequency range from 0.1 Hz to 100 kHz. Inset: Randles circuit model. (b) CV response of (i) bare GCE, (ii) MoSe2/GCE, and (iii) SrMoSe2/GCE in 5 mM of [Fe(CN)6]3-/4- contain 0.1 M of KCl at scan rate of 50 mV s-1. (c) CV response of SrMoSe2/GCE in 5 mM of [Fe(CN)6]3-/4- contain 0.1 M of KCl by varying scan rate of from 10 to 100 mV s -1 (inset: corresponding linear calibration plot). (d) CV response of (i) bare GCE, (ii) MoSe2/GCE, and (iii) SrMoSe2/GCE with presence and (iv) absence of MTZ (0.47 mM) in N2 saturated 0.05 M PBS (pH 7) at scan rate of 50 mV s-1.



Scheme 1: Possible electrochemical reduction mechanism of MTZ

Figure. 6 (a) CV response of SrMoSe2/GCE for varying the concentration of MTZ (0.09-0.740 mM) in N2 saturated 0.05 M PBS (pH 7) at scan rate of 50 mV s-1. (b) corresponding linear calibration plot for reduction peak current vs. concentration of MTZ. (c) corresponding linear calibration plot for logarithm of reduction peak current vs. logarithm of concentration of MTZ.


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Figure. 7 (a) CV response of SrMoSe2/GCE for varying the scan rate on SrMoSe2/GCE with presence of MTZ (0.47 mM) in N2 saturated 0.05 M PBS (pH 7). (b) corresponding linear calibration plot for reduction peak current vs. scan rate. (c) corresponding linear calibration plot for reduction peak potential vs. logarithm of scan rate. (d) Tafel plot for logarithm of reduction peak current vs. peak potential measured by using CV curve obtained at 10 mV s-1 for MTZ (0.47 mM). (e) CV response of SrMoSe2/GCE for varying the pH of 0.05 M PBS for reduction of MTZ (0.47 mM). (f) Corresponding calibration plot for pH vs. peak current.

Figure. 8 (a) DPV response of SrMoSe2/GCE for linear addition of MTZ (0.05 to 914.92 µM) on SrMoSe2/GCE in N2 saturated 0.05 M PBS (pH 7). (b) corresponding linear calibration plot for reduction concentration of MTZ. (c) DPV response of interference with presence of MTZ (100 µM) and interference 3− 2+ species such as 200 fold higher concentration of metal ions (NO− 3 , PO4 , and Zn ) and 100 fold higher concentration of organic compounds (Dopamine (DA), 4-Nitrophenol (NP), Alanine (Ala), Nifedipine (NIF), Cystine (Cys), Citric acid (CA), Ascorbic acid (AA), and Tartaric acid (TA)) . (d) Corresponding relative error percentage bar diagram. 32 ACS Paragon Plus Environment

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Figure. 9 (a) DPV response (1st and 20th run) for stability test of SrMoSe2/GCE in N2 saturated 0.05 M PBS (pH 7) with presence of MTZ (100 µM). (b) Relative bar diagram for reproducibility test of SrMoSe2/GCE. (c) DPV response for real sample analysis with the addition of MTZ spiked human urine sample in N 2 saturated 0.05 M PBS (pH 7).



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Tables Table 1: Comparison of the different modified electrode for electrochemical sensing of MTZ.

Linear range (µM)

LOD (µM)

Sensitivity (µA µM-1 cm2 )

Ref

Modified electrode

Method

MWCNTs/CTS-Ni/GCE

DPV

0.1 - 150

0.025

0.695

(34)

SWV

0.02-1.6

0.004

-

(57)

Polyfurfural film/GCE

DPV

2.5-500

0.05

-

(58)

CPE

SWV

1.0-500

0.297

1.0

(59)

CV

5-161.0

58

-

(60)

poly(AMTEOS)/CPE

DPV

0.4-200

0.09

-

(45)

Chit/CuTsPc/GCE

DPV

0.00087200 0.17205.39

0.004 1

-

(61)

0.049

-

(62)

Cu(II)-CNP/CPE

Ni/Fe-LDH/GCE

MNZMIP/MWCNT/GCE

CV

BDD/GCE

SWV

0.2-4.2

0.065

-

(63)

Hi Q Sil C18 column

HPLC

-

1.908

-

(67)

HPLC with UV

0.0153.18 63625440

0.006

-

(68)

25.44

-

(69)

C18 Sep-Pak Platinum electrode

Flow injection

3-Amino-9ethylcarbazole probe

Fluorescence

20-1000

0.009

-

(70)

DPV

0.05914.92

0.001

1.13

This work

SrMoSe2/GCE


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Table 2. Determination of MTZ in spiked urine sample using SrMoSe2/GCE electrode (n = 5)

Added

Found

Recovery

RSD

(µM)

(µM)

(%)

(%)

0.2

0.22

110

2.36

10

9.2

92

2.12

15

16.8

112

2.43

20

21.2

106

2.75

Sample Urine sample



TOC Graphic


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Synthesis of Two-Dimensional Sr-Doped MoSe2 Nanosheets and its

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