Synthesis and Characterization of Samarium-Substituted Molybdenum

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Synthesis and Characterization of Samarium Substituted Molybdenum Diselenide and its Graphene Oxide Nanohybrid for Enhancing Selective Sensing of Chloramphenicol in Milk Sample Mani Sakthivel, Ramaraj Sukanya, and Shen-Ming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12006 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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ACS Applied Materials & Interfaces

Synthesis and Characterization of Samarium Substituted Molybdenum Diselenide and its Graphene Oxide Nanohybrid for Enhancing Selective Sensing of Chloramphenicol in Milk Sample Mani Sakthivela,b, Ramaraj Sukanyaa, Shen-Ming Chena* and Kuo-Chuan Hob* a

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and

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

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]

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ABSTRACT: The electronic conductivity and electrocatalytic activity of metal chalcogenides are normally enhanced by following the ideal strategies such as substitution/doping of heterogeneous atoms and hybridization of highly conductive carbon supportive materials. Here, a rare earth element (samarium) was substituted with MoSe2 by using the simple hydrothermal method. The lattice distortion due to the substitution of Sm3+ with MoSe2 was clearly observed by using HRTEM analysis. As a consequence, the prepared SmMoSe2 nanorod was encapsulated with graphene oxide sheets (GO) using ultrasonication process. Furthermore, the GO encapsulated SmMoSe2 nanocomposite modified GCE (GO@SmMoSe2/GCE) was used for the sensing of chloramphenicol (CAP). The results show that the GO@SmMoSe2/GCE revealed the superior electrocatalytic activity with low detection (LOD: 5 nM) and sensitivity (20.6 µA µM-1 cm-2) to electrochemical detection of an analyte. It indicates that the substitution of Sm3+ and encapsulation of GO significantly increased the electrical conductivity and electrocatalytic activity of MoSe2.

KEYWORDS: Molybdenum diselenide, Graphene oxide, Samarium, Electrochemical sensor, Chloramphenicol.


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INTRODUCTION The modern research in electrochemical applications is focused on the synthesize of promising active electrode materials with improved electrochemical properties. Recently, molybdenumbased metal chalcogenides especially MoS2 and MoSe2 have been deliberated as the host electrode materials with intriguing electrocatalytic activity. Both MoS2 and MoSe2 possess sandwich structure consisting of one Mo covalently bonded with two chalcogen atoms (S or Se), and each monolayer is attracted by weak forces. The layered structure with predominant catalytic active sites offers superior electrochemical properties1, 2. The MoSe2 potentially exhibits superior electrocatalytic reaction and specific conductance than MoS2 owing to the twenty order greater metallic conductivity and the more functional unsaturated sites of Se, attractive volumetric energy density, faster electrochemical reaction rate, higher metallic binding with transition metals, larger atomic size, and polarizability3-5. These emerging properties of MoSe2 make it an extremely useful for various electrochemical applications6-13. Currently, the different methods have been exploited to enrich the electrocatalytic reaction of MoSe2. For this thing, the different strategy has been developed and followed such as (i) create the atomic lattice distortion or defect through introduce the electropositive cation with heterogeneous spin states and (ii) integration of carbon conductive materials with abundant active sites. Both atomic lattice distortion/defect and carbon materials facilitate more active sites for electrochemical reactions14,

15

. For example,

Xiaoyan Ma et al., designed Co and Ni-substituted molybdenum disulfide nanosheets as the superior electrocatalysts for solar H2 production16. Shengjie Xu et al., demonstrated the synthesis of 3D MoS2/MoSe2 nanosheets incorporated with graphene network as a catalytic promoter for the HER17. Encouraged by these studies, we attempted to synthesis GO nanosheets encapsulated Sm-doped MoSe2 and developed as an excellent catalyst for sensing of CAP. Recently, substitution of lanthanide earth elements (Ce, Nd, Eu, Sr, Pr, Yb, Lu, and Sm etc.,) is evoked as a striking method to achieve a prospective enhancement in electrical conductivity and electrocatalytic activity18-20. Among these rare earth elements, doped/substituted Sm3+ exhibits distinctive catalytic performance, dramatically increase the activation energy, lower critical voltage, increase the ionic conductivity and form associated defects owing to the high surface basicity, and large ionic radius of 1.08 Å with mixed oxidation states (+2 and +3 states). Therefore, the doping/substitution of Sm3+ has been achieved extreme performances in the aforementioned electrochemical applications21-24. For example, Yan Xiang et al., designed planar 3 ACS Paragon Plus Environment


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perovskite photovoltaic cell using Sm-doped TiO2 compact electron transport layer and achieved higher power conversation efficiency, where Sm doping increases the carrier transport ability and an upward shift in Fermi energy level25. Rahman et al., prepared Sm-doped Co3O4 nanokernel and used as the electrocatalysts for a superior ethanol sensor, where Sm doping not only alter the electrical property and also enhancing the electrocatalytic activity26. Jieyu Liu et al., achieved the high oxygen reduction reaction catalytic performance based on SmMn2O5

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.

Therefore, the doping/substitution of Sm is believed as an exceptional way to improve the electrochemical properties of MoSe2. As a consequence, the electrocatalytic property of SmMoSe2 was further improved by hybridizing with a suitable conductive carbon material, which facilitates more active sites and fast electron transfer process. Several carbon nanomaterials, namely carbon nanotubes28, carbon nanofiber29, porous carbon materials30, graphene and its derivatives31 are widely interested as the active materials of different electronic devices. Among all, graphene and its derivatives generally exhibit high electrochemical activities due to their substantial physical and chemical properties. Especially, graphene oxide is a well-known derivative of graphene, also have an enormous surface area, extraordinary electronic conductivity and mechanical strength owing to the existence of the number of structural defects and functional groups (epoxy, hydroxyl, and carbonyl) on basal plane and edges. Chemically modified graphene oxide is hydrophilic in nature owing to the existence of oxygen vacancies which enables to surface functionalization through covalent or non-covalent bonding. The defective sheets of GO may change its chemical and electronic properties and easily dispersed in solvents for various electrochemical applications32,33. Highly functionalized graphene oxide provides active sites for the effective immobilization of other metals or species for electrochemical sensing application. Moreover, graphene oxide exhibits a direct electron transfer between electrode and species due to the existences of oxygen vacant sites which imparts an improved electrocatalytic towards the detection of biomolecules34. In the past decades, GO and its composites based electrocatalysts have been developed for the sensing of numerous biomolecules and heavy metal ions. For instance, Wong et al., explained the role of graphene oxide in the detection of various analyte molecules such as isoproterenol, paracetamol, vitamin B9, propranolol, and guaranine35. Thus, it is found that the hybridization/integration of GO can efficiently enhance the electrocatalytic performances of SmMoSe2 towards the electrochemical determination of CAP. 4 ACS Paragon Plus Environment

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As from the broad spectrum of antibiotics, chloramphenicol (CAP) is an active antibiotic36. It has a tendency to act against gram-positive and gram-negative bacteria37. Thus, CAP is used as a drug to function against the infections in humans and animals. However, the overdose of CAP causes various side effects to both human and animals such as myelotoxicity, anemia, leukemia, gray-baby syndrome38. Due to the harmful side effects, CAP has been banned by the European Union and the United States39. Still, the use of CAP is not fully avoided in the animal husbandry which is due to the availability and low-cost. Therefore, the sensitive and selective determination of CAP is considered as an important concern to avoid these serious health problems. Several methods

namely high-performance

chromatography (HPLC)40,

liquid

immunoassay41,

Fluorescence method42, capillary zone electrophoresis43, and piezoelectric quartz crystal-based techniques44 have been applied for the detection of CAP. However, the above-mentioned methods are expensive and highly time-consuming techniques. In order to resolve such problems, electrochemical sensor methods have been developed for the sensing of CAP due to their simple electrode fabrication, low cost, and effective detection at a low quantity of analyte45,46. The main attention of the present work is the synthesis of GO encapsulated SmMoSe2 nanocomposite and analysis its electrochemical characteristic for the sensing of CAP. The results of the structural and morphological studies evidently confirmed the formation of GO/SmMoSe2 nanocomposite. In addition, the electrocatalytic properties of GO/SmMoSe2/GCE have been recorded by using cyclic voltammetry (CV), and linear sweep voltammetry (LSV) methods. As the recorded data, both the substitution of Sm3+ and hybridization of GO considerably enrich the electrocatalytic activity of MoSe2. Fortunately, the proposed sensor exhibited a substantial electrochemical activity towards the detection of CAP. EXPERIMENT SECTION Materials and Reagents. The samarium (III) nitrate hexahydrate (Sm(NO3)3·6H2O), molybdate disodium dihydrate (Na2MoO4·2H2O), 1,2-diaminomethane C2H4(NH2)2, and selenide powder (Se) and diphenylamine (C12H11N), graphite flakes were bought from Sigma-Aldrich. The hydrazinium hydroxide solution (N2H4.xH2O) acquired from ACROS chemicals. NaNO3 was purchased from Katayama Chemicals. H2SO4 (98%) were purchased from ACS reagent. The phosphate buffer (0.05 M) was used as the working electrolyte in all electrochemical experiment. For this, the buffer was made by mixing of 0.05 M sodium phosphate dibasic (Na2HPO4) and



sodium phosphate monobasic (NaH2PO4). The addition of NaOH/H2SO4 was followed to adjust the pH of the buffer. Hydrothermal Synthesis of SmMoSe2 Nanorod. In the synthesis of SmMoSe2 nanorod, Sm(NO3)3·6H2O (0.02 M, 0.15 g) and of Na2MoO4·2H2O (0.02 M, 0.09 g) were added in H2O (20 mL) under continuous magnetic stirring process for 10 min. Then, of Se powder (0.04 M, 0.12 g) in N2H4· xH2O (10 mL) was dropped into the above mixture whereas the dark brown color precipitation was obtained. Then, the resultant precipitation was poured into Teflon supported stainless steel autoclave and fixed the reaction temperature of about 180 oC for twelve hours in an air furnace. After the hydrothermal treatment, the reaction mixture was transfered at ambient temperature for the cooling process. Finally, the resultant product was washed with water/ ethanol several times then desiccated at 45 oC for 24 hr26,47. The synthesis mechanism of SmMoSe2 nanorod is evidently demonstrated by using the equations, Sm(NO3)3.6H2O + Na2(MoO4).2H2O → Sm(MoO4) Se (0) → Se2Sm(MoO4) + Se2- + Se → SmMoSe2 Preparation of Graphene Oxide (GO) and GO Encapsulated SmMoSe2 Nanorod (GO@SmMoSe2). The modified Hummers method was followed to chemically synthesize the graphene oxide (GO) from graphite flakes. In this process, the graphite flakes were treated with NaNO3 and H2SO4 under the reaction temperature of 0-5 oC in an ice bath with continuous stirring. After 1 h, the KMnO4 was mixed slowly into the suspension in stirring where the rate of addition was prudently monitored and kept the reaction temperature lower than 15 oC to avoid the unwanted detonation. Afterward, the resultant mixture was weakened by adding the deionized water and maintained at continuous stirring for 30 min. After the reaction time, H2O2 was slowly mixed to the above-diluted mixture and observed the color variations from dark black to light yellow. It confirms the formation of GO. Then, the reaction product was collected and then washed with 10 % of hydrochloric acid and DD water. This process was continued for sometimes until the pH of the product attained to neutral. The final slurry-like GO product was obtained and dried in the oven for overnight48.


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To synthesis GO@SmMoSe2, the as-prepared GO (0.005 g/mL) was mixed in 100 mL of DD water and kept in ultrasonication for 20 min. Consequently, 0.2 g of SmMoSe2 powder was mixed to the above GO suspension under continuous ultrasonication process. After 1 hr reaction time, the mixture solution was allowed to rest for some time. Then, the GO@SmMoSe2 precipitate was collected by simply removing the supernate. Finally, the resultant precipitate was desiccated at ambient temperature and then the resultant GO@SmMoSe2 powder was used for the further electrochemical studies49. The proposed synthesis procedure is obviously demonstrated in Scheme 1. Scheme 1. Synthesis procedures for GO@SmMoSe2 nanocomposite

Characterization Techniques. The surface morphology of as-synthesized SmMoSe2 and the successful encapsulation of GO were clearly confirmed by using transmission electron microscopy (JEOL 2100F) analysis. Field emission scanning electron microscope (FESEM) was recorded on ZEISS Sigma 300 microscope. The elemental distribution of as-synthesized SmMoSe2 was preliminarily studied by using Hitachi S-3000 H scanning electron microscope attached energy dispersive X-ray spectroscopy (EDX, HORIBA EMAX X-ACT). The crystallographic phase of the GO@SmMoSe2 nanocomposite was studied through X-ray 7 ACS Paragon Plus Environment


crystallography technique (XRD, XPERT-3 analyzer attached Cu Kα X-ray energy (K= 1.54 Å)). The electron spectroscopy measurement was studied by using an X-ray photoelectron spectrometer (Thermo scientific multi-lab 2000). The interfacial charge transfer resistance (Rct) between GO@SmMoSe2/GCE and working electrolyte was detected by performing the impedance analysis (IM6ex ZAHNER analyzer). Consequently, the electrocatalytic properties of SmMoSe2/GCE for the sensing of CAP were analyzed by performing the CV and LSV techniques by using a signal analyzer (CHI 611A). In all electrochemical experiment, glassy carbon electrode (GCE), Ag/AgCl/Sat.KCl electrode and Pt wire were performed as the operational electrode, reference electrode, and auxiliary electrode, respectively. Fabrication of GO@SmMoSe2/GCE. The electrode fabrication is an important procedure to identify the true and reproducible electrochemical information/data of an active material. In this modification process, 1 mg GO@SmMoSe2 nanocomposite in ethanol (1 mL) solution was ultrasonicated for 10 min. Consequently, the dispersed GO@SmMoSe2 was coated on polished GCE surface and kept at ambient temperature. After, the GO@SmMoSe2/GCE was rinsed by using water to eliminate the weakly attached GO@SmMoSe2 on GCE surface. The fabricated GO@SmMoSe2/GCE was applied as an operational electrode in all followed electrochemical analysis. For the comparison studies, the other operational electrodes such as GO@GCE, SmMoSe2/GCE, and MoSe2/GCE were made-up by following the similar procedures. RESULTS AND DISCUSSION Characterization of SmMoSe2 Nanorod and GO@SmMoSe2 Nanocomposite. The size and structure of as-prepared SmMoSe2 nanorod and GO@SmMoSe2 nanocomposite were clearly shown in the representative TEM images. Figure 1(A-B) evidently displays the rod-like a morphology of SmMoSe2 with even size and shape, whereas an average thickness of SmMoSe2 nanorod was measured to be 27 nm. In addition, the distorted atomic arrangement in SmMoSe2 was visually observed by using HRTEM as displayed in Figure 1C. It clearly shows the relative interplanar distance of MoSe2 around 0.65 nm, which can be assigned to lattice planes (002), respectively. Interestingly, the atomic arrangement distortion was identified, which is marked in the yellow dotted sphere. It is evidently confirmed the successful integration of heterogeneous spin states of Sm3+ into the plane of MoSe2. It is our main attention because the introduction of Sm3+ creates the distortion into the plane of MoSe2 8 ACS Paragon Plus Environment

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Figure 1 (A, B) TEM, and (C) high resolution TEM images of SmMoSe2 nanorod. and consequently increases the active sites (edge and terrace sites) for the effective electrocatalytic activity. For comparison, the TEM and high-resolution TEM images of MoSe2 sheets are displayed in Figure S1(A, B), which show the stacking arrangement of MoSe2 layers with the interplanar distance of MoSe2 as 0.67 nm. In addition, the uniform size of the asprepared SmMoSe2 nanorod was further studied by using FESEM analysis as shown in Figure S2. It clearly shows the uniform distribution of SmMoSe2 nanorods with uniform size and shape. In order to get further preliminary confirmation for the formation and study the elemental distribution of SmMoSe2 nanorod, EDX analysis was performed and demonstrated in Figure 2. The SEM image of SmMoSe2 used for EDX analysis is shown in Figure 2(A) and the corresponding EDX spectra with define and sharp characteristic peaks for major elements (Sm, Mo, and Se) present in SmMoSe2 nanorod is displayed in Figure 2(B). From this EDX spectra, the atomic percentage were measured for Sm, Mo, and Se elements to be 22.88, 24.67, and 52.45%, respectively. In addition, the possible elemental mapping of the SmMoSe2 nanorod is shown in Figure 2(C-E), which clearly indicates that the major constituents such as Sm, Mo, and Se are uniformly distributed in SmMoSe2 nanorod. The TEM images of prepared GO (Figure 3(A-C)) show its large wrinkled sheet-like structure with the thick fold, it is suggesting the successful formation of GO. Finally, Figure 3(D-F) shows the maximized TEM images of GO nanosheets encapsulated SmMoSe2 nanorod. It was interesting to observe that the nanorod like a structure of SmMoSe2 was fully covered and well integrated by wrinkle like a structure of GO nanosheets. The observed such type of GO



integration can facilitate the more active sites to SmMoSe2 and play the major role in electrochemical applications.

Figure 2 (A) SEM image used for EDX analysis (B) corresponding EDX spectra and (C-E) elemental mapping of SmMoSe2 nanorod. Further intensive crystalline information of GO@SmMoSe2 nanocomposite was estimated by performing XRD and XPS studies as shown in Figure 4. The XRD pattern of GO obtained via the modified Hummers’ method (Figure 4A) consists of a diffraction peak at 2θ = 10.22 ̊, which is strongly assigned to the crystal plane of (002)50. This is typically confirming the successful functionalization of oxygen moieties on graphite due to the oxidation process. From the XRD pattern of GO@SmMoSe2 nanocomposite, the strong intensive diffraction peaks at 2θ of 10.69 ̊, 14.95 ̊, 16.43 ̊, 23.53 ̊, 28.36 ̊, 29.35 ̊, 34.74 ̊, 42.28 ̊, 46.05 ̊, 52.59 ̊, 56.52 ̊ and 70.42 ̊ for matching atomic planes are (002), (102), (104), (010), (214), (100), (103), (006), (105), (416), (110) and (116), respectively. It strongly agrees with the hexagonal crystal phase of MoSe2 and well matched with standard data (JCPDS card number: 29-0914)51. Meanwhile, an atomic interaction of Sm3+ into the crystal lattice of MoSe2 was confirmed through the presence of diffraction peaks at 22.21 ̊, 23.53 ̊, 29.35 ̊, 34.74 ̊, 42.28 ̊, 42.93 ̊ and 49.31 ̊ for corresponding crystal planes (012), (011), (014), (015), (115), (008) and (009) which are typically related to the 10 ACS Paragon Plus Environment

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tetragonal planes of SmSe2 and the standard data (JCPDS card number; 98-042-0790). It is indicating that an effective atomic integration of Sm3+ with MoSe2.

Figure 3 Different magnified TEM of GO nanosheets (A-C) and GO@SmMoSe2 nanocomposite (D-F). Furthermore, the result of XRD was proved by using XPS analysis through observing the chemical/surface electronic states of corresponding elements (Sm, Mo, and Se). Figure 4B shows the wide XPS survey spectrum of the GO@SmMoSe2 nanocomposite, it shows the characteristic peaks for Sm 3d, Sm (Auger), Sm 4f, Mo 3p, Mo 3d, Mo 4p, Se 3d, Se 3p, Se (Auger), C 1s and O 1s at appropriate energies (eV). The observed survey spectrum clearly confirmed that the major elements (Sm, Mo, Se, C, and O) only present on the near-surface range of the GO@SmMoSe2 nanocomposite. Figure 4(C-F) displays the high-resolution scan of Sm 3d, Mo 3d, Se 3d, and C 1s, respectively. The high-resolution scan of Sm 3d (Figure 4C) shows the two major peaks for Sm 3d5/2 and Sm 3d3/2 at an energy level of 1086.5 and 1113.4 eV, respectively. It is due to the spin-orbit doublet (Sm3+ and Sm2+) states24. As given in Figure 4D, the spectrum of Mo 3d contains two main peaks at 233.04 and 236.07 eV for 3d5/2/3d3/2 states of Mo, respectively. It is strongly associated with the +3 state of Mo. The resultant peak at 238.93 is 11 ACS Paragon Plus Environment


Figure 4 XRD patterns of (A) GO nanosheets and (B) GO@SmMoSe2 nanocomposite. (C) Wide scan XPS spectra and (C-F) high resolution XPS spectra of GO@SmMoSe2 nanocomposite. 12 ACS Paragon Plus Environment

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ensuring the presence of Mo6+ due to MoO reduction in the followed synthesis process. Meanwhile, the peak at 228.66 is ascribed to Mo0. Figure 4E displays the high-resolution scan of Se 3d with spin-orbit metal selenium bonds such as Se 3d5/2/Se 3d3/2 at an energy level of 55.44 and 57.90 eV, respectively52. The high-resolution scan of deconvoluted C 1s (Figure 4F) was observed with the peaks at 285.66, 286.75, 289.56 and 291.18 eV for different oxygen moieties (C-C/C=C, C-O, C=O and O-C=O)53. Interestingly, the all followed studies have evidently confirmed the formation of the SmMoSe2 nanorod and GO@SmMoSe2 nanocomposite.

Figure 5 (A) Nyquist plot of different operational electrodes in ferricyanide system. Inset: (a) Randles circuit model (b) corresponding bar diagram for Rct vs. different electrodes. (B) CV result of different operational electrodes with CAP (0.384 mM) in N2 purged buffer (pH 7) at 50 mV s-1. Electrochemical Properties of GO@SmMoSe2/GCE. In electrochemistry, the interfacial charge transfer between solid conductor (working electrode) and an electrolyte is studied by using EIS technique. It provides the exact quantitative measurement of reaction resistance against to the electron transfer at an electrode surface. Hence, EIS measurement was carried out for various operational electrodes such as bare GCE, MoSe2/GCE, SmMoSe2/GCE, GO/GCE, and GO@SmMoSe2, where 5 mM of ferricyanide system ([Fe(CN)6]3-/ [Fe(CN)6]4-) in 0.1 M of KCl was used as the working electrolyte, the range of frequency was applied from 0.1 to 100 kHz and applied the fixed AC potential of 10 mV. Figure 5A displays the resultant Nyquist plot, in which the radius of the semicircle is equal to the electron transfer resistance (Rct). In addition, the obtained Nyquist plot curve was interpreted and equivalent Randles circuit provided in Figure 13 ACS Paragon Plus Environment


5A (insert (a)). Finally, the Rct of bare GCE, MoSe2/GCE, SmMoSe2/GCE, GO/GCE, and GO@SmMoSe2/GCE were detected to be 207.9, 150.9, 35.1, 15 and 5.83 Ω, respectively. As the result, SmMoSe2/GCE is showing the lower Rct value than that of MoSe2/GCE and bare GCE, and GO/GCE. Consequently, the GO@SmMoSe2/GCE is further showing the lower Rct than that of all other modified electrodes. The corresponding bar diagram is demonstrated in Figure 5A (insert (b)). It evidently proved the main investigation of this work that the substitution of heterogeneous spin Sm3+ and the hybridization of GO with the oxygen-containing functional groups are readily decreasing the Rct of MoSe2, which might be related to the formation of lattice distortion and increasing the active sites as discussed in TEM images (Figure 1). Finally, GO@SmMoSe2/GCE was identified as an excellent electrocatalyst with fast electron transfer activity, it is more favorable for the electrochemical detection of CAP. Electrochemical Properties of GO@SmMoSe2/GCE for Sensing of CAP. The electrochemical responses of CAP at GO@SmMoSe2/GCE was estimated and compared to that obtained at bare GCE, MoSe2/GCE, SmMoSe2/GCE, and GO/GCE. For this, CV technique was performed for bare GCE, MoSe2/GCE, SmMoSe2/GCE, and GO@SmMoSe2/GCE, GO/GCE with CAP (0.384 mM) in N2 purged buffer (pH 7) at 50 mV s-1 and the obtained CV response is shown in Figure 5B. From this CV curves, bare GCE showed only a small reduction peak (R1) for CAP at -0.69 V. Similarly, MoSe2/GCE exhibited the small reduction peak and oxidation peak at -0.72 (R1) and 0.32 V (O1) respectively. For SmMoSe2/GCE, the reduction peak and very small pair of redox peak were observed at -0.65 V (R1) and 0.05 V (R2) and 0.12 (O1), respectively. The GO/GCE also showed the reduction peak (R1) at -0.59 V with a pair of redox peak (R2 & O1). Interestingly, GO@SmMoSe2/GCE showed the very sharp reduction peak and a pair of redox peak were observed at -0.55 (R1), -0.081 (R2) and -0.023 V (O1), respectively. The R1 at GO@SmMoSe2/GCE is relatively 4.87, 2.26, 1.74, and 1.1 fold enhanced than that of bare GCE, MoSe2/GCE, SmMoSe2/GCE, and GO/GCE respectively. This obtained result is clearly demonstrating that both Sm3+ substitution and hybridization of GO are not only increasing the electronic conductivity and also enhancing the electrocatalytic activity of MoSe2.


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Scheme 2. Possible electrochemical redox process of CAP

Herein, the R1 is ascribed owing to the reduction of nitrophenyl group and formation of hydroxylamine as the result of the four electron transfer process. Meanwhile, the reversible redox peak (R2 & O1) is attributed to the oxidation of hydroxylamine into a nitroso derivative as the result of the two electron redox reaction process. It is clearly demonstrated in the scheme 2. In addition, the response of GO@SmMoSe2/GCE to CAP was recorded for the different quantity of GO@SmMoSe2 (2, 4, 6, and 8 µL) on the surface of GCE. The resultant CV curve is displayed in Figure S3. It evidently reveals that the different amount of GO@SmMoSe2 on the surface of GCE is greatly influenced the sensing of CAP. In this optimization study, the 6 µL of GO@SmMoSe2 on the surface of GCE shows the higher current of CAP. For changing amount from 2 to 6 µL is display the progressively enhancing current of CAP. At the time, 8 µL coating of GO@SmMoSe2 showed the decreased current responses due to the enormous aggregation of the GO@SmMoSe2. Thus, the 6 µL of GO@SmMoSe2 on the surface of GCE was fixed as an optimized amount of active material for promising sensing of CAP.

Figure 6 (A) CV of GO@SmMoSe2/GCE by changing CAP (0-0.384 mM) in N2 purged buffer (pH 7) at 50 mV s-1. Linear plot for (B) current vs. quantity of CAP. (C) log of current vs. log of quantity of CAP.



Influences of CAP Concentration at GO@SmMoSe2/GCE. To clarify the higher electrochemical activity of GO@SmMoSe2/GCE for the sensing of CAP, the CV studies was performed by changing the quantity of CAP from 0 to 0.384 mM in N2 purged buffer (pH 7) at 50 mV s-1 (Figure 6A). It clearly displays the increasing current (R1) by increasing the concentration of CAP respectively. The linear response between current (R1) vs. concentration is shown in Figure 6B, where the corresponding linear equation of Ipc (µA) = 90.7 CAP (mM) + 31.5 with coefficient (R2) to be 0.965. Furthermore, the linear plot for the log of reduction current vs. log of the CAP concentration is displayed in Figure 6C. The regression formula and R2 were detected as log (Ipc (µA)) = 0.878 CAP (log (mM)) + 1.97 and 0.9987, respectively. The detected slope (m) was relatively equal to 1. It is suggesting that the reaction of CAP at GO@SmMoSe2/GCE is resulting 1st order kinetic process. Thus, the GO@SmMoSe2/GCE is a promising electrocatalyst for an excellent sensing of CAP. Influences of Scan Rate and pH Level at GO@SmMoSe2/GCE for Sensing of CAP. It is known that the scan rate at the electrode and pH level of working electrolyte are playing the essential part in the electrochemical properties of an operational electrode towards the sensing of the target analyte. Thus, the optimization studies were carried out at GO@SmMoSe2/GCE for changing the scan rate and pH of the supportive electrolyte. Figure 7A displays the CV of GO@SmMoSe2/GCE for changing the scan rate from 20 to 200 mV s-1, where the concentration of CAP is 0.384 mM in N2 purged buffer (pH 7). In the observed CV curve, both irreversible reduction peak (R1) current and reversible redox peak (R2 & O1) current increased by changing the scan rate. As consequence, the Figure 7B shows the plot of reduction (R1) current vs. scan rate with Ipc (µA) = 1.2746 (mV s-1) -1.80, R2 = 0.9977. It reveals that the over-all sensing process of CAP at GO@SmMoSe2/GCE followed a surface controlled reaction. Whereas, the change in reduction (R1) potential (Epc) by changing the scan rate is attributed to the electron transfer limitation during the sensing of CAP at the proposed electrode. It is evidently shown in the plot of Epc (V) vs. log of scan rate (Figure 7C).


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Figure 7 (A) CV for GO@SmMoSe2/GCE by changing the scan rate (20 - 200 mV s-1) with CAP (0.384 mM) in N2 purged buffer (pH 7). Linear plot for (B) current vs. scan rate. (C) Epc vs. log of scan rate. Moreover, the electrocatalytic property of GO@SmMoSe2/GCE by changing the pH of the 0.05 M phosphate buffer, whereas the concentration of CAP is constant of about 0.384 mM and the scan rate is fixed of about 50 mV s-1. Figure 8A displayed the Epc was varied to the negative side by changing the pH from 7 to 11. Meanwhile, the change in pH from 7 to 3 displays the change in peak potential to a positive side. In both cases, the reduction peak current is comparatively lower than the current obtained for pH 7. It is clearly revealed that the CAP is easily hydrolyzed in concentrated alkaline or acidic conditions. And, GO@SmMoSe2/GCE also shows the sharp reduction peak (R1) for pH 7. It evidently showed in the corresponding bar diagram (Figure 8B). Finally, pH 7 was detected as the suitable pH level of 0.05 M PBS for the electrochemical detection of CAP.



Figure 8 (A) CV of GO@SmMoSe2/GCE for changing the pH of buffer with CAP (0.384 mM), (B) corresponding bar diagram for reduction current vs. pH. LSV Technique for Electrochemical Detection of CAP at GO@SmMoSe2/GCE. To clarify the advantage of GO@SmMoSe2/GCE, LSV technique was recorded by changing the concentration of CAP (0.01 - 244 µM) in buffer (pH 7) as displayed in Figure 9A. The obtained LSV show the linear increment in the current of R1 for varying the concentration of CAP. Whereas, Figure 9A

Figure 9 (A) LSV response of GO@SmMoSe2/GCE for CAP (0.01 to 244 µM) in N2 purged phosphate buffer (pH 7). (B) plot for concentration vs. current. (C) LSV curve for interference analysis with CAP (50 µM) and hundred times higher amount of interfering compounds such as


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mercury, zinc, copper, lead, cadmium, 4-nitrobenzene, 4-nitroaniline, 4-aminophenol, 4acetoaminophenol, 2-aminophenol, and 4-nitrophenol. (D) Corresponding bar diagram. (inset) shows the LSV response for initial/low concentration of CAP. The Figure 9B displayed the linear plot of current vs. concentration. The regression equation and R2 was calculated for both low and higher concentration range of CAP such as Ipc(µA) = 1.509 CAP (µM) + 9.388; R2 = 0.9596 and Ipc (µA) = 0.1899 CAP (µM) + 26.46; R2 = 0.9898 respectively. By following the resultant slope, the sensitivity and LOD of the CAP at GO@SmMoSe2/GCE were detected of about 5 nM and 20.6 µA µM-1cm-2, respectively. Finally, the resultant sensitivity and LOD were matched with already published CAP sensors and demonstrated in Table 1. It reveals that the proposed GO@SmMoSe2/GCE shows the improved electrocatalytic property with ultra-low LOD and excellent sensitivity than that of previously reported molybdenum and carbon-based electrode materials. Finally, GO@SmMoSe2/GCE is concluded as a better electrocatalyst with satisfied electrochemical properties for the electrochemical sensor applications. Table 1: Analytical parameters for sensing of CAP at different modified electrodes. Modified electrode

Method

LOD (µM)

Linear range (µM)

Ref

3DRGO/GCE

DPV

0.15

1-113

45

Ti-rGO/GCE

DPV

0.02

0.05–100

46

CV

10

10-500

54

MIP

DPV

0.01

-

55

Fe3O4/GCE

SWV

0.09

0.09-47

56

BSA-AuNCs

Fluorescence

0.03

0.10–70.00

42

LSV

0.005

0.01-244

This Work

3D CNTs@Cu NPs/GCE

GO/SmMoSe2/GCE

Reproducibility, Stability, and Selectivity of GO@SmMoSe2/GCE for the Sensing of CAP. The selectivity, reproducibility, and stability studies provide an exact electrochemical performance of GO@SmMoSe2/GCE, especially for developing to real-time detection of CAP. According to this aspect, the selectivity of the GO@SmMoSe2/GCE for the sensing of CAP was studied by using LSV in the similar experimental condition as kept in above studies. For this selectivity study, LSV technique was performed with CAP (50 µM) and hundred-times more concentration of the 19 ACS Paragon Plus Environment


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potential interfering compounds such as mercury, zinc, copper, lead, cadmium, 4-nitrobenzene, 4-nitroaniline, 4-aminophenol, 4-acetaminophen, 2-aminophenol, and 4-nitrophenol. The recorded LSV curve for interference studies is shown in Figure 9C and demonstrated the bar graph for different interfering compounds vs. Relative error in Figure 9D. The selected interference species slightly interfere with the signal obtained for CAP. The resultant interfere signal was calculated to less than 5%, which is negligible. Thus, GO@SmMoSe2/GCE showed the response (reduction peak current signal) for interference compounds also, but the signal change is less than 5% only. Thus, GO@SmMoSe2/GCE showed the considerable selectivity for the electrochemical sensing of CAP.

Figure 10 (A) LSV response for reproducibility test of GO@SmMoSe2/GCE in N2 purged buffer (pH 7) with CAP (100 µM). (B) CV curve for stability studies of GO@SmMoSe2/GCE with 0.384 mM of CAP. (C) LSV curve for real sample studies of CAP spiked milk ((a) 2.5, (b) 7 and (c) 20 µM). For the reproducibility studies of GO@SmMoSe2/GCE, five GO@SmMoSe2/GCE were fabricated and estimated the LSV for the reduction of CAP (100 µM). The bar diagram for cathodic current vs. number of electrodes is displayed in Figure 10A. As the result, the standard RSD was calculated of to be 3.8%. In addition, the stability of GO@SmMoSe2/GCE was tested by using CV technique for CAP (0.384 mM) in N2 purged buffer (pH 7) at 50 mV s-1. Herein, the CV current responses were measured before and after storage at 4 oC in the refrigerator for 7 days interval. The CV data is shown in Figure 10B, from which the proposed sensor retains 94.1% of its initial current after stored for 7 days. The all reported experimental studies are revealing

that

the

proposed

GO@SmMoSe2/GCE

showed


considerable

selectivity,

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reproducibility, and stability towards the sensing of CAP. It is so favorable for further development in real-time detection of CAP from the food sample. Detection of CAP in Milk Sample. To identify the practicability of the reported GO@SmMoSe2/GCE, which was tested in the known concentration of CAP spiked milk sample and acquired the estimated recovery value. In this experiment, a milk was diluted in buffer (pH 7) and then the recognized concentration of CAP was added into the milk sample. Subsequently, the LSV of GO@SmMoSe2/GCE was detected for CAP spiked milk sample as displayed in Figure 10C. The calculated recovery values were detected in the range from 96 to 102 % and displayed in Table 2. The real sample study implies that the considerable practicality of GO@SmMoSe2/GCE for the sensing of CAP in milk. In addition, the response of spiked milk sample was compared with the standard method and demonstrated in the Figure S4. As a result, the background signal of the real sample shifted with standard response signal due to the unknown interfering compounds in milk. However, the peak current response not affects by any unknown interferences. As expected the spiked milk sample showed significant improvement in peak current response. It indicates that the proposed sensor showed the feasible electrochemical response for CAP spiked milk sample. Table 2. Detection of CAP in spiked milk at GO@SmMoSe2/GCE (n = 3) Sample Milk

Added (µM)

Found (µM)

Recovery (%)

RSD (%)

0

Not found

Not found

Not found

2.5

2.4

96

1.18

7

6.9

98.5

1.34

20

20.4

102

1.21

CONCLUSIONS In this present investigation, SmMoSe2 nanorod and GO encapsulated SmMoSe2 nanocomposite were synthesized by following a hydrothermal process and ultrasonication techniques. The followed characterizations such as XRD, TEM, XPS, and EDX provided the strong confirmation for the substitution/introduction of Sm3+ with MoSe2 and encapsulation of GO nanosheets. The 21 ACS Paragon Plus Environment


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Sm3+ with MoSe2 and encapsulation of GO nanosheets are significantly enhancing the electrical conductivity, which was evidently proved in EIS experiment. As expected, the substitution of Sm3+ and encapsulation of GO nanosheets not only enhanced the electrical conductivity and also improved the electrocatalytic activity. The recorded CV and LSV techniques reveal the superior electrocatalytic property of GO@SmMoSe2/GCE for the sensing of CAP. As the result of this experiments, the ultra-low LOD (5 nM) and sensitivity (20.6 µA µM-1 cm-2) were calculated. Moreover,

the

feasible

GO@SmMoSe2/GCE

were

selectivity, recorded.

excellent Remarkably,

reproducibility,

better

stability

of

GO@SmMoSe2/GCE

exhibits

the

considerable recovery range from the sensing of CAP in milk, It reveals the substantial practicality of the proposed sensor.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM images of MoSe2, Different magnified FESEM images of SmMoSe2 nanorod, optimization of the amount of electrocatalysts, and the direct comparison of LSV response for spiked mil sample vs. standard method (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Shen-Ming Chen) *E-mail: [email protected] (Kuo-Chuan Ho) NOTES The authors declare no competing financial interest ACKNOWLEDGEMENTS Financial supports of this work by the Ministry of Science and Technology, Taiwan (MOST 1072113-M-027-005-MY3 to SMC) and (MOST-107-2221-E-002-173-MY3 to KCH) is gratefully acknowledged.


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Dissipative soliton generation in an all-normal dispersion

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Synthesis and Characterization of Samarium-Substituted Molybdenum

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