Praseodymium Vanadate Decorated Sulfur-doped Carbon Nitride

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Energy, Environmental, and Catalysis Applications

Praseodymium Vanadate Decorated Sulfur-doped Carbon Nitride Hybrid Nanocomposite: The Role of Synergistic Electrocatalyst for the Detection of Metronidazole Thangavelu Kokulnathan, and Shen-Ming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

Praseodymium Vanadate Decorated Sulfur-doped Carbon Nitride Hybrid Nanocomposite: The Role of Synergistic Electrocatalyst for the Detection of Metronidazole Thangavelu Kokulnathan, Shen-Ming Chen* 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, ROC.

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ABSTRACT The construction of efficient and superior nanostructured materials for the precise determination of contaminants that are hazardous to the environment has gained significant attention by the scientific community. In this regard, we fabricated a nanocomposite consisting of praseodymium vanadate (PrVO4; PrV) anchored to sulfur-doped carbon nitride (PrV/SCN) and applied it to the electrochemical detection of the antibiotic drug metronidazole (MTZ). The structural and crystalline features of the as-prepared PrV/SCN nanocomposite were characterized by various analytical and spectroscopic methods. More distinctly, the PrV/SCN-nanocompositemodified glassy carbon electrode (GCE) exhibits an outstanding linear range (0.001–2444 μM), high sensitivity (1.386 μA/µM cm2), low detection limit (0.8 nM), good reproducibility and strong anti-interference ability. Notably, the PrV/SCN sensor can determine MTZ in spiked urine and water samples with high recoveries, suggesting its feasibility for real-time applications. Our findings establish PrV/SCN as a robust and promising platform for electrochemical detection. It promotes innovative design for the synthesis of novel functional nanocomposites.

KEYWORDS: Heteroatom, Binary Metal Oxide, Environment Pollutant, Antibiotics, Biological sample.


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INTRODUCTION Over the past decades, pharmaceutical compounds such as antibiotics, antimicrobials, analgesics, antipyretics, anti-inflammatories and hormones have been widely administered to both humans and animals. Amongst these compounds, antibiotics are broadly used as nonprescription/prescription therapeutics for human beings and domesticated animals as well as growth promoters in aquaculture and livestock operations.3 In particular, metronidazole (MTZ; 1(2hydroxyethyl)-2methyl-5nitroimidazole)), which belongs to a class of nitroimidazole derivatives, presents antibacterial and antiprotozoal properties.4 MTZ is widely used today for the treatment of infections caused by anaerobic bacteria (Clostridium and Bacteroides) and protozoa (trichomoniasis, giardiasis, and amoebiasis).5 MTZ is associated with several significant side effects such as seizures, genotoxicity, ataxia, gastric mucus irritation, neutropenia, carcinogenicity, encephalopathy, spermatozoid damage, mutagenicity and hematuria.6,7 During breastfeeding, it is excreted into breast milk in large amounts of up to 20%. The long-term usage of MTZ leads to drug resistance, which causes serious a health threat to human beings. Notably, due to its cost-effectiveness and high efficacy in the treatment of infections, MTZ is still illegally used as an additive in some cosmetic products by numerous commercial companies.8 Due to its widespread usage, MTZ has been frequently detected in emerging toxic contaminants from various aquatic and soil environments and poses a serious health risk to living organisms and the ecosystem.1 For these reasons, the identification of MTZ in water and soil environments is of importance to researchers. Until now, many approaches have been reported for analysis of MTZ, such as fluorescence techniques, gas chromatography, ion mobility spectrometry, polarography detection, high-performance liquid chromatography, flame ionization detection, mass spectrometry and UV spectrophotometry.6–10 However, the aforementioned methods are restrictive in being time-consuming, costly, limited in selectivity and complex. Conversely, electrochemical techniques are a superior choice due to the ease of onsite methodologies, the possibility of miniaturization, low expense, amenable monitoring, sensitivity, portability and speed which have attracted considerable attention.17–22 Nanostructured materials are poised to address societal challenges in the variety of scientific disciplines including pollution, medicine, electronics, energy and water treatment.23 Additionally, binary metal oxides (BMOs) demonstrate superior physiochemical properties when compared to those of single-phase oxides, resulting in a fast electron diffusion pathway. 3 ACS Paragon Plus Environment


Recently, metal vanadates have attracted substantial interest for use in various fields due to their multi-valency, high ionic conductivity, good chemical stability, layered structure, wide band gap, interfacial properties and their synergistic effects in multiple metal components. Especially, tetragonal zircon-type metal vanadates (AVO4, A= cations) have been broadly investigated owing to their fast electrochemical reaction rate, superior chemical stability, excellent strength, large atomic size, volumetric energy density and polarizability, which makes them an efficient candidate for a wide number of applications.28,

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As an important rare-earth metal,

praseodymium (Pr3+) promotes a large internal surface area, volumetric energy density, facilitates electrolyte infiltration and ionic conductivity. It also exhibits a large ionic radius (1.179 Å) and electron negativity (1.13 Å)

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Consequently, some studies have developed

praseodymium-based vanadates (PrVO4 and PrV) with a zircon-type structure for use in optoelectronic and luminescence applications.32,33 However, there is no available report on the electrochemical performance of PrV due to the intricacy in synthesizing a well-defined surface morphology with a large active surface area. Therefore, the synthesis of PrV with a distinctive morphology is vital task to research. Moreover, the addition of foreign elements into the semiconductor could prevent self-aggregation, increase the active surface area and enhance electron transfer channels as well as improve the stability. According to the literature, carbonaceous materials are considered as one of the best choices for supporting materials in electrochemical applications due to their peculiar topography, structural properties, superior electrical properties and chemical stability under ambient conditions.35–37 In particular, graphiticcarbon nitride (g-C3N4; CN) displays a two-dimensional (2D) morphological structure with a narrow bandgap of 2.7 eV and has received increased interest owing to its unique properties such as cost-effectiveness, thermal stability, low-cost synthesis, nontoxicity, molecular tenability and optoelectronic attributes.38–40 CN is made-up of nitrogen-substituted graphite hexatomic ring units with abundant nitrogen active sites, which show special properties applicable to photocatalysis, chemiluminescence sensing, electrochemical sensing and the oxygen reduction reaction.40 To date, many studies have suggested doping heteroatoms (such as nitrogen, sulfur, boron, iodine, etc.) into CN as an effective way to enhance the electrochemical activity.41 Moreover, heteroatom dopants can modify the electronegativity of CN atoms to facilitate electron transfer and conductivity. Sulfur (S) is an essential heteroatom due to its small atomic size, electronegativity (2.58Å), strong valence bonding, large radius (1.09 Å) and characteristics 4 ACS Paragon Plus Environment

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similar to those of carbon atoms.43 Sulfur-doped CN (SCN) has attracted intensive attention owing to its ionic conductivity, high specific surface area, desirable electronic structure, and improved electron transport rate.44 Inspired by the above achievements, we proposed a straightforward simple strategy for the rational design of a PrV/SCN nanocomposite and employed it as a highly selective electrochemical sensor for the determination of MTZ in biological samples. The as-prepared PrV/SCN nanocomposite was characterized by various analytical and spectroscopic methods. The PrV/SCN-modified electrode shows a wide linear range, very low detection limit, high selectivity, reproducibility and good sensitivity. The electrochemical performance towards MTZ detection is well improved when compared with that of previously reported analytical methods. The reasonable results in real sample determinations clearly demonstrated that the PrV/SCNbased electrochemical sensor has promise for real-time usage. EXPERIMENTAL SECTION Chemicals and Reagents. Ammonium metavanadate (NH4VO3), Praseodymium (III) nitrate hexahydrate (Pr(NO3)3·6H2O), ethylene glycol (C2H6O2), thiourea (CH4N2S), ethanol and dicyandiamide (C2H4N4) and other chemicals were purchased from Sigma-Aldrich (Taiwan) utilized without any further purification. The required supporting electrolyte used for the electrochemical studies (phosphate buffer solution; PBS) for assay was prepared by mixing NaH2PO4 and Na2HPO4 as well as pH values was adjusted by using H2SO4/NaOH. All the stock and required solutions were prepared by using de-ionized (DI) water without purification. Synthesis of PrV Hierarchical Nanostructure. First, 0.1 M of Pr(NO3)3·6H2O and 0.1 M of NH4VO3 was dissolved in 60 mL of DI H2O under constant magnetic stirring for 30 mins. After a few minutes, 7 mL of C2H6O2 was slowly added to the mixture. The above mixture was constantly heated at 80 °C for 2 h. Subsequently, the obtained precipitate was filtered, rinsed several times with DI water/ethanol and dried overnight. Finally, the dried powder was calcined at 600 °C at a rate of 10 °C /min for 3 h to obtain the PrV hierarchical nanostructure. Synthesis of SCN. Sulfur-doped carbon nitride (SCN) nanosheets were prepared via the direct thermal polymerization of dicyandiamide. Typically, 5 g of C2H4N4 and 2 g of CH4N2S were mixed together, put into a crucible covered by a lid, and then heated at 550 °C in a single-zone tubular furnace for 3 h under an argon atmosphere. Herein, CH4N2S acted as the sulfur source for 5 ACS Paragon Plus Environment


sulfur doping. During the synthesis, the –SH functional group (thiol) in thiourea reacted with the amine group in C2H4N4 to replace N atoms and form S–C bonds. Afterwards, the as-prepared SCN was collected and washed with water & ethanol to remove impurities. For comparative studies, C2H4N4 was separately used to produce carbon nitride (CN) in a similar way except without CH4N2S.

Scheme 1 The schematic synthesis procedure of PrV/SCN nanocomposite Synthesis of PrV/SCN Nanocomposite. In typical recipe, the as-prepared PrV (5 mg) and SCN (4 mg) were dispersed in DI water and were sonicated for 30 min to obtain a homogeneous suspension. The overall synthesis procedure for the PrV/SCN hybrid nanocomposite is represented in Scheme 1. To scrutinize the concentration-dependent electrochemical sensing performance, the proposed nanocomposite-modified sensors were fabricated using different ratios of PrV-SCN ranging from 1% to 5%. The detailed electrochemical activity of the different ratios is displayed in the optimization of experimental parameters section. Fabrication of PrV/SCN-modified GCE. Before modification, the electrode was well-polished with variously sized α-Al2O3 slurries for several minutes and thoroughly rinsed with DI water. Then, 6 µL of the PrV/SCN (optimized) nanocomposite suspension was dropped onto the pretreated GCE and allowed to dry in a hot-air oven. The PrV/SCN sensor should be gradually 6 ACS Paragon Plus Environment

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rinsed with DI water before performing the electrochemical analysis to remove lightly attached nanoparticles. The fabricated PrV/SCN-modified electrode was used for the electrochemical analysis.

Figure 1 (A) XRD patterns and (B) FTIR spectra of (a) PrV, (b) SCN, (c) PrV/ SCN nanocomposite. RESULTS AND DISCUSSION Physical Characterization of PrV/SCN nanocomposite. The crystallinity and phase purity of the as-prepared samples were investigated by XRD patterns. Fig. 1A displays the XRD patterns of pristine PrV, SCN and the PrV/SCN nanocomposite. Fig. 1A(a) shows the XRD pattern of the pure PrVO4 (PrV) with a tetragonal crystal structure. Furthermore, the lattice parameters were determined (a=7.36; b=7.36; c=6.46), and these are well matched to JCPDS No 78-076. No redundant peaks from other crystalline forms are detected, indicating the high crystal purity of the PrV nanostructure. Moreover, the characteristics diffraction peaks at 13.1° and 27.4° can be ascribed to SCN.45–47 The low-angle (13.1°) peak reveals the interlayer stacking, corresponding to an interlayer distance of 0.673 nm, and is indexed as the (100). Subsequently, the well-defined high-angle diffraction peak infers the interlayer stacking of the aromatic material, corresponding to an interlayer distance of 0.364 nm, and is indexed as the (002). For the PrV/SCN nanocomposite, the well-characterized diffraction peaks of PrV and SCN are observed and reveals the formation of the nanocomposite (Fig. 1A(c)). A further detailed analysis is mandatory to comprehend this phenomenon fully. The surface chemistry of the as-prepared nanocomposite was examined by using FT-IR analysis (Fig. 1B). For PrV (Fig. 1B(a)), characteristic bands at 806 and 446 cm-1 correspond to the vibrational modes of Pr and V-O bonds, respectively. In addition, weak vibration bands related to hydroxyl groups are noticed at 1631 and 3450 cm-1. 48 7 ACS Paragon Plus Environment


In the curve of SCN (Fig. 1B(b)), vibrational peaks in the range of 2700-3500 cm-1 correspond to the stretching vibrations of N-H and O-H, which are attributed to residual surface N-H groups and adsorbed H2O. Similarly, aromatic C-N heterocycle-related vibration bands are observed in the range of 1200-1630 cm-1. The observed band at 807 cm-1 is related to the breathing mode of tri-s-triazine units.49 The FTIR spectrum of the PrV/SCN nanocomposite reveals the existence of PrV and SCN in the as-prepared composite (Fig. 1B(c)). The XRD patterns and FT-IR spectra confirm the PrV /SCN nanocomposite formation. XPS was performed to determine the electronic structure of the PrV/SCN nanocomposite (Fig. 2). The XPS survey spectrum of the PrV/SCN nanocomposite (Fig. S1B) clearly demonstrates the presence of Pr, V, O, C, N, S, and the obtained results are in good agreement with the FT-IR and XRD results. No other elements or impurity are present in the survey spectrum, which clearly confirms the high purity of the as-prepared sample. The high-resolution spectrum (Fig. 2A) of the Pr 3d component consists of two peaks at binding energies of 928 eV and 948 eV that correspond to the Pr 3d5/2 and Pr 3d3/2. Likewise, the peaks at 518.7 and 523.5 eV (Fig. 2B) belong to the V 2p3/2 and V 2p1/2, while the ones at 517.8 and 522.4 eV are the satellite peaks of V 2p species.52 A broader O 1s (Fig. 2B) spectrum consists of lattice oxygen (Pr and V-O) (531.8 eV) and chemically adsorbed hydroxy groups (533.2 eV). The high-resolution O 1s spectrum (Fig. 2C) can be deconvoluted into three peaks located at 530.9 eV, 532.6 eV and 535 eV corresponding to VO43- bonds, C=O and C-OH. It can be seen that two characteristic peaks at 163 eV and 165 eV belong to the S 2p3/2 and S 2p1/2 binding energy values (Fig. 2D). The peaks at 169 and 170 eV correspond to SO3H– and SO4H–. It indicating that the S elements are partially doped into the CN.

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The C 1s spectrum (Fig. 2E) shows a significant peak at

284.7 eV, which is ascribed to the C-C/C=C bond, and two other major peaks, which are represent the C-O bond (286.6 eV) and C–N coordination of SCN/C–S–C (288.6 eV). Fig. 2F shows the N 1s peak revealing the successful doping of N in the forms of pyridinic (398.1 eV) and graphitic (401.2 eV) N species, which are also confirmed by the C-N-C peak. 42 Hence, the XPS spectra clearly clarify the formation of the PrV/SCN nanocomposite. The morphology of the as-prepared nanomaterials was analyzed by using FESEM & HRTEM. As presented in Fig. 3A & D, the PrV exhibits an oval and polyhedron-shaped particles with a smooth surface. The SCN (Fig. 3B & E) is made of a thick packing of nanosheets in a structure with stacking, wrinkles and folds at the sheet edges. In addition, the presence of pores 8 ACS Paragon Plus Environment

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in the SCN, which can be efficiently utilized to improve catalytic performance and faster ion diffusion, is observed. Subsequently, Fig. 3C, F & G illustrates the decoration of PrV particles on the SCN surface. The good attachment between the PrV particles and SCN sheets favor fast electron transport as well as strengthen electrode/electrolyte interactions. On the other hand, it is expected that the doped S atoms modify the PrV electronic structure and enhance the electrochemical reactions. Overall, this kind of morphology is expected to deliver a good electrochemical performance and improve stability. The porous SCN nanosheet-like structure and PrV particles can also provide an effective route for electrolyte penetration, enabling effective interaction between the active electrode material and the supporting electrolyte. Furthermore, the elemental mapping (Fig. 3G-M) images corresponding to the PrV/SCN nanocomposite confirm the uniform distribution of Pr, V, O, C, S and N elements in the sample. The HR-TEM (Fig. 3O) image reveals the good crystallinity of PrV in the composite. The lattice spacing is found to be 0.274 nm, corresponding to the (112) lattice plane of PrV. The SAED (Fig. 3P) pattern shows diffraction bright spots indexed to the (020), (132), (001) and (031) planes of the PrV/SCN nanocomposite. Furthermore, the formation of the PrV/SCN nanocomposite is also confirmed by the FFT (Fig. 3Q). Therefore, the SAED patterns & FFT demonstrate that the PrV/SCN nanocomposite obtained via this synthetic route is highly crystalline. The atomic structure and electronic properties of the PrV/SCN was analyzed by Raman spectroscopy, as shown in Fig. S1A. The Raman active modes of PrV (Fig. S1A(a)) at 218, 254, 368, 461, 773, 787 and 851 cm−1 are ascribed to the Eg, B1g, A1g, B1g, Eg, and A1g. The Raman result for the as-prepared PrV is in perfect agreement with the XRD and FT-IR results. All the characteristic bands of PrV particles are observed in the Raman spectrum of the PrV/SCN nanocomposite (Fig. S1A(b)). Additionally, characteristic peaks at 471, 705, 767, 977, and 1308 cm-1 are obtained for SCN nanosheets.49 Due to the high intensity of peaks assigned to PrV particles, the characteristic peaks of SCN are dramatically reduced in intensity. All the characteristic Raman peaks are red shifted, and some peaks are absent in the Raman spectrum of the PrV/SCN nanocomposite, indicating the entrapment of SCN in the PrV.



Figure 2 XPS spectra of PrV/ SCN composite (A) Pr 3d, (B) V 2p, (C) O 1s, (D) S 2p, (E) C 1s and (F) N 1s. 10 ACS Paragon Plus Environment

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Figure 3 FESEM and HR-TEM images of (A & D) PrV, (B & E) SCN, (C, F & G) PrV/ SCN composite, (G) elemental mapping for of PrV/ SCN composite, (H) Pr, (I) V, (J) O, (K) C, (L) S, (M) N, (O) lattice fringe, (P) SAED pattern, (Q) FFT of PrV/ SCN nanocomposite. ELECTROCHEMICAL CHARACTERIZATIONS Electrochemical Activity of PrV/SCN-modified Electrode. The modified and non-modified electrode surfaces and the electrochemical behaviors were investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammogram (CV) curves. The respective figures and detailed information are given in the Electronic Supporting Material (Fig. S1). The EIS and CV responses clearly confirm that the PrV/SCN-modified GCE shows a very high surface area



(0.107 cm2), low Rct value (36 Ω) and short cathodic to anodic peak current ratio (0.98) compared to those of the bare GCE, PrV/GCE, CN/GCE, SCN/GCE, PrV/CN/GCE. The optimization of working conditions. The experimental parameters were optimized to improve and enhance the electrochemical sensing performance. The working parameters of the PrV/SCN-modified electrode were optimized by the mass ratio, loading amount, accumulation time, pH value and scan rate. All the experimental parameters were examined through CV experiments.

Figure 4 (A) The various weight ratio of PrV and SCN, (B) The effect of modifier amount and (C) Influence of accumulation time on PrV/SCN modified GCE. The effect of the PrV and SCN ratio in the nanocomposite was examined. The detailed procedure was as follows: we prepared 1:1, 1:2 2:3, 5:4, and 5:5 PrV and SCN ratios for the PrV/SCN nanocomposite (Fig. 4A). Thus, different ratios of the PrV/SCN nanocomposite suspension were dripped onto a fresh electrode surface and dried in a hot-air oven. The reduction current response increases with the ratio of PrV and SCN up to 5.4. Afterwards, the electrochemical current response decreases. This result may be attributed to the fact that excessive PrV/SCN reduces the interaction between the electrolyte and electrode. Therefore, 5:4 of PrV to SCN was selected as the optimal ratio for further electrochemical experiments. The loading amount of the PrV/SCN nanocomposite was further optimized. The cathodic peak current of MTZ reduction increases gradually with the suspension volume of the nanocomposite from 2.0 to 6.0 µL and then decreases (Fig. 4B). The higher concentration of the nanocomposite shows a low current response and a wide reduction peak response. Additionally, the higher concentration reduces the mass transport as well as the electron transfer between the electrode and MTZ. As a result, the 6.0 μL PrV/SCN suspension was employed to modify the 12 ACS Paragon Plus Environment

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electrode. The accumulation time is also an important electrochemical condition that directly affects the detection owing to the adsorption of MTZ onto the electrode surface. The effect of accumulation time was examined at a time interval of every 30 s. As seen from Fig. 4C, the CV response of MTZ increases with the accumulation time from 30 to 150 s. With increasing accumulation time, more MTZ adsorbed on the PrV/SCN-modified electrode surface, so the reduction peak current significantly increases. However, the reduction peak current remains nearly constant when the accumulation time is longer than 90 s, suggesting that the amount of MTZ at the PrV/SCN-modified electrode surface tends to saturate. Therefore, 90 s was selected as the optimal accumulation time in the subsequent electrochemical experiments. Electrochemical Characterizations Electrochemical Characterization of Different Electrodes. The electrochemical activity of MTZ on the various modified and non-modified electrodes was examined by means of CV techniques. Fig. 5A displays the CVs obtained on the bare electrode and the electrodes modified with PrV, CN, SCN, PrV/CN, PrV/SCN in 0.05 M PBS (pH 7.0) containing 200 μM of MTZ. The bare electrode shows a poor response towards the electrochemical reduction of MTZ (-0.76 V, 6.5 μA). The high reduction potential and low peak current reveal the poor electron transfer towards MTZ. After electrode modification with PrV, a good improvement in the MTZ reduction peak is revealed (-0.73 V, 12.6 μA). PrV can be observed as a suitable electrocatalyst to increase the electrochemical response. Additionally, the Pr ions that have the special characteristic of incompletely occupied 4f and empty 5d orbitals can electron capture centers and increase the electrochemical activity. The CN-modified electrode shows a well-increased peak current due to its high conductivity (-0.66 V, 19.6 μA). Most interestingly, S-doped CN exhibits an enhanced current for MTZ detection compared to that of other electrodes (-0.64 mV, 22.8 μA). The introduction of SCN, with a high surface area and active sites, can facilitate the MTZ reduction reaction greatly. Such distinctive features improves the fast access of MTZ molecules to the catalytic sites and can favor electron transfer to the modified electrode. The PrV/CNmodified electrode reveals an excellent peak current of 29.5 µA with a low reduction peak potential of -0.60 V for the reduction of MTZ. The obtained results indicate that the PrV/CN nanocomposite can be used as a highly efficient electrocatalyst towards MTZ detection and that the synergistic effect of PrV and CN can evidently improve the electrochemical performance. Furthermore,

the

PrV/SCN-nanocomposite-modified 13 ACS Paragon Plus Environment

electrode

shows

superior


electroconductivity to stimulate the electron transfer between the electrolyte and electrode surface (-0.59 mV, 32.8 μA). The integration of PrV on the surface of SCN efficiently increases the active sites and promotes the formation of π- π stacking interactions within the nanocomposite. This interaction was already conformed by the various spectroscopic techniques. Additionally, EIS results shows a very low charge transfer resistance (Rct = 36 Ω) for PrV/SCN modified GCE compare to other electrodes. Meanwhile, the CV curves of ferricyanide system at proposed PrV/SCN electrode showed a high active surface area (A = 0.107 cm2), low peak potential separation (ΔEp = 74 mV) and excellent redox current response (0.98). Apparently, the low charge transfer resistance, highest redox peak response with lowest peak potential separation facilitates the rapid electron transfer towards the MTZ sensing. The electron clouds on the heteroatom interact with PrV ions and apparently accelerate the electron transfer kinetics. PrV, as a novel electrode nanomaterial, can offer high active sites, which can effectively enhance the electrocatalytic activity towards MTZ. SCN, as a supporting electrocatalyst for PrV, is attractive due to the vital electrochemical properties, such as a high surface area and electronic conductivity. Such results clearly indicate that the PrV/SCN-nanocomposite-modified GCE shows better electrochemical activity than do other materials for MTZ reduction. 12-16, 19, 20-22, 54-61 Effect of Concentration. The CV responses of the PrV/SCN-modified electrode with various additions of MTZ in N2-saturated 0.05 M PBS (pH 7.0) at a scan rate of 50 mV/s are shown in Fig. 5B, and the reduction peak potential is at approximately -0.591 V. Furthermore, the reduction peak current of MTZ using the electrochemical sensor based on the PrV/SCN-modified electrode gradually increases with the MTZ concentration from 50.0 to 500.0 µM, suggesting that increasing amounts of MTZ are recognized by the proposed sensor. The CV results show the good electrocatalytic behavior of the PrV/SCN-modified GCE towards the reduction of MTZ. The plot of the reduction peak current (µA) vs. the concentration of MTZ (µM) from 50.0 to 500.0 µM expresses a linear regression equation of Ipc = -0.0097 [MTZ/µM]-12.60 with a correlation coefficient of R² = 0.995 (Fig. 5C). This result represents the high electrochemical activity of the PrV/SCN-modified GCE towards the electrochemical reduction of MTZ. The calibration plot for the log of the cathodic peak current and the log of the MTZ concentration is shown in Fig. 5D. From the plot, the regression equation was calculated as Ipc (log (μA)) = 0.611 (log (µM)) + 0.118 (R² = 0.996). This result expressively demonstrates that the electrochemical detection of MTZ by the PrV/SCN-modified electrode follows first-order kinetics. 14 ACS Paragon Plus Environment

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Figure 5 (A) The CV curves of bare GCE, PrV/GCE, CN/GCE, SCN/GCE, PrV/CN/GCE and PrV/SCN nanocomposite modified GCE in the presence of 200 µM MTZ containing 0.05 M PBS (pH 7.0) at a scan rate of 50 mV/s. (B) The CV response of PrV/SCN modified GCE in the presence of various addition of MTZ (50–500 µM) at a scan rate of 50 mV/s. (C) The plot for reduction current (µA) vs. concentration of MTZ (µM). (D) The log of cathodic current (µA) vs. log of concentration (µM). Effect of Scan Rate. We examined the influence of various scan rates on the proposed PrV/SCN-modified GCE towards MTZ detection. Fig. 6A shows the CV curves recorded at different scan rates from 20 to 200 mV/s for the PrV/SCN-modified GCE in N2-saturated 0.05 M PBS (pH 7.0) containing 200 μM of MTZ. The reduction peak current of MTZ increases with the scan rate from low to high. Moreover, an apparent negative movement phenomenon of the 15 ACS Paragon Plus Environment


cathodic peak potential is also observed. This relocation of the cathodic potential is influenced by the size of the diffusion, layer which depends on the scan rate. The regression equation can be expressed as Ipc (µA) = 0.419 (mV/s)–23.28 (R2 = 0.996), as shown in Fig. 6B, and indicates that the cathodic reduction of MTZ is a typical adsorption-controlled process. However, the cathodic peak current is linearly proportional to the square root of the scan rate from 20 to 200 mV/s (Fig. 6C). The linear regression equation can be expressed as Ipc (µA) 7.940 (mV/s)1/2 + 10.46 (R2 = 0.990), revealing that the electrochemical behavior of MTZ is also a diffusion controlled process. The above result clearly reveals that the electrochemical kinetic behavior of MTZ at the PrV/SCN nanocomposite-modified GCE is controlled by a mixed kinetic process.20

Figure 6 (A) The CV response of PrV/SCN modified electrode in 0.05M PBS (pH 7.0) at various scan rates 20–200 mV/s. (B) The calibration plot for the reduction peak current (μA) vs. scan rate. (C) The plots of peak current of MTZ vs square root of scan rate. (D) The plot for the reduction peak potential vs. log of scan rate. 16 ACS Paragon Plus Environment

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The electrocatalytic activity of PrV/SCN/GCE was examined by calculating the charge transfer coefficient (α). To estimate the α value, a Tafel plot was plotted with the cathodic peak potential against the log value of the scan rate and is depicted in Fig. 6D. The linear relationship observed for the electrochemical reduction of MTZ is Epc (V) = -0.129 (log v (mV/s)) -0.391, with a correlation coefficient of 0.994. From this calibrated Tafel plot, the slope value (Δ) was calculated to be -0.129 and applied in equation 1.51 Δ = n (1- α) F/2.3RT

(1)

where, n, R, T, and F are their normal meanings. Finally, the α value was estimated to be 0.99 (n = 4). Moreover, the number of electrons (n) involved in the electrochemical behavior of MTZ was estimated from equation 2. Ip = (2. 99 ×105) n [(1 −α) na]1/2 ACo Do1/2 ν1/2

(2)

where A is the electrode active surface area, Co (200 µM) is the bulk concentration of MTZ, Do (cm2 s−1) is the diffusion coefficient of MTZ and the other symbols have their normal meanings. By applying all the constant values to the above equation, the n value was calculated to be four. The adsorption amounts of MTZ on the surface of PrV/SCN-nanocomposite-modified electrode was further calculated by following equation (3) ip = n2F2AΓν / 4RT

(3)

where, n is the number of electron transferred during electrochemical reaction, F is the Faraday constant (96485 C/mol), ν is the scan rare, T is the room temperature (295 K), Γ is the surface concentration of proposed electrode. The surface concentration of absorbed electro active species on PrV/SCN/GCE was estimated to be 3.1 x 10-6 mol cm2. Effect of pH. The influence of pH on the electrochemical behavior of MTZ at the PrV/SCNnanocomposite-modified electrode was examined in the pH range from 3.0 to 11.0 (Fig. 7A). Note that the reduction peak potential of MTZ shifts towards more cathodic potentials with rising pH values from 3.0 to 11.0, which can be clearly explained by the protonation-deprotonation properties of MTZ. The electrochemical reduction of MTZ at the PrV/SCN-modified GCE is clearly found to be pH-dependent. The calibration plot was drawn for the pH against the cathodic peak current (Fig. 7B). This plot shows that the highest cathodic peak current of MTZ is obtained 17 ACS Paragon Plus Environment


at the pH value of 7.0. The cathodic reduction peak potential exhibits linearity for pH values ranging from 3.0 to 11.0 and a linear regression equation of Epc = -0.046pH – 0.246 (R2 = 0.994), (Fig. 7B). The calibration slope of -0.046 V/pH is nearly close to the theoretical value of -0.059 V/pH according to the Nernst equation54, which indicates the involvement of equal numbers of electrons and protons in the electrochemical reduction of MTZ. A possible reduction mechanism is displayed in Scheme 2. Usually, the electrochemical reduction of nitro compounds follows the EC mechanism. First, the nitro group undergoes a reduction process by obtaining an electron and is converted into a nitroso group. Later, this group will convert into a hydroxylamine group by chemical reactions.

Figure 7 (A) The CV response of PrV/SCN/GCE in presence of 200 µM MTZ N2 saturated 0.05M PBS at various pH values from 3.0 to 11.0 at a scan rate of 50 mV/s. (B) The effect of pH values vs. peak current & calibration plot for peak potential vs. pH values.

Scheme 2 The electrochemical reduction mechamism of MTZ Electrochemical Detection of MTZ. To confirm the ability of the designed PrV/SCN sensor to detect MTZ, a series of various concentrations of MTZ was measured under the optimum 18 ACS Paragon Plus Environment

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electrochemical parameters, and the analytical calibrations were obtained to estimate the sensitivity. The amperometric technique is a very effective electrochemical method for the analysis of the performance of the proposed sensor due to its trace level detection limit (LOD) and high sensitivity. Fig. 8 shows the amperometric response of the PrV/SCN-modified rotating disc glassy carbon electrode (RDGCE) towards various concentrations of MTZ at a fixed cathodic peak of -0.59 V. The cathodic current response of the PrV/SCN-modified electrode is found to increase with the concentration of MTZ in the range of 0.001 - 4338 µM, which may be due to the binding of more MTZ molecules to the electrode surface. The calibration curve in the form of ipc vs the concentration of MTZ (µM) shows a linear region from 0.001 - 2444 μM with a correlation coefficient of 0.991. Notably, the reduction current decreases for the higher concentrations of MTZ because the reduction product does not discharge easily from the modified electrode surface. The higher concentration of MTZ limits the diffusion of the analyte. Therefore, it creates an internal resistance at the electrode surface. From the linear slope value, the LOD was calculated by modern IUPAC recommendation method based on the type I error and type II error.54 The LOD was determined to be 0.8 nM. Furthermore, the sensitivity (Slope of calibration plot/Active surface area) was estimated to be 1.386 μA/µM cm2. To analyze the electrochemical performance of the newly developed sensor, a comprehensive comparison of the PrV/SCN-modified electrode with previously reported modified electrodes for the electrochemical detection of MTZ is summarized in Table 1. The results presented in this table clearly show that the designed electrochemical sensor, which is based on the PrV/SCN-modified electrode as an ideal platform, exhibits excellent response characteristics compared with those of the previously reported materials. The superior electrocatalytic activity of the proposed sensor may be due to the active sites of PrV easily interacting with MTZ molecules, resulting in a more active catalyst than the individual metals. The heteroatom-doped SCN displaying excellent properties, such as a high surface area and active sites on the modified electrode surface. Moreover, the obtained excellent electrocatalytic activity attributed to the nanometer size and high conductivity of SCN integrated with PrV. PrV providing numerous anchor sites that remedy the defects in SCN, thus retaining the excellent catalytic activity towards MTZ sensing. Hence, we conclude that the PrV/SCN-modified electrode is very suitable for the electrochemical detection of MTZ.



Figure 8 Amperometric response of the PrV/SCN modified RDGCE with various injection of MTZ into N2 saturated 0.05M PBS (pH 7.0) at -0.59 potential (Inset: The linear calibration plot for reduction current vs. concentration of MTZ)


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Figure 9 The amperometric response of MTZ reduction in the presence of co-interfering compounds such as (b) ODE, (c) OXN, (d) CFX, (e) TDE, (f) 5-NIS, (g) IZE, (h) NIL, (i) NFD, (j) CAP, (k) 4-NB, (l) 4-NP, (m) DA, (n) UA and (o) Hg. (B) The stability study of PrV/SCN in presence of 200 µM MTZ N2 saturated 0.05 M PBS at a scan rate of 50 mV/s. (C) Repetitive CV response of PrV/SCN /GCE with the same electrode in N2 saturated 0.05M PBS (pH 7.0) presence of 200 μM MTZ at a scan rate of 50 mV/s. (D) CV response of PrV/SCN/GCE for three different electrodes containing 0.05M PBS in the presence of 200 μM MTZ at a scan rate of 50 mV/s. Table 1: The comparison of the proposed PrV/SCN modified electrode with previously reported MTZ electrochemical Sensors. Electrodes

Techniques

SDS-GR MWCNTs/CTS-Ni Cu-poly(Cys) BDDE DMIP Cu(II)-CNP CPE ASPCE

DPSV DPV LSV SWV DPV SWV SWV DPV

MIP/AuNPs Ni/Fe-LDH GR/IL Poly(C2B) ɑ-CD poly(AMTEOS) Pt NS/PFFF MMIP SrMoSe2 PrV/SCN

DPV CV DPV DPV DPV DPV DPV DPSV DPV i-t

Linear range (μM) 0.08–200 0.1–150 0.5–400 0.2–4.2 0.04–200 0.02–1.6 1.0-500 0.05 - 563; 753 - 2873 0.5–1000 5-161.0 0.1–25 10–400 0.5–103 0.4-200 2.5–500 0.05–1.0 0.05–914.92 0.01–2444.0

LOD (μM) 0.0085 0.025 0.37 0.065 0.0091 0.004 0.297 0.01

Sensitivity (μA µM-1 cm2) – 0.695 – – – – 1.0 –

Ref.

0.12 58.0 0.047 0.33 0.02 0.09 0.05 0.016 0.001 0.0008

– – – – – – – – 1.13 1.386

22 55 56 57 58 59 60 61 62 This work

12 13 14 15 16 19 20 21

Interference, Storage Stability, Repeatability and Reproducibility. Anti-interference is critically important in view of the feasibility of using the PrV/SCN-modified electrode. To evaluate the injection of common interfering compounds including antibiotic, nitroaromatic, 21 ACS Paragon Plus Environment


anticancer, environmental pollutant, and physiological substances as well as metal ions, ornidazole (ODE), ofloxacin (OXN), ciprofloxacin (CFX) tinidazole (TDE), 5-nitroimidazole (5NIS), imidazole (IZE), nilutamide (Nil), nifedipine (NFD), chloramphenicol (CAP), 4nitrobenzene (4-NB), 4-nitrophenol (4-NP), dopamine (DA), mercury (Hg), and uric acid (UA) were added at concentrations 10-fold higher than that of MTZ and the obtained results are illustrated in Fig. 9A. The interference results of MTZ reduction suggest that the proposed PrV/SCN sensor shows superior selectivity for the electrochemical detection of MTZ. Thus, the newly proposed PrV/SCN for the determination of MTZ in real samples is perfectly suitable. The storage stability of the PrV/SCN-modified electrode was examined by measuring the CV response to 200 μM of MTZ (Fig. 9B). The fabricated PrV/SCN-modified GCE was stored at 4o C in PBS, and further tests of the storage stability of the PrV/SCN-modified electrode were carried out after one week. The reduction current response to MTZ retains at least 95 % of the initial current response, indicating the long-term stability of the sensor. To evaluate the repeatability of the proposed sensor, a solution of MTZ at a concentration 200 μM was analyzed using the same PrV/SCN-modified electrode under the same experimental parameters (Fig. 9C). The relative standard deviation (RSD) of the measurements is 1.9 % for three consecutive assays, indicating the excellent accuracy of the fabricated PrV/SCN sensor. Furthermore, the reproducibility of individually prepared PrV/SCN-modified electrodes was also estimated (Fig. 9D). Five different PrV/SCN-modified electrodes were fabricated, and their peak current responses for 200 μM MTZ were used to estimate an RSD of 2.3%. These results clearly indicate the acceptable reproducibility of the proposed MTZ sensor. It is clear that the fabricated electrochemical sensor possesses a good anti-interference property, excellent stability, good repeatability and acceptable reproducibility. Analytical Application. The detailed electrochemical studies determining interference, reproducibility, repeatability and storage stability confirm that the PrV/SCN-modified electrode is perfectly suited to the determination of MTZ in the analysis of real samples (human urine and water). The human urine sample was collected from a healthy person. The human urine sample was diluted and centrifuged for 30 min as well as filtered to remove some of the unwanted impurities. Similarly, water was collected from the Taipei Tech campus. Afterwards, a known amount of MTZ was spiked into pretreated human urine and water samples. Furthermore, the known concentration of MTZ in the spiked human urine and pond water samples was used for 22 ACS Paragon Plus Environment

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N2-saturated electrochemical experiments, and the obtained results demonstrate that the proposed sensor exhibits an excellent detection of MTZ. The standard addition method was used to examine the recovery percentage. Good recoveries are obtained in the range between 98.5 % and 99.8 %, representing the proposed new strategy has excellent potential for detection in real samples (Table S1). The observed result demonstrates that the sensing ability of the PrV/SCNmodified electrode is not limited to analyzing standard MTZ samples but is applicable to real sample analysis with excellent reliability. CONCLUSIONS In summary, a PrV/SCN nanocomposite was prepared by using a facile and cost-effective physical method. The XRD patterns, Raman spectra, FTIR spectra, XPS, HRTEM, FESEM, EDX and elemental mapping prove the formation of the PrV/SCN nanocomposite. In addition, the electrocatalytic properties of the PrV/SCN modified electrode were examined by using several electrochemical methods. The as-fabricated PrV/SCN-modified electrode was applied to MTZ detection. The electrode exhibits an excellent electrochemical performance characterized by a LOD, high sensitivity, dynamic linear range, good reproducibility, repeatability and longterm stability. Additionally, MTZ in the spiked human urine and water samples was determined using the proposed sensor, yielding satisfactory recovery values. The observed results prove that the PrV/SCN nanocomposite is a promising probe for clinical diagnosis and environmental safety. This simple strategy for the fabrication of metal vanadate-based nanocomposites will potentially become more popular and find numerous applications in the future. ■ ASSOCIATED CONTENT Supporting Information Raman Spectra, XPS Survey, BET, electrochemical impedance spectroscopy, cyclic voltammogram for surface area analysis and real sample analysis table. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (S-M Chen) Phone: +886-2270-17147. Fax: +886-2270- 25238.



ORCID Shen-Ming Chen: 0000-0002-8605-643X ■ NOTE The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS This project was supported by the Ministry of Science and Technology (MOST 106-2113-M027-003), Taiwan, ROC. REFERENCES 1. Min, X.; Li, W.; Wei, Z.; Spinney, R.; Dionysiou, D.D.; Seo, Y.; Tang, C.J; Xiao, R. Sorption and biodegradation of pharmaceuticals in aerobic activated sludge system: A combined experimental and theoretical mechanistic study. Chem. Eng. J. 2018, 342, 211– 219. 2. Manjunath, S.V; Kumar, M. Evaluation of single-component and multi-component adsorption of metronidazole, phosphate and nitrate on activated carbon from Prosopıs julıflora. Chem. Eng. J. 2018, 346, 525–534. 3. Ramavandi, B.; Akbarzadeh, S. Removal of metronidazole antibiotic from contaminated water using a coagulant extracted from Plantago ovata. Desalin Water Treat. 2015, 55(8), 2221–2228. 4. Negi, B.; Poonan, P.; Ansari, M.F.; Kumar, D.; Aggarwal, S.; Singh, R.; Azam, A.; Rawat, D.S. Synthesis, antiamoebic activity and docking studies of metronidazole-triazole-styryl hybrids. Eur. J. Med. Chem. 2018, 150, 633–641. 5. Goolsby, T.A.; Jakeman, B.; Gaynes, R.P. Clinical relevance of metronidazole and peripheral neuropathy: a systematic review of the literature. Int. J. Antimicrob. Agents. 2017, 51, 319–325. 6.

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Praseodymium Vanadate Decorated Sulfur-doped Carbon Nitride

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