Manganese Molybdenum Oxide Micro Rods Adorned Porous Carbon Hybrid Electrocatalyst for Electrochemical Determination of Furazolidone in Environmental Fluids

The frequent occurrence of furazolidone (FZD) in environmental fluids reveals the ongoing increase in use and raises concerns about the need of monitoring it. To investigate the electrochemical behavior of FZD, a novel sensor of manganese molybdenum oxide (MMO) micro rods adorned three-dimensional porous carbon (PC) electrocatalyst was constructed. The crystalline structure and surface morphology of the MMO/PC composite was characterized by XRD, Raman, FESEM, and HR-TEM. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and amperometric(i-t) methods were used to assess the electrocatalytic activity of modified electrodes. In the presence of FZD, the as-fabricated MMO/PC modified glassy carbon electrode (GCE) performed better at lower potentials with a greater peak current than other modified GCE. These results emanate from the synergistic effect of the MMO/PC suspension on the GCE. The electrochemical behavior of the amperometric(i-t) technique was used to determine FZD. Amperometric(i-t) detection yielded linear dynamic ranges of 150 nM to 41.05 µM and 41.05 to 471.05 µM with detection limits of 30 nM. The MMO/PC hybrid sensor was also effectively used to detect FZD in environmental fluids, yielding ultra-trace level detection.


Introduction
Furazolidone, 3-(5-nitrofurfurylideneamino)-2-oxazolidinone, is a class of nitrofuran, which are Schiff's base derivatives of nitrofuraldehyde. It can be used to treat ulcers and gastrointestinal diseases which are triggered by bacteria and protozoa. However, some research has suggested that furazolidone has pathogen infection in poultry, pigs, rabbits, fish, and human health [1,2]. The economically farmed animals' death is prevented by furazolidone and it is illegally added to animal food in most of the countries [3,4]. Hence, the environmental level of usage must be monitored with highly sensitive techniques for the detection of residual furazolidone. Regular detecting approaches for the detection of furazolidone include high-performance liquid chromatography (HPLC) [5], high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS) [6], enzyme-linked immunosorbent assays (ELISAs) [7], spectrophotometry [8], and electrochemical methods [9]. Even though these methods have some benefits such as high sensitivity and accuracy, they also have disadvantages, such as being expensive, stability issues, interference issues, and being tedious [10,11]. When compared to the other methods, an electrochemical sensor is an alternative one, which is low-cost, simple to process, with elevated sensitivity and superior reproducibility, and is appropriate for real-time monitoring [12,13].
The electrochemical analysis has covered the measurement of electrical values as well as their connection to chemical variables. Electrochemical analysis has been applied in industrial quality control, environmental research, and medical analysis. In addition, electrodes have played a crucial role in electrochemical oxidation and reduction processes [14,15], particularly in their effect on electron transport between electroactive species and the electrode surface. Electrochemical behaviors for many types of electroactive substances on the surfaces of different electrodes could be seen. By varying the size and chemical composition of the electrode surface, the analytical performance may be enhanced by attaining electrocatalytic characteristics, such as hypersensitivity and selectivity [16,17]. Chemically modified electrodes would make use of the characteristics of the modifier reagents to tackle a variety of electrochemical issues. Because of their exceptional characteristics, micro rods and hybrid composites have a wide range of uses, particularly in electrode modification for electrochemical sensing [18,19].
Recently, porous carbon has been utilized for electrocatalyst materials, due to their spectacular property such as porous structure, chemical stability, plenteous conductivity, and large specific surface area for ion adsorption. Among these properties include engineering of the pore structures with a higher electrochemical accessible surface area, faster electron-transport path, and a more robust interface with the electrolyte and electrode surface [20,21]. Therefore, it has been considered as an innovative advanced material to find utility in multiple applications related to the catalyst [22,23], electronic [24], supercapacitors [25], medicines [26], and electrochemical sensor [27].
Metal molybdates are a family of inorganic materials that have gotten a lot of interest recently because of their widespread use in fields like lithium storage [28], HER [29], hybrid devices [30], luminescence property [31], and heavy metal detection sensors [32]. In particular, MnMoO 4 has received special attention due to its inexpensive cost, exceptional stability, outstanding chemical properties, low toxicity, and abundant oxidation, as well as the original price state of Mo and Mn compared to that of binary transition metal oxides such as FeMoO 4 [33], CuMoO 4 [34], NiMoO 4 [35], CoMoO 4 [36], and others. Thus, manganese molybdate (MnMoO 4 ) as a superior electrocatalytic electrode material has an electrochemical application.
Despite the performance during electrochemical redox reactions, it has portrayed very low conductivity and instability of the electron transfer rate. To get rid of this issue, combining MnMoO 4 with highly conductive carbonaceous material (such as carbon nanotubes, nanofibers, nanocarbon, and porous carbon) could portray enhanced electrical conductivity as well the electrochemical activity of the reduction reaction. Currently, the use of binary transition metal oxide decorated carbon hybrid electrocatalysts is expanding the electrochemical sensing sector. Among these, hybrid electrocatalysts have a larger active electrode surface area, improved electrochemical amplification signal, lower potential with higher current, superior sensitivity, and electrode durability. Recently, a binary transition metal oxide hybrid composite was developed with higher electrochemical performance and used various fields of application. Li [40]. Ranjith et al. reported an electrochemical sensing platform with simultaneous detection of 1D-2D MnMoO 4 -MXene nanocomposites and modified GCE [41]. In that order, we developed co-precipitation synthesis of MnMoO 4 micro rods at the initial stage. Then, the synthesized MnMoO 4 micro rods were treated with 3D network porous carbon through the ultrasonication process. The crystalline structure and surface morphology of ultrasonication-assisted MnMoO 4 micro rods in assembled porous carbon composites were examined. The as-prepared MMO/PC (1:3 ratio) hybrid electrocatalyst modified GC electrode exhibited significantly enhanced electrocatalytic performance in the detection of furazolidone. The as-prepared MMC/PC hybrid electrocatalyst revealed lower electron transfer resistance, excellent stability, selectivity, good sensitivity, and optimum environmental samples with recoveries.

Results and Discussion
The simple co-precipitation technique was used to architect MnMoO 4 micro rods. X-ray diffraction analysis confirmed the crystal structure of MMO, porous carbon, and MMO/PC composite powder. As depicted in Figure 1A, XRD patterns show architectured MMO (a), porous carbon (b), and MMO/PC composite (c) samples. The diffraction peaks of as-prepared MMO shows clearly the monoclinic phase of pure crystal structure with space group C2/m. The measured diffraction peaks and interplanar spacing matched the typical MMO diffraction pattern (JCPDS card no: 01-72-0285) [42]. All the observed major diffraction peaks at 2θ values of 12.  (100), respectively [43]. The XRD patterns of the MMO/PC composite have exhibited no change when compared to the diffraction patterns of pure MMO. Aside from a small peak shift, the intensity has been reduced, indicating confirmation of the composite. Raman spectroscopy is used to determine the bonding nature, defect, and vibration states of metal oxide on the surface. The structure of MMO (a), porous carbon (b), and the as-prepared MMO/PC composite (c) were further revealed by Raman spectroscopy, as shown in Figure 1B. The Raman active bands detected at 932, 884, 820, 350, 330, and 287 cm −1 are representative bands for MMO [44]. The stretching vibrations and bending vibrations of tetrahedral MoO 4 − ions are attributed to the higher and lower wavenumber frequency region. In addition, the porous carbon exhibited two peaks at (D-band) 1355 cm −1 and (G-band) 1585 cm −1 , respectively [45]. The D-band is associated with crystal defects and the disordered nature of porous carbon. The G-band is ascribed to the tangential stretching (E 2g ) mode of graphite. It provides robust evidence that carbon is present in the MMO/PC composite ( Figure 1B(c)). Finally, all the above-mentioned Raman active bands were present in the as-prepared MMO/PC composite.
The chemical state of elements in the as-fabricated MMO/PC composite was analyzed by XPS measurement. The wide scan survey revealed the presence of elements Mn 2p, Mo 3d, O 1s, and C 1s. They can be seen in Figure 2A. In Figure 2B the high-resolution core-level spectrum of Mn 2p is displayed which can be deconvoluted into the four peaks. The main binding energy of the Mn 2p 3/2 occurred at 641.14 eV and Mn 2p 1/2 at 653.26 eV, respectively [46]. The main binding energy of Mn 2p 3/2 and 2p 1 2 are separated by 11.6 eV, which corresponds to the Mn 2+ oxidation state. The chemical state of the Mo 3d core-level spectrum ( Figure 2C) was deconvoluted into two binding energy values at 232 and 235 eV corresponding to Mo 3d 5/2 and Mo 3d 3/2, respectively, which reveals the MoO 4 − oxidation state [46]. The O 1s binding energy peak was observed at 529 and 530 eV [47], which belongs to the oxygen species of chemisorbed and the hydroxyl group of surface adsorbed water molecules. The state of C 1s binding energy values at 284.1, 285.1, and 28.6 eV are assigned to C-C, C=C, and C-O respectively [48]. Based on the XPS result, the novel MMO/PC composite chemical state elements are confirmed. The surface morphology of as-prepared MMO, porous carbon, and as-fabricated MMO/PC composite were examined by FESEM images and are presented in Figure 3. As shown in Figure 3A,B, the formation of the micro rod-like structures of MMO can be observed at a lower magnification view in the range of 1 µm. From this view, the other structure formation has not been produced, it confirms the formation of pure micro rods MMO particles in this study. In the higher magnification view growth of micro rods has larger and thicker arrays, as seen in Figure 3C. In addition, the large and uniform 3D porous structures like carbon morphology were observed in the higher and lower magnification view image, which can be seen in Figure 3D,E. Structurally, this three-dimensional holelike structure has been used to interconnect with electrode materials, it could serve to enhance electron transfer performance. As shown in Figure 3F-H, as-fabricated MMO/PC portrays higher composite surface morphology and lower angles can be observed. The assynthesized MMO enriched with a three-dimensional interconnected porous carbon hybrid electrocatalyst could increase the electrochemical rate performance. More importantly, EDS and elemental mapping confirmed the presence of each element, Mn, Mo, O, and C in Figure 3I-N. Further, information about the internal structure of as-fabricated MMO/PC composite was obtained from HR-TEM, as shown in Figure 4. As illustrated in Figure 4A,B, it clearly shows as-synthesized micro rods structure formation. The porous carbon nature exhibits hole-like 3D porous structure formation, as demonstrated in Figure 4C. The novel properties of one-dimensional micro rods and three-dimensional porous carbon hybrid composite at different angle views are depicted in Figure 4D-G. The 3D porous carbon has the merits of high surface area, fast electron transfer path, and higher conductivity. As stated earlier, novel properties are connected with micro to enhance the electrochemical activity of the modified composite electrode. From the SAED pattern, the as-synthesized MMO singlecrystal nature was confirmed, as shown in Figure 4H. The crystal and amorphous layer can be seen and the lattice fringe, as shown in Figure 4I.

Electrochemical Characterization and Application
To investigate electrochemical properties of electron transfer resistance (R ct ) of different modified electrodes, electrochemical impedance spectroscopy was performed. From EIS data results, the semicircle occurred at a higher frequency and linear curve at a lower frequency region. As shown in Figure 5A Figure 5A shows that the EIS measurement revealed the Randles circuit model which describes the electrical network at the modified electrodes. The obtained R ct value can be seen, the as-prepared MMO/PC/GCE exhibited a much smaller charge transfer resistance value than bare, MMO and porous carbon/GCE. The results exhibited the as-prepared MMO/PC/GCE has excellent conductivity and enhances fast charge transfer processes. The investigation of the electrocatalytic activity of the catalytic material was performed with cyclic voltammetry. As shown in Figure 5B Due to an open pore-like 3D porous carbon having a large active surface area with a nanoporous structure network combination with one-dimensional hybrid rods, it would enhance electronic conductivity between MMO/PC/GCE and FZD species at the electrode/solution interface and their synergy effect. The FZD detection of a reduction peak in the reverse scan and the lack of an oxidation peak demonstrated that the system is irreversible. This represents the reduction of the nitro group transformed into its hydroxylamine group with four electrons (4e − ) and four protons (4H + ) transfer reaction processes. Finally, the EIS and CV data demonstrate that MMO/PC/GCE is an effective and promising electrode material with strong catalytic activity for detecting FZD.
The electrocatalytic activity of the MMO/PC/GCE was evaluated by CV at a sweep rate of 50 mV/s in the presence of different concentrations of FZD ranging from 30 to 200 µM in 0.1 M PBS (pH-7). As shown in Figure 6A, the increasing FZD concentration resulted in a significant increase in cathodic peak current, it indicates the extraordinary electrocatalytic activity at the MMO/PC/GCE electrocatalyst. The corresponding calibration linear plot is displayed in Figure 6B, the linear plot between the concentration of FZD vs. cathodic current, yielding the following linear equation I pc [µA] = -0.092 [µM] -9.350, and the correlation coefficient R 2 = 0.99 respectively. The observed results prove that the as-prepared MMO/PC/GCE is electrocatalytic active sensing towards FZD. To investigate the reaction kinetic composite, the CV curve ( Figure 7A) of MMO/PC/G CE in FZD 200 µM was observed at different scan speeds ranging from 20 to 300 mV/s. The obtained result shows that changing the scan rate linearly increases the cathodic current appropriately. The calibrated linear plot is shown in Figure 7B, the square root of scan rate vs. cathodic current (µA) with corresponding linear regression equation and correlation coefficient expressed as I pc = -3.855 (ν) + 7.711, R 2 = 0.995. It correctly identifies FZD reduction as a diffusion-controlled electron transfer mechanism on the surface of an MMO/PC/GCE. Further, the CV method was conducted to analyze the electrochemical activity of the asprepared MMO/PC/GCE under acidic to basic medium control. Figure 8A shows the CV curve exhibit for FZD 100 µM at pH (3,5,7,9,11) electrolyte medium on the MMO/PC/GCE at a sweep rate of 50 mV/s. It shows that at pHs 3.0 to 11.0, peak potential shifts towards more negative, also in the basic medium the peak current is reduced. Because the presence of additional hydroxide ions (-OH) may promote the further reduction of hydroxylamine (-NH-OH) into its equivalent nitroso (-NO) compound. Figure 8B shows the bar diagram of the pH 3.0-11.0 calculated cathodic peak current, this result suggested the maximum cathodic peak current attained at pH 7.0 (0.1 M PBS) neutral medium, which was selected as optimum for further experiments.

Amperometric(i-t) Determination of FZD at MMO/PC Composite
The amperometric(i-t) current responses were obtained in a three-electrode system. The MMO/PC modified rotating ring disk (RRD) electrode as working electrode, Ag/AgCl as reference electrode, and platinum as counter electrode and 0.1 M PBS (pH-7.0) was used as electrolyte solution up to 10 min and N 2 gas saturated. The electrode input potential was held at -0.38 V, and the RRD electrode rotation speed was fixed at 1800 rpm. Figure 9A shows the amperometric(i-t) current response obtained for MMO/PC RRD electrode upon the successive addition of FZD various concentrations (1,10 mM) at every 50 sec interval. The obtained result revealed the cathodic current response increased with increasing FZD concentration. Additionally, the calculated steady-state current was less than 6 s, suggesting that the as-fabricated MMO/PC RRD electrode had a fast and highly sensitive response towards the reduction of FZD. Figure 9B shows (I and II) the linear range obtained for FZD detection from 150 nM to 41.05 µM and 41.05 to 471.05 µM with a regression equation of I pc = −0.100 [µM] − 1.108; correlation coefficient R 2 = 0.991 and I pc = −0.024 [µM] − 4.486; correlation coefficient R 2 = 0.991, respectively. The limit of detection and sensitivity was derived from the formula of (LOD = 3σ/m). Here, (σ) is the standard deviation of the steady-state current signal; (m) indicates a lower linear range slope value and the calculated results of LOD is 30 nM with an obtained sensitivity of 0.510 µA µM −1 cm −2 towards FZD. The MMO/PC hybrid composite electrode towards detection of FZD result is much better when compared previously reported and summarized in Table 1. Table 1. Previously reported modified detection limit of FZD to compare to our result.

Selectivity and Stability Test
The amperometric(i-t) test was performed to study selectivity test towards MMO/PC modified RRD electrode for FZD detection. The interferent species of nitro compounds ((a) nitrofuratonin (b) nimisulide (c) ornidazole (d) metronidazole (e) 2-nitroaniline), metal ions (f) Fe 2+ , (g) Mg 2+ , (h) pb 2+ , (i) Zn 2+ , (j) Na + , and biological compounds (k) uric acid (l) isoniazid, (m) dopamine, (n) clorampinical, (o) fructose, (p) naproxen, and (q) glucose were added every 50 s of intervals in 0.1 M PBS (pH 7.0). As can be seen in Figure 9C, there is no significant current response except the FZD detection current. Therefore, the proposed sensor showed excellent anti-interference capability, and the as-fabricated MMO/PC modified RRD electrode proved highly selective to FZD sensing. The electrode stability test was carried out in the same experimental condition as the amperometric(i-t) steady-state current response by adding 100 µM FZD of the first three injections. The steady-state current response was detected up to 2750 s after the initial three additions of FZD, and it retained 98% of its original current signal, as shown in Figure 9D. This result indicates as-prepared MMO/PC modified RRD electrode has excellent operation stability towards the detection of FZD.
2.4. Repeatability, Reproducibility, and Real-Sample Analysis Figure 10A displays the MMO/PC/GCE, which have been employed to detect 200 µM FZD by CV consecutive four times to evaluate its repeatability. The relative standard deviation (RSD) of their CV current was calculated to be 1.32% which indicates MMO/PC/GCE has excellent retest reliability. To estimate, four individual MMO/PC/GCE were prepared and analyzed ( Figure 10B). The calculated RSD value was 1.47%. This result demonstrates the as-fabricated MMO/PC composite has a good reproduction ability. In addition, the real-time monitoring analysis is performed by the amperometric(i-t) method. Real-sample analysis of the fabricated MMO/PC modified RRD electrode electrochemical sensor was investigated by detecting FZD in the lake and river water samples. In this experiment, river and lake water samples were obtained from the local area of Taipei City, Taiwan (ROC). The collected samples were filtered with a PVDF syringe filter. After that, the obtained samples were diluted with 0.1 M PBS (pH 7), then we conducted the electrochemical analysis. As shown in Figure 10C,D, the as-prepared MMO/PC modified RRD electrode was performed and spiked peaks are obtained. The obtained recovery result is given in Table 2, it is clearly indicated that the modified electrode has a potential application for the detection of FZD in environmental water samples. This validates the accuracy of this approach and also demonstrates that the suggested electrochemical sensor has adequate dependability for determining FZD in environmental water samples.  HPO 4 , and all other chemicals were obtained from Sigma Aldrich Pvt. All chemical reagents were utilized without further purification. Deionized water (DI water-18.2 MΩ millipore) was used to synthesize and all electrolyte buffer solutions.

Synthesis Procedure of MMO Micro Rods
The facile co-precipitation method has been utilized for the synthesis of MnMoO 4 micro rods. Initially, manganese chloride tetrahydrate (0.18 g) and ammonium heptamolybdate (1.74 g) were combined separately with 30 DI water and constantly agitated for 30 min. The subsequently prepared two solutions were mixed dropwise one by one. Then, the stoichiometric sodium hydroxide solution was added to the above-mentioned mixing solution. The precipitate was formed and continuously stirred for up to 3 h. The obtained MnMoO 4 precipitate was centrifuged by washing with millipore DI water and ethanol several times. Continuously, the obtained product was dried at 50 • C in a heat oven for up to 24 h. Finally, the collected MnMoO 4 (denoted as MMO) powder was calcined in an air oven at 600 • C for up to 3 h.

Fabrication of MMO@PC Composite
Next up, MnMoO 4 micro rods assembled porous carbon composites were prepared through a wet chemical synthesis approach. The 50 mg MnMoO 4 micro rods and 10 mg porous carbon (PC) were added into the 10 mL (1:1 ratio of water and ethanol) solution. Then the mixture solution was ultrasonicated for 1h (34/70 kHz frequency and 60 W power of ultrasonication at 20 • C). Afterward, the obtained solution was dried at 50 • C in a heat oven for 24 h. Finally, the collected MnMoO 4 micro rod assembled porous carbon composites (mentioned as MMO/PC composite) product was used for characterization. Scheme 1 represents the preparation of MMO/PC/GCE and the reduction mechanism of FZD processes. Scheme 1. The schematic representation of synthesis and electrochemical reduction mechanism of FZD.

Preparation of MMO/PC/GCE
Before the modification processes, the blank GCE was precleaned by 0.05 M alumina fine powder as stated in the draft. Then the blank GCE was washed with DI water, ethanol, and dried at room temperature. Then the as-prepared MMO/PC composite powder of about 1 mg was mixed with 1 mL of DI water. After, the solution was sonicated for 30 min. From that obtained solution, the 6 µL slurry was dropped cast onto the mirrorlike surface on the GCE. Then, the MMO/PC composite modified GCE was dried at ambient temperature. Finally, the obtained MMO/PC/GCE was performed for further electrochemical applications.

Characterization and Electrochemical Measurement
The powder X-ray diffraction (XRD, PANalytical X'pert PRO MRD, Almelo, Netherlands) with Cu K radiation (λ = 0.15406 nm) was used to examine the crystal structures of the as-prepared MMO/PC composite samples. Raman spectra were recorded with a micro-Raman Dongwoo Ramboss 500i with an incident laser at the wavelength of 514 nm. The X-ray photoelectron spectroscopy (XPS) experiment was undertaken on the surface composition of MMO/PC composite to investigate the chemical state of components. Field emission scanning electron microscopy (FESEM, JSM-7610F, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy were used to analyze the surface morphology of MMO micro rods and porous carbon (HR-TEM, Shimadzu JEM-1200 EXSTEM, Tokyo, Japan). The electronic conductivity and electron transfer activity of the as-prepared MMO/PC composite were investigated by electrochemical impedance spectroscopy (EIS), IM6ex ZAHNER instrument from Kronach, Germany. The electrocatalytic activity of the MMO/PC composite was investigated and measurement was carried out by CV (cyclic voltammetry; CHI1205A, and CHI 750A CH Instruments, Austin, TX, USA) and three-electrode system of glassy carbon electrode (GCE) as a working electrode (electrode area = 0.07 cm 2 ), Ag/AgCl was used as reference electrode and platinum electrodes used as the counter electrode.

Conclusions
The absence of inexpensive and reliable sensing instruments is making it difficult to evaluate water quality in environmental fluids. In this present work, we have prepared MMO micro rods that were synthesized in the co-precipitation method. The sonication assisted the MMO/PC and a composite was obtained. The as-prepared MMO/PC were characterized by XRD, RAMAN, XPS, FESEM, and HR-TEM. The as-prepared MMO/PC modified GCE was utilized for the highly sensitive and selective detection of FZD. As per CV results, the MMO/PC/GCE was showed a very good analytical performance in an optimized condition. The amperometric(i-t) method was utilized for the as-prepared MMO/PC modified RRD electrode resulting in a good linear range, lower LOD, higher stability, and reliability. The MMO/PC modified RRD electrode in practical application results exhibited good recovery obtained in environmental fluids. The proposed sensor is a significant step in developing a simple and low-cost method for detecting FZD in environment water resources on-site.