Fabrication of Carbon Nanofiber Incorporated with CuWO4 for Sensitive Electrochemical Detection of 4-Nitrotoluene in Water Samples

In the current work, copper tungsten oxide (CuWO4) nanoparticles are incorporated with carbon nanofiber (CNF) to form CNF/CuWO4 nanocomposite through a facile hydrothermal method. The prepared CNF/CuWO4 composite was applied to the electrochemical detection of hazardous organic pollutants of 4-nitrotoluene (4-NT). The well-defined CNF/CuWO4 nanocomposite is used as a modifier of glassy carbon electrode (GCE) to form CuWO4/CNF/GCE electrode for the detection of 4-NT. The physicochemical properties of CNF, CuWO4, and CNF/CuWO4 nanocomposite were examined by various characterization techniques, such as X-ray diffraction studies, field emission scanning electron microscopy, EDX-energy dispersive X-ray microanalysis, and high-resolution transmission electron microscopy. The electrochemical detection of 4-NT was evaluated using cyclic voltammetry (CV) the differential pulse voltammetry detection technique (DPV). The aforementioned CNF, CuWO4, and CNF/CuWO4 materials have better crystallinity with porous nature. The prepared CNF/CuWO4 nanocomposite has better electrocatalytic ability compared to other materials such as CNF, and CuWO4. The CuWO4/CNF/GCE electrode exhibited remarkable sensitivity of 7.258 μA μM−1 cm−2, a low limit of detection of 86.16 nM, and a long linear range of 0.2–100 μM. The CuWO4/CNF/GCE electrode exhibited distinguished selectivity, acceptable stability of about 90%, and well reproducibility. Meanwhile, the GCE/CNF/CuWO4 electrode has been applied to real sample analysis with better recovery results of 91.51 to 97.10%.


Introduction
4-Nitrotoluene (4-NT), also known as para nitrotoluene, is one of the noticeable intermediates of nitro-aromatic compounds (NAC). It has been widely used in the production of dyes, medicaments, synthetic fibers, pesticides, paints, rubber, and explosives [1]. In addition, 4-NT has been used as an intermediate in the synthesis of chemicals, such stilbene, p-nitrobenzoic acid, p-nitro benzaldehyde, and p-toluidine, which are consumed as starting materials for agricultural, drug, and dye industries. Owing to the extensive industrial application, 4-NT discharged into water and soil lead to contamination of soil and water causing hazardous effects on the ecological system either directly or indirectly [2]. Further, the prolonged consumption of 4-NT leads to severe health issues such as cardiovascular troubles, emphysema, dizziness, skin irritation, intoxication, endocrine disruption, and distraction in the central nervous system. Thus, the detection of 4-NT has become a serious concern [3]. So far, numerous analytical techniques, such as chemiluminescence [4], gas chromatography [5], fluorescence spectroscopy [6], liquid chromatography [7], and electrochemical analysis, have been employed in the determination of 4-NT [2]. Among all, and CNF/CuWO 4 nanocomposite have been studied by various analytical techniques. The prepared CuWO 4 /CNF/GCE electrode successfully detects 4-NT at a low LOD with high sensitivity, selectivity, repeatability, and reproducibility. The CuWO 4 /CNF/GCE electrode was tested in water samples for exhibiting good recovery results.

Materials and Methods
All the chemicals used for this experiment were analytical grade without further purification. The chemicals Copper (II) Nitrate Hexahydrate ((Cu (NO 3 ) 3 ·6H 2 O) and Sodium tungstate dihydrate (Na 2 WO 4 ·2H 2 O) were obtained from Sigma Aldrich Chemical Company, Taiwan. The PBS solution was prepared by NaH 2 PO 4 and Na 2 HPO 4 . Different characterization studies were used to explore the prepared materials. The pH level was monitored by using a Suntex pH meter at room temperature (SP-2100). Voltammetry experiments were conducted using a three-electrode system, with an Ag/AgCl electrode acting as a reference electrode, GCE as a working electrode (surface area = 0.071 cm 2 ), and a platinum wire acting as an auxiliary electrode. A scanning electron microscope was used for surface morphology analysis, and energy-dispersive X-ray spectroscopy was used for the elemental analyses (FESEM-EDX, JEOL JSM-7610F, Tokyo, Japan, and Hitachi Regulus 8100), X-ray diffraction tests (XRD, D2 Phaser, Bruker, Billerica, MA, USA, λ = 1.540 Å) were used for the phase structure. The CHI 1211B (CH Instruments Co., Austin, TX, USA) electrochemical workstation with cyclic voltammetry (CV), differential pulse voltammetry (DPV), and amperometry technique (i-t) was used for electrochemical studies.

Preparation of CuWO 4 /CNF Nanocomposite
Firstly, 1M of Cu (NO 3 ) 2 was dissolved in 20 mL of DI water and subjected to thorough stirring. Simultaneously, 1M of Na 2 WO 4 solution was prepared using the aforementioned procedure. Then, the prepared Na 2 WO 4 solution was added dropwise to the Cu (NO 3 ) 2 solution and kept for homogeneous stirring to form the CuWO 4 solution. To improve the status of the reaction, the homogeneous CuWO 4 solution is transferred to a Teflon-lined, stainless-steel autoclave and placed in the hydrothermal oven at 120 • C for 5 h. After the hydrothermal reaction, the hot autoclave was allowed to cool down to room temperature, and the collected solution was washed with ethanol and DI water several times to obtain the precipitate. Then, the precipitate was dried at 60 • C for 24 h to obtain CuWO 4 nanoparticles. Secondly, the synthesized CuWO 4 nanoparticles were incorporated with carbon nanofibers through the facile ultra-sonication method to make up CuWO 4 /CNF nanocomposite.

Fabrication of GCE/CNF/CuWO 4 Electrode
The following methodology was adopted to modify the CuWO 4 /CNF/GCE electrode. CuWO 4 nanoparticles (1 mg) and CNF (0.25 mg) were dispersed evenly in DMF solution in the ratio of 4:1 and sonicated for 1 h to generate CuWO 4 /CNF nanocomposite via noncovalent interaction. On the other hand, the GCE is polished with an alumina slurry well for 15 min and washed with ethanol through ultra-sonication to obtain a cleaned GCE surface. Following these steps, about 6 µM is drop cast onto the precleaned GCE surface and dried at room temperature to acquire CuWO 4 /CNF/GCE electrode. The fabricated electrode is imposed for further electrochemical studies of 4-NT detection.

XRD Studies
The XRD analysis was applied to obtain the crystallinity of the prepared material, which is portrayed in Figure 1. Crystallinity and phase purity of CuWO 4   . These characteristic peaks indicated that the prepared materials have better crystallinity. Moreover, the CNF was introduced into the CuWO 4 materials the same peaks were observed. However, the characteristic peak position was shifted with decreasing the peak intensity. As per the XRD result, the CNF materials have successfully been incorporated with CuWO 4 . However, we cannot observe the CNF characteristic peaks due to the lower concentration of CNF. The average crystallite size of the CuWO 4 was 48.25 nm and the CuWO 4 /CNF was~46.73 nm, respectively.  . These characteristic peaks indicated that the prepared materials have better crystallinity. Moreover, the CNF was introduced into the CuWO4 materials the same peaks were observed. However, the characteristic peak position was shifted with decreasing the peak intensity. As per the XRD result, the CNF materials have successfully been incorporated with CuWO4. However, we cannot observe the CNF characteristic peaks due to the lower concentration of CNF. The average crystallite size of the CuWO4 was ~48.25 nm and the CuWO4/CNF was ~46.73 nm, respectively. The Raman characterization was used to analyze the properties of CuWO4, CNF, and CuWO4/CNF. The Raman spectra of CuWO4 showcase intense peaks at 593, 770, and 900 cm −1 in Figure 1b. Hardcastle and Wachs's rules state that there are six internal and external modes connected to WO4 octahedra in the composite [41]. The Raman shift at 900 cm −1 matches to W-O stretching vibration mode present in the anorthic structure of CuWO4 [39]. The peaks at 1374 cm −1 and 1609 cm −1 correspond to the D and G modes of CNF spectra. The amount of disorder in the carbon material can be determined by comparing the intensity of the D peak and the G peak. Because the metal on the surface of the carbon fiber disturbs the interior atomic ordering, the ID/IG value is 1.138 [42]. Therefore, the formation of CuWO4/CNF has been confirmed using Raman spectroscopy.

Structure and Morphology Analysis by FESEM and HRTEM Studies
Figure 2a-c shows the FESEM analysis was employed to examine the structural morphology of the synthesized CuWO4 and CuWO4/CNF composite. The CuWO4 particle has a spherical shape structure with uniform size within the nanometer range. Figure 2ac clearly shows that the CuWO4/CNF composite has successfully formed, and CuWO4 materials have been placed on the CNF surface. In addition, the presence of all elements was examined by FESEM-EDX analysis with elemental mapping. The elemental mapping has revealed that all elements are uniformly distributed to the materials (Figure 2d-g). The EDX spectra of the CuWO4/CNF composite confirm the presence of Cu (16.4%), W The Raman characterization was used to analyze the properties of CuWO 4 , CNF, and CuWO 4 /CNF. The Raman spectra of CuWO 4 showcase intense peaks at 593, 770, and 900 cm −1 in Figure 1b. Hardcastle and Wachs's rules state that there are six internal and external modes connected to WO 4 octahedra in the composite [41]. The Raman shift at 900 cm −1 matches to W-O stretching vibration mode present in the anorthic structure of CuWO 4 [39]. The peaks at 1374 cm −1 and 1609 cm −1 correspond to the D and G modes of CNF spectra. The amount of disorder in the carbon material can be determined by comparing the intensity of the D peak and the G peak. Because the metal on the surface of the carbon fiber disturbs the interior atomic ordering, the ID/IG value is 1.138 [42]. Therefore, the formation of CuWO 4 /CNF has been confirmed using Raman spectroscopy.

Structure and Morphology Analysis by FESEM and HRTEM Studies
Figure 2a-c shows the FESEM analysis was employed to examine the structural morphology of the synthesized CuWO 4 and CuWO 4 /CNF composite. The CuWO 4 particle has a spherical shape structure with uniform size within the nanometer range. Figure 2a-c clearly shows that the CuWO 4 /CNF composite has successfully formed, and CuWO 4 materials have been placed on the CNF surface. In addition, the presence of all elements was examined by FESEM-EDX analysis with elemental mapping. The elemental mapping has revealed that all elements are uniformly distributed to the materials (Figure 2d    Therefore, the synthesized materials have a uniform structure with the size of nanorange. The topology of synthesized nanocomposites was studied by HRTEM analysis, which is shown in Figure 3a-d. Figure 3a shows The HR-TEM analysis revealed that the CuWO 4 materials have formed a potato-like structure with uniform size. The average particle size of the prepared CuWO 4 has~100 nm. Figure 3b shows the HRTEM image of CNF, which highlights that the CNF has a hollow-tube-like structure with a diameter of~50 nm. In addition, Figure 3c,d shows that CuWO 4 and CNF have successfully incorporated and formed the CNF/CuWO 4 composite, and CuWO 4 nanoparticles have been successfully placed on the surface of the CNF hollow tube. Therefore, the synthesized materials have a uniform structure with the size of nanorange. The topology of synthesized nanocomposites was studied by HRTEM analysis, which is shown in Figure 3a-d. Figure 3a shows The HR-TEM analysis revealed that the CuWO4 materials have formed a potato-like structure with uniform size. The average particle size of the prepared CuWO4 has ~100 nm. Figure 3b shows the HRTEM image of CNF, which highlights that the CNF has a hollow-tube-like structure with a diameter of ~50 nm. In addition, Figure 3c,d shows that CuWO4 and CNF have successfully incorporated and formed the CNF/CuWO4 composite, and CuWO4 nanoparticles have been successfully placed on the surface of the CNF hollow tube.

Electrochemical Behavior of Different Modified Electrodes
The electrochemical properties of synthesized CuWO4/CNF/GCE electrodes were probed using the CV technique. Figure 4a presents the attained CV signals of bare GCE, CNF/GCE, CuWO4/GCE, and CuWO4/CNF/GCE at the sweep rate of 50 mV/s in 5.0 mM of [Fe(CN)6] 3−/4− constituting 0.1 KCl as an electrolyte solution. It can be seen that bare GCE displays a weak, reversible, redox peak which is negligible due to lower current density. Concurrently, CNF/GCE (Ipc = −290, Ipa = 284), CuWO4/GCE (Ipc = −229, Ipa = 237), CuWO4/CNF/GCE (Ipc = −213, Ipa = 220) contributes to the well-determined redox behavior with a higher current density. These outcomes reveal that there is elevated electrochemical behavior when the GCE is modified with CuWO4/CNF nanocomposite. Further, the electroactive surface area (EASA) of the fabricated electrodes was calculated using the Randles-Sevick equation. Figure 4b shows the bar diagram of redox currents for CNF/GCE, CuWO4/GCE, and CuWO4/CNF/GCE electrodes. Figure 4c shows the CV curves for different sweep rates from 20-240 mV/s for CuWO4/CNF/GCE. Figure 4d shows the linear relation of peak current versus the square root of the scan rate. On substitution of these values in the Randles-Sevick equation, the EASA of the CuWO4/CNF/GCE nanocomposite electrode was found to be 1.107 cm 2 , which is greater than other fabricated electrodes. Hence, the CuWO4/CNF/GCE electrodes hold a greater surface area with more active sites, which can promote the electrochemical interaction toward the detection of 4-NT.

Electrochemical Behavior of Different Modified Electrodes
The electrochemical properties of synthesized CuWO 4 /CNF/GCE electrodes were probed using the CV technique. Figure 4a presents the attained CV signals of bare GCE, CNF/GCE, CuWO 4 /GCE, and CuWO 4 /CNF/GCE at the sweep rate of 50 mV/s in 5.0 mM of [Fe(CN)6] 3−/4− constituting 0.1 KCl as an electrolyte solution. It can be seen that bare GCE displays a weak, reversible, redox peak which is negligible due to lower current density. Concurrently, CNF/GCE (I pc = −290, I pa = 284), CuWO 4 /GCE (I pc = −229, I pa = 237), CuWO 4 /CNF/GCE (I pc = −213, I pa = 220) contributes to the well-determined redox behavior with a higher current density. These outcomes reveal that there is elevated electrochemical behavior when the GCE is modified with CuWO 4 /CNF nanocomposite. Further, the electroactive surface area (EASA) of the fabricated electrodes was calculated using the Randles-Sevick equation. Figure 4b shows the bar diagram of redox currents for CNF/GCE, CuWO 4 /GCE, and CuWO 4 /CNF/GCE electrodes. Figure 4c shows the CV curves for different sweep rates from 20-240 mV/s for CuWO 4 /CNF/GCE. Figure 4d shows the linear relation of peak current versus the square root of the scan rate. On substitution of these values in the Randles-Sevick equation, the EASA of the CuWO 4 /CNF/GCE nanocomposite electrode was found to be 1.107 cm 2 , which is greater than other fabricated electrodes. Hence, the CuWO 4 /CNF/GCE electrodes hold a greater surface area with more active sites, which can promote the electrochemical interaction toward the detection of 4-NT.

Different Films and Effects of Different Concentration Studies
The electrocatalytic response of CuWO4/CNF nanocomposite was evaluated using cyclic voltammetry to approach the detection of 4-NT. Figure 5A depicts the CV response of (b) bare GCE, (c) CNF/GCE, (d) CuWO4/GCE, (e) CuWO4/CNF/GCE nanocomposite modified electrode in the presence of 100 µM 4-NT, containing N2 saturated 0.05 M PBS at the fixed scan rate of 50 mV/s. (a) shows the CV response in the non-existence of 4-NT. Although, the CuWO4/CNF nanocomposite-modified electrode depicts a greater reduction-peak current towards the reduction of 4-NT, and at the same time, there is no prominent reduction-peak current perceived for the non-existence of 4-NT, while the bare GCE reduced at the lowest 4-NT reduction potential (Epc) and cathodic current (Ipc) of −0.73 V and 29.07 µA, respectively. In addition, the CNF shows the negative shift of Epc at −0.74 V and enhanced Ipc at about 45.65 µA, which accounts for the interaction of analyte molecules with a larger surface area. Furthermore, the EPC and Ipc of CuWO4 were observed at −0.76 V and 49.73 µA, distinctively, which is attributed to the highelectrocatalytic property of the CuWO4 nanoparticles. Over and above that, there was an excellent upraised Epc at −0.80 V and Ipc around 118. 83 µA ( Figure 5B). The combined effect, such as good electrocatalytic activity of CuWO4 and the larger surface area with high-electron mobility of CNF, exhibited higher Ipc. The irreversible cathodic peak occurs due to the addition of 4-NT, which is further reduced to N-p-tolyl hydroxylamine (4hydroxylamine-toluene) (Scheme 1) [43].

Different Films and Effects of Different Concentration Studies
The electrocatalytic response of CuWO 4 /CNF nanocomposite was evaluated using cyclic voltammetry to approach the detection of 4-NT. Figure 5A depicts the CV response of (b) bare GCE, (c) CNF/GCE, (d) CuWO 4 /GCE, (e) CuWO 4 /CNF/GCE nanocomposite modified electrode in the presence of 100 µM 4-NT, containing N 2 saturated 0.05 M PBS at the fixed scan rate of 50 mV/s. (a) shows the CV response in the non-existence of 4-NT. Although, the CuWO 4 /CNF nanocomposite-modified electrode depicts a greater reduction-peak current towards the reduction of 4-NT, and at the same time, there is no prominent reduction-peak current perceived for the non-existence of 4-NT, while the bare GCE reduced at the lowest 4-NT reduction potential (E pc ) and cathodic current (I pc ) of −0.73 V and 29.07 µA, respectively. In addition, the CNF shows the negative shift of E pc at −0.74 V and enhanced I pc at about 45.65 µA, which accounts for the interaction of analyte molecules with a larger surface area. Furthermore, the E PC and I pc of CuWO 4 were observed at −0.76 V and 49.73 µA, distinctively, which is attributed to the highelectrocatalytic property of the CuWO 4 nanoparticles. Over and above that, there was an excellent upraised E pc at −0.80 V and I pc around 118. 83 µA ( Figure 5B). The combined effect, such as good electrocatalytic activity of CuWO 4 and the larger surface area with high-electron mobility of CNF, exhibited higher I pc . The irreversible cathodic peak occurs due to the addition of 4-NT, which is further reduced to N-p-tolyl hydroxylamine (4hydroxylamine-toluene) (Scheme 1) [43]. To examine the sensing ability of the nanocomposite towards different concentrations of the 4-NT analyte, independent studies are executed. Figure 6a shows the apparent sharp cathodic peak of 4-NT as the concentration was raised from 10 µM to 100 µM; there was a linear responsive current. Figure 6b shows the linear plot of the reduction peak current vs. 4-NT concentration.  To examine the sensing ability of the nanocomposite towards different concentrations of the 4-NT analyte, independent studies are executed. Figure 6a shows the apparent sharp cathodic peak of 4-NT as the concentration was raised from 10 µM to 100 µM; there was a linear responsive current. Figure 6b shows the linear plot of the reduction peak current vs. 4-NT concentration. To examine the sensing ability of the nanocomposite towards different concentrations of the 4-NT analyte, independent studies are executed. Figure 6a shows the apparent sharp cathodic peak of 4-NT as the concentration was raised from 10 µM to 100 µM; there was a linear responsive current. Figure 6b shows the linear plot of the reduction peak current vs. 4-NT concentration. To examine the sensing ability of the nanocomposite towards different concentrations of the 4-NT analyte, independent studies are executed. Figure 6a shows the apparent sharp cathodic peak of 4-NT as the concentration was raised from 10 µM to 100 µM; there was a linear responsive current. Figure 6b shows the linear plot of the reduction peak current vs. 4-NT concentration.

Effect of Different pH and Different Scan Rate Studies
The supporting electrolyte pH has a remarkable effect on the electrochemical reduction of 4-NT and was examined in the presence of 100 µM with different pH values from  Figure 7a. It can be seen that the cathodic peak of 4-NT gradually increases from pH 3.0 to 7.0 and attained the maximal cathodic peak current at pH 7.0. Moreover, there is a considerable decrease in peak current on increasing pH from 7.0 to 9.0. Therefore, a high current is found in pH 7.0, which is attributed to the Zwitterion effect [44]. Figure 7b shows the calibration curve of reduction peak current vs. different pH. Hence, pH 7.0 is chosen as the optimal pH for further investigation of electrochemical studies.

Effect of Different pH and Different Scan Rate Studies
The supporting electrolyte pH has a remarkable effect on the electrochemical reduction of 4-NT and was examined in the presence of 100 µM with different pH values from 3.0-7.0 of 0.05 M phosphate-buffered solution at the scan rate of 50 mV/s, as shown in Figure 7a. It can be seen that the cathodic peak of 4-NT gradually increases from pH 3.0 to 7.0 and attained the maximal cathodic peak current at pH 7.0. Moreover, there is a considerable decrease in peak current on increasing pH from 7.0 to 9.0. Therefore, a high current is found in pH 7.0, which is attributed to the Zwitterion effect [44]. Figure 7b shows the calibration curve of reduction peak current vs. different pH. Hence, pH 7.0 is chosen as the optimal pH for further investigation of electrochemical studies.  Figure  8a displays that the cathodic peak current (Ipc) gradually raised when the scan rate increased from 20 mV/s to 260 mV/s. It can be seen that by increasing the scan rate, there is a noticeable shift in negative direction. Figure 8b shows the calculation plot reduction peak current versus the scan rate. (IPC, R 2 = 0.99). In low sweep rates, there is an easy interaction of the CuWO4/CNF/GCE electrode with the analyte 4-NT through electrostatic interaction. At the same time, for high-sweep rates, there is no sufficient time for electrostatic interaction between CuWO4/CNF/GCE electrode and the analyte 4-NT. The results show that the reduction of 4-NT is a surface-controlled process.  Figure 8a displays that the cathodic peak current (I pc ) gradually raised when the scan rate increased from 20 mV/s to 260 mV/s. It can be seen that by increasing the scan rate, there is a noticeable shift in negative direction. Figure 8b shows the calculation plot reduction peak current versus the scan rate. (IPC, R 2 = 0.99). In low sweep rates, there is an easy interaction of the CuWO 4 /CNF/GCE electrode with the analyte 4-NT through electrostatic interaction. At the same time, for high-sweep rates, there is no sufficient time for electrostatic interaction between CuWO 4 /CNF/GCE electrode and the analyte 4-NT. The results show that the reduction of 4-NT is a surface-controlled process.

Effect of Different pH and Different Scan Rate Studies
The supporting electrolyte pH has a remarkable effect on the electrochemical reduction of 4-NT and was examined in the presence of 100 µM with different pH values from 3.0-7.0 of 0.05 M phosphate-buffered solution at the scan rate of 50 mV/s, as shown in Figure 7a. It can be seen that the cathodic peak of 4-NT gradually increases from pH 3.0 to 7.0 and attained the maximal cathodic peak current at pH 7.0. Moreover, there is a considerable decrease in peak current on increasing pH from 7.0 to 9.0. Therefore, a high current is found in pH 7.0, which is attributed to the Zwitterion effect [44]. Figure 7b shows the calibration curve of reduction peak current vs. different pH. Hence, pH 7.0 is chosen as the optimal pH for further investigation of electrochemical studies.  Figure  8a displays that the cathodic peak current (Ipc) gradually raised when the scan rate increased from 20 mV/s to 260 mV/s. It can be seen that by increasing the scan rate, there is a noticeable shift in negative direction. Figure 8b shows the calculation plot reduction peak current versus the scan rate. (IPC, R 2 = 0.99). In low sweep rates, there is an easy interaction of the CuWO4/CNF/GCE electrode with the analyte 4-NT through electrostatic interaction. At the same time, for high-sweep rates, there is no sufficient time for electrostatic interaction between CuWO4/CNF/GCE electrode and the analyte 4-NT. The results show that the reduction of 4-NT is a surface-controlled process.

Differential Pulse Voltammetry Studies
Differential pulse voltammetry is extremely sensitive for the effective quantitative determination of 4-NT. Accordingly, DPV is nominated for the determination of 4-NT. Figure 9a demonstrates the electrochemical examination of 4-NT from lower to higher concentrations (10 µM to 150 µM) in 0.05 M PBS. It can be seen that a sharp and apparent peak was observed on a lower to higher concentration increase. Figure 9b shows a linear relation between the 4-NT concentration and the cathodic peak current. In addition, the limit of detection (LOD) and sensitivity was calculated from the slope of the cathodic current. The CuWO 4 /CNF/GCE electrode exhibited the value of LOD and sensitivity was calculated to be 86.15 nM and 1.1485 µAµM −1 cm −2 , respectively. Finally, the CuWO 4 /CNF/GCE electrode performance was compared with the previously reported 4-NT sensor and mentioned in Table 1. As per Table 1 Table 1 showcases a comparison of various parameters of previous works toward the detection of 4-NT by the CuWO 4 /CNF/GCE electrode.

Differential Pulse Voltammetry Studies
Differential pulse voltammetry is extremely sensitive for the effective quantitative determination of 4-NT. Accordingly, DPV is nominated for the determination of 4-NT. Figure 9a demonstrates the electrochemical examination of 4-NT from lower to higher concentrations (10 µM to 150 µM) in 0.05 M PBS. It can be seen that a sharp and apparent peak was observed on a lower to higher concentration increase. Figure 9b shows a linear relation between the 4-NT concentration and the cathodic peak current. In addition, the limit of detection (LOD) and sensitivity was calculated from the slope of the cathodic current. The CuWO4/CNF/GCE electrode exhibited the value of LOD and sensitivity was calculated to be 86.15 nM and 1.1485 µAµM −1 cm −2 , respectively. Finally, the CuWO4/CNF/GCE electrode performance was compared with the previously reported 4-NT sensor and mentioned in Table 1. As per Table 1 Table 1 showcases a comparison of various parameters of previous works toward the detection of 4-NT by the CuWO4/CNF/GCE electrode.

Interference Studies
For a newly constructed sensor, selectivity is a key component. The selectivity of the as-prepared CuWO 4 /CNF/GCE-modified electrode was examined in the presence of anions such as (a) F − , (b) Cl − , (c) NO 3 − , (d) CO 2 − , (e) CO 3 − , (f) Cl − , and cations such as (g) Na + , (h) Cr 2+ , (i) Mg 2+ , (j) Zn + , (k) NH 4 + , using the amperometric technique. Figure 10 shows that even after the 25-fold excess concentration of other interfering ions than 4-NT, there was a sharp prominent response only for the reduction of 4-NT; there is no response for the addition of other aforementioned interfering ions. Hence, the above outcomes indicate that the as-synthesized CuWO 4 /CNF/GCE electrode can be used for the selective detection of 4-NT even in the presence of other ions.

Interference Studies
For a newly constructed sensor, selectivity is a key component. The selectivity of the as-prepared CuWO4/CNF/GCE-modified electrode was examined in the presence of anions such as (a) F − , (b) Cl − , (c) NO3 − , (d) CO2 − , (e) CO3 − , (f) Cl − , and cations such as (g) Na + , (h) Cr 2+ , (i) Mg 2+ , (j) Zn + , (k) NH4 + , using the amperometric technique. Figure 10 shows that even after the 25-fold excess concentration of other interfering ions than 4-NT, there was a sharp prominent response only for the reduction of 4-NT; there is no response for the addition of other aforementioned interfering ions. Hence, the above outcomes indicate that the as-synthesized CuWO4/CNF/GCE electrode can be used for the selective detection of 4-NT even in the presence of other ions.

Repeatability, Reproducibility, and Stability Performance of GCE/CuWO4/CNF Electrode
The repeatability, reproducibility, and stability of CuWO4/CNF/GCE-modified electrodes toward the detection of 4-NT were studied. Figure 11a shows that the repeatability was examined by the addition of 100 µM of 4-NT in N2 saturated 0.05 M PBS solution at the scan rate of 50 mV/s for five successive runs. Further, Figure 11b exhibits the reproducibility studied for five different electrodes in N2 saturated 0.05 M PBS solution at the scan rate of 50 mV/s in the presence of 100 µM. In addition, Figure 11c shows the stability of the as-synthesized CuWO4/CNF nanocomposite, and was investigated for 100 cycles. The results showed that there was less than 10% current loss. From these stability studies, we can conclude that the CuWO4/CNF/GCE-modified electrode shows better repeatability, reproducibility, and stability results.

Repeatability, Reproducibility, and Stability Performance of GCE/CuWO 4 /CNF Electrode
The repeatability, reproducibility, and stability of CuWO 4 /CNF/GCE-modified electrodes toward the detection of 4-NT were studied. Figure 11a shows that the repeatability was examined by the addition of 100 µM of 4-NT in N 2 saturated 0.05 M PBS solution at the scan rate of 50 mV/s for five successive runs. Further, Figure 11b exhibits the reproducibility studied for five different electrodes in N 2 saturated 0.05 M PBS solution at the scan rate of 50 mV/s in the presence of 100 µM. In addition, Figure 11c shows the stability of the as-synthesized CuWO 4 /CNF nanocomposite, and was investigated for 100 cycles. The results showed that there was less than 10% current loss. From these stability studies, we can conclude that the CuWO 4 /CNF/GCE-modified electrode shows better repeatability, reproducibility, and stability results.

Real Sample Analysis
To investigate the practical sensibility and applicability of the samples towards the detection of 4-NT, real sample analysis was performed, which is demonstrated in Figure  12a,b, and the results are mentioned in Table 2. Before executing the real sample analysis, water samples were collected from rivers and taps. The collected water samples are

Real Sample Analysis
To investigate the practical sensibility and applicability of the samples towards the detection of 4-NT, real sample analysis was performed, which is demonstrated in Figure 12a,b, and the results are mentioned in Table 2. Before executing the real sample analysis, water samples were collected from rivers and taps. The collected water samples are centrifuged and then diluted with PBS to circumvent any interference that is present in the river and tap water samples. Then, the calculated amount of 4-NT was spiked into the solution and real sample analysis was performed using DPV. Furthermore, using the standard addition method, the recovery percentage (91.51-97.10%) was determined with an RDS value of less than 10%. Thus, the prepared CuWO 4 /CNF/GCE electrode can be a purposeful electroactive material for the detection of 4-NT in water samples with a good recovery percentage.

Real Sample Analysis
To investigate the practical sensibility and applicability of the samples towards the detection of 4-NT, real sample analysis was performed, which is demonstrated in Figure  12a,b, and the results are mentioned in Table 2. Before executing the real sample analysis, water samples were collected from rivers and taps. The collected water samples are centrifuged and then diluted with PBS to circumvent any interference that is present in the river and tap water samples. Then, the calculated amount of 4-NT was spiked into the solution and real sample analysis was performed using DPV. Furthermore, using the standard addition method, the recovery percentage (91.51-97.10%) was determined with an RDS value of less than 10%. Thus, the prepared CuWO4/CNF/GCE electrode can be a purposeful electroactive material for the detection of 4-NT in water samples with a good recovery percentage.

Conclusions
In this study, CuWO 4 was synthesized using the hydrothermal method. Furthermore, the CuWO 4 particles were incorporated into CNF using the ultra-sonication method. To confirm the formation, crystallinity, phase purity, structure morphology, and topology of the as-synthesized nanoparticles, XRD, FESEM, EDS, and HR-TEM analyses were performed. The electrochemical ability was conducted using cyclic voltammetry and differential pulse voltammetry studies. The GCE/CNF/CuWO 4 electrode possessed an appreciable sensibility towards the detection of 4-NT with a better LOD of 86.16 nM with a sensitivity of 7.258 µA µM −1 cm −2 , also, a wide linear range of 0.2-100 µM was observed. To add on, the developed sensor exhibited impressive repeatability, reproducibility, and stability for 4-NT detection. We also achieved good recovery results in real-time analysis of 4-NT. Hence, it can be applied as a potential electrochemical sensor for the detection of 4-NT in water samples.