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Article

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

by
Ganesh Abinaya Meenakshi
1,2,
Subramanian Sakthinathan
1,2 and
Te-Wei Chiu
1,2,*
1
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan
2
Institute of Materials Science and Engineering, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan
*
Author to whom correspondence should be addressed.
Sensors 2023, 23(12), 5668; https://doi.org/10.3390/s23125668
Submission received: 15 May 2023 / Revised: 5 June 2023 / Accepted: 8 June 2023 / Published: 17 June 2023
(This article belongs to the Special Issue Electrochemical Sensors and Applications)
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Versions Notes

Abstract

:
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%.

Graphical Abstract

1. 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, electrochemical analysis has various merits such as cost-effectiveness, online monitoring, accuracy, easy fabrication of electrodes, high sensitivity, and quick response [2].
Previous studies on electrochemical detection of 4-NT were engaged with carbon-based metal/metal oxide ion composite materials [8], molecularly imprinted polymers [9], metallic/mixed-metallic nanoparticles [10], and aptamer-based materials [11]. Nevertheless, most of these catalysts possess a limit of detection (LOD) at the micromolar level, electrode fouling, immoderate cost, and poor selectivity [12]. To overcome this issue, heterogeneous catalysts are considered a suitable material for electrochemical sensing [13]. In recent years, bimetallic tungstate is used in various fields as water splitting [14], photocatalysis [15], electrochemical sensors [16], hydrogen-evolution reactions [17], supercapacitors [18], and scintillators [19]. Different kinds of metal tungstate have been reported in the sensor application [20]. Metal tungstate has a superior electrical conductivity due to the occupation of the W atoms (10−7 to 10−3 S cm−1). CuWO4 is one of them, an n-type semiconductor with a small band gap of 2.2 eV and advantageous qualities such as being inexpensive, readily available, nontoxic, cost-effective electrodes for clean, well-renewable, and maintainable energy devices, as well as stable electrode material for electrocatalytic applications [21,22,23]. The water-splitting and stable photocatalytic properties of CuWO4 decorated with polypyrrole were reported by P. H. Dinh et al. [24]. CuWO4 has been used to modify an electrode material due to the wide surface area, high resistance, and high sensitivity, which opens up the possibility of investigating the potential of this material for use in electrochemical sensing applications. By utilizing the hybridization of carbon-based materials, the high-resistive characteristics of CuWO4 were lowered [25,26,27].
To improve the sensitivity and selectivity of the bare GCE electrode, metal oxides are combined with different carbonaceous materials, which can adequately promote charge transfer through a synergetic effect [28]. Generally, carbonaceous nanomaterials have a large specific surface area and a high surface–volume ratio. Moreover, these materials establish excellent biocompatibility, a massive rate of electron transfer, electrocatalytic properties, and interfacial adsorption features, which makes them distinguished from other materials in electrochemical sensing [29]. For instance, carbonaceous materials, viz., fullerenes, graphite, graphene, graphene oxide, carbon nano horns, carbon nanofibers (CNF), carbon nanotubes (CNT), and carbon quantum dots, were reported [30]. For electrochemical sensing applications, the combination of bimetallic tungstate with carbon-based materials, such as graphene oxide (GO), carbon nanofibers (CNFs), carbon nanotubes (CNTs), ordered mesoporous carbon (OMC), and graphitic carbon nitride (g-C3N4), is becoming more and more common [31,32,33].
In recent times, carbon nanofiber has emerged as an ideal material because of its properties, such as good electrical and thermal conductivity [34,35]. Moreover, the CNF can easily bond with high-conductive metal materials to generate composite materials, which are often used as an electrocatalyst for electrochemical sensing [36]. Jeyaraman et al. have reported the CuWO4/Sg-C3N4 by ultrasonication method for the electrochemical detection of nitrofurazone [37]. Pavithra et al., have investigated the electrochemical sensing and photocatalysis of nitrofurazone by the ZnWO4-CuWO4 composite [38]. The electrochemical detection of nicotine using the composite CuWO4/rGO was demonstrated by A. Karthiga et al. [39]. CuWO4@MoS2 and chitosan-Au nanoparticles were proposed to detect cortisol by C. Nong et al. [40]. Subsequently, it is essential to fabricate a nanocomposite with remodeled electrodes together with a preferable surface area, sensitivity, reproducibility, and more notable electrocatalytic materials for the detection of 4-NT.
To achieve the aforementioned properties, GCE is modified with CNF/CuWO4 nanocomposites made to act as an electrochemical sensor for the detection of 4-NT. Herein, we reported the preparation of a CuWO4 composite by a facile, novel, and inexpensive hydrothermal method. On the other hand, CuWO4/CNF nanocomposites were prepared by a simple ultrasonication method which is less time-consuming, more convenient, free of solvent and a more productive method. To my best of knowledge, CuWO4/CNF nanocomposites have not reported yet towards the detection of 4-NT. The prepared CNF, CuWO4, and CNF/CuWO4 nanocomposite have been studied by various analytical techniques. The prepared CuWO4/CNF/GCE electrode successfully detects 4-NT at a low LOD with high sensitivity, selectivity, repeatability, and reproducibility. The CuWO4/CNF/GCE electrode was tested in water samples for exhibiting good recovery results.

2. Materials and Methods

All the chemicals used for this experiment were analytical grade without further purification. The chemicals Copper (II) Nitrate Hexahydrate ((Cu (NO3)3·6H2O) and Sodium tungstate dihydrate (Na2WO4·2H2O) were obtained from Sigma Aldrich Chemical Company, Taiwan. The PBS solution was prepared by NaH2PO4 and Na2HPO4. 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 cm2), 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.

2.1. Preparation of CuWO4/CNF Nanocomposite

Firstly, 1M of Cu (NO3)2 was dissolved in 20 mL of DI water and subjected to thorough stirring. Simultaneously, 1M of Na2WO4 solution was prepared using the aforementioned procedure. Then, the prepared Na2WO4 solution was added dropwise to the Cu (NO3)2 solution and kept for homogeneous stirring to form the CuWO4 solution. To improve the status of the reaction, the homogeneous CuWO4 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 CuWO4 nanoparticles. Secondly, the synthesized CuWO4 nanoparticles were incorporated with carbon nanofibers through the facile ultra-sonication method to make up CuWO4/CNF nanocomposite.

2.2. Fabrication of GCE/CNF/CuWO4 Electrode

The following methodology was adopted to modify the CuWO4/CNF/GCE electrode. CuWO4 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 CuWO4/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 CuWO4/CNF/GCE electrode. The fabricated electrode is imposed for further electrochemical studies of 4-NT detection.

3. Results

3.1. 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 CuWO4 and CuWO4/CNF were compared. The CuWO4 exhibited related strong diffraction peaks were attributed at 2θ values of 15.28°, 18.92°, 19.02°, 22.8°, 23.3°, 24.13°, 26.01, 28.63°, 30.15°, 30.91°, 31.67°, 32.0°, 34.42°, 35.61°, 36.48°, 36.48°, 36.92°, 38.55°, 39.86°, 41.16°, 42.93°, 48.70°, 51.55°, 53.29°, and 53.54°, corresponding to (010), (001), (100), (110), (0 1 − 1), (011), ( 1 − 01), ( 1 − 1 − 1), (111), (020), ( 1 − 11), (1 1 − 1), (120), (021), (021), (002), ( 1 − 20), (210), ( 1 − 02), ( 1 − 21), (130), (221), (-202), and ( 1 − 30). 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.

3.2. 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 2a–c 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 (32.2%), O (39.1%), and C (10.3%) atom base in mass fraction of C and other elements, as shown in Figure 2h.
Therefore, the synthesized materials have a uniform structure with the size of nano-range. 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.

4. Electrochemical Reduction Studies of 4-NT on the GCE/CNF/CuWO4 Electrode

4.1. 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 cm2, 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.

4.2. 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 high-electrocatalytic 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 (4-hydroxylamine-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.

4.3. 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.
The effect of the scan rate was determined by the cyclic voltammetry technique with a CuWO4/CNF/GCE electrode in 0.05 M PBS, on the addition of 100 µM of 4-NT. 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, R2 = 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.

4.4. 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−1cm−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, Mohammad. A. et al. used the ZnO-CN/GCE electrode, which exhibited the LOD of 100 nm by employing the CV technique [43]. However, Rani. S. et al. reported using AgNPs/Au electrodes with a linear range of 0.010–0.100 and LOD of 92 nm, using an amperometric technique [45]. Moreover, Yuan. S et al. used an NH2-Fe-MIL-88B@OMC-3 electrode and displayed the linear range of 20.00–225.0 with the LOD 8000 nm under the DPV technique [46]. Furthermore, Ahmad. K. et al. fabricated a GCE electrode with α-MnO2 that reached the linear range of 0.162–48.80 with an LOD of 144.0 using the CV technique [47]. Yang. R et al. obtained the linear range of 0.5–4, 0.2–1.2 and LOD of 442, 282.2 using the Pn-MWCNTs/Po-MWCNTs electrode [48]. On comparing the aforementioned studies, this CuWO4/CNF/GCE electrode shows a wide linear range of 0.2–100 with better sensitivity and LOD of 86.16 nM. Table 1 showcases a comparison of various parameters of previous works toward the detection of 4-NT by the CuWO4/CNF/GCE electrode.

4.5. 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) Cr2+, (i) Mg2+, (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.

4.6. 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.

4.7. 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.

5. Conclusions

In this study, CuWO4 was synthesized using the hydrothermal method. Furthermore, the CuWO4 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/CuWO4 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.

Author Contributions

G.A.M.: Data curation, Methodology, Formal analysis, Writing—original draft, S.S.: Data curation, Formal analysis, Writing—original draft, T.-W.C.: Supervision, Funding acquisition, Investigation, validation, Writing—review, and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and Technology (MOST 109–2221-E348 027–059, 110–2221-E-027–041, and NSTC 111–2221–E–027–104) and National Taipei University of Technology (NTUT) through their financial encouragement.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available on request due to privacy/ethical restrictions.

Conflicts of Interest

The authors declare no competing interest.

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Figure 1. (a) XRD patterns of CuWO4 and CNF/CuWO4 nanocomposite. (b) Raman spectra of CuWO4, CNF, and CNF/CuWO4 nanocomposite.
Figure 1. (a) XRD patterns of CuWO4 and CNF/CuWO4 nanocomposite. (b) Raman spectra of CuWO4, CNF, and CNF/CuWO4 nanocomposite.
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Figure 2. FESEM analysis for (a) CuWO4, (b) CuWO4/CNF, (c) EDX color mapping for CuWO4/CNF, and (d) Cu, (e) W, (f) O, and (g) C elemental mapping (h) EDX spectra of CuWO4/CNF composite.
Figure 2. FESEM analysis for (a) CuWO4, (b) CuWO4/CNF, (c) EDX color mapping for CuWO4/CNF, and (d) Cu, (e) W, (f) O, and (g) C elemental mapping (h) EDX spectra of CuWO4/CNF composite.
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Figure 3. HR-TEM analysis of (a) CuWO4, (b) CNF, and (c,d) CuWO4/CNF nanocomposite.
Figure 3. HR-TEM analysis of (a) CuWO4, (b) CNF, and (c,d) CuWO4/CNF nanocomposite.
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Figure 4. (a) Cyclic voltammogram of bare GCE, CNF/GCE, CuWO4/GCE, CuWO4/CNF/GCE modified electrodes in 5 mM K3[Fe (CN)6]3−/4− 0.1 M KCl system at 10 mV/s. (vs. Ag/AgCl). (b) Bar diagram of redox currents for different electrodes. (c) CV profiles were recorded for CuWO4/CNF/GCE at scan rates of 20–240 mVs−1. Different colors peak mentioned different scan rates studies. (d) The linear plot for scan rate vs. peak current (Ip).
Figure 4. (a) Cyclic voltammogram of bare GCE, CNF/GCE, CuWO4/GCE, CuWO4/CNF/GCE modified electrodes in 5 mM K3[Fe (CN)6]3−/4− 0.1 M KCl system at 10 mV/s. (vs. Ag/AgCl). (b) Bar diagram of redox currents for different electrodes. (c) CV profiles were recorded for CuWO4/CNF/GCE at scan rates of 20–240 mVs−1. Different colors peak mentioned different scan rates studies. (d) The linear plot for scan rate vs. peak current (Ip).
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Figure 5. (a) Cyclic voltammetry detection of 4-NT using different electrodes (b) GCE, (c) CNF/GCE, (d) CuWO4/GCE, (e) CuWO4/CNF/GCE in 0.05 M PBS (pH = 7.0) with 100 µM 4-NT. (A) shows the CV response in the non-existence of 4-NT, (B) Bar diagram of different electrode peak currents.
Figure 5. (a) Cyclic voltammetry detection of 4-NT using different electrodes (b) GCE, (c) CNF/GCE, (d) CuWO4/GCE, (e) CuWO4/CNF/GCE in 0.05 M PBS (pH = 7.0) with 100 µM 4-NT. (A) shows the CV response in the non-existence of 4-NT, (B) Bar diagram of different electrode peak currents.
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Scheme 1. The electrocatalytic reduction of 4-nitro toluene to 4-hydroxylamine toluene on the CuWO4/CNF/GCE electrode surface.
Scheme 1. The electrocatalytic reduction of 4-nitro toluene to 4-hydroxylamine toluene on the CuWO4/CNF/GCE electrode surface.
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Figure 6. (a) Cyclic voltammetry response of CNF/CuWO4 at different concentrations of 4-NT from 10 μM to 100 μM in 0.05 M PBS (pH = 7.0) at the scan rate 50 mV/s. Different colors mentioned for different concentration. (b) Linear plot of 4-NT peak current vs. concentration.
Figure 6. (a) Cyclic voltammetry response of CNF/CuWO4 at different concentrations of 4-NT from 10 μM to 100 μM in 0.05 M PBS (pH = 7.0) at the scan rate 50 mV/s. Different colors mentioned for different concentration. (b) Linear plot of 4-NT peak current vs. concentration.
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Figure 7. (a) Cyclic voltammetry response of CuWO4/CNF modified electrode in different pH-3, 5, 7, 9 conditions with 100 µM at the scan rate of 50 mV/s. (b) Calibration plot of reduction peak current vs. different pH.
Figure 7. (a) Cyclic voltammetry response of CuWO4/CNF modified electrode in different pH-3, 5, 7, 9 conditions with 100 µM at the scan rate of 50 mV/s. (b) Calibration plot of reduction peak current vs. different pH.
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Figure 8. (a) Cyclic voltammetry response of CuWO4/CNF nanocomposite electrode in PBS with 100 µM of 4-NT at different scan rates. Different scan rates peaks mentioned in different colors. (b) Calibration plot of the reduction peak current vs. scan rate.
Figure 8. (a) Cyclic voltammetry response of CuWO4/CNF nanocomposite electrode in PBS with 100 µM of 4-NT at different scan rates. Different scan rates peaks mentioned in different colors. (b) Calibration plot of the reduction peak current vs. scan rate.
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Figure 9. (a) DPV response of the CuWO4/CNF/GCE for consecutive addition of 4-NT in 0.05M PBS (pH = 7). Different concentration peaks mentioned in different colors. (b) The linear plot for cathodic current peak vs. concentration of 4-NT.
Figure 9. (a) DPV response of the CuWO4/CNF/GCE for consecutive addition of 4-NT in 0.05M PBS (pH = 7). Different concentration peaks mentioned in different colors. (b) The linear plot for cathodic current peak vs. concentration of 4-NT.
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Figure 10. i–t response of reduction of 4-NT in the presence of various interfering ions anions such as (a) F−, (b) Cl−, (c) NO3−, (d) CO2−, (e) CO3−, (f) Cl−, and cations such as (g) Na+, (h) Cr2+, (i) Mg2+, (j) Zn+, (k) NH4+.
Figure 10. i–t response of reduction of 4-NT in the presence of various interfering ions anions such as (a) F−, (b) Cl−, (c) NO3−, (d) CO2−, (e) CO3−, (f) Cl−, and cations such as (g) Na+, (h) Cr2+, (i) Mg2+, (j) Zn+, (k) NH4+.
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Figure 11. (a) Repetitive runs of a single GCE/CuWO4/CNF electrode. (b) Reproducibility of CuWO4/CNF/GCE electrode for five differently fabricated electrodes. (c) Cyclic stability of the CuWO4/CNF/GCE electrode in 0.05 M PBS with 100 µM at the scan rate of 50 mV/s for 100 cycles.
Figure 11. (a) Repetitive runs of a single GCE/CuWO4/CNF electrode. (b) Reproducibility of CuWO4/CNF/GCE electrode for five differently fabricated electrodes. (c) Cyclic stability of the CuWO4/CNF/GCE electrode in 0.05 M PBS with 100 µM at the scan rate of 50 mV/s for 100 cycles.
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Figure 12. DPV voltammogram for various concentrations from 10 to 30 µM in water samples of (a) river water and (b) tap water.
Figure 12. DPV voltammogram for various concentrations from 10 to 30 µM in water samples of (a) river water and (b) tap water.
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Table
Table 1. Comparison of electrochemical detection 4-NT by of GCE/CuWO4/CNF electrode and various modified electrodes.
Table 1. Comparison of electrochemical detection 4-NT by of GCE/CuWO4/CNF electrode and various modified electrodes.
ElectrodesTechniquesLinear RangeLOD (nM)Reference
ZnO-CN/GCECV-100.0[43]
Ag NPs/AuAmperometry0.010–0.10092.00[45]
NH2-Fe-MIL-88B@OMC-3DPV20.00–225.08000[46]
GCE/α-MnO2CV0.162–48.80144.0[47]
Pn-MWCNTs/Po-MWCNTsSWASV0.5–4, 0.2–1.2442, 282.2[48]
CuWO4/CNF/GCEDPV0.2–10086.15This work
Table
Table 2. It shows the calculation of the recovery percentage and RSD results of the collected real samples: river and tap water samples.
Table 2. It shows the calculation of the recovery percentage and RSD results of the collected real samples: river and tap water samples.
SamplesAdded (µM)Found (µM)Recovery (%)RSD (%)
River water109.1591.510.121
2019.0895.420.102
3029.0896.950.070
Tap Water109.1891.800.079
2019.2796.350.108
3029.1397.100.089
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Meenakshi, G.A.; Sakthinathan, S.; Chiu, T.-W. Fabrication of Carbon Nanofiber Incorporated with CuWO4 for Sensitive Electrochemical Detection of 4-Nitrotoluene in Water Samples. Sensors 2023, 23, 5668. https://doi.org/10.3390/s23125668

AMA Style

Meenakshi GA, Sakthinathan S, Chiu T-W. Fabrication of Carbon Nanofiber Incorporated with CuWO4 for Sensitive Electrochemical Detection of 4-Nitrotoluene in Water Samples. Sensors. 2023; 23(12):5668. https://doi.org/10.3390/s23125668

Chicago/Turabian Style

Meenakshi, Ganesh Abinaya, Subramanian Sakthinathan, and Te-Wei Chiu. 2023. "Fabrication of Carbon Nanofiber Incorporated with CuWO4 for Sensitive Electrochemical Detection of 4-Nitrotoluene in Water Samples" Sensors 23, no. 12: 5668. https://doi.org/10.3390/s23125668

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