Application of MnO2 Nanorod–Ionic Liquid Modified Carbon Paste Electrode for the Voltammetric Determination of Sulfanilamide

The current work introduced a convenient single-phase hydrothermal protocol to fabricate MnO2 nanorods (MnO2 NRs). Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX) and field-emission scanning electron microscopy (FE-SEM) were used to determine the characteristics of MnO2 NR. Then, ionic liquid (IL) and MnO2 NRs were utilized to modify a carbon paste electrode (CPE) surface (MnO2NR-IL/CPE) to voltammetrically sense the sulfanilamide (SAA). An enhanced voltammetric sensitivity was found for the as-developed modified electrode toward SAA when compared with a bare electrode. The optimization experiments were designed to achieve the best analytical behavior of the SAA sensor. Differential pulse voltammetry (DPV) in the optimized circumstances portrayed a linear dependence on various SAA levels (between 0.07 and 100.0 μM), possessing a narrow detection limit (0.01 μM). The ability of the modified electrode to be used in sensor applications was verified in the determination of SAA present in the actual urine and water specimens, with impressive recovery outcomes.


Introduction
Sulfonamides (SAs) are agents that were able to show a selective effect on bacteria for the first time. They are systematically prescribed to control bacterial infections. The structure of sulfonamides contains an amino group bound with a phenyl ring having an alkyl sulfonamide in the para direction [1,2]. 4-aminobenzenesulfonamide, also called sulfanilamide or SAA, with the formula C 6 H 8 N 2 O 2 S is an antibiotic with antibacterial activity. The antibacterial response of SAA is related to the competitive inhibition of dihydropterase synthetase relative to paminobenzoate [3,4]. Despite the numerous benefits of antibiotics, they are potential contaminants. Hence, there are numerous reports of adverse effects on human health and the formation of resistant bacteria following long-term and over-administration of antibiotics. The biotic and abiotic degradability of SAA is weak in nature, so its residues can be found in various aquatic solutions like waste treatment plant effluents, surface water and groundwater, which is a public health problem worldwide [5,6]. This introduction highlights the need to take a robust approach to quantifying SAA at trace levels in different environments.
There are multiple accurate techniques for sensing the SAA, some of which are liquid chromatography [7], high performance liquid chromatography [8,9], spectrophotometry [10], chemiluminescence [11], and capillary electrophoresis-mass spectrometry [12]. However, some of these methods have disadvantages such as a high cost, a time-consuming operation, unsuitability for on-site analysis, and the need for qualified operators and complicated instrumentations.
Among all sensing systems, great attention has been focused on the electrochemical determinations owing to their impressive merits like simple use, affordability, portability, exceptional selectivity, and sensitivity and rapidity [13][14][15][16][17][18][19]. Voltammetry has had a great role in the detection of various analytes in environmental, biomedical and food matrices. Numerous studies reported the direct detection of analytes by voltammetry owing to accuracy, convenience, simplicity, sensitivity, and rapidity [20][21][22][23]. Hence, the SAA concentrations can also be detected by voltammetric methods. A major problem in this field is the need for a high overpotential due to the direct redox reaction of analytes on the bare electrode, as a result of which the appeared fouling impact leads to a weakness in reproducibility and selectivity [24,25].
Electrode surface modification can increase the sensitivity of the produced sensors due to the establishment of appropriate and adjustable features. Diverse modifiers in this field have offered a lower limit of detection (LOD), excellent sensitivity, overpotential reduction and surface fouling resistance [26][27][28][29][30][31][32].
The use of appropriate electrochemical methods in conjunction with carbon paste electrodes (CPEs) has yielded acceptable results. A variety of analytes have been detected by CPEs because of their unique properties, some of which include residual currents 10 times lower than glassy carbon electrodes (GCEs), a wider potential window, and facile preparation steps [33][34][35][36][37][38]. The selectivity and sensitivity of the target analyte determination can be significantly enhanced by chemically modified CPEs using modifiers [39][40][41][42].
One of the most successful modifiers used in electrochemical sensors is nanoparticles (NPs), and many special features of the working electrode, like the huge surface area and small size, reproducibility, peak current and sensitivity, can be enhanced in the presence of NPs [43,44].
Another modifiers are metal oxides, owing to their admirable traits, some of which include an adjustable production, huge specific surface area, direct paths for electron transfer, empty space to reduce volume expansion, proper adhesion of active substance to current collector and low cost [45,46]. A popular oxide material is manganese dioxide (MnO 2 ), whose behavior can be enhanced by changing its morphology and surface area. MnO 2 is a polymorph (1D α-, βand γ-MnO 2 , and 2D δ-MnO 2 ) owing to an octahedral [MnO 6 ] spatial arrangement [47,48]. Nano-sized MnO 2 exhibits commendable benefits, due to a larger surface-to-volume ratio and further reactive surface for electrochemical reactions. The diverse application of this substance in electrochemistry and sensor fabrication can be attributed to the simple reduction of MnO 2 to Mn 2 O 3 and MnO and, at the proper potential, the re-oxidation to MnO 2 as a catalytic circle for electrochemical detection [49][50][51].
Electroanalysis and electrochemistry have recently benefited greatly from room temperature ionic liquids (RTILs), which have astonishing physicochemical properties such as an impressive thermal and chemical stability, ionic architecture, proper extraction capacity, low equilibrium vapor pressure, ion exchange activity, potent conductivity and broad electrochemical window. ILs have a better conductivity than paraffin oil, which makes them suitable for making paste electrodes in voltammetric determinations due to a reduced charge current and consequent intensification of sensitivity and a decrease in the LOD value [52,53].
Accordingly, the present study utilized these two compounds to modify the CPE surface. The current work introduced a convenient single-phase hydrothermal protocol to fabricate MnO 2 NR. Then, IL and MnO 2 NR were utilized to modify the CPE surface to voltammetrically sense the SAA. An enhanced voltammetric sensitivity was found for the as-developed modified electrode toward SAA when compared with the bare CPE. The ability of the modified electrode for use in sensor applications was verified in the determination of SAA present in the real specimens, with an impressive relative standard deviation (RSD).

Chemicals and Equipment
An X-ray diffractometer (Panalytical X'Pert Pro, Almelo, The Netherlands) using copper/Kα radiation (λ = 1.5418 Å) captured XRD patterns. A Tensor II spectrometer (Bruker, Denkendorf, Baden-Württemberg, Germany) was utilized to record the FT-IR spectra. A scanning electron microscope (MIRA3, Tescan, Brno, Czech Republic) provided the FE-SEM images and the EDX patterns. An autolab potentiostat/galvanostat (PGSTAT-302N, Eco Chemie, Utrecht, The Netherlands) recorded all electrochemical determinations. The General Purpose Electrochemical System (GPES) as selected software monitored all testing protocols. A conventional three-electrode system was used at 25 ± 1 • C. An Ag/AgCl/KCl (3.0 M) electrode, a platinum wire and MnO 2 NR-IL/CPE were used as the reference, auxiliary and working electrodes, respectively. A pH meter (Metrohm type 713) was utilized to determine all solution pH values. All solutions were prepared freshly by deionized water (DIW, Millipore Direct-Q ® 8 UV water purification system, Darmstadt, Germany). Sulfanilamide, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF 6 ) ionic liquid, and all materials were purchased from Merck (Darmstadt, Germany) with analytical research purity. Orthophosphoric acid as well as relevant salts have been applied to achieve all phosphate buffer solutions (PBS), set in a pH range of 2.0-9.0.

Hydrothermal Synthesis of MnO 2 NRs
The MnO 2 NRs were obtained by dissolving KMnO 4 (0.316 g) in deionized water (30 mL) while vigorously stirring, followed by the addition of 3 M HCl (1.4 mL) under vigorous stirring for another half hour. Then, the solution was placed in a 50-mL Teflon-lined autoclave at 160 • C for six hours. Next, the products were cooled down to room temperature and subsequently centrifuged and thoroughly rinsed with ethanol and deionized water to clean any impurity, followed by drying at 60 • C for 12 h.

Process of Electrode Fabrication
As the composition of the carbon paste is very important in electrochemical responses, we optimized all of them. To produce an MnO 2 NR and ionic liquid-modified carbon paste electrode (MnO 2 NR-IL/CPE), firstly, ionic liquid (0.8 mL), MnO 2 NR (0.4 g), and graphite powder (0.9 g) were blended well together in a mortar to achieve a uniformly wetted paste. Then, the paste was compressed into the bottom of a glass tube. A copper wire was embedded into the back of the mixture down in a glass tube to establish an electrical contact. Next, excess paste, if present, was expelled from the tube, and a weighing paper was used for polishing to achieve a new surface. The EDS analysis was performed to check the elemental composition of the MnO 2 NRs. Figure 2 shows the presence of O (42.16 wt%) and Mn (57.84 wt%) in the structure of MnO 2 NRs without any impurity. Figure 3 displays the FT-IR spectrum captured for as-fabricated MnO 2 NRs, with the characteristic peaks at 476, 532 and 729 cm −1 corresponding to Mn-O α-phase vibration. The FT-IR spectrum of the as-prepared sample was consistent with the previous report [54].  The EDS analysis was performed to check the elemental composition of the MnO2 NRs. Figure 2 shows the presence of O (42.16 wt%) and Mn (57.84 wt%) in the structure of MnO2 NRs without any impurity.

Electrochemical Response of SAA on the MnO2NR-IL/CPE Surface
According to the obtained findings, the SAA electro-oxidation was based on electron and proton exchange. Moreover, it was crucial to optimize the pH value in detecting the analyte. Hence, the DPV technique was applied to clarify the electrochemical response of SAA on the MnO2NR-IL/CPE surface in PBS (0.1 M) at various pH values between 2.0 and 9.0. Reportedly, the SAA electro-oxidation on the MnO2NR-IL/CPE surface was higher in neutral conditions than in alkaline or acidic medium ( Figure 5). Consequently, the optimal pH was chosen as 7.0 for SAA electro-oxidation on the MnO2NR-IL/CPE surface.

Electrochemical Response of SAA on the MnO 2 NR-IL/CPE Surface
According to the obtained findings, the SAA electro-oxidation was based on electron and proton exchange. Moreover, it was crucial to optimize the pH value in detecting the analyte. Hence, the DPV technique was applied to clarify the electrochemical response of SAA on the MnO 2 NR-IL/CPE surface in PBS (0.1 M) at various pH values between 2.0 and 9.0. Reportedly, the SAA electro-oxidation on the MnO 2 NR-IL/CPE surface was higher in neutral conditions than in alkaline or acidic medium ( Figure 5). Consequently, the optimal pH was chosen as 7.0 for SAA electro-oxidation on the MnO 2 NR-IL/CPE surface.  Figure 6. The MnO2NR-IL/CPE voltammetric behavior (Figure 6c) exhibited the relatively strongest oxidation peak at 925 mV, with the oxidation peak current at 16.0 μA, sequentially. The MnO2NR/CPE voltammetric behavior ( Figure  6b) exhibited the relatively strongest oxidation peak at 960 mV, with the oxidation peak current at 9.3 μA, sequentially, and the bare CPE voltammetric behavior (Figure 6a) exhibited a relatively weak oxidation peak with less intensity at 1000 mV, with the oxidation peak current at 3.8 μA, sequentially. Consequently, the MnO2NR-IL/CPE has an obviously better electrocatalytic behavior towards the SAA with a relatively strong current response. Additionally, bare in PBS without SAA did not show any peak.   (Figure 6b) exhibited the relatively strongest oxidation peak at 960 mV, with the oxidation peak current at 9.3 µA, sequentially, and the bare CPE voltammetric behavior (Figure 6a) exhibited a relatively weak oxidation peak with less intensity at 1000 mV, with the oxidation peak current at 3.8 µA, sequentially. Consequently, the MnO 2 NR-IL/CPE has an obviously better electrocatalytic behavior towards the SAA with a relatively strong current response. Additionally, bare in PBS without SAA did not show any peak.  Figure 6. The MnO2NR-IL/CPE voltammetric behavior (Figure 6c) exhibited the relatively strongest oxidation peak at 925 mV, with the oxidation peak current at 16.0 μA, sequentially. The MnO2NR/CPE voltammetric behavior ( Figure  6b) exhibited the relatively strongest oxidation peak at 960 mV, with the oxidation peak current at 9.3 μA, sequentially, and the bare CPE voltammetric behavior (Figure 6a) exhibited a relatively weak oxidation peak with less intensity at 1000 mV, with the oxidation peak current at 3.8 μA, sequentially. Consequently, the MnO2NR-IL/CPE has an obviously better electrocatalytic behavior towards the SAA with a relatively strong current response. Additionally, bare in PBS without SAA did not show any peak.

Effect of Scan Rate
The scan rate performance on the SAA oxidation peak current was investigated to analyze electrode processes. The linear sweep voltammograms (LSVs) were recorded for 50.0 µM of SAA in PBS (0.1 M) at various scan rates, and the results indicated that anodic peak currents (Ipa) were linearly related to the scan rate square root (Figure 7). Accordingly, the diffusion-controlled process was seen for the SAA oxidation on the MnO 2 NR-IL/CPE.

Effect of Scan Rate
The scan rate performance on the SAA oxidation peak current was investiga analyze electrode processes. The linear sweep voltammograms (LSVs) were record 50.0 μM of SAA in PBS (0.1 M) at various scan rates, and the results indicated that peak currents (Ipa) were linearly related to the scan rate square root (Figure 7). A ingly, the diffusion-controlled process was seen for the SAA oxidation on the Mn IL/CPE. Tafel graph was plotted to achieve data on the rate-determining step, as ob from points of the Tafel area in linear sweep voltammetry. In this condition, the transfer coefficient (α) can be estimated from the slope Tafel plot: Tafel graph was plotted to achieve data on the rate-determining step, as obtained from points of the Tafel area in linear sweep voltammetry. Figure 8   An average slope of 0.1307 V is obtained, indicating a one-electron transfer for limiting step assuming a transfer coefficient of α = 0.55. Figure 9 shows the chronoamperometric analysis to calculate the diffusion coe of SAA. The plots of I against t −1/2 have been utilized in relation to the optimal different SAA contents in PBS (0.1 M), as seen in Figure 9A. The chronoamper measurements were carried out for various SAA contents on MnO2NR-IL/CPE working electrode potential of 975 mV. The slopes of the obtained straight lines SAA contents were drawn by the Cottrell equation in order to obtain the below eq  Figure 9 shows the chronoamperometric analysis to calculate the diffusion coefficient of SAA. The plots of I against t −1/2 have been utilized in relation to the optimal fits for different SAA contents in PBS (0.1 M), as seen in Figure 9A. The chronoamperometric measurements were carried out for various SAA contents on MnO 2 NR-IL/CPE at the working electrode potential of 975 mV. The slopes of the obtained straight lines versus SAA contents were drawn by the Cottrell equation in order to obtain the below equation:

Chronoamperometric Determinations
with D (cm 2 /s) being the diffusion coefficient, n the number of electrons for oxidation of SAA (n = 1) and C b (mol/cm 3 ) the bulk content. There was a linearity for the I against t −1/2 plot under diffusion control over various SAA contents. The slopes could be used to compute the mean D value for SAA (see Figure 9B), which was 2.2 × 10 −5 cm 2 /s.

Detection Limit and Standard Plot
The SAA content was measured by the DPV technique. The DPVs captured for MnO2NR-IL/CPE at various SAA contents in PBS (0.1 M) are shown in Figure 10. There was a stepwise enhancement in the SAA oxidation current by gradually increasing the SAA contents, which is to say the applicability of MnO2NR-IL/CPE for electrochemically sensing the SAA. Figure 10 (inset) represents the alterations in the oxidation signal for MnO2NR-IL/CPE as a function of various SAA contents (0.07-100.0 μM), showing a low detection limit of 0.01 μM. Moreover, Table 1 shows that MnO2NR-IL/CPE can be successfully used for the determination of SAA, compared with other sensors.

Detection Limit and Standard Plot
The SAA content was measured by the DPV technique. The DPVs captured for MnO 2 NR-IL/CPE at various SAA contents in PBS (0.1 M) are shown in Figure 10. There was a stepwise enhancement in the SAA oxidation current by gradually increasing the SAA contents, which is to say the applicability of MnO 2 NR-IL/CPE for electrochemically sensing the SAA. Figure 10 (inset) represents the alterations in the oxidation signal for MnO 2 NR-IL/CPE as a function of various SAA contents (0.07-100.0 µM), showing a low detection limit of 0.01 µM. Moreover, Table 1 shows that MnO 2 NR-IL/CPE can be successfully used for the determination of SAA, compared with other sensors.

Interference Studies
Interference studies were investigated to know how the results for the SAA analysis were affected by the presence of various compounds. According to the used definition, the tolerance limit was defined as the ratio of the concentration of the interfering compounds to the SAA, which led to a relative error of less than ±5.0%. The possible interference was investigated by the addition of various compounds such as Mg 2+ , Na + , K + , Ca 2+ , Cl − , ascorbic acid, uric acid, glucose, sucrose, L-cystine, and dopamine to PBS (pH 7.0) in the presence of 50.0 µM SAA. It was found that the addition of these interfering species had no remarkable effect on the DPV signal of SAA. These results indicate that the modified electrode has a good selectivity for SAA determination.

Real Sample Analysis
The ability of MnO 2 NR-IL/CPE to be used for sensor applications in the detection of SAA was determined in real specimens of urine and water according to the standard addition method, as seen in Table 2. The recorded recovery rates ranged from 97.0% to 104.2%, thereby highlighting the appreciable applicability of MnO 2 NR-IL/CPE in sensing the SAA in real specimens.

Conclusions
A facile hydrothermal method was applied to prepare uniform and pure manganese dioxide NRs. Then, ionic liquid and MnO 2 NRs were utilized to modify a carbon paste electrode surface (MnO 2 NR-IL/CPE) to voltammetrically sense the SAA. An enhanced electrochemical performance was found for the as-developed modified electrode toward SAA oxidation when compared with a bare electrode, with a lengthy linear range (0.07-100.0 µM) and a narrow limit of detection (0.01 µM). The as-fabricated sensor possessed an exceptional electrocatalytic behavior, excellent sensitivity and low limit of detection. Additionally, the MnO 2 NR-IL/CPE showed a good selectivity for the detection of SAA in the presence of various compounds. The ability of the modified electrode to be used for sensor applications was verified by the determination of SAA present in actual human urine and serum specimens, with impressive recovery outcomes.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest.