Molybdenum Disulfide/Nickel-Metal Organic Framework Hybrid Nanosheets Based Disposable Electrochemical Sensor for Determination of 4-Aminophenol in Presence of Acetaminophen

The toxicity of commonly used drugs, such as acetaminophen (ACAP) and its degradation-derived metabolite of 4-aminophenol (4-AP), underscores the need to achieve an effective approach in their simultaneous electrochemical determination. Hence, the present study attempts to introduce an ultra-sensitive disposable electrochemical 4-AP and ACAP sensor based on surface modification of a screen-printed graphite electrode (SPGE) with a combination of MoS2 nanosheets and a nickel-based metal organic framework (MoS2/Ni-MOF/SPGE sensor). A simple hydrothermal protocol was implemented to fabricate MoS2/Ni-MOF hybrid nanosheets, which was subsequently tested for properties using valid techniques including X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transformed infrared spectroscopy (FTIR), and N2 adsorption-desorption isotherm. The 4-AP detection behavior on MoS2/Ni-MOF/SPGE sensor was followed by cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV). Our experimental findings on the generated sensor confirmed a broad linear dynamic range (LDR) for 4-AP from 0.1 to 600 μM with a high sensitivity of 0.0666 μA/μM and a low limit of detection (LOD) of 0.04 μM. In addition, an analysis of real specimens such as tap water sample as well as a commercial sample (acetaminophen tablets) illuminated the successful applicability of as-developed sensor in determining ACAP and 4-AP, with an impressive recovery rate.


Introduction
HOC 6 H 4 NH 2 , p-Aminophenol or 4-aminophenol is known as a raw material with wide and varied applications in the manufacture of pharmaceutical products, thermal dyes, black and white photographic developer, antioxidants, polymer stabilizers, petroleum additives, fungicides, herbicides, and insecticides [1,2]. Such extensive applications have inevitably led to large concentrations of 4-AP being introduced into the environment and in particular water sources, which can leave toxic effects due to the presence of structural phenol and aniline. The penetration of 4-AP into the human body can be associated with serious health consequences such as dermatitis, eczema, nephrotoxicity, and teratogenic complications [3,4]. Therefore, some European countries and the United States have recommended that the maximum allowable dose of 4-AP in pharmacy should be up to 50 ppm [5]. Thus, both biochemically and environmentally hazardous 4-AP can contaminate and threaten both environmental resources and the health of life by easily penetrating the skin and membranes of plants.
N-acetyl-p-aminophenol, Paracetamol, Acetaminophen or ACAP, is known as one of the most common antipyretic and analgesic drugs that impact mainly the management of migraine pain, headache, arthritis, cancer pain, back pain, and postoperative pain [6][7][8]. ACAP can also be prescribed to aspirin-sensitive patients, such as those with hemophilia,

Fabrication of MoS 2
A typical protocol was followed to prepare MoS 2 NSs [58,59]. Thus, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (3 mmol) and thiourea (2.3 g) were poured in deionized water (30 mL) and then heated for 24 h at 200 • C inside a 50-mL Teflon autoclave. The product was thoroughly rinsed with deionized water/ethanol (with a volume ratio of 1:1), and finally dried at 50 • C for six hours under vacuum condition.

Fabrication of MoS 2 /Ni-MOF Hybrid Nanosheets
In situ growth of Ni-MOF on MoS 2 surface led to the formation of the hybrid NSs of MoS 2 /Ni-MOF. Thus, MoS 2 (0.32 g) was poured in DMF (15 mL) under 30-min ultrasonication, followed by adding nickel (II) nitrate hexahydrate (0.87 g, 3 mmol) dispersed in deionized water (10 mL) under another 30-min ultra-sonication. The obtained solution was then slowly appended with DMF solution (10 mL) including terephthalic acid (0.17 g, 1 mmol) and 0.4 M KOH (5 mL). Following another sonication for two hours, the obtained solution was subjected to solvothermal reaction inside the 50-mL Teflon stainless steel autoclave at 120 • C for 24 h. Next, the product was cooled down to room temperature, followed by centrifugation to obtain the precipitate that was subsequently rinsed with deionized water/DMF (with a volume ratio of 1:1) thoroughly, and finally dried at 65 • C for 24 h under vacuum condition.
Moreover, Ni MOF NSs were prepared as control, similar to the method used to produce MoS 2 /Ni MOF hybrid NSs, except for the addition of MoS 2 prior to solvothermal treatment.

SPGE Modification with MoS 2 /Ni MOF Hybrid Nanosheets
In this step, SPGE was coated by the MoS 2 /Ni MOF hybrid NSs. Thus, 1 mg of MoS 2 /Ni MOF hybrid NSs was dispersed in 1 mL of aqueous solution through a 45-min ultrasonication to prepare a stock solution of the hybrid nanosheets of MoS 2 /Ni MOF. Then, the graphite working electrodes were used to cast 5 µL of the suspension solution of the MoS 2 /Ni MOF nanohybrid. Finally, we put the solvent at room temperature to evaporate to achieve the MoS 2 /Ni-MOF/SPGE.

Preparation of Real Specimens
In order to prepare the real sample of an ACAP tablet (labeled 325 mg), the first five tablets of the ACAP were powdered with a mortar and pestle. Then, 325 mg of this powder Biosensors 2023, 13, 524 4 of 17 was dissolved in 25 mL deionized water under ultra-sonication. Then, in order to remove impurities and fillers in the tablet, the resulting sample was filtered by using filter paper. Then, a specific volume of this solution was transferred to a volumetric flask (25 mL) and diluted with 0.1 M PBS (pH = 7.0). Finally, the standard addition method was used to determine the ACAP and 4-AP content in tablet samples.
Tap water specimens filtrated with a membrane filter and added into 0.1 M PBS (pH = 7). At last, the ACAP and 4-AP contents were measured in the tap water samples using the developed protocol according to standard addition method.

Characterizations
The FE-SEM images captured from Ni-MOF (Figure 1a) shows approximately transparent 20-nanometer NSs and also well-defined two-dimensional layer of fabricated Ni-MOF. A flower-like MoS 2 nanostructure constructed by the assembly of 2D NSs is shown in Figure 1b. The NSs cut with a smooth surface show an identical thickness and a distinct gap of the layers. Figure 1b illustrates the FE-SEM image captured from MoS 2 , highlighting a 13-nanometer nanosheet morphology. Moreover, the obtained MoS 2 /Ni-MOF hybrid NSs exhibit a hierarchical structure while retaining the 2D NS property (Figure 1c,d), so that the MoS 2 NS surface is covered densely by Ni-MOF, leading to a relatively uneven surface.

Preparation of Real Specimens
In order to prepare the real sample of an ACAP tablet (labeled 325 mg), the first five tablets of the ACAP were powdered with a mortar and pestle. Then, 325 mg of this powder was dissolved in 25 mL deionized water under ultra-sonication. Then, in order to remove impurities and fillers in the tablet, the resulting sample was filtered by using filter paper. Then, a specific volume of this solution was transferred to a volumetric flask (25 mL) and diluted with 0.1 M PBS (pH = 7.0). Finally, the standard addition method was used to determine the ACAP and 4-AP content in tablet samples.
Tap water specimens filtrated with a membrane filter and added into 0.1 M PBS (pH = 7). At last, the ACAP and 4-AP contents were measured in the tap water samples using the developed protocol according to standard addition method.
The composition of as-produced hybrid NSs was explored using the Fourier transform infrared spectroscopy ( Figure 4). The FT-IR spectrum revealed the peaks at 614.55 and 884 cm −1 for MoS 2 ( Figure 4; curve a) respectively corresponding to Mo-S and S-S vibrations. The peaks formed at 1100, 1642 and 3448 cm −1 corresponded to hydroxyl stretching vibration resulting from water molecules absorbed [63,64]. The FT-IR spectrum obtained for Ni-MOF ( Figure 4; curve c) showed strong peaks at 3490 and 3320 cm −1 corresponding to hydroxyl (−OH) stretching vibration, and at 2960, 811 and 743 cm −1 corresponding to aromatic units' C-H bonds, as well as at 1382 and 1581 cm −1 respectively corresponding to terephthalic anions' Vs(COO) and Vas(COO). The peak at 545 cm −1 corresponding to ν(Ni-O) verifies a metal-oxo bond between the terephthalic acid's carboxylic group and Ni atoms [62]. When comparing to Ni-MOF and MoS 2 , a new peak appeared strongly at 1650.6 cm −1 related to MoS 2 /Ni-MOF hybrid NSs ( The composition of as-produced hybrid NSs was explored using the Fourier transform infrared spectroscopy ( Figure 4). The FT-IR spectrum revealed the peaks at 614.55 and 884 cm −1 for MoS2 ( Figure 4; curve a) respectively corresponding to Mo-S and S-S vibrations. The peaks formed at 1100, 1642 and 3448 cm −1 corresponded to hydroxyl stretching vibration resulting from water molecules absorbed [63,64]. The FT-IR spectrum obtained for Ni-MOF ( Figure 4; curve c) showed strong peaks at 3490 and 3320 cm −1 corresponding to hydroxyl (−OH) stretching vibration, and at 2960, 811 and 743 cm −1 corresponding to aromatic units' C-H bonds, as well as at 1382 and 1581 cm −1 respectively corresponding to terephthalic anions' Vs(COO) and Vas(COO). The peak at 545 cm −1 corresponding to ν(Ni-O) verifies a metal-oxo bond between the terephthalic acid's carboxylic group and Ni atoms [62]. When comparing to Ni-MOF and MoS2, a new peak appeared strongly at 1650.6 cm −1 related to MoS2/Ni-MOF hybrid NSs (   Figure 5 shows the calculation of MoS 2 @Ni-MOF pore size and surface areas based on nitrogen adsorption−desorption isotherms adopted from Barrett-Joyner-Halenda and Brunauer-Emmett-Teller methods [62,65]. In accordance with the graphical isotherm of typical H3 hysteresis loop in Figure 5A, MoS 2 /Ni-MOF hybrid NSs follows the type-IV isotherms. The accumulation of pores is evident based on the hysteresis loop of MoS 2 /Ni-MOF at relative high pressure (p/p 0 ). The surface area of 27.19 m 2 /g was computed for the MoS 2 /Ni-MOF. According to the distribution of pore size in Figure 5B, the MoS 2 /Ni-MOF had the pore diameter of 1.85 nm.   Figure 5 shows the calculation of MoS2@Ni-MOF pore size and surface areas based on nitrogen adsorption−desorption isotherms adopted from Barrett-Joyner-Halenda and Brunauer-Emmett-Teller methods [62,65]. In accordance with the graphical isotherm of typical H3 hysteresis loop in Figure 5A, MoS2/Ni-MOF hybrid NSs follows the type-IV isotherms. The accumulation of pores is evident based on the hysteresis loop of MoS2/Ni-MOF at relative high pressure (p/p0). The surface area of 27.19 m 2 /g was computed for the MoS2/Ni-MOF. According to the distribution of pore size in Figure 5B, the MoS2/Ni-MOF had the pore diameter of 1.85 nm.

Electrochemical Performance of 4-AP on the Surface of MoS2/Ni-MOF/SPGE
Research indicates that the pH of the electrolytes is one of the fundamental factors influencing the 4-AP response on the MoS2/Ni-MOF/SPGE. Hence, researchers cautiously used DPV in the pH ranges between 2.0 and 9.0 through 0.1 M PBS to determine the impact of the pH value on the electro-chemical behaviors of 4-AP. Analyses revealed that the peak current 4-AP of oxidation enhanced when pH value elevated to 7.0 but it was reduced with an increase in pH. For achieving higher sensitivity, we chose pH 7.0 as an optimal pH to electrochemically detect the 4-AP on the MoS2/Ni-MOF/SPGE.
The CVs of 4-AP at the scanning rate of 50 mV s −1 in the 0.1 M PBS (pH = 7.0) on the bare SPGE (curve a) as well as the MoS2/Ni-MOF/SPGE (curve b) are depicted in Figure  6. As seen in Figure 6 (curve a), the cathodic and anodic peaks of 4-AP on the bare SPGE were observed at −70 and 145 mV, respectively, while the separation between peak potentials (ΔEp) was observed at 213 mV. Figure 6 (curve b) demonstrates the further enhancement of the 4-AP oxidation peak current on the MoS2/Ni-MOF/SPGE compared to that of the bare SPGE, negative shift of the anodic peak potential to 105 mV as well as positive shift of the cathodic peak potential to −35 mV. Furthermore, ΔEp of 4-AP on the MoS2/Ni-MOF/SPGE equaled 140 mV. Consequently, we found a considerable increase of the 4-AP redox peak currents on the MoS2/Ni-MOF/SPGE caused by acceptable conductivity and electrocatalytic feature of MoS2/Ni-MOF.  Figure 6. As seen in Figure 6 (curve a), the cathodic and anodic peaks of 4-AP on the bare SPGE were observed at −70 and 145 mV, respectively, while the separation between peak potentials (∆Ep) was observed at 213 mV. Figure 6 (curve b) demonstrates the further enhancement of the 4-AP oxidation peak current on the MoS 2 /Ni-MOF/SPGE compared to that of the bare SPGE, negative shift of the anodic peak potential to 105 mV as well as positive shift of the cathodic peak potential to −35 mV. Furthermore, ∆Ep of 4-AP on the MoS 2 /Ni-MOF/SPGE equaled 140 mV. Consequently, we found a considerable increase of the 4-AP redox peak currents on the MoS 2 /Ni-MOF/SPGE caused by acceptable conductivity and electrocatalytic feature of MoS 2 /Ni-MOF.

The Effect of the Scanning Rate
We used CV in this step to determine the impact of the scanning rate on the redox reaction of 4-AP on the surface of MoS2/Ni-MOF/SPGE. Figure 7 represents the CV curves of 400.0 µ M 4-AP on the MoS2/Ni-MOF/SPGE with diverse scanning rates from 5-900 mVs −1 . Considering the figure, the cathodic and anodic peak currents (Ipc, Ipa) of 4-AP were elevated by enhancing the scanning rates. Therefore, values of Ipc and Ipa exhibit an acceptable linear correlation to the square root of the scanning rate (υ 1/2 ) (see Inset in Figure 7). Thus, it could be concluded that the electrode reaction of 4-AP on the MoS2/Ni-MOF/SPGE would be a process controlled by diffusion.

The Effect of the Scanning Rate
We used CV in this step to determine the impact of the scanning rate on the redox reaction of 4-AP on the surface of MoS 2 /Ni-MOF/SPGE. Figure 7 represents the CV curves of 400.0 µM 4-AP on the MoS 2 /Ni-MOF/SPGE with diverse scanning rates from 5-900 mVs −1 . Considering the figure, the cathodic and anodic peak currents (I pc , I pa ) of 4-AP were elevated by enhancing the scanning rates. Therefore, values of I pc and I pa exhibit an acceptable linear correlation to the square root of the scanning rate (υ 1/2 ) (see Inset in Figure 7). Thus, it could be concluded that the electrode reaction of 4-AP on the MoS 2 /Ni-MOF/SPGE would be a process controlled by diffusion.

Chronoamperometric Measurements
Chronoamperometry was employed to study the 4-AP electro-oxidation using a MoS2/Ni-MOF/SPGE (see Figure 8). Therefore, the potential of the working electrode was set at 0.14 V as the first-step potential to measure the chronoamperometry of several concentrations of 4-AP on the MoS2/Ni-MOF/SPGE sensor. In this way, the diffusion coefficient, D, of 4-AP in 0.1 M PBS was identified by chronoamperometric tests. Then, we applied the experimental plots of Ip vs. t −1/2 with the best fits for several concentrations of 4-AP (see Figure 8A). Moreover, the slopes shown for the final straight lines were drawn vs. 4-AP concentrations (refer to Figure 8B) and thus D = 3.2 × 10 −5 cm 2 /s by the Cottrell equation and the obtained slopes.

Chronoamperometric Measurements
Chronoamperometry was employed to study the 4-AP electro-oxidation using a MoS 2 /Ni-MOF/SPGE (see Figure 8). Therefore, the potential of the working electrode was set at 0.14 V as the first-step potential to measure the chronoamperometry of several concentrations of 4-AP on the MoS 2 /Ni-MOF/SPGE sensor. In this way, the diffusion coefficient, D, of 4-AP in 0.1 M PBS was identified by chronoamperometric tests. Then, we applied the experimental plots of Ip vs. t −1/2 with the best fits for several concentrations of 4-AP (see Figure 8A). Moreover, the slopes shown for the final straight lines were drawn vs. 4-AP concentrations (refer to Figure 8B) and thus D = 3.2 × 10 −5 cm 2 /s by the Cottrell equation and the obtained slopes.

Calibration Plot and Detection Limit
According to the research design, we employed DPV at optimum conditions to measure several concentrations of 4-AP in the 0.1 M PBS (pH = 7.0) for evaluating the analytical functions of the MoS2/Ni-MOF/SPGE sensor. With regard to Figure 9 the increased peak current was observed as the concentration of 4-AP elevated from curve 1 to 14, reflecting a satisfactory linear correlation to the 4-AP concentration (as depicted in the inset of Figure 9) in ranges 0.1-600.0 μM, which followed the correlation equation of Ipa = 0.0666 C4-AP + 0.9904 (R 2 = 0.9995). It should be noted that LOD equaled 0.04 µM. Two-dimentional Ni-MOFs have shown special attributes such as high porosity and nano-sized thickness that predispose excellent electron transfer and rapid mass transport. Also, the synergistic effect between Ni-MOF and Mos2 increases the conductivity and electro-catalytic activity of the proposed MoS2/Ni-MOF/SPG sensor. The electro-analytical performance for 4-AP at MoS2/Ni-MOF/SPGE was compared with the other chemically modified electrodes (Table 1). These findings revealed the acceptable analytical function of MoS2/Ni-MOF/SPGE sensor for 4-AP detection.

Calibration Plot and Detection Limit
According to the research design, we employed DPV at optimum conditions to measure several concentrations of 4-AP in the 0.1 M PBS (pH = 7.0) for evaluating the analytical functions of the MoS 2 /Ni-MOF/SPGE sensor. With regard to Figure 9 the increased peak current was observed as the concentration of 4-AP elevated from curve 1 to 14, reflecting a satisfactory linear correlation to the 4-AP concentration (as depicted in the inset of Figure 9) in ranges 0.1-600.0 µM, which followed the correlation equation of Ipa = 0.0666 C 4-AP + 0.9904 (R 2 = 0.9995). It should be noted that LOD equaled 0.04 µM. Two-dimentional Ni-MOFs have shown special attributes such as high porosity and nanosized thickness that predispose excellent electron transfer and rapid mass transport. Also, the synergistic effect between Ni-MOF and Mos 2 increases the conductivity and electrocatalytic activity of the proposed MoS 2 /Ni-MOF/SPG sensor. The electro-analytical performance for 4-AP at MoS 2 /Ni-MOF/SPGE was compared with the other chemically modified electrodes (Table 1). These findings revealed the acceptable analytical function of MoS 2 /Ni-MOF/SPGE sensor for 4-AP detection.

Determination of 4-AP in Presence ACAP
Upon the creation of the optimized conditions, as a result of its more acceptable resolution and greater current sensitivity, DPV was employed for the quantitative simultaneous detection of 4-AP and ACAP. Figure 10 presents DPVs of various concentrations of 4-AP and ACAP on the MoS 2 /Ni-MOF/SPGE surface. Considering Figure 10, we observe 2 complete oxidation peaks at 100 mV and 440 mV for 4-AP and ACAP. Moreover, 4-AP and ACAP concentrations exhibit an acceptable linear relationship to the respective oxidation peak currents in ranges between 1.0 µM and 400.0 µM (see Insets A and B). In addition, sensitivity to 4-AP in the case of the presence and absence of ACAP equaled 0.0674 µA/µM (see Figure 10 Inset A) and 0.0666 µA/µM (see inset of Figure 9), respectively. Hence, we inferred that it is possible to use MoS 2 /Ni-MOF/SPGE successfully for the simultaneous detection of 4-AP and ACAP with higher sensitivity and better selectivity.

Determination of 4-AP in Presence ACAP
Upon the creation of the optimized conditions, as a result of its more acceptable resolution and greater current sensitivity, DPV was employed for the quantitative simultaneous detection of 4-AP and ACAP. Figure 10 presents DPVs of various concentrations of 4-AP and ACAP on the MoS2/Ni-MOF/SPGE surface. Considering Figure 10, we observe 2 complete oxidation peaks at 100 mV and 440 mV for 4-AP and ACAP. Moreover, 4-AP and ACAP concentrations exhibit an acceptable linear relationship to the respective oxidation peak currents in ranges between 1.0 µ M and 400.0 µM (see Insets A and B). In addition, sensitivity to 4-AP in the case of the presence and absence of ACAP equaled 0.0674 µA/µ M (see Figure 10 Inset A) and 0.0666 µ A/µM (see inset of Figure 9), respectively. Hence, we inferred that it is possible to use MoS2/Ni-MOF/SPGE successfully for the simultaneous detection of 4-AP and ACAP with higher sensitivity and better selectivity.

ACAP and 4-AP Detection in the Real Samples
Since our research aimed for assessing the new approach practically, we employed MoS 2 /Ni-MOF/SPGE for ACAP and 4-AP detection in the acetaminophen tablet and tap water samples with the standard addition method. Hence, each measurement was iterated give times in similar conditions. See Table 2 for more results. As seen in the table, recovery of ACAP and 4-AP equaled 97.0-103.3% and 96.2-104.3%, indicating the possible use of our electrochemical sensor to detect ACAP and 4-AP in the real samples.

Conclusions
We produced new MoS 2 /Ni-MOF hybrid nanosheets as the modifier for screen-printed graphite electrode surface modification to fabricate an ultra-sensitive sensor for the electrochemical detection of 4-AP. Findings presented an admirable electro-catalytic behavior for as-developed hybrid nanosheets towards the oxidation of 4-AP owing to commendable mass transfer, abundant active sites, impressive conductivity and huge surface area. As-developed MoS 2 /Ni-MOF/SPGE had a broad LDR (0.1-600.0 µM) and a narrow LOD towards the oxidation of 4-AP. Moreover, 4-AP and ACAP were electrochemically detected concurrently on the modified electrode surface, with a peak potential separation of 340 mV. In addition, the analysis of real specimens such as tap water sample as well as a commercial sample (acetaminophen tablets) illuminated the successful applicability of the as-developed sensor in determining ACAP and 4-AP, with an impressive recovery rate.

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

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