Fabrication of an Electrocatalyst Based on Rare Earth Manganites Incorporated with Carbon Nanoﬁber Hybrids: An Efﬁcient Electrochemical Biosensor for the Detection of Anti-Inﬂammatory Drug Mefenamic Acid

: Pharmaceutical and personal care products are emerging as a new category of environmental pollution. Analytical drug detection from a biological sample for detection is still crucial today. Mefenamic acid (MA) is an anti-inﬂammatory drug utilized for its antipyretic and analgesic properties, which is harmful to patients at higher dosages and is also recognized as a chemical pollutant that harms the environment. In this view, Dysprosium manganite/carbon nanoﬁber (DMO/CNF) was prepared by hydrothermal method for the electrochemical detection of MA. DMO/CNF/GCE exhibits high selectivity, excellent anti-interference, good stability, and reproducibility toward the detection of MA. The enhanced electrochemical performance of DMO/CNF/GCE was attributed to their synergetic interaction. Under optimized conditions, DMO/CNF/GCE shows a wide linear range of 0.01–741 µ M and a low LOD of 0.009 µ M. Satisfactory recoveries were obtained for human blood and tablet samples. Thus, the proposed DMO/CNF nanocomposite emerges as a promising material for the detection of MA.


Introduction
Mefenamic acid (MA) is chemically an N-substituted phenylantharanilic acid (N-(2,3-dimethyl phenyl)-2-aminobenzoic acid), and it is a non-steroidal anti-inflammatory drug (NSAID) available in the form of tablets, capsules, and suspensions. MA compound exists as white or light gray crystalline powder and has extremely low solubility in water. MA exhibit antipyretic, anti-inflammatory, and analgesic activities [1]. The pathophysiological functions of MA are implemented by its ability to inhibit the biosynthesis of prostaglandin involved in pathogen diseases. MA is used to treat rheumatoid arthritis, osteoarthritis, autoimmune hemolytic anemia, sports injuries, gastrointestinal disorders, primary dysmenorrhea muscle pain, back pain, and dental pain [2][3][4][5]. As MA works on cyclooxygenase (COX) pathways and suppresses prostaglandin, the sensations of pain are momentarily lessened. Due to its massive production and presence in sewage water, MA is evolving into an environmental pollutant that is found in surface water. MA is C 2023, 9, 6 ]), potassium chloride (KCl, 99.0-100.5%), hydrochloric acid (HCl, 36.5-38%), and mefenamic acid were purchased from Sigma Aldrich and used without further purification. Deionized (DI) water and ethanol were used for solutionmaking and other washing purposes throughout the experiments.

Materials Characterization
The morphology of the synthesized material was investigated by various techniques, such as field emission scanning electron microscopy (FESEM, Hitachi S-3000 H) and highresolution transmission electron microscopy (HRTEM, H-7600, Hitachi, Japan). The chemical composition and the individual elemental percentage were investigated through an energy-dispersive X-ray (EDX, HORIBA EMAX X-ACT, Sensor +24 V = 16 W, resolution at 5.9 k eV) attached to the HRTEM. Powder X-ray diffraction (XRD, XPERT-PRO, PAN analytical B.V., Breda, The Netherlands) and the diffractometer with Cu Kα radiation (k = 1.54 Å) were used to investigate the crystallographic structure of all synthesized materials. The data of the Raman spectrum was collected from the Micro-Raman spectrometer (Raman Dong Woo 500 I, Seoul, Republic of Korea). Electrochemical impedance spectroscopy (EIS) was used to identify the materials and electrochemical measurements were performed in the electrochemical workstation in (5 mM [Fe(CN) 6 ] 3−/4− ) FC solution and 0.1 M KCl solution, and cyclic voltammetry (CV CHI 1205a) and differential pulse voltammetry (DPV, CHI 900), both made in the United States. These experiments were carried out using a three-electrode system with a glassy carbon electrode (GCE) as the working electrode (electrode surface area = 0.071 cm 2 ), a saturated Ag/AgCl as the reference electrode, and platinum as the counter electrode in the presence of N 2 gas saturated 0.1 M PB supporting electrolyte (pH 7.0).

Synthesis Procedure of DMO/CNF
Initially, distilled water was thoroughly mixed with Dy(NO 3 ) 3 ·H 2 O and Mn(NO 3 ) 2 ·6H 2 O were dissolved in a stoichiometric ratio. Then, PEG was added to the aforementioned solution as a capping agent. By combining with NH 3 , the solution's ultimate pH was set at 8. The mixture was put in an autoclave lined with Teflon and aged at 180 • C for 18 h. An as-prepared sample was then calcined for 4 h to eliminate the organic component. The prepared DMO sample and CNF were dissolved in 1 mL of DMF and sonicated for 30 min to get a homogeneous mixture of DMO/CNF nanocomposite. The specific synthesis procedure was represented in Scheme 1.  36.5-38%), and mefenamic acid were purchased from Sigma Aldrich and used without further purification. Deionized (DI) water and ethanol were used for solution-making and other washing purposes throughout the experiments.

Materials Characterization
The morphology of the synthesized material was investigated by various techniques, such as field emission scanning electron microscopy (FESEM, Hitachi S-3000 H) and highresolution transmission electron microscopy (HRTEM, H-7600, Hitachi, Japan). The chemical composition and the individual elemental percentage were investigated through an energy-dispersive X-ray (EDX, HORIBA EMAX X-ACT, Sensor +24 V = 16 W, resolution at 5.9 k eV) attached to the HRTEM. Powder x-ray diffraction (XRD, XPERT-PRO, PAN analytical B.V., The Netherlands) and the diffractometer with Cu Kα radiation (k = 1.54 Å) were used to investigate the crystallographic structure of all synthesized materials. The data of the Raman spectrum was collected from the Micro-Raman spectrometer (Raman Dong Woo 500 I, Republic of Korea). Electrochemical impedance spectroscopy (EIS) was used to identify the materials and electrochemical measurements were performed in the electrochemical workstation in (5 mM [Fe(CN)6] 3−/4− ) FC solution and 0.1 M KCl solution, and cyclic voltammetry (CV CHI 1205a) and differential pulse voltammetry (DPV, CHI 900), both made in the United States. These experiments were carried out using a three-electrode system with a glassy carbon electrode (GCE) as the working electrode (electrode surface area = 0.071 cm 2 ), a saturated Ag/AgCl as the reference electrode, and platinum as the counter electrode in the presence of N2 gas saturated 0.1 M PB supporting electrolyte (pH 7.0).

Synthesis Procedure of DMO/CNF
Initially, distilled water was thoroughly mixed with Dy(NO3)3.H2O and Mn(NO3)2.6H2O were dissolved in a stoichiometric ratio. Then, PEG was added to the aforementioned solution as a capping agent. By combining with NH3, the solution's ultimate pH was set at 8. The mixture was put in an autoclave lined with Teflon and aged at 180 °C for 18 h. An as-prepared sample was then calcined for 4 h to eliminate the organic component. The prepared DMO sample and CNF were dissolved in 1 ml of DMF and sonicated for 30 min to get a homogeneous mixture of DMO/CNF nanocomposite. The specific synthesis procedure was represented in Scheme 1.

Fabrication of DMO/f-CNF/GCE
Deionized water is used to clean the carbon surface of the glassy carbon electrode. The prepared DMO/f-CNF/GCE was then dispersed in 1 ml of water and sonicated for 15 min. Following that, 6 × 10 −6 L of DMO/f-CNF/GCE suspension was drop-casted on the carbon surface of GCE and dried in a 50 °C oven. Lastly, the electrochemical characterization was performed on the as-prepared DMO/f-CNF/GCE.

Fabrication of DMO/f-CNF/GCE
Deionized water is used to clean the carbon surface of the glassy carbon electrode. The prepared DMO/f-CNF/GCE was then dispersed in 1 mL of water and sonicated for 15 min. Following that, 6 × 10 −6 L of DMO/f-CNF/GCE suspension was drop-casted on the carbon surface of GCE and dried in a 50 • C oven. Lastly, the electrochemical characterization was performed on the as-prepared DMO/f-CNF/GCE.

Morphological and Structural Analysis
The morphology of the synthesized material DMO/CNF was characterized by SEM and HR-TEM, depicted in Figure 1. Figure 1A shows the FESEM image of DMO, which appears to be aggregated nanoparticle with irregular size, randomly oriented. Figure 1B shows the FESEM image of nanofibers aligned partially with a smooth surface. Figure 1C shows the FESEM image DMO/CNF in which the DMO nanoparticle was aligned on the surface of CNF. When DMO is anchored to the surface of CNF, their tight integration creates synergism, which increases the electrocatalytic activity of the prepared nanocomposite. Figure 1D,E shows the TEM images of DMO/CNF at different magnifications, which resemble the topography-like nanoparticle, which resembles the structure as depicted in FESEM, which confirms the structure of DMO/CNF. Figure 1F shows the SAED pattern of DMO/CNF revealing the crystallinity through the appearance of lattice planes (221) and (311) corresponding to the DMO/CNF as confirmed through XRD results. Figure 1G shows the lattice fringes image of DMO/CNF, which reveals the d-spacing value to be 0.318 nm. The spatial distribution of different elements present in the DMO/CNF was found by mapping ( Figure 2A-E). The elemental constitution was analyzed by EDX, which reveals the presence of Dy, Mn, O, and C with weight percentages of 27.7%, 23.3%, 20.4%, and 28.6%, respectively.

Morphological and Structural Analysis
The morphology of the synthesized material DMO/CNF was characterized by SEM and HR-TEM, depicted in Figure 1. Figure 1A shows the FESEM image of DMO, which appears to be aggregated nanoparticle with irregular size, randomly oriented. Figure 1B shows the FESEM image of nanofibers aligned partially with a smooth surface. Figure 1C shows the FESEM image DMO/CNF in which the DMO nanoparticle was aligned on the surface of CNF. When DMO is anchored to the surface of CNF, their tight integration creates synergism, which increases the electrocatalytic activity of the prepared nanocomposite. Figure 1D,E shows the TEM images of DMO/CNF at different magnifications, which resemble the topography-like nanoparticle, which resembles the structure as depicted in FESEM, which confirms the structure of DMO/CNF. Figure 1F shows the SAED pattern of DMO/CNF revealing the crystallinity through the appearance of lattice planes (221) and (311) corresponding to the DMO/CNF as confirmed through XRD results. Figure  1G shows the lattice fringes image of DMO/CNF, which reveals the d-spacing value to be 0.318 nm. The spatial distribution of different elements present in the DMO/CNF was found by mapping ( Figure 2A-E). The elemental constitution was analyzed by EDX, which reveals the presence of Dy, Mn, O, and C with weight percentages of 27.7%, 23.3%, 20.4%, and 28.6%, respectively.   Figure 3A depicts the XRD pattern of as-prepared DMO, CNF, and DMO/CNF nanocomposite. In the diffraction of CNF ( Figure 3A(c)), the peak appears at 25.4, which indicates the graphite-like carbon corresponds to the (002) plane [44]. As in Figure 3A(a), the crystalline nature of the DMO reduces the intense peak of CNF, thus resulting in the exfoliation of the CNF in the composite DMO/CNF. Figure 3A Figure 3A depicts the XRD pattern of as-prepared DMO, CNF, and DMO/CNF nanocomposite. In the diffraction of CNF ( Figure 3A(c)), the peak appears at 25.4, which indicates the graphite-like carbon corresponds to the (002) plane [44]. As in Figure 3A(a), the crystalline nature of the DMO reduces the intense peak of CNF, thus resulting in the exfoliation of the CNF in the composite DMO/CNF. Figure 3A  The micro-Raman spectra were evaluated to confirm the presence of CNF in the DMO/CNF nanocomposite, as depicted in Figure 3B. The Raman spectra indicated that the D and G bands were around 1365 cm −1 and 1590 cm −1 . Where the ID/IG intensity ratio of CNF and   Figure 3A depicts the XRD pattern of as-prepared DMO, CNF, and DMO/CNF nanocomposite. In the diffraction of CNF ( Figure 3A(c)), the peak appears at 25.4, which indicates the graphite-like carbon corresponds to the (002) plane [44]. As in Figure 3A(a), the crystalline nature of the DMO reduces the intense peak of CNF, thus resulting in the exfoliation of the CNF in the composite DMO/CNF. Figure 3A  The micro-Raman spectra were evaluated to confirm the presence of CNF in the DMO/CNF nanocomposite, as depicted in Figure 3B. The Raman spectra indicated that the D and G bands were around 1365 cm −1 and 1590 cm −1 . Where the ID/IG intensity ratio of CNF and The micro-Raman spectra were evaluated to confirm the presence of CNF in the DMO/CNF nanocomposite, as depicted in Figure 3B. The Raman spectra indicated that the D and G bands were around 1365 cm −1 and 1590 cm −1 . Where the I D /I G intensity ratio of CNF and DMO/CNF is 0.99 and 1.03. The I D /I G ratio of DMO/CNF was higher for DMO/CNF. The effective interaction between the CNF and DMO further increases the defects with the I D /I G ratio of DMO/CNF to 1.03.

Electrochemical Behavior of DMO/CNF
The interfacial properties of the electrodes were examined using the EIS study in the FC system, which includes a semicircle at a high frequency and a straight line at a low frequency which relates to the electron transfer limiting process and diffusion process, respectively. Figure 4A depicts the electron transfer ability of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE. The charge transfer resistance taking part in the solution interface (R ct ) of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was calculated to be 803.9, 207, 132.6, 128.4Ω. DMO/CNF/GCE shows a smaller semi-circle and low R ct value, which means the CNF promotes the mass electron transfer at DMO/CNF/GCE electrode. This confirms that the DMO/CNF/GCE has a better electron transfer ability. As a result, DMO/CNF nanocomposite can make good electrical contact with electrodes and electrolytes, allowing high-speed electrocatalytic detection of target analytes. In a further step, we can use the R ct value to calculate the charge-transfer rate (Kapp) [46].
The equation shows that C represents the concentration of the solution, and A represents the surface of the electrodes transferred. R gas constant, a room temperature, and a reaction faradic constant are represented by R, T, and F, respectively. On the basis of Eq, the Ks values for bare GCE, CNF/GE, DMO/GCE, and DMO/CNF/GCE are 5.55 × 10 −9 , 2.16 × 10 −8 , 3..37 × 10 −8 , and 3.48 × 10 −8 , respectively. Compared to other electrodes, DMO/CNF/GCE have the highest Kapp value, indicating fast electron transport.
frequency which relates to the electron transfer limiting process and diffusion process, respectively. Figure 4A depicts the electron transfer ability of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE. The charge transfer resistance taking part in the solution interface (Rct) of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was calculated to be 803.9, 207, 132.6, 128.4Ω. DMO/CNF/GCE shows a smaller semi-circle and low Rct value, which means the CNF promotes the mass electron transfer at DMO/CNF/GCE electrode. This confirms that the DMO/CNF/GCE has a better electron transfer ability. As a result, DMO/CNF nanocomposite can make good electrical contact with electrodes and electrolytes, allowing high-speed electrocatalytic detection of target analytes. In a further step, we can use the Rct value to calculate the charge-transfer rate (Kapp) [46].

Kapp = RT/F 2 R ct AC
The equation shows that C represents the concentration of the solution, and A represents the surface of the electrodes transferred. R gas constant, a room temperature, and a reaction faradic constant are represented by R, T, and F, respectively. On the basis of Eq, the Ks values for bare GCE, CNF/GE, DMO/GCE, and DMO/CNF/GCE are 5.55 × 10 −9 , 2.16 × 10 −8 , 3..37 × 10 −8 , and 3.48 × 10 −8 , respectively. Compared to other electrodes, DMO/CNF/GCE have the highest Kapp value, indicating fast electron transport.  Cyclic voltammetry was utilized to assess the electroanalytical behavior of the bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE. Utilizing reference redox couple FC system, their performance was observed, displayed in Figure 4B. It revealed a significant improvement in redox peak current and reduced peak separation for DMO/CNF/GCE composite. This could be owing to the increased electrode surface of DMO and CNF, which could act as an electron transfer pathway. Figure 4C depicts the CV response of DMO/CNF/GCE by various scan rates. As shown in the figure, the CV was performed at varying scan rates of 20 mVs −1 -300 mVs −1 . A linear relationship was observed between the redox peak current and the square root of the scan rate, which was depicted in Figure 4D. As the scan rate increases, the redox peak current also increases. The active surface area (EASA) was calculated by using Randles-Sevcik equation [47]: The EASA of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was calculated to be 0.074, 0.085, 0.098, and 0.117 cm 2 , respectively. The obtained results revealed that DMO/CNF/GCE had a larger EASA value; therefore, DMO/CNF/GCE will be more advantageous for the electrochemical response towards MA detection.

Modified Electrodes and Different Ph
The electrocatalytic performance of DMO/CNF/GCE was analyzed. Initially, the electrochemical performance of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was measured in the existence of 100 µM of MA in 0.01 M PB supporting electrolyte at 50 mVs −1 depicted in Figure 5A; corresponding bar diagram was shown in Figure 5B. From the results, we observed that the bare GCE has poor catalytic activity compared to all other modified electrodes. The electrocatalytic activity of DMO/GCE was increased after the introduction of CNF, and the DMO/CNF/GCE resulted in a high oxidation current response. However, a better current response towards MA detection was obtained for the DMO/CNF/GCE, which shows the better catalytic activity of the as-synthesized nanocomposite. Figure 4C depicts the CV response of DMO/CNF/GCE by various scan rates. As shown in the figure, the CV was performed at varying scan rates of 20 mVs −1 -300 mVs −1 . A linear relationship was observed between the redox peak current and the square root of the scan rate, which was depicted in Figure 4D. As the scan rate increases, the redox peak current also increases. The active surface area (EASA) was calculated by using Randles-Sevcik equation [47]: The EASA of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was calculated to be 0.074, 0.085, 0.098, and 0.117 cm 2 , respectively. The obtained results revealed that DMO/CNF/GCE had a larger EASA value; therefore, DMO/CNF/GCE will be more advantageous for the electrochemical response towards MA detection.

Modified Electrodes and Different Ph
The electrocatalytic performance of DMO/CNF/GCE was analyzed. Initially, the electrochemical performance of bare GCE, CNF/GCE, DMO/GCE, and DMO/CNF/GCE was measured in the existence of 100 μM of MA in 0.01 M PB supporting electrolyte at 50 mVs −1 depicted in Figure 5A; corresponding bar diagram was shown in Figure 5B. From the results, we observed that the bare GCE has poor catalytic activity compared to all other modified electrodes. The electrocatalytic activity of DMO/GCE was increased after the introduction of CNF, and the DMO/CNF/GCE resulted in a high oxidation current response. However, a better current response towards MA detection was obtained for the DMO/CNF/GCE, which shows the better catalytic activity of the as-synthesized nanocomposite.   Figure 5C. Moreover, the calibration plot representing the relationship between the different pH against potential and current response is shown in Figure 5D. Figure 5E represents the calibration plot of pH against the potential. As the pH rises from 3 to 7, the anodic peak current increases, whereas above 7, the pH is lowered. Therefore, pH 7 was used for the MA detection process. The reaction mechanism of mefenamic acid is shown in Scheme 2. The oxidative reaction of MA is a one-electron, one-proton transfer process resulting in the radical species = N radical dot, which can be outlined as the following expression [6]: analyzed. The Ipa response of MA in different PB-supporting electrolytes is depicted in Figure 5C. Moreover, the calibration plot representing the relationship between the different pH against potential and current response is shown in Figure 5D. Figure 5E represents the calibration plot of pH against the potential. As the pH rises from 3 to 7, the anodic peak current increases, whereas above 7, the pH is lowered. Therefore, pH 7 was used for the MA detection process. The reaction mechanism of mefenamic acid is shown in Scheme 2. The oxidative reaction of MA is a one-electron, one-proton transfer process resulting in the radical species = N radical dot, which can be outlined as the following expression [6]: Scheme 2. Schematic representation of oxidation mechanism of mefenamic acid.

Influence of Concentration and Scan Rate
The effect of the concentration of MA at fabricated DMO/CNF/GCE sensor was analyzed by CV curves, with an increasing MA concentration of 20-100 μM in 0.01 M PB supporting electrolyte. From Figure 6A Figure 7A. Under optimized conditions, MA concentration addition shows a response of increasing current. The linear relationship between the MA concentration and the peak current was represented in Figure 7B, which shows a regression equation of I pa (µA) = 0.030 (MA) µM + 3.404, R 2 = 0.9966 concentration ranging from 0.01 to 741 µM. By using the formula LOD = 3 S/q [48], the LOD was assessed to be 0.009 µm, and the sensitivity of DMO/CNF/GCE to be 0.4309 µA µM −1 cm −2 . Table 1 summarizes the electrocatalytic efficiency of different sensing materials for the detection of MA. In comparison with the literature reports, our sensing nanomaterial DMO/CNF/GCE is better in terms of determination, detection limit, and linear range for the detection of MA. As a result, DMO/CNF/GCE shows an acceptable linear range and better LOD in comparison with other methods.

Selectivity Assay
Anti-interfering ability is one of the essential factors in assessing the potential to interfere with the signal generated by a sensor. The interfering agents, such as inorganic ions and organic molecules to MA, such as glucose (GLU), urea (UA), ascorbic acid (AA), Cl -, NH 4+ , and K + , were analyzed. Figure 8A shows the negligible change in the current of MA upon the addition of interfering reagents to 0.1 M PB supporting electrolyte containing 100 μM of MA. No considering changes were found, even in the presence of some interfering agents. Hence, this proves the selectivity of the proposed sensor DMO/CNF/GCE toward the detection of MA. The stability of the DMO/CNF/GCE is also

Selectivity Assay
Anti-interfering ability is one of the essential factors in assessing the potential to interfere with the signal generated by a sensor. The interfering agents, such as inorganic ions and organic molecules to MA, such as glucose (GLU), urea (UA), ascorbic acid (AA), Cl -, NH 4+ , and K + , were analyzed. Figure 8A shows the negligible change in the current of MA upon the addition of interfering reagents to 0.1 M PB supporting electrolyte containing 100 µM of MA. No considering changes were found, even in the presence of some interfering agents. Hence, this proves the selectivity of the proposed sensor DMO/CNF/GCE toward the detection of MA. The stability of the DMO/CNF/GCE is also an important category. The stability of the DMO/CNF/GCE proposed by DPV analysis was evaluated in 0.1 M PB solution at a scan rate of 50 mVs −1 in the presence of 100 µM of MA for about 15 days ( Figure 8B). The stability response of DMO/CNF/GCE was tested by storing the electrode at an ambient temperature. Based on the observation, the peak current of DMO/CNF/GCE retained 98.4% of the original current even after 20 days, which proves the stability of the proposed sensor. To ensure the ability of the proposed DMO/CNF/GCE, reproducibility was assessed by DPV analysis. The DPV response of five different modified electrodes was shown in Figure 8C, which was constructed for monitoring of 100 μM concentration of MA in 0.1 M PB supporting electrolyte at a scan rate of 50 mVs −1 . The results of which confirmed the impressive reproducibility of DMO/CNF/GCE. According to the results, our proposed sensor has better reproducibility.

Real Sample Analysis
DPV technique was employed for the real-time analysis of our proposed sensor DMO/CNF/GCE (Figure 9). The prepared nanocomposite was used to determine the existence of MA in human blood and tablet. To ensure the ability of the proposed DMO/CNF/GCE, reproducibility was assessed by DPV analysis. The DPV response of five different modified electrodes was shown in Figure 8C, which was constructed for monitoring of 100 µM concentration of MA in 0.1 M PB supporting electrolyte at a scan rate of 50 mVs −1 . The results of which confirmed the impressive reproducibility of DMO/CNF/GCE. According to the results, our proposed sensor has better reproducibility.

Real Sample Analysis
DPV technique was employed for the real-time analysis of our proposed sensor DMO/CNF/GCE (Figure 9). The prepared nanocomposite was used to determine the existence of MA in human blood and tablet. The tablets were purchased from a pharma shop in Taipei, Taiwan. The human urine samples were collected from the Chang-Gung memorial hospital in Taiwan. In addition, the institutional review board of Chang-Gung memorial hospital (IRB No.201801660B), Taiwan, approved this study.
For the preparation of pharmaceutical samples, the tablet (5 mg) was manually homogenized to a fine powder. An adequate amount of this powder, equivalent to 1 mM, was weighed and dissolved in pure ethanol. The solution was sonicated for 15 min to achieve complete dissolution. The solution was filtered and stored for preparing working solutions for pharmaceutical formulations by taking suitable aliquots and diluting them with the same solvent and phosphate buffer.The human blood sample was collected from a healthy volunteer. The collected blood samples were centrifuged for 15 min at 1600 rpm. Finally, the supernatant generated was carefully separated using a clean pipette. Now, the blood sample was diluted 100 times with 1.0 × 10 −1 M phosphate buffer and stored in a refrigerator for further analysis.
Thus, the prepared sample was taken for DPV analysis, depicted in Figure 9. Table 2 summarizes the recovery results of the human blood and tablet as 99.8-98.4 % and 99.8-98.2%. It was clear that the DMO/CNF/GCE was reliable and feasible for real sample analysis.

Conclusions
In this present work, DMO/CNF was prepared by the facile hydrothermal method. The crystal structure, composition, and morphology were confirmed by XRD, XPS, FE-SEM, and HR-TEM. Thus, our proposed sensor exhibits excellent electrocatalytic activity and selectivity toward the detection of MA. Added to that, CNF improves the electron transfer property, which results in the enhancement of the electrocatalytic property. The developed sensor shows a low LOD of 0.009 μM with a wide linear range of 0.01-741 μM. Satisfactory results were obtained for the real sample, such as human blood and tablet. Due to the synergetic effect between the DMO and CNF, which provides a higher surface The tablets were purchased from a pharma shop in Taipei, Taiwan. The human urine samples were collected from the Chang-Gung memorial hospital in Taiwan. In addition, the institutional review board of Chang-Gung memorial hospital (IRB No.201801660B), Taiwan, approved this study.
For the preparation of pharmaceutical samples, the tablet (5 mg) was manually homogenized to a fine powder. An adequate amount of this powder, equivalent to 1 mM, was weighed and dissolved in pure ethanol. The solution was sonicated for 15 min to achieve complete dissolution. The solution was filtered and stored for preparing working solutions for pharmaceutical formulations by taking suitable aliquots and diluting them with the same solvent and phosphate buffer.The human blood sample was collected from a healthy volunteer. The collected blood samples were centrifuged for 15 min at 1600 rpm. Finally, the supernatant generated was carefully separated using a clean pipette. Now, the blood sample was diluted 100 times with 1.0 × 10 −1 M phosphate buffer and stored in a refrigerator for further analysis.
Thus, the prepared sample was taken for DPV analysis, depicted in Figure 9. Table 2 summarizes the recovery results of the human blood and tablet as 99.8-98.4% and 99.8-98.2%. It was clear that the DMO/CNF/GCE was reliable and feasible for real sample analysis.

Conclusions
In this present work, DMO/CNF was prepared by the facile hydrothermal method. The crystal structure, composition, and morphology were confirmed by XRD, XPS, FE-SEM, and HR-TEM. Thus, our proposed sensor exhibits excellent electrocatalytic activity and selectivity toward the detection of MA. Added to that, CNF improves the electron transfer property, which results in the enhancement of the electrocatalytic property. The developed sensor shows a low LOD of 0.009 µM with a wide linear range of 0.01-741 µM. Satisfactory results were obtained for the real sample, such as human blood and tablet. Due to the synergetic effect between the DMO and CNF, which provides a higher surface area and effective electron transport, the electrochemical performance for the MA detection was very good. DMO/CNF is, therefore, a promising material for sensitive, inexpensive, and efficient sensors, making it possible for practical applications in anti-inflammatory medication sample monitoring. Data Availability Statement: All data generated or analyzed during this study are included in this published article.

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