SrMnO 3 /Functionalized h-BN Composite Modiﬁed Disposable Sensor for the Voltammetric Determination of Furaltadone Antibiotic Drug

: Improper disposal of pharmaceutical drugs, including antibiotics, can affect the ecological system and generate serious health problems for living organisms. In this work, we have developed an electrochemical sensor based on a strontium manganese oxide/functionalized hexagonal boron nitride (SrMnO 3 / f -BN) electrocatalyst for the detection of the antibiotic drug furaltadone (FLD). Various analytical techniques were used to characterize the physicochemical properties of the as-prepared SrMnO 3 /f-BN composite. The as-fabricated SrMnO 3 / f -BN composite electrode showed excellent sensing activity towards FLD, with a wide linear range (0.01–152.11 µ M) and low detection limit (2.0 nM). The sensor exhibited good selectivity towards FLD for detection in the presence of various interfering species (nitro compounds, metal ions, and biological compounds). Interestingly, real-time analysis using the proposed SrMnO 3 / f -BN composite was able to determine the FLD content in human urine and wastewater samples with good recovery. Hence, the as-developed SrMnO 3 / f -BN modiﬁed sensor could be viable in practical applications to target the antibiotic drug FLD in both human ﬂuids and environmental samples


Introduction
Furaltadone (FLD) is a synthetic antibiotic drug used in the veterinary field to treat bacterial diseases such as fowl cholera, coccidiosis, and blackhead disease [1,2]. High-level doses of FLD in animals lead to severe side effects, including mutagenesis, hemolytic anemia, thrombopenia, insanity, and chronic toxicity, and causes cancer in rats and mice [3,4]. Due to the adverse effects of this drug, most countries have banned it; however, FLD is used in several food products at the quantitative level [5,6]. Thus, the determination of FLD in food samples is an essential need; detection follows Rapid Alert System for Food and Feed (RASFF) standards for its residues in poultry, aquatic, and all animal products due

Materials Characterization
The functional group analysis of the as-prepared SrMnO 3 , f -BN, and SrMnO 3 /f -BN composite was confirmed by FT-IR spectroscopy, as shown in Figure 1A. The FTIR spectrum of SrMnO 3 ( Figure 1A(a)) exhibited a stretching frequency band at 660 cm −1 , ascribed to the Mn-O bond formation. The bending vibration mode of the O-B-O bond appeared at 545 cm −1 , which resembles the BO 6 octahedron in the SrMnO 3 perovskite structure. The bands at 580 and 450 cm −1 originate from the vibration of metal-oxygen bonds with a spinel cubic structure of tetrahedral and octahedral sites. The broad bands at 3100 and 3400 cm −1 are attributed to the hydroxyl groups. In the FT-IR spectrum of f -BN ( Figure 1A(b)), the observed bands at 520, 620, 820, 1580, and 3180 cm −1 correspond to B-N, B-O, B-N-B, B-NH 2 , and B-OH bond formation, respectively. The FT-IR spectrum of the SrMnO 3 /f -BN composite showed similar characteristic vibrational peaks, with high intensity and slight shift as compared to the FT-IR spectrum of SrMnO 3 and f -BN, as shown in Figure 1A(c). The FT-IR spectrum of the SrMnO 3 /f -BN composite revealed the shift in peak values due to the strong interaction between SrMnO 3 and f -BN.
The XRD pattern of SrMnO 3 , f -BN, and SrMnO 3 /f -BN composite is depicted in Figure 1B. The XRD pattern of SrMnO 3 ( Figure 1B , and (220) planes, respectively. All these diffraction planes are well matched with the standard pattern of JCPDS card No. 84-1612, ascribed to the hexagonal perovskite structure with a space group of P6 3 /mmc. The (110) plane shows the maximum intensity, with the d-spacing of 2.46 Å, and the corresponding lattice parameters are obtained at about a = b = 5.44 Å and c = 9.07 Å. Furthermore, the crystallite size of the as-prepared SrMnO 3 perovskite particle was calculated to be 13 nm based on the (110) plane using the Scherrer method. In addition, Figure 1B(b) shows the XRD pattern of f -BN, which exhibits the diffraction of 2θ angles at 26.96 • , 41.96 • , 50.23 • , and 55.33 • , corresponding to the hexagonal structure of boron nitride with the crystal planes of (002), (100), (102), and (004), respectively. The corresponding pattern is well-matched with the standard file of JCPDS card No. 73-2095 and the space group of P-6m2. In addition, the XRD pattern of the SrMnO 3 /f -BN composite shows a slight decrease in crystallinity planes, with a lower angle shift in Figure 1B(c). However, the peaks at (002) and (100) planes continue to exist in the pattern, indicating the incorporation of the f -BN nanosheet with the SrMnO 3 microsphere. Figure 1C shows the Raman spectra of SrMnO 3 , f -BN, and SrMnO 3 /f -BN composite. The hexagonal phase of the SrMnO 3 ( Figure 1C(a)) exhibits eight Raman active modes, namely, modes 2A 1g + 2E 1g + 4E 2g [12,41,42] The peak at 339 cm −1 is assigned to the E 1g mode, suggesting Mn ion displacements. At the same time, the peaks at higher frequencies indicate that the oxygen ion is involved in the MnO 6 octahedral. This suggests that the oxygen ions can be distinguished as O (1) and O (2) ions, with both involved in face sharing and corner-sharing of the adjacent octahedral. The O (1) oxygen ions alone provide Raman active modes. Significantly, the peak around 435 cm −1 is assigned to the octahedral tilting E 1g and bending E 2g modes. The high-intensity peaks at 593 and 642 cm −1 are ascribed to the asymmetric E 2g and symmetric A 1g octahedral stretching modes associated with the Sr and Mn ions, respectively. The Raman spectrum of f-BN ( Figure 1C(b)) exhibits a sharp peak at 1369 cm −1 , assigned to the E 2g mode and comparable to the G-band of graphene. The Raman spectrum of the SrMnO 3 /f -BN composite ( Figure 1C(c)) shows the  Figure 1C shows the Raman spectra of SrMnO3, f-BN, and SrMnO3/f-BN composite. The hexagonal phase of the SrMnO3 ( Figure 1C(a)) exhibits eight Raman active modes, namely, modes 2A1g + 2E1g + 4E2g [12,41,42] The peak at 339 cm −1 is assigned to the E1g mode, suggesting Mn ion displacements. At the same time, the peaks at higher frequencies indicate that the oxygen ion is involved in the MnO6 octahedral. This suggests that the oxygen ions can be distinguished as O (1) and O (2) ions, with both involved in face sharing and corner-sharing of the adjacent octahedral. The O (1) oxygen ions alone provide Raman active modes. Significantly, the peak around 435 cm −1 is assigned to the octahedral tilting E1g and bending E2g modes. The high-intensity peaks at 593 and 642 cm −1 are ascribed to the asymmetric E2g and symmetric A1g octahedral stretching modes associated with the Sr and Mn ions, respectively. The Raman spectrum of f-BN ( Figure 1C(b)) exhibits a sharp peak at 1369 cm −1 , assigned to the E2g mode and comparable to the G-band of graphene. The Raman spectrum of the SrMnO3/f-BN composite ( Figure 1C(c)) shows the typical characteristic bands of SrMnO3 and f-BN mentioned above, which display a decrease in intensity due to the strong interaction of the f-BN nanosheet with the SrMnO3 microspheres.
The thermal stability of SrMnO3, f-BN, and SrMnO3/f-BN was characterized by the thermogravimetric analysis (TGA) method. Figure 1D(a) displays the three different The thermal stability of SrMnO 3 , f -BN, and SrMnO 3 /f -BN was characterized by the thermogravimetric analysis (TGA) method. Figure 1D(a) displays the three different weight-loss stages: (i) the first weight loss of 11.4% in the temperature range up to 150 • C corresponds to the loss of surface adsorbed water molecules' evaporation; (ii) the second weight loss between 250 to 400 • C involves the carbonyl of SrMn(CO 3 ) 2 followed by decomposition of organic contents; and (iii) the final weight loss at 400-700 • C is attributed to the strontium precursor and manganese decomposition. In addition, Figure 1D(b) shows f -BN nanosheet thermal degradation at 200 • C, which can be attributed to disclosing of the peroxide moiety and decomposition of the oxygen group. Interestingly, the thermal decomposition behavior of the SrMnO 3 /f -BN composite ( Figure 1D(c) exhibits similar weight loss as the pristine SrMnO 3 and f -BN, confirming the incorporation of f -BN into the SrMnO 3 microsphere particles.
Furthermore, BET measurements were performed for SrMnO 3 , f -BN, and SrMnO 3 /f -BN composite materials based on the N 2 adsorption/desorption isotherm method. The results can be seen in Figure S1. From the BET analysis, the specific surface area of the SrMnO 3 and SrMnO 3 /f -BN is 8.76 and 10.25 m 2 /g, respectively, while the pore size and pore volume of the SrMnO 3 /f -BN composite is calculated as 13.23 nm and 1.37 cm 3 /g, respectively. The larger surface area and porous are attributed to the formation of different active sites for enhancing the electrocatalytic activity of the material due to the mobility of ions/electrons. FE-SEM was used to analyze the surface morphology of SrMnO 3 , f-BN, and SrMnO 3 /f-BN composite samples. Figure 2A-C shows a homogeneous microsphere morphology formed through the integration of SrMnO 3 primary nanoparticles (particle size range of 100-250 nm. However, the average size of microsphere particles is calculated at approximately 6 µm. Figure 2D-F shows the FE-SEM images of the f -BN, which exhibit the stacked flakes of exfoliated BN due to the combination of oxygen functionalities. The elemental mapping proves the oxygen functionalities, and their atomic percentages are provided in Figure S3. Figure 2G,H displays FE-SEM images of the SrMnO 3 /f -BN composite, confirming the decoration of SrMnO 3 microsphere particles on f -BN flakes. This could be valuable in the creation of a substantial surface area in the composite for improving its electrochemical performance. Furthermore, the elemental mapping and EDX spectrum confirmed the distribution of elements such as Sr (Strontium), Mn (Manganese), O (Oxygen), B (boron), and N (Nitrogen) in the SrMnO 3 /f -BN composite. The corresponding results are shown in Figure S2.

Electrochemical Properties
The charge transfer resistance property of the SrMnO3/f-BN nanocomposite electrode was investigated by electrochemical impedance spectroscopy, for which it was examined in

Electrochemical Properties
The charge transfer resistance property of the SrMnO 3 /f-BN nanocomposite electrode was investigated by electrochemical impedance spectroscopy, for which it was examined in 5 mM [Fe(CN) 6

Electrochemical Behavior of FLD
Cyclic voltammetry (CV) is an essential technique in electrochemical methods, as it is easier to understand the initial electrochemical behavior, and the resulting information is rather useful for performing electrochemical studies about complicated electrode reactions. Therefore, preliminary electrochemical experiments were carried out using the CV technique to examine the electrochemical behavior of FLD. Beforehand, different operational electrodes were fabricated, such as bare SPCE, pristine boron nitride modified SPCE (BN/SPCE), f-BN/SPCE, and SrMnO3/f-BN/SPCE. The CV curves were recorded at a fixed scan rate of 50 mV/s in pH 7 containing 200.0 µM FLD sensing sample. The obtained CV signals are shown in Figure 4A. The CV results show that the bare SPCE has a weak reduction peak at −0.475 V with a current intensity of −8.01 µA, which is due to poor electron transfer mobility between the unmodified SPCE surface and FLD. On the other hand, the BN/SPCE shows a slightly better cathodic current response than the bare SPCE, at −0.473 V, with a current density of about −8.66 µA. This might be due to the electrical insulating property of BN, which bears a wide bandgap; this may resist the transfer of electrons towards the reduction of FLD. Despite this, the BN consists of covalently bounded B (boron) and N (nitrogen) atoms, which disturb the electronic states of symmetry as well as their electronic arrangement, suggesting the narrowing of the sp 2 -derived π bands. The SrMnO3/SPCE shows a well-resolved cathodic current amplitude of −14.85 µA at −0.5311 V, corresponding to the superior electrical conductivity of the SrMnO3 nanospheres, and it retains the covalent bonding of Mn-O bonds and Sr 2+ ions in the crystal lattice, providing faster electron transferability for FLD sensing. Along with this, the SrMnO3 nanospheres access to the transition of manganese ionic oxidation states between Mn 3+ Mn 4+ is able to accelerate the electrocatalysis property, promoting the reduction of FLD. f-BN/SPCE exhibits better electrocatalytic activity than BN/SPCE thanks to the effective functionalization of the BN surface with OH functional groups, tremendously increasing the boron radicals at the edges to create defective sites. This electrochemically enables advantageous

Electrochemical Behavior of FLD
Cyclic voltammetry (CV) is an essential technique in electrochemical methods, as it is easier to understand the initial electrochemical behavior, and the resulting information is rather useful for performing electrochemical studies about complicated electrode reactions. Therefore, preliminary electrochemical experiments were carried out using the CV technique to examine the electrochemical behavior of FLD. Beforehand, different operational electrodes were fabricated, such as bare SPCE, pristine boron nitride modified SPCE (BN/SPCE), f -BN/SPCE, and SrMnO 3 /f -BN/SPCE. The CV curves were recorded at a fixed scan rate of 50 mV/s in pH 7 containing 200.0 µM FLD sensing sample. The obtained CV signals are shown in Figure 4A. The CV results show that the bare SPCE has a weak reduction peak at −0.475 V with a current intensity of −8.01 µA, which is due to poor electron transfer mobility between the unmodified SPCE surface and FLD. On the other hand, the BN/SPCE shows a slightly better cathodic current response than the bare SPCE, at −0.473 V, with a current density of about −8.66 µA. This might be due to the electrical insulating property of BN, which bears a wide bandgap; this may resist the transfer of electrons towards the reduction of FLD. Despite this, the BN consists of covalently bounded B (boron) and N (nitrogen) atoms, which disturb the electronic states of symmetry as well as their electronic arrangement, suggesting the narrowing of the sp 2 -derived π bands. The SrMnO 3 /SPCE shows a well-resolved cathodic current amplitude of −14.85 µA at −0.5311 V, corresponding to the superior electrical conductivity of the SrMnO 3 nanospheres, and it retains the covalent bonding of Mn-O bonds and Sr 2+ ions in the crystal lattice, providing faster electron transferability for FLD sensing. Along with this, the SrMnO 3 nanospheres access to the transition of manganese ionic oxidation states between Mn 3+ ⇔Mn 4+ is able to accelerate the electrocatalysis property, promoting the reduction of FLD. f -BN/SPCE exhibits better electrocatalytic activity than BN/SPCE thanks to the effective functionalization of the BN surface with OH functional groups, tremendously increasing the boron radicals at the edges to create defective sites. This electrochemically enables advantageous FLD sensing, with a current response of about −15.71 µA at −0.46 V. Finally, the SrMnO 3 /f -BN/SPCE shows higher cathodic current waves of about −23.01 µA at a cathodic potential of-0.465 V. This cathodic CV signal is attributed to the direct electrocatalytic reduction of the nitro group (NO 2 ) of FLD into the hydroxylamine group, with the elimination of the water molecule accompanied by the participation of an equal number of electron (4H + ) and proton (4e − ) transfer processes (Scheme 1).
Catalysts 2022, 12,1494 FLD sensing, with a current response of about −15.71 µA at −0.46 V. Finally, the SrM BN/SPCE shows higher cathodic current waves of about −23.01 µA at a cathodic p of-0.465 V. This cathodic CV signal is attributed to the direct electrocatalytic re of the nitro group (NO2) of FLD into the hydroxylamine group, with the elimin the water molecule accompanied by the participation of an equal number of electro and proton (4e -) transfer processes (Scheme 1).

Scheme 1. Electrochemical reduction mechanism of FLD.
At the same time, no oxidation peak current response was observed while p ing the anodic scan, suggesting an irreversible electron transfer chemical reaction p From the above-obtained results and findings, it is clear that the SrMnO3/f-BN/SP livered a beneficial reduction in peak current response, and the lowest cathodic p for FLD detection over that of other electrodes (bare SPCE, BN/SPCE, f-BN/SPCE retained. Additionally, the electrochemical reduction of FLD occurs at the su SrMnO3/f-BN/SPCE due to the excellent electrostatic interaction between the FLD Scheme 1. Electrochemical reduction mechanism of FLD. prominent modification of the electronic properties of f-BN aided on the 3d, 4d, and 5d transition metal surfaces, which enhances the combinational electrocatalytic activity of the composite and in turn facilitates the effective sensing of FLD. The obtained current response for different electrodes is represented as a bar diagram in Figure 4B.

Effect of Concentration
The influence of concentration on SrMnO3/f-BN/SPCE was examined using CV by adding FLD concentration from 50.0 to 300.0 µM. Figure 4C depicts the CV curves of various concentrations of FLD; it can be seen that the cathodic peak current of FLD increases linearly with increasing concentration of FLD. This is due in part to the good electrodeelectrolyte interface, as the optimal diffusion length favors sensing of the FLD in the electrochemical system. In addition, an excellent linear relationship can be observed in Figure  4D, demonstrating that the reduction of current signal waves is directly proportional to the concentration of FLD. There is a linear regression equation of Ipc = −0.081 FLD [µM] −6.138 with correlation coefficient of R 2 = 0.988. From the concentration CV studies, the asprepared composite acts as a potential electrode material as well as an excellent electron At the same time, no oxidation peak current response was observed while performing the anodic scan, suggesting an irreversible electron transfer chemical reaction process. From the above-obtained results and findings, it is clear that the SrMnO 3 /f -BN/SPCE delivered a beneficial reduction in peak current response, and the lowest cathodic potential for FLD detection over that of other electrodes (bare SPCE, BN/SPCE, f -BN/SPCE) can be retained. Additionally, the electrochemical reduction of FLD occurs at the surface of SrMnO 3 /f -BN/SPCE due to the excellent electrostatic interaction between the FLD and the electrode surface, confirming the improved electrocatalytic property of the SrMnO 3 /f -BN composite. This synergistic activity is due to the interaction of the d z 2 of the transition metal orbitals and the B-p z , N-p z orbitals of f-BN. These bonds are responsible for the prominent modification of the electronic properties of f -BN aided on the 3d, 4d, and 5d transition metal surfaces, which enhances the combinational electrocatalytic activity of the composite and in turn facilitates the effective sensing of FLD. The obtained current response for different electrodes is represented as a bar diagram in Figure 4B.

Effect of Concentration
The influence of concentration on SrMnO 3 /f -BN/SPCE was examined using CV by adding FLD concentration from 50.0 to 300.0 µM. Figure 4C depicts the CV curves of various concentrations of FLD; it can be seen that the cathodic peak current of FLD increases linearly with increasing concentration of FLD. This is due in part to the good electrode-electrolyte interface, as the optimal diffusion length favors sensing of the FLD in the electrochemical system. In addition, an excellent linear relationship can be observed in Figure 4D, demonstrating that the reduction of current signal waves is directly proportional to the concentration of FLD. There is a linear regression equation of I pc = −0.081 FLD [µM] −6.138 with correlation coefficient of R 2 = 0.988. From the concentration CV studies, the as-prepared composite acts as a potential electrode material as well as an excellent electron mediator, as it possesses chemically electroactive sites via the functionalization of BN with active functional groups. In addition, the electrical conductivity is further enhanced by the SrMnO 3 nanospheres, which allow faster electron transfer processes for FLD detection.

Effect of pH and Scan Rates
In electrochemical detection systems, pH is one of the most important parameters for developing an electrochemical sensor, as it affects the sensitivity and electrochemical behavior of FLD at the surface of SrMnO 3 /f -BN/SPCE. Figure 5A displays the CV response of 200.0 µM FLD for different pH values (pH 3.0, pH 5.0, pH 7.0, pH 9.0, and pH 11.0) recorded at a scan rate of 50 mV/s. It can be seen that the cathodic current response of FLD gradually increases from pH 3.0 to 7.0, and gains a maximum current intensity at pH 7.0; subsequently, the current amplitude decreases, moving towards a higher pH value in Figure 5B. Following this pH test, pH 7.0 was chosen as a supporting electrolyte for electrochemical experiments. The slope of the linear plot between cathodic peak potential (E p ) and pH ( Figure S4) demonstrates that the electrochemical performance of FLD is strongly pH-dependent [46] and the hydrogen ion (H + ) concentration influences the reaction rate of the constructed electrode. The electrode kinetics property of FLD at the SrMnO 3 /f -BN/SPCE was studied by recording the CV signals in pH 7.0 at different scan rates from 20 to 200 mV/s in the electrochemical cell containing 200.0 µM FLD. Figure 5C shows the CV signal wave concerning the applied potential, which is equivalent to the cathodic reduction reaction of FLD. It can be observed that the cathodic current density ramps up accordingly upon increasing the scan rate from 20 to 200 mV/s. In addition, the cathodic peak potential shift towards the negative side of the potential window affirms the irreversible electrode process. Figure 5D represents a good linear relationship between the FLD peak current intensity and the scan rate (ν), with the correlation coefficient R 2 = 0.997 and the corresponding linear regression equation expressed as I pc = −0.078 ν −10.77. These results show that the reduction process of FLD catalyzed by SrMnO 3 /f -BN/SPCE is controlled by the adsorption process. The process was further evaluated by plotting the linear relation between the log scan rate (log υ(mV/s) and log current (log I (µA)), as shown in Figure S5A. The slope of the linear plot clearly confirms that the electrode processes are neither adsorption-controlled (slope = 1) nor diffusion-controlled (slope = 0.5). The slope value (0.35) of Figure S5A shows that FLD detection on SrMnO 3 /f-BN modified SPCE favors a diffusion-controlled reaction process. Another linear relation ( Figure S5B) is plotted against E p vs. lnυ based on the Laviron Equations (1) and (2): where R is the gas constant (8.314 J mol −1 K −1 ), T is the temperature (298 K), n is the number of transferred electrons, α is the electron transfer coefficient (0.5), and F is the Faraday constant is 96,485 C mol −1 . The slope of the rate of the linear plot is shown in Figure S5B.

Differential Pulse Voltammetry (DPV) Analysis
To evaluate the limits of detection and sensitivity, the electroanalytical behavior of SrMnO3/f-BN composite fabricated SPCE was investigated using the DPV technique; the obtained DPV signals are shown in Figure 6A. Initially, the electrolyte solution (pH 7.0) was deoxygenated with massive purity of N2 gas. Following this process, the voltammogram experiment was recorded by the consecutive addition of analyte concentration from 0.01 to 332.11 µM. Figure 6A represents the DPV amplification waves concerning the successive injection of the target FLD analyte. It can be seen that the cathodic current intensity accelerates linearly with each addition of FLD, governed by the linear regression equation Ipc = −0.1715 FLD [µM]-4.269 with the correlation coefficient R 2 = 0.991 ( Figure 6B). The developed sensor exhibits a wide dynamic linear range of detection, from 0.01 to 152.11 µM. The limit of detection of the sensor is calculated using the following equation: where σ is the standard deviation of the background of the three DPV signals and the slope value obtained from the linear plot of peak current (µA) vs. the concentration of

Differential Pulse Voltammetry (DPV) Analysis
To evaluate the limits of detection and sensitivity, the electroanalytical behavior of SrMnO 3 /f -BN composite fabricated SPCE was investigated using the DPV technique; the obtained DPV signals are shown in Figure 6A. Initially, the electrolyte solution (pH 7.0) was deoxygenated with massive purity of N 2 gas. Following this process, the voltammogram experiment was recorded by the consecutive addition of analyte concentration from 0.01 to 332.11 µM. Figure 6A represents the DPV amplification waves concerning the successive injection of the target FLD analyte. It can be seen that the cathodic current intensity accelerates linearly with each addition of FLD, governed by the linear regression equation I pc = −0.1715 FLD [µM]-4.269 with the correlation coefficient R 2 = 0.991 ( Figure 6B). The developed sensor exhibits a wide dynamic linear range of detection, from 0.01 to 152.11 µM. The limit of detection of the sensor is calculated using the following equation: where σ is the standard deviation of the background of the three DPV signals and the slope value obtained from the linear plot of peak current (µA) vs. the concentration of FLD (µM). The calculated LOD of the FLD sensor is about 2.0 nM, and its sensitivity (sensitivity = slope/electrode area) is 2.45 µA µM −1 cm −2 . In addition, the modified electrode reduces the mass transfer resistance property due to the synergistic electrocatalytic effect between the f -BN and SrMnO 3 nanospheres. From the DPV studies, the fabricated SrMnO 3 /f -BN/SPCE sensor reveals its dramatic sensing application towards FLD, with a good catalytic effect affirmed by the rapid electron transfer facilitation of the composite. The electrochemical performance of SrMnO 3 /f -BN/SPCE in the detection of FLD was compared with the previously reported FLD sensors, and the results are outlined in Table 1 [9,10,[56][57][58][59].
reduces the mass transfer resistance property due to the synergistic electrocatalytic effect between the f-BN and SrMnO3 nanospheres. From the DPV studies, the fabricated SrMnO3/f-BN/SPCE sensor reveals its dramatic sensing application towards FLD, with a good catalytic effect affirmed by the rapid electron transfer facilitation of the composite. The electrochemical performance of SrMnO3/f-BN/SPCE in the detection of FLD was compared with the previously reported FLD sensors, and the results are outlined in Table 1 [9,10,56-59]. Note: ADSV -Anodic differential stripping voltammetry; DPV -Differential pulse voltammetry; A -Amperometry; LOD -Limit of detection.

Selectivity, Repeatability, and Storage Stability Studies
Selectivity is one of the key factors for the developed sensor, and seems to be the basic requirement for practical applications. Here, the selectivity test was performed using DPV in the existence of interfering compounds, namely, metal ions (Fe 2+ , Mg 2+ , Na + , NO 2− , Zn 2+ , Co 2+ ), biomolecules such as ascorbic acid (AA), uric acid (UA), sucrose (SUC), dopamine (DOP), glucose (GLU), hydrogen peroxide (H2O2), lactose (LAC), and citric acid (CA), and nitro compounds such as flutamide (FLU), chloramphenicol (CAP), nitrofurantoin (NFD), 4-nitroaniline (4-NA), and furazolidone (FZ). The DPV signals of SrMnO3/f-BN/SPCE towards 10.0 µM FLD in the presence of interfering compounds are displayed in Figure 7A-C. It can be seen that the interfering compounds show negligible changes with the main peak current DPV signal of FLD, with an RSD of about 5.95%. Therefore, the developed sensor possesses anti-interference capability towards the selective sensing of FLD. The repeatability experiment was carried out by adding 10.0 µM of FLD in the electrolyte cell, then the DPV signal was recorded continuously over five repeated runs, with each consecutive measurement obtained as DPV signal waves. The  Note: ADSV-Anodic differential stripping voltammetry; DPV-Differential pulse voltammetry; A-Amperometry; LOD-Limit of detection.

Selectivity, Repeatability, and Storage Stability Studies
Selectivity is one of the key factors for the developed sensor, and seems to be the basic requirement for practical applications. Here, the selectivity test was performed using DPV in the existence of interfering compounds, namely, metal ions (Fe 2+ , Mg 2+ , Na + , NO 2− , Zn 2+ , Co 2+ ), biomolecules such as ascorbic acid (AA), uric acid (UA), sucrose (SUC), dopamine (DOP), glucose (GLU), hydrogen peroxide (H 2 O 2 ), lactose (LAC), and citric acid (CA), and nitro compounds such as flutamide (FLU), chloramphenicol (CAP), nitrofurantoin (NFD), 4-nitroaniline (4-NA), and furazolidone (FZ). The DPV signals of SrMnO 3 /f -BN/SPCE towards 10.0 µM FLD in the presence of interfering compounds are displayed in Figure 7A-C. It can be seen that the interfering compounds show negligible changes with the main peak current DPV signal of FLD, with an RSD of about 5.95%. Therefore, the developed sensor possesses anti-interference capability towards the selective sensing of FLD. The repeatability experiment was carried out by adding 10.0 µM of FLD in the electrolyte cell, then the DPV signal was recorded continuously over five repeated runs, with each consecutive measurement obtained as DPV signal waves. The results are displayed in Figure 7D, showing that the fashioned electrode exhibits excellent repeatability with an RSD of about 0.39%. Furthermore, the storage stability of the electrode was tested by adding 10 µM, then obtaining the DPV signal. Afterwards, the DPV signal was measured again over the duration of 10 to 30 days. Interestingly, the cathodic current signal shows only a slight drop in the current response from the initial FLD peak current signal ( Figure 7E), with an RSD of about 2.25%. From the above DPV results, the fabricated electrode retains good selectivity, repeatability, and admirable storage stability towards the sensing of FLD. results are displayed in Figure 7D, showing that the fashioned electrode exhibits excellent repeatability with an RSD of about 0.39%. Furthermore, the storage stability of the electrode was tested by adding 10 µM, then obtaining the DPV signal. Afterwards, the DPV signal was measured again over the duration of 10 to 30 days. Interestingly, the cathodic current signal shows only a slight drop in the current response from the initial FLD peak current signal ( Figure 7E), with an RSD of about 2.25%. From the above DPV results, the fabricated electrode retains good selectivity, repeatability, and admirable storage stability towards the sensing of FLD.

Real Sample Analysis
The practical usability of the designed sensor was examined using the DPV technique for the real-time detection of FLD in lake water and human urine samples. The lake water samples were collected from nearby Taipei Lake, and a human urine sample was collected from a healthy person. The collected real samples were free from FLD. Following the initial testing process, a known concentration of FLD was spiked into the samples. The prepared samples were used as a stock solution for further experimental analysis. The DPV amplification waves were recorded for the consecutive addition of FLD concentrations from 20.0 to 100.0 µM from the prepared stock solutions. The obtained DPV results are displayed in Figure 8A,B; it can be seen that the developed sensor retains excellent electrocatalytic activity for the detection of FLD in real sample analysis, with good recovery results of about to 98.1-99.9%. The real sample analysis findings reveal that SrMnO 3 /f-BN/SPCE possesses ideal sensing properties for the detection of FLD. The recovery results are calculated using the standard addition method with three replicative measurements, and the obtained results are outlined in Table 2. and the obtained results are outlined in Table 2.

Synthesis of SrMnO 3 Perovskite Microspheres
SrMnO 3 perovskite microspheres were synthesized by the co-precipitation method. In brief, about 50 mL each of strontium nitrate (0.05 M; 1.058 g) and manganese nitrate (0.05 M; 1.255 g) aqueous solution were sequentially added to a beaker with 100 mL of potassium bicarbonate (0.1 M; 2.7642 g) solution and then subjected to magnetic stirring at ambient conditions for 3 h. During this time, the color of the mixed solution turned to a brown residue. The precipitate was collected by filtration and washed with deionized water (DI) and ethanol several times. The collected precipitate was dried in a hot oven at 80 • C for 12 h. Later, it was calcined at 700 • C (5 • C/min) for 2 h in an air atmosphere. The final obtained powder was denoted as SrMnO 3 .

Functionalization of BN
Briefly, the as-received h-BN powder (1 mg/mL) was added to 200 mL H 2 O 2 solution and then sonicated for 30 min to obtain a homogeneous dispersion. Then, the suspension was stirred at room temperature using a magnetic stirrer for 12 h. Then, the mixture was transferred to an autoclave container for thermal reaction at 120 • C, for 24 h. The final product was centrifuged and washed several times with DI water and then dried at 80 • C in an air oven for 6 h. The resultant powder was referred to as f -BN.

Synthesis of SrMnO 3 /f-BN Composite
About 50 mg of as-prepared SrMnO 3 microsphere particles were dispersed in 50 mL DI water along with 10 mg of f -BN and ultra-sonicated for 1 h. The composite suspension was centrifuged and washed with DI water and ethanol several times. Afterwards, the resultant residue was dried at 60 • C for 24 h. The final product was called SrMnO 3 /f-BN composite. The synthesis details are clearly described in Scheme 2.

Synthesis of SrMnO3 Perovskite Microspheres
SrMnO3 perovskite microspheres were synthesized by the co-precipitation method. In brief, about 50 mL each of strontium nitrate (0.05 M; 1.058 g) and manganese nitrate (0.05 M; 1.255 g) aqueous solution were sequentially added to a beaker with 100 mL of potassium bicarbonate (0.1 M; 2.7642 g) solution and then subjected to magnetic stirring at ambient conditions for 3 h. During this time, the color of the mixed solution turned to a brown residue. The precipitate was collected by filtration and washed with deionized water (DI) and ethanol several times. The collected precipitate was dried in a hot oven at 80 °C for 12 h. Later, it was calcined at 700 °C (5 °C/min) for 2 h in an air atmosphere. The final obtained powder was denoted as SrMnO3.

Functionalization of BN
Briefly, the as-received h-BN powder (1 mg/mL) was added to 200 mL H2O2 solution and then sonicated for 30 min to obtain a homogeneous dispersion. Then, the suspension was stirred at room temperature using a magnetic stirrer for 12 h. Then, the mixture was transferred to an autoclave container for thermal reaction at 120 °C, for 24 h. The final product was centrifuged and washed several times with DI water and then dried at 80 °C in an air oven for 6 h. The resultant powder was referred to as f-BN.

Synthesis of SrMnO3/f-BN Composite
About 50 mg of as-prepared SrMnO3 microsphere particles were dispersed in 50 mL DI water along with 10 mg of f-BN and ultra-sonicated for 1 h. The composite suspension was centrifuged and washed with DI water and ethanol several times. Afterwards, the resultant residue was dried at 60 °C for 24 h. The final product was called SrMnO3/f-BN composite. The synthesis details are clearly described in Scheme 2.

Instrumentation
The X-ray diffraction (XRD) pattern study was analyzed using a Bruker D2 PHASER diffractometer (Karlsruhe, Germany) with Cu-Kα radiation (K = 1.54 Å). Fourier transforminfra red (FT-IR) spectral data were recorded using FTIR-6600 spectroscopy (Easton, MD, USA). Raman spectra were examined using a confocal micro-Renishaw, 632 nm He-Ne laser source spectrometer (Gloucestershire, UK). The surface morphologies of the as-synthesized materials were analyzed using a field emission scanning electron microscope (FE-SEM: JEOL JSM-7610F Plus, Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDAX). Brunauer-Emmett-Teller (BET) analysis for specific surface area measurement was conducted using Micromeritics, Gemini VII (Monchegladbach, Germany) A conventional three-electrode system was employed using a CHI1205C and CHI900 potentiostat (CH Instruments, Inc., Austin, TX, USA), where Ag/AgCl (sat. KCl) was used as a reference electrode, modified screen-printed carbon electrode (SPCE) as a working electrode, and Pt-wire as a counter electrode.

Fabrication of the Sensor Electrode
The as-prepared SrMnO 3 /f-BN composite powder (5 mg/mL) was added to a DI water-containing vial. A homogenous suspension was obtained after ultrasonication treatment for 30 min. Meanwhile, the surface of SPCE was gently washed with DI water and ethanol to remove the surface adsorbed dust particles, then dried naturally. About 6 µL of the catalyst suspension was drop-coated on the cleaned SPCE surface, followed by drying in an air oven at 80 • C for 10 min. The fabricated electrode was named SrMnO 3 /f-BN/SPCE. Electrodes such as SrMnO 3 /SPCE, BN/SPCE, and f-BN/SPCE were fabricated similarly for the control experiment. The fabricated electrodes were immersed in a deoxygenated electrolyte (0.1 M PBS, pH 7) system for sensing measurements.

Conclusions
In summary, we synthesized a facile and inexpensive composite based on SrMnO 3 /f -BN for FLD detection. The above results obtained from electrochemical detection indicate that the developed sensor has superior catalytic activity and excellent analytical performance, with a wide linear range of 0.01-152.11 µM and a very low detection limit of 2.0 nM. Interestingly, the SrMnO 3 /f -BN composite could be a promising candidate for the detection of FLD in both biological and pharmaceutical applications.  Figure S4: The linear relation between cathodic peak potential (Ep) and various pH. Figure S5: (A) The linear relation between log scan rate (log υ (mV/s) and log current (log I (µA)). (B) The linear relation between Ep and ln υ.