Methylene blue (Chemical formula: C₁₆H₁₈ClN₃S; Purity > 98.0) was obtained from Aladdin Chemicals Co. Ltd. (Shanghai, China). Sulfuric acid (H₂SO₄) and sodium sulfate were produced by Merck. Sodium chloride (NaCl) and sodium hydroxide (NaOH) were manufactured by Merck. Stock solutions were prepared with distilled water. All experiments were carried out at laboratory temperature. All the chemicals used in this work were of analytical grade.

Methylene blue solutions were prepared in different supporting electrolytes by dissolving the desired amount of MB in distilled water. Voltammograms were then recorded using an Autolab PGSTAT 20 connected to a computer.

The working electrode is a boron-doped diamond electrode with an active surface area of 1 cm². This electrode was prepared by hot filament chemical vapor deposition (HF-CVD) on low-resistivity p-type silicon wafers (1 – 3 mΩ·cm) (Siltronix, 10 cm diameter, 0.5 mm thickness). The process gas was a mixture of 1% CH₄ in H₂ containing trimethylborane. The film growth rate was 0.24 μm·h⁻¹. The film thickness was approximately 1 μm. Further details concerning the preparation of BDD electrodes are available in references [19] [20] . Scanning electron microscopy (SEM) images of the BDD electrode were taken using a JEOL LJMS-6300-F instrument.

The experimental setup consists of a three-electrode electrochemical cell composed of a working electrode (BDD) (working area was 1 cm²), a counter electrode (Pt wire), and a reference electrode (saturated calomel electrode, SCE). These electrodes, immersed in a 100 mL solution, are connected to an Autolab potentiostat (ECHOCHEMIE PGSTAT 20) controlled by a computer (GPES software) allowing computerized data acquisition ( Figure 1 ). The working electrode is fixed in a cage, facilitating contact between it and the electrolyte through an opening. The reference electrode is placed in a luggin capillary, the tip of which is positioned approximately 2 mm from the working electrode to minimize ohmic drop.

Before any voltammetric measurements, the electrodes are pretreated in a 0.1 M sulfuric acid solution by performing several cyclic voltammetry cycles at a scan rate of 50 mV/s during 10 minutes, with the goal of ensuring reproducibility and reliability of the results.

The surface morphology of the boron-doped diamond electrode is shown in Figure 2 . Scanning Electron Microscopy (SEM) reveals that the BDD has a polycrystalline structure [21] . The grain size ranges from 0.3 to 0.6 µm, and the grains are tightly bonded to each other. Additionally, a dark area is observed at the base of the diamond grains, particularly at the grain boundaries, likely related to graphitic carbon (Csp²) formed during the preparation of the BDD.

Figure 3 shows the Raman spectrum of the boron-doped diamond (BDD) sample, obtained under laser excitation at a wavelength of 514.5 nm. This spectrum reveals two intense peaks located at 500 cm⁻¹ and 1332 cm⁻¹, attributed to silicon and diamond-type carbon (Csp³), respectively. In addition, a broad band centered around 1550 cm⁻¹, characteristic of non-diamond carbon impurities (Csp²), is also observed [22] , which is consistent with the observation made using scanning electron microscopy (SEM). This observation suggests that graphitic carbon is present on the boron-doped diamond electrode, likely located at the grain boundaries of the diamond.

Figure 4 shows the cyclic voltammogram of the BDD electrode in 0.1 M H₂SO₄ medium in the absence of organic compound at 50 mV/s. The onset potentials for hydrogen and oxygen evolution can be clearly distinguished at −1.38 V and 1.84 V vs. SCE, respectively. Between the onset potentials of hydrogen and oxygen evolution, there is the electrochemical window of water stability at BDD electrode stability region, as well as an anodic wave between 1.40 and 1.85 V (highlighted in Figure 3 ). This wave could likely indicates the formation of oxidative sulfate species such as $S{O}^{·}{}_4^{-}$ and $S_2{O}_8^{2-}$ [23] - [25] in the supporting electrolyte. Species formation is described by equations (1) through (7). The formation of hydroxyl radicals results from the electrochemical decomposition of water on the BDD electrode (Equation 8). According to the literature, a sulfuric acid solution simultaneously contains sulfate ions, hydrogen sulfate anions, and undissociated sulfuric acid. However, only the latter two species react with hydroxyl radicals to generate sulfate radicals, as shown in equations (6) and (7) [25] . Moreover, according to Gnamba et al., this anodic wave may be attributed to the oxidation of surface redox species such as Csp² especially surface quinone functional group [14] .

$S{O}_4^{2-}\to S{O}^{·}{}_4^{-}+e^{-}$(1)

$HS{O}_4^{-}\to S{O}^{·}{}_4^{-}+H^{+}+e^{-}$ (2)

$S{O}^{·}{}_4^{-}+HS{O}_4^{-}\to S_2{O}_8^{2-}+H^{+}+e^{-}$ (3)

$S{O}_4^{2-}+S{O}^{·}{}_4^{-}\to S_2{O}_8^{2-}+e^{-}$ (4)

$S{O}^{·}{}_4^{-}+S{O}^{·}{}_4^{-}\to S_2{O}_8^{2-}$ (5)

$HS{O}_4^{-}+{}^{·}OH\to S{O}^{·}{}_4^{-}+H_{2}O$ (6)

$H_{2}S{O}_4+{}^{·}OH\to S{O}^{·}{}_4^{-}+H_{3}O$ (7)

$H_{2}O\to {}^{·}OH+H^{+}+e^{-}$ (8)

Figure 5(A) shows the behavior of the boron-doped diamond electrode in the absence and presence of methylene blue (0.4 mM). This figure reveals that the oxidation of MB is characterized by four (04) anodic peaks and one cathodic peak located at 0.035, 0.94, 1.34, 1.57, and −0.063 V, respectively. All of these peaks appear before the oxygen evolution potential (1.84 V). However, Peak 4 appears within the potential domain associated with the formation of oxidative sulfate species, which may contribute to the oxidation of MB. These observations suggest that the oxidation of MB may occur via direct electron transfer and/or via oxidative species electro-generated in the reaction medium. The MB oxidation could also be catalyzed the non-diamond carbon impurities such as Csp2 presents at the BDD’s grain boundaries [14] . Peaks 1 and 1’ are attributed to the redox peaks of MB [26] [27] .

In the following sections of this work, the investigations will focus exclusively on these two peaks. The difference between the anodic potential (Epa = 0.0497 V) and the cathodic potential (Epc = −0.0626 V) gives ΔEp = 0.1123 V or 112.3 mV, which is greater than 59/n mV. This is characteristic of a quasi-reversible electrochemical process. This result is confirmed by the ratio of the anodic peak current (Ipa = 4.91 × 10⁻⁵ A) to the cathodic peak current (|Ipc| = 1.13 × 10⁻⁴ A) is less than 1.

The plot of the redox peak currents (1 and 1’) as a function of concentration ( Figure 5B ) shows linear relationships over the entire concentration range (0.1 - 0.6 mM), with determination coefficients (R² = 0.993 and R² = 0.9985) close to 1. This linearity confirms that the oxidation and reduction peak currents are proportional to the MB concentration. Therefore, the calibration equations obtained (Equations 9 and 10) indicate that this relationship can be used for the quantification of MB using the BDD electrode.

$Ipa=0.0001\left[BM\right]+2\times {10}^{-06},R^2=0.993$ (9)

$Ipc=-0.0002\left[BM\right]+1\times {10}^{-06},R^2=0.999$ (10)

The effect of scan rate on the oxidation of MB (0.6 mM) was studied in a 0.1 M sulfuric acid electrolyte solution over a scan rate range of 20 to 200 mV/s, within a potential window from −1 to 1.8 V vs. SCE. The obtained voltammograms were used to determine the peak potentials and peak currents (for peaks 1 and 1’). Figure 6(A) shows the plot of Ip as a function of v^1/2. This representation allows for the analysis of the electrochemical reaction kinetics, identifying whether the process is controlled by diffusion and/or adsorption.

Indeed, if the relationship Ip = f(v^1/2) is linear and passes through the origin, the process at the electrode surface is governed by the diffusion of electroactive species to the electrode surface. Conversely, any deviation from linearity or significant intercept away from the origin would likely indicate an adsorption-controlled or mixed-control process.

The curves Ipa = f(v^1/2) and Ipc = f(v^1/2) for peaks 1 and 1’ yield straight lines that nearly intersect the origin (Equations 11 and 12). This observation means that the oxidation and reduction processes at the electrode surface are diffusion-controlled.

$Ipa=0.0002v^{1/2}+3\times {10}^{-06},R^2=0.9889$(11)

$Ipc=-0.0003v^{1/2}-7\times {10}^{-06},R^2=0.9916$(12)

The diffusion-controlled nature of the processes occurring at peaks 1 and 1’ is confirmed by the plots of ln Ip vs. ln v ( Figure 6(B) ), which yield straight lines with slopes of 0.5002 and 0.4417 for the anodic and cathodic peaks, respectively (Equations 13 and 14). In fact, a slope close to 0.5 indicates a diffusion-controlled process, whereas a slope close to 1 suggests an adsorption-controlled process [28] [29] .

$\ln Ipa=0.5002\ln v-8.593,R^2=0.9884$(13)

$\ln Ipc=0.4417\ln v-8.1009,R^2=0.9914$(14)

The speed of the electrochemical system was evaluated by analyzing the relationship between the potential and the logarithm of the scan rate. Figure 6(C) shows the resulting plots, which are straight lines with non-zero slopes (Equations 15 and 16) and determination coefficients close to 1, suggesting a slow electrochemical process characteristic of a non-reversible system.

$Epa=0.0185\ln v+0.0694,R^2=0.9998$(15)

$Epc=-0.0071\ln v-0.0808,R^2=0.9712$(16)

Figure 7 presents the cyclic voltammograms of the BDD electrode in 0.1 M Na₂SO₄ medium, in the absence and presence of the organic compound, at 50 mV/s. In the cyclic voltammogram recorded in the absence of MB (inset of Figure 7 ), an anodic wave is observed between 1.33 and 1.80 V. The same observation was made in the H₂SO₄ medium. Additionally, the onset potentials for hydrogen and oxygen evolution are approximately −1.33 V and 1.80 V, respectively.

The voltammetric profile of the electrolyte solution containing MB (0.4 mM) exhibits the same general characteristics as those obtained in the sulfuric acid medium. The oxidation of MB in this medium is indicated by peaks, with four anodic peaks located at −0.33 V, 0.79 V, 1.2 V, and 1.43 V, and a cathodic peak at −0.44 V. All anodic peaks appear before the oxygen evolution potential (1.80 V), indicating a direct electron transfer oxidation process at the BDD surface. However, Peak 4 appears in the potential region corresponding to the anodic wave of the BDD in Na₂SO₄ alone, suggesting the involvement of oxidative sulfate species in the oxidation process observed at Peak 4. The MB oxidation could also be catalyzed the non-diamond carbon impurities such as Csp2 presents at the BDD’s grain boundaries.

Moreover, in the presence of MB, a decrease in the oxygen evolution current at high potentials is observed, which suggests the involvement of oxidative species generated from water decomposition in the oxidation of MB.

According to the values obtained from the voltammogram in the presence of the organic compound ( Figure 7 ), the difference between the anodic and cathodic peak potentials is greater than 59/n mV, and the Ipa/Ipc ratio is less than 1, characteristics of a quasi-reversible electrochemical process.

The plots of the redox peak currents (1 and 1’) as a function of concentration ( Figure 8 ) shows linear relationships over the entire concentration range from 0.1 to 0.4 mM for peak 1, and from 0.1 to 0.6 mM for peak 1’. The determination coefficients are close to 1, indicating excellent correlation between peak intensity and MB concentration (Equations 17 and 18). This linearity confirms that the oxidation and reduction peak currents are proportional to the MB concentration. However, in the case of anodic peak 1, it is observed that the current no longer increases when the methylene blue concentration exceeds 0.4 mM. Similar observations were reported by Ju et al in a supporting medium with a pH of 7.2 [30] .

According to them, this phenomenon could be explained by the formation of dimers or larger aggregates at higher concentrations.

$Ipa=0.1066\left[BM\right]+2\times {10}^{-06},R^2=0.9923$(17)

$Ipc=-0.1455\left[BM\right]-3\times {10}^{-07},R^2=0.9992$(18)

The effect of scan rate on the oxidation of MB (0.6 mM) was also studied in a 0.1 M Na₂SO₄ electrolyte solution over a scan rate range of 10 to 200 mV/s within a potential window from −1 to 1.8 V vs. SCE. The obtained voltammograms were used to determine the peak potentials and peak currents for peaks 1 and 1’. Figure 9(A) shows the plots of Ipa = f(v^1/2) and Ipc = f(v^1/2) for peaks 1 and 1’ over the scan rate range of 10 to 200 mV/s. The resulting curves are linear with determination coefficients close to 1 (Equations 19 and 20). This observation indicates that the oxidation and reduction processes at the electrode surface are diffusion-controlled [31] [32] .

$Ipa=0.0001v^{1/2}+2\times {10}^{-06},R^2=0.993$(19)

$Ipc=-0.0002v^{1/2}-8\times {10}^{-07},R^2=0.9961$(20)

This result is consistent with that obtained from the plots of ln Ip versus ln v for the anodic and cathodic peaks [33] [34] , where all slopes are less than 0.5 ( Figure 9(B) ).

The plot of Ep as a function of ln (v) is a straight line with a non-zero slope described by Equations 21 and 22 ( Figure 9(C) ):

$Epa=0.0378\ln Ipa-0.197,R^2=0.9587$(21)

$Epc=-0.0067\ln Ipc-0.435,R^2=0.9971$(22)

This result suggests that the reaction is not fast. Moreover, the plot of Ip = f(v^1/2) is a straight line, which is characteristic of a quasi-reversible reaction.

The electrochemical behavior of MB (0.6 mM) on boron-doped diamond was studied within a potential range from −1 to 1.8 V in 0.1 M NaOH at a scan rate of 50 mV/s ( Figure 10 ). The inset of Figure 10 shows the cyclic voltammograms of the BDD electrode in the absence and presence of MB. In the absence of the compound, the cyclic voltammogram exhibits an anodic large peak located around at 1.44 V and an oxygen evolution potential started at 1.60 V. The anodic peak can be attributed to oxygen production through the oxidation of hydroxide, according to the redox couple O₂/OH⁻ (Equation 23).

$O{H}^{-}\to 1/2O_2+H^{+}+2e^{-}$(23)

In the forward potential scan, and the presence of MB in the reaction medium, an anodic wave appears starting at 0.33 V, indicating the oxidation of the organic compound. This oxidation occurs before the onset of oxygen evolution, suggesting that a direct oxidation process occurs at the surface of the BDD electrode. The

lower current observed in the presence of MB compared to that obtained in the supporting electrolyte alone, in the high potential region, is characteristic of a mediated oxidation process involving oxidative species generated from the electrochemical discharge of water.

Unlike in H₂SO₄ and Na₂SO₄ media, the oxidation of MB in the basic NaOH (0.1 M) medium is characterized by a single anodic wave. This difference in electrochemical behavior can be attributed to the nature of the supporting electrolyte. In sulfuric acid and sodium sulfate supporting electrolytes, the oxidative species formed, notably the sulfate radical ( $S{O}^{·}{}_4^{-}$) and the persulfate ion ( $S_2{O}_8^{2-}$), exhibit higher selectivity toward electron-rich organic compounds, such as aromatic molecules and sulphur and nitrogen containing molecules. The oxidative species derived from sulfates react selectively with MB (C₁₆H₁₈ClN₃S) due to the presence of an aromatic ring and sulfur and nitrogen atoms in its structure [35] . This interaction leads to a multi-step oxidation mechanism based on direct electron transfers, without the involvement of strong oxidants before the onset of water oxidation.

Figure 11 illustrates the plot of current as a function of concentration at a fixed potential of 0.5 V. The MB concentration ranged from 0.1 to 0.4 mM, and voltammetric measurements were carried out at 50 mV/s. The resulting plot is a straight line described by the equation (Equation 24):

$Ip=0.0005\left[MB\right]+6\times {10}^{-5},R^2=0.9931$(24)

The determination coefficient (R²), close to 1, indicates excellent linearity of the relationship. This confirms that the BDD electrode can be used in NaOH medium for the quantification of MB.

In this study, the effect of scan rate was investigated over a potential scan range from 30 to 500 mV/s. The resulting cyclic voltammograms are shown in Figure 12 . It can be observed that the oxidation wave of methylene blue increases with the scan rate.

To determine the oxidation process at the electrode, Ip versus the square root of scan rate was plotted at a fixed potential of 0.5 V.

The plot of current versus the square root of the scan rate yields a straight line that does not pass through the origin, indicating that the oxidation of MB is diffusion-controlled, likely coupled with a chemical reaction or adsorption ( Figure 13(A) ).

In contrast, the plot of ln Ip = f(ln v), shown in Figure 13(B) (Equation 25), gives a straight line with a slope of 0.5567, close to 0.5, which is characteristic of a diffusion-controlled process.

$\ln Ip\left(A\right)=0.5567\ln v-7.0653,R^2=0.9957$(25)

Previous studies have highlighted a polymerization phenomenon occurring at the surface of the working electrode during the electrochemical oxidation of certain organic compounds [14] . The formation of this polymeric film can hinder the oxidation reaction or alter the electrochemical behavior of the electrode. In the present study, the influence of the number of scan cycles on the shape of the voltammograms in the presence of methylene blue (MB) was investigated in different supporting electrolytes. The results obtained in Na₂SO₄ and NaOH media are presented in Figure 14 , for a fixed scan rate of 50 mV/s. In both media, a gradual decrease in the anodic current is observed from the first cycles, until a superposition of the curves is reached, an indication of a stable electrochemical behaviour, starting from the 5ᵗʰ cycle in NaOH and the 12ᵗʰ cycle in Na₂SO₄. These observations suggest that the oxidation of MB on BDD leads to the formation of a polymeric film on the electrode surface. Moreover, the stabilization of the anodic current may result from the low adsorption capacity of BDD, which is related to the inert nature of its surface. A similar behavior was also observed in H₂SO₄ medium (figure not shown), where the current decreases up to the 4ᵗʰ cycle, followed by a superposition of the voltammetric curves in the anodic domain.