## 3 Results and Discussion
### Electrochemical Behaviors of the SY onto GCE/ERGO-NiNPs Surface  
The effect of the pH value on the electrochemical response was studied using the CV method in buffer solutions with pH values ranging from 3 to 8. The intensity of the current corresponding to the SY anodic peak changes with pH, and is obtained at its highest value for a pH = 4.0 (Figure 5A). This pH value is selected as an optimized buffer solution for further investigations. Otherwise, both anodic and cathodic peak potentials are decreased with an increase in pH. This result indicates the oxidation mechanism of SY involving electron transfers coupled with proton exchange. The anodic peak potential was linearly changed to pH value following the equation (Figure 5B):  
\[E_{\text{pa}}\left(\text{mV}\right)=(-33.5\pm 1.2)\times\text{pH}+(888.5\pm 6.6) \ (R^{2}=0.995). \tag{4}\]  
The slope of the 33.5 mV pH\({}^{-1}\) unit is estimated to a half of the theoretical Nernstian value of 59 mV pH\({}^{-1}\), indicating that the proton/electron ratio involved in the SY oxidation reaction is approximately being to 0.5. The effect of the scan rate on the response of 10 uM SY on the GCE/ERGO-NiNPs is investigated. The CVs show that the peaks exhibit an increase in the currents and shift with respect to the formal potential toward increasing the scan rate (Figure 6A). This behaviour is involved in the kinetics of the electrochemical process. The anodic and cathodic peak currents are linearly varied to the scan rate 0.02-0.4 V s\({}^{-1}\) following the equation:  
\[I_{\text{pa}}\left(\text{\mu A}\right)=(110.5\pm 3.62)\times v \tag{5}\]  
\[I_{\text{pc}}\left(\text{\mu A}\right)=(-73.0\pm 3.42)\times v \tag{6}\]  
The current increases slowly over the range of 0.5-1.0 V s\({}^{-1}\) and deviates from linearity (Figure 6B). This observation indicates that the mechanism of the electrochemical process of SY was changed gradually from an adsorption to a diffusion process. Such a property is in good agreement with some previous works which used ionic liquid functionalised reduced graphene oxide-AuNPs and polypyrrole-carbon nanotube nanocomposite-modified electrodes [22; 58]. Using [PERSON]'s theory, the peak potentials were plotted towards the logarithm of the scan rate, leading to two linear lines over the range 0.07-0.4 V s\({}^{-1}\) using the following equations (Figure 6C):  
\[E_{\text{pa}}\left(\text{V}\right)=(0.0315\pm 0.0020)\times\text{ln}v(\text{V s${}^{-1}$})+(0.839\pm 0.0037) \tag{7}\]  
\[E_{\text{pc}}=(-0.0279\pm 0.0022)\times\text{ln}v(\text{V s${}^{-1}$})+(0.0637 \pm 0.0042) \tag{8}\]  
Figure 5: (**A**) CVs at the scan rate v = 50 mV s\({}^{-1}\) at GCE/ERGO-NiNP surface of 10 uM SY in buffer solution with different pH values: pH = 3 (black), pH = 4 (red), pH = 5 (green), pH = 6 (blue), pH = 7 (cyan), pH = 8 (magenta). (**B**) Linear variation of anodic peak potential toward the pH value. From these slope values, the values of \(n\) and \(\alpha\) were estimated at 1.73 and 0.55 following equations RT/(1-\(\alpha\))nF and -RT/\(\alpha\)nF for anodic and cathode lines, respectively. From the intercept of these linear lines (vi), the apparent electron transfer rate constant kET was calculated to 1.65 s\({}^{-1}\) following equation kET = (1-\(\alpha\))nFv\({}_{\rm{vi}}\)/RT. These results suggest that the oxidation mechanism of SY on GCE/ERGO-NiNPs involves two electrons and one proton [58]. This result is in good agreement with recent research thoroughly investigating the electrochemical oxidation pathway of SY by using theoretical calculations and spectro-electrochemical analyses [59]; our previous studies were also based on using GCE/ERGO-modified electrodes [55]. Hence, the electrochemical oxidation mechanism of SY onto GCE/ERGO-NiNPs was proposed thorough two consecutive electro-transfer processes separated by a chemical deprotonation (denoted as an E1 CE2 mechanism) (Scheme 1). Normally, it is expected that the oxidation potential of a second electron be higher than the first one.