# Induced Effect of Gold Nanoparticles (AuNPs) and Halide Ions on Pyridoxine Molecule Stability
## 3 Results and Discussion
### The Influence of Fluoride Ions on Electrochemical Behavior of Pyridoxine  
Figure 1 presents a comparison of the voltammograms recorded on the gold electrode in the supporting electrolyte of 0.1 mol \(\times\) L\({}^{-1}\) NaF, in the absence and in the presence of 75 mg \(\times\) L\({}^{-1}\) AuNPs and 2.9 \(\times\) 10\({}^{-4}\) mol \(\times\) L\({}^{-1}\) pyridoxine (vitamin B6). As can be seen, the supporting electrolyte leads to a higher peak corresponding to the potential range between 1.5 and 2.0 V. However, the addition of gold nanoparticles in the electrolyte solution leads to the appearance of a new wide peak of the current density recorded between 0.8 and 1.5 V vs. Ag I AgCl reference electrode. At high values of the anodic overvoltage, oxides/hydroxides (AuOH, AuO, Au\({}_{2}\)O\({}_{3}\)) are formed on the surface of the gold electrode [56]. The electrogeneration of oxides on the surface of the gold electrode modifies the catalytic properties of the surface [56], which can lead to various mechanisms of electrochemical degradation of pyridoxine molecules. The onset potential for initiating the growth of the mixed oxide/hydroxide layer is about 1.35 V. The irreversible feature in the gold response is the huge anodic peak that extends in the range of 1.5 to 2.0 V.  
In terms of electrochemical properties, the potential range from 1.5 to 2.0 V is very important because, in addition to oxygen depolarization, there is also the continuous change of the electrode/electrolyte interface. In this range of potential, gold nanoparticles oxidize (NaF_AuNP curve). The properties of gold nanoparticles are highlighted so that the anodic current densities are considerably reduced in their presence, decreasing from 37 (NaF curve) to 22 mA \(\times\) cm\({}^{-2}\) (NaF_AuNP curve). According to the cyclic voltammogram NaF_B6, lower values of anodic current densities are recorded, compared to that of the supporting electrolyte. This is due to the strong adsorption processes of organic vitamin molecules on the surface of the gold electrode. The electrochemical analysis highlights the heterogeneous oxidation processes corresponding to the oxidation of organic molecules by the appearance of an anodic peak, well defined by low intensity at anodic potential of 0.9 V. In the detail of the NaF_B6 curve, shown in Figure 2, there is a clear increase in current densities starting with potential greater than 0.8 V. This is also the onset potential corresponding to the charge transfer in which the pyridoxine molecules are involved. The simultaneous presence in the electrolyte solution of pyridoxine and nanoparticles (NaF_B6-AuNP curve) has an antagonistic effect, thus reducing the anodic current densities corresponding to oxygen depolarization (NaF curve), but at the same time has higher current densities compared to the NaF_AuNP curve. The characteristics and appearance of cyclic voltammograms also changes during cathodic polarization. The  
Figure 1: Cyclic voltammetry responses of gold electrode in 0.1 mol \(\times\) L\({}^{-1}\) NaF support electrolyte in the absence and in the presence of 75 mg \(\times\) L\({}^{-1}\) gold nanoparticles (AuNPs) and 2.9 \(\times\) 10\({}^{-4}\) mol\(\cdot\)L\({}^{-1}\) pyridoxine (vitamin B6). cathodic polarization highlights the appearance of a peak of the cathodic current density in the support electrolyte, with low intensity (0.7 mA \(\times\) cm\({}^{-2}\)), due to the reduction of oxides/hydroxides on the surface of the gold electrode. Characteristic to this peak is that, in the presence of gold nanoparticles, the peak intensity increases to 7.5 mA \(\times\) cm\({}^{-2}\), and in the ternary system (NaF_B6_AuNPs) it has the highest value of 12.6 mA \(\times\) cm\({}^{-2}\).