## 3 Results and Discussion
### Effect of Concentration of 4-NBD\({}^{+}\) on Surface Coverage  
The SPE was electrografted with 4-NBD\({}^{+}\) concentrations from 5 \(\upmu\)M to 5 mM for SPCE and from 5 \(\upmu\)M to 10 mM for AuBT-SPE (included for comparison), following the methodology described in Section 2.4.1. Figure 4a shows the behavior of the cathodic peak potential (ascribed to peak \"1\" in Figure 3b and depicted as blue dots) as a function of the 4-NBD\({}^{+}\) concentration in solution on activated SPCE (grafting step in the potential window from +0.5 V to \(-\)1.0 V). The results indicate a shift in the cathodic peak potential towards more negative values as the 4-NBD\({}^{+}\) concentration increases up to 1 mM, typically for irreversible systems. Beyond this concentration, the potential shifts towards more positive values, making reduction more difficult. The cathodic peak current (represented as black dots in Figure 4a) increases with 4-NBD\({}^{+}\) concentration up to 2 mM, followed by a nearly constant value up to 5 mM 4-NBD\({}^{+}\). At concentrations above 3 mM, it can be assumed that the number of aryl amino functionalities on the electrode surface remains relatively constant, suggesting a saturation of the surface [63]. This fact can be corroborated by the decrease in the cathodic peak current of the redox probe, Fe(CN)\({}_{6}\)\({}^{3-}\) (illustrated as red dots in Figure 4a), whose values achieve the residual current (25 mM HCl, green dots in Figure 4a), while the peak current of 4-NBD\({}^{+}\) remains almost constant. Figure 4b illustrates the evolution of the electrode surface coverage (\(\Gamma\)), calculated using [PERSON]'s law [64], as a function of the aryldiazonium ion concentration. The SPCE surface coverage (illustrated as black dots) increases with the 4-NBD\({}^{+}\) concentration up to a constant value, corresponding to the maximum coverage of 8.51 \(\times\) 10\({}^{-10}\) molcm\({}^{-2}\), which is obtained at 2 mM 4-NBD\({}^{+}\). The theoretical surface coverage for a compact monolayer of a 4-substituted phenyl group was calculated by [PERSON] and [PERSON] [57], with a result of \(\Gamma\) = 13.5 \(\times\) 10\({}^{-10}\) molcm\({}^{-2}\). The experimental surface coating obtained is in good agreement with the theoretical value and the small difference found can be attributed to two critical parameters: electrolysis time and grafting potential. On the one hand, the surface coating decreases as the grafting potential becomes more anodic potential [65]. On the other hand, the roughness and the real surface area (quite different from the geometric area) achieved after electrochemical treatment play an important role as well, being the lower values of surface coverage than those corresponding to the theoretical surface coverage [57]. Then, the comparison between the experimentally obtained coverage on SPCE and the theoretical value confirms that the electrografting process successfully forms a compact monolayer on the electrode surface. This is also supported by the non-observation of the CV signal of the redox probe, [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\), illustrated as a red line in Figure 3d. In contrast, higher 4-NBD\({}^{+}\) concentrations are needed to achieve the maximum coating on the AuBT-SPE surface (depicted as blue dots in Figure 4b). In this case, the potential window is narrower than that used with SPCE with a more positive grafting potential (grafting step in the potential window from +0.5 V to \(-\)0.5 V), achieving less surface coating for the same concentration of 2 mM 4-NBD\({}^{+}\). To achieve maximum coverage on AuBT-SPE, at least 5 mM 4-NBD\({}^{+}\) is needed, but the possibility of multilayer formation is expected. When this occurs, some aryl groups can be covalently bonded together and others are grafted to the surface, while the remainder of the film can be the result of the adsorption [57].