## 3 Results and Discussion
### Electrografting of Carbon Electrodes with 4-NBD\({}^{+}\): Electrochemical Characterization  
Grafting onto commercial bare carbon and MWCNT modified SPCEs was performed by electrochemical reduction of 4-NBD\({}^{+}\) on the electrode surface using CV over a wide potential window (+0.5 V to \(-\)1.0 V). Figure 3a shows the CVs of the electrografting of 2 mM 4-NBD\({}^{+}\) in 25 mM HCl on an activated MWCNT-SPCE. This first scan exhibits an irreversible cathodic peak (illustrated as \"(1)\") with a peak potential at 0.225 V, corresponding to the formation of an aryl radical which covalently binds to the electrode surface, accompanied by the loss of N\({}_{2}\). Furthermore, a second wide cathodic peak (illustrated as \"(2)\") with two maximums is observed; the first one at \(-\)0.605 V, followed by the other at \(-\)0.720 V, which can be assigned to the electrochemical reduction in the nitrophenyl group. This process results in the formation of an aminophenyl group through a two-step reduction process with an overall 6 e\({}^{-}\) and 6 H\({}^{+}\) exchange, leaving the electrode surface functionalized to nanometric scale with arylamine functionalities oriented towards the bulk solution [51]. The anodic scan (illustrated as \"(3)\") shows a peak at 0.180 V, which can be associated with the oxidation of the aminophenyl to hydroxyaminophenyl groups [52; 53; 54]. The narrow peak observed close to 0.0 V is attributed to the carbonyl groups generated on carbon surface during the acidic electrochemical treatment (activation step). Figure 3b,c show the CVs of the electrografting of 2 mM 4-NBD\({}^{+}\) in 25 mM HCl on an activated SPCE during five scans, and the detail of the fifth scan on both electrodes, SPCE and MWCNT-SPCE, respectively. The first scan (depicted as black line in Figure 3b) shows the irreversible cathodic peak \"(1)\" and the second cathodic wave \"(2)\" at negative potentials (0.006 V and \(-\)0.820 V, respectively) with lower peak currents when compared to MWCNT-SPCE (Figure 3a), demonstrating the high catalytic effect of the MWCNT modifier on the electrochemical reduction processes of 4-NBD\({}^{+}\). The cathodic peaks were completely inhibited in the subsequent scans (second to fifth, illustrated as dashed lines in Figure 3b), confirming the formation of a layer with blocking properties that inhibit the electronic transfer between the solution and the surface [27; 55]. This inhibition is not complete on MWCNT-SPCE, as can be seen in its CV illustrated in Figure 3c (dashed line). The electrochemical properties of SPCE-GRAFT and MWCNT-SPCE-GRAFT with aminophenyl functionalities on the surface were investigated by CV in the presence of 1 mM K\({}_{3}\)Fe(CN)\({}_{6}\) as an electroactive redox system or redox probe. Figure 3d shows the CVs of 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) in an aqueous solution containing 50 mM PBS (pH 7.4), before and after the electrochemical reduction in the 4-NBD\({}^{+}\) on activated SPCE and on MWCNT-SPCE. The electrochemical response of the redox probe on bare activated SPCE (illustrated as a black line) exhibited a cathodic peak of \(-16.62\) uA at 0.007 V and the corresponding anodic peak of 17.13 uA at 0.168 V. This corresponds to the transformation of Fe(CN)\({}_{6}\)\({}^{3-}\) into Fe(CN)\({}_{6}\)\({}^{4-}\) and vice versa through a one-electron process [56]. The response of the redox probe on bare activated SPCE displayed a quasi-reversible system with a peak-to-peak separation (\(\Delta\)E) of 161 mV, which is significantly higher than the ideal 59 mV value, and the ratio between the current peaks was \(\mathrm{i}_{\mathrm{p}a}/\mathrm{i}_{\mathrm{p}c}=1.03\). After the electrografting process, the CV of the Fe(CN)\({}_{6}\)\({}^{3-}\) in 50 mM PBS on SPCE-GRAFT (illustrated as a red line) showed no peaks in 50 mM PBS, indicating that a compact organic layer was formed (passivating the electrode surface) that completely blocked the electron transfer [27; 54].