## 3 Results and Discussion
### Electrochemical Characterization of Catalytic Surface: DA-Sensor
Figure 6 shows the voltammograms of 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) in 50 mM PBS (pH 7.4) recorded in the potential window between \(-\)0.5 V and +0.5 V at 100 \(\mathrm{mVs^{-1}}\). The electrochemical response of the redox probe on activated SPCE BARE (black line in Figure 6a and amplified in Figure 6b), shows an anodic peak of 22.42 \(\upmu\)A at 0.168 V and a cathodic peak of 20 \(\upmu\)A at \(-\)0.052 V, with a \(\Delta\)E of 116 mV (E\({}^{0\prime}\) = 58 mV) and \(\mathrm{i_{pa}/i_{pc}}\) = 1.1, obtaining a quasi-reversible one-electron transfer process. As described in Section 3.2, the CV of 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) in 50 mM PBS on SPCE-GRAFT (red line in Figure 6a and amplified in Figure 6b) showed no discernible peaks. Therefore, the absence of a signal indicates that the conductive surface (activated SPCE) has turned into a non-conductive, confirming the functionalization to the nanometric scale with aminophenyl groups. In contrast, the signals of [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\) were recovered for the SPCE-GRAFT-GLU (green line in Figure 6a and amplified in Figure 6b) to become a  
Figure 6: (**a**) CVs of 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) in 50 mM PBS (pH 7.4) on activated SPCE BARE (black line), SPCE-GRAFT (red line), SPCE-GRAFT-GLU (green line), SPCE-GRAFT-GLU-DA (blue line) and non-grafted SPCE-GLU-DA (blue dashed line). (**b**) Amplification of activated SPCE BARE, SPCE-GRAFT and SPCE-GRAFT-GLU signals. Scan rate: 100 \(\mathrm{mVs^{-1}}\). conductive surface again. Moreover, a shift in both anodic and cathodic peaks to more negative potentials was observed, which is ascribed to the catalytic effect of the ketonic groups of GLU, oriented towards the solution (Figure 1a, step 3). The voltammogram displayed an anodic peak of 18.57 \(\upmu\)A at 0.128 V and a corresponding cathodic peak of 18.79 \(\upmu\)A at 0.040 V. The electrochemical process can be considered quasi-reversible as well, with a \(\Delta\)E of 88 mV (E\({}^{0\prime}\) = 44 mV) and \(\mathrm{i_{pa}/i_{pc}}\) = 0.99. After the immobilization of DA on the SPCE-GRAFT-GLU surface to obtain the SPCE-GRAFT-GLU-DA (Figure 1a, step 4), this named surface exhibited excellent catalytic properties that can be associated with the presence of the terminal hydroquinone/benzoquinone couple or CC moieties at the electrode surface (blue line in Figure 6a). The CV of the 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) showed an anodic peak of 316.7 \(\upmu\)A at 0.218 V and a cathodic peak of 100 \(\upmu\)A at \(-\)0.059 V (blue line) with \(\Delta\)E = 277 mV and \(\mathrm{i_{pa}/i_{pc}}\) = 3.17. Consequently, the electrochemical response of the redox probe on SPCE-GRAFT-GLU-DA demonstrated an increase in the irreversibility of the process, as evidenced by an increase in \(\Delta\)E and \(\mathrm{i_{pa}/i_{pc}}\) ratio. Conversely, the surface showed high catalytic activity, starting the oxidation process at negative values of 100 mV, as well as an increase in the anodic and cathodic peak currents by a factor of 14 and 5, respectively, compared to the values obtained on activated SPCE BARE (unmodified, black line in Figure 6a and amplified in Figure 6b). This demonstrates the great performance of the DA-sensor under near-physiological conditions, specifically, at 25\({}^{\circ}\)C, 0.05 M ionic strength and pH 7.4, highlighting the sensor's adaptability to biochemical environments. Furthermore, the redox probe response on the SPCE-GLU-DA surface was recorded, where DA immobilization was performed by direct incubation on GLU (adsorption) in an activated SPCE, thus obviating the need for an electrografting step. The CV of 1 mM Fe(CN)\({}_{6}\)\({}^{3-}\) in 50 mM PBS at pH 7.4 (blue dashed line in Figure 6a) shows an anodic peak of 100.5 \(\upmu\)A at 0.242 V and a cathodic peak of \(-\)72.97 \(\upmu\)A at \(-\)0.021 V, with a \(\Delta\)E of 263 mV (E\({}^{0\prime}\) = 111 mV) and \(\mathrm{i_{pa}/i_{pc}}\) = 1.38.