## 3 Results and Discussion
### Electrochemical Characterization of Catalytic Surface: DA-Sensor  
SPCE modified with aminophenyl groups was selected as a surface for DA immobilization via covalent bonding, using GLU as the cross-linking molecule. As illustrated in Figure 1a, the electrode surface immobilized with DA shows the characteristic CC structure in the terminal layer. The electrochemical behavior of the DA-modified SPCE was investigated using CV with the redox probe [Fe(CN)\({}_{6}\)]\({}^{3-4/-}\) and CC solutions. The electrochemical response of 50 mM PBS at pH 7.4 was recorded at each step of the procedure, showing overlapped residual current in the CVs for activated SPCE BARE, SPCE-GRAFT, and SPCE-GRAFT-GLU (gray dashed line in Figure 5a). However, the voltammogram of the 50 mM PBS on SPCE-GRAFT-GLU-DA displayed an irreversible anodic signal due to oxidation of the CC groups on the electrode surface (blue line in Figure 5a), following the electrochemical reaction of Figure 5b. This is clear evidence of the existence of the immobilized CC functionalities on the functionalized carbon surface. In detail, three anodic signals were observed, a pre-wave of 150 \(\upmu\)A at 0.108 V, a second peak of 161.7 \(\upmu\)A at 0.200 V and a third post-peak of 140 \(\upmu\)A at 0.454 V. No cathodic peak was observed in the reverse scan, indicating that the surfaces remained functionalized with terminal o-benzzoquinones oriented towards the solution. The oxidation process of o-hydroxylphenyl group of 3 mM CC in solution to o-quinone group on activated SPCE (red line in Figure 5a and reaction in Figure 5b) can be considered as quasi-reversible, involving the exchange of 2 e\({}^{-}\) and 2 H\({}^{+}\)[66]. The CV showed an anodic peak of 73.33 \(\upmu\)A at 0.238 V and a cathodic peak of 50 \(\upmu\)A at \(-\)0.054 V, with a \(\Delta\)E of 292 mV (E\({}^{0\prime}\)= 146 mV) and \(\text{i}_{\text{pa}}/\text{i}_{\text{pc}}=1.4\). A comparative analysis between the oxidation of CC in PBS on activated SPCE (red line in Figure 5a) and the CC functionalities on SPCE-Graft-GLU-DA (blue line in Figure 5a)  
Figure 5: (a) CVs of 50 mM PBS (pH 7.4) on SPCE-GRAFT-GLU (gray dashed line) and SPCE-GRAFT-GLU-DA (blue line). CV of 3 mM CC in 50 mM PBS on activated SPCE BARE (red line) and SPCE-GRAFT-GLU-DA (black line). Scan rate: 100 mVs\({}^{-1}\). (b) Electrochemical reaction of o-hydroxylphenyl group of CC to o-quinone either in solution or onto the modified surface. shows that the oxidation of the CC functionalities on the modified electrode surface starts to have less positive values by 170 mV than the oxidation of CC in the solution, accompanied by a 60% increase in the peak current. These phenomena can be ascribed to a high catalytic activity because of CC functionalities on the surface, confirming the formation of a covalent bond between the different self-assembled monolayers and the electrode surface, which enhances direct electron transfer. The addition of 3 mM CC in 50 mM PBS (pH 7.4) (black line in Figure 5a) on SPCE-GRAFT-GLU-DA resulted in the characteristic voltammetric response of the CC redox process. The CV presented one anodic peak of 150 \(\upmu\)A at 0.319 V and the corresponding cathodic peak of \(-\)90 \(\upmu\)A at \(-\)0.070 V, giving a \(\Delta\)E of 389 mV (E\({}^{0\prime}\) = 125 mV) and an \(\mathrm{i_{pa}/i_{pc}}\) of 1.67. As a result, the irreversibility of the process increased and the anodic and cathodic peak currents were enhanced by two times compared to the CC signal obtained on activated SPCE BARE (unmodified electrode), resulting in a remarkable improvement in the electrocatalytic activity towards the oxidation of CC with the SPCE-GRAFT-GLU-DA modified electrode. To investigate the electrochemical properties of the surface SPCE-GRAFT-GLU-DA, its response to an electrochemical probe, [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\), was evaluated by CV in all steps of the procedure (described in Section 2.4.2 and depicted in Figure 1a).