## 4 Conclusions  
The electrochemical reduction in the aryldiazonium ion represents a rapid, simple and versatile methodology for the surface grafting of organic layers on any type of material. In addition, the rich chemistry of these compounds applied to achieve the immobilization of biomolecules by covalent bonds is impressive. This work demonstrates that SPE functionalized with 4-NBD\({}^{+}\) ion provides robust and compact platforms capable of anchoring delicate biological components, such as DA or TYR, or serving as a protective insulating layer to prevent corrosion by passivation of the electrode surface. Through the optimization of critical parameters, including scan rate, potential window and 4-NBD\({}^{+}\) concentration, the formation of a thin organic layer on a rough and highly porous surface with the highest surface coverage was achieved when five cyclic scans were applied to a 2 mM 4-NBD\({}^{+}\) solution in the potential window of +0.5 V to \(-\)1.0 V at 100 mVs\({}^{-1}\). \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{1}{c}{**Electrode \({}^{*}\)**} & \multicolumn{1}{c}{**Technique \({}^{**}\)**} & \multicolumn{1}{c}{**LDR ***} & \multicolumn{1}{c}{**LOD**} & \multicolumn{1}{c}{**Sensitivity**} & \multicolumn{1}{c}{**Ref.**} \\ \hline SPCE/GNPs & DPV & 23–200 & 6.9 & 150 & [67] \\ SPCE & SWV & 20–220 & 5.9 & 31.0 & [68] \\ GCE/Fe3O4-TiO2 & CV & 150–500 & 45 & 5.6 & [69] \\ GCE/MWCNT/NiO & DPV & 10–400 & 2.5 & 196 & [70] \\ GCE/GR/CdTe-QDs & DPV & 80–1000 & 18.3 & 6.4 & [71] \\ GCE/PTA & DPV & 26–500 & 7.8 & 120 & [72] \\ GCE/RGO-MWNTs & DPV & 6–540 & 1.8 & 70 & [73] \\ SPCE/Cu-CMCS2 & CA & 7.3–40 & 7.3 & 10.0 & [33] \\ SPCE/TYR & CA & 29–40 & 29 & 3.3 & [33] \\ GCE/CMS-g- & DPV & 83–100 & 25 & 78.5 & [38] \\ PANI@MWCNTs/TYR & DPV & 10–80 & 7.6 & 4.8 & [74] \\ SPCE/MNP/MWCNTs/TYR & Amp. & 2.5–95 & 0.7 & 115 & [31] \\ GCE-MLN/AO/TYR & Amp. & 1.7–80 & 0.5 & 31.5 & [75] \\ GCE/PS-ND/TYR & DPV & 5–740 & 0.9 & 23.0 & [32] \\ GCE/ZnO/TYR/Nafion & Amp. & 13–400 & 6 & 2.14 & [76] \\ CBPE/GLU/TYR & Amp. & 0.05–8.5 & 0.015 & 460 & [77] \\ CGGE/ & CA & 0.5–50 & 0.3 & 231 & [78] \\ GCE/MWCNT-IT-YYR & LSV & 4.9–1100 & 0.6 & 32.8 & [79] \\ SPCE/CB4/TYR & CV & 6.3–100 & 1.9 & 130 & [80] \\ SPCE/BSA-GLU/TYR & DPV & 19–103 & 5.6 & 6.2 & [36] \\ GCE/pTN-GLU/TYR & CV & 20–300 & 6.0 & 5.04 & [81] \\ SPCE-GRAFT-GLU-DA & CV & 62–10,000 & 18.6 & 32.2 & This work \\ SPCE-GRAFT-Diazo- & CV & 14.9–400 & 4.5 & 317 & This work \\ TYR & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Analytical performance comparison of different unmodified and modified carbon electrodes for CC sensing. Briefly, the DA-sensor is stable, robust, simple and low cost. It exhibited noteworthy catalytic activity towards [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\), producing analytical signals 14 times higher than those observed on unmodified surfaces under near-physiological conditions. The enhanced electron transfer kinetics and stability of the modified surface underscores its potential for diverse electrochemical applications. Furthermore, when utilized for the detection of CC, the DA-sensor demonstrated a limit of detection of 4.6 \(\upmu\)M and a linear range extending three concentration units. Additionally, the immobilization of TYR on a grafted surface via in situ diazotation followed by a coupling reaction was also conducted. The TYR-biosensor, in addition to being highly selective due to the presence of the enzyme, also demonstrated high sensitivity. Specifically, the observed catalytic effect in the [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\) mediator detection was 10 times higher than the already sensitive DA-sensor. Furthermore, immobilization by covalent bond confers additional stability to the (bio)sensor, thus counteracting the problem of the low stability of the enzymes and ensuring the reproducibility of the results. When applying the TYR-biosensor to the determination of CC, a narrower linear range was obtained than with the SPCE-GRAFT-GLU-DA.