## 3 Results and Discussion
### Characterization of Screen-Printed Electrodes Modified with Copper and Copper Oxide Particles  
Figure 3 depicts the SEM morphology of the synthesized sensors. The image shows the presence of copper and copper oxide particles on carbon surfaces. The particle size for copper and copper oxide is 2 \(\upmu\)m, but particle agglomeration was more present in the case of copper-based electrodes. The size is in accordance with previous articles and promotes a high increment in the electrode surface area [20]. The particles were homogeneously dispersed as shown by the white dots on both images (Figure 3a,b). Figure S1 in the Supplementary Materials shows the rough surface of carbon screen printed electrodes, where the copper microparticles are electrodeposited. Figure 3: SEM image of copper and copper oxide-based electrodes: (**a**) copper-based electrode; (**b**) copper oxide electrode. Figure 2: Screen-printed electrodes (**a**) carbon printed electrode—before copper electrodeposition; (**b**) copper-based electrode—before oxidation step; (**c**) copper oxide electrode. The composition of both electrodes was analyzed by using Energy X-ray Powder Diffraction (EDX). Figure 4. illustrates the spectrum for the copper and copper oxide profiles. As it is shown in Figure 4 copper was present in both electrodes in more than 50% in terms of weight percentage, 71% for copper-based electrode and 57% for copper oxide electrode. Up to 30% of the weight of copper oxide corresponds to oxygen, while in the case of copper-based electrodes the percentage is only 5%. These results are in good agreement with previously reported results in the literature [21]. [PERSON] et al., performed an electrode composed of CuO spindle-like nanosheets grown on a carbon cloth [22]. The electroactive area of the electrode was estimated from cyclic voltammetry performed at different scan rates. Commercial gold, copper, and copper oxide particles' active areas were compared by using different reactants, [Ru(NH\({}_{3}\))\({}_{6}\)]\({}^{3+}\) and [Fe(CN)\({}_{6}\)]\({}^{4-}\). These species follow the next reactions when the potential is applied on the electrode surface [23,24]:  
\[[\text{Ru(NH${}_{3}$)}_{6}]3^{+}+\text{e}^{-} \leftrightarrow [\text{Ru(NH${}_{3}$)}_{6}]2^{+} \tag{6}\] \[[\text{Fe(CN)}_{6}]3^{-}+\text{e}^{-} \leftrightarrow [\text{Fe(CN)}_{6}]4^{-} \tag{7}\]  
Peak to peak separation (\(\Delta\)Ep) analysis of the cyclic voltammetry profiles is useful to determine if the redox process is reversible or irreversible. For those cases where \(\Delta\)Ep--57 mV and is independent of the scan rate, the process is considered reversible. On the contrary, when \(\Delta\)Ep increases with the scan rate, the redox process is classified as quasi- or irreversible. In the latter case, the Randles Equation (Equation (8)) can be applied. This equation relates the effect of scan rate on the current peak. \[i_{p}=0.4463\cdot\left(\frac{F^{3}}{RT}\right)\cdot\cdot\cdot n^{3/2}\cdot A\cdot C \cdot D^{1/2}\cdot v^{1/2} \tag{8}\]  
where \(F\) is Faraday constant (96,500 C mol\({}^{-1}\)), \(R\) is the universal gas constant (8143 J mol\({}^{-1}\) K\({}^{-1}\)) and \(T\) is the absolute temperature (K). The other parameters are: \(n\), number of electrons involved in the redox half-reaction (-); \(D\), the diffusion coefficient for the redox-active species on the solution media (cm\({}^{2}\) s\({}^{-1}\)), \(C\) is the solution molar concentration of the redox species (mol cm\({}^{-3}\)) and \(A\) is the surface area (cm\({}^{2}\)) and \(v\) is the scan rate of the experiment (V s\({}^{-1}\)). The Randles-Sevcik equation is often written in an abbreviated form under the assumption that the temperature is fixed at 298.15 K (25 \({}^{\circ}\)C) [25].