## 3 Results and Discussion
### Characterization of Screen-Printed Electrodes Modified with Copper and Copper Oxide Particles
For this specific temperature, the constants appearing at the beginning of the equation, 0.4463, can be combined allowing the equation to be written in a simplified form as follows [25]:  
\[i_{p}=\left(2.99\times 10^{5}\right)\cdot\cdot n^{3/2}\cdot A\cdot C\cdot D^{1/2 }\cdot v^{1/2} \tag{9}\]  
Figure 4: EDX spectra of copper and copper oxide-based electrodes: (a) copper-based electrode; (b) copper oxide electrode. The constant appearing at the beginning of this simplified equation, 2.99 \(\times\) 10\({}^{5}\) is known to have units \(C\) mol\({}^{-1}\)\(v^{-1/2}\). For gold-based electrodes, 1 mM K\({}_{4}\)Fe(CN)\({}_{6}\) in 0.1 M KCl was used as redox solution for active area determination. The scan rate varies from 100 to 10 mV s\({}^{-1}\). Diffusion coefficient of K\({}_{4}\)Fe(CN)\({}_{6}\) in 0.1 M KCl were obtained from [PERSON] et al., which reported a value of 7.6 \(\times\) 10\({}^{-6}\) cm\({}^{2}\) s\({}^{-1}\)[26]. Figure 5 shows cyclic voltammetry profiles and Randles Equation linear regression for gold-based electrodes. As expected gold commercial visual area coincided with the active area determined by the Randles-Sevcik equation, 0.12 cm\({}^{2}\). This result demonstrates that there are no significant nanostructures that contribute to increasing the surface area of the commercial electrode. The experiment was repeated twice obtaining the same result, no variation between visual and active area. Figure 5: Active area calculation for the gold commercial-based electrode: (**a**) Cyclic voltammetry with 1 mM K\({}_{4}\)Fe(CN)\({}_{6}\) in 0.1 M KCl with a scan rate from 100 to 10 mV s\({}^{-1}\); (**b**) Randles Equation linear regression. Selecting the correct probe is crucial to gain accuracy in calculating the active area of the electrode. For copper-based electrodes, K4 Fe(CN)6 is not recommended because iron ions interact with copper atoms leading to unstable cyclic voltammetry profiles, therefore different probes should be used. In this work, [Ru(NH3)6]3+ salt has been selected for the characterization of the active area of copper-based electrodes. In this case, the diffusion coefficient of 5 mM Ru(NH3)6 Cl3 in 0.1 M KCl was obtained from [PERSON] et al. which reported a value of 8.43 \(\times\) 10\({}^{-6}\) cm\({}^{2}\) s\({}^{-1}\)[24]. Cyclic voltammetry experiment displayed from 500 mV s\({}^{-1}\) to 10 mVs\({}^{-1}\) also was used to determine the active area by the Randles-Sevcik equation. In this case, the surface area was increased from 0.12 cm\({}^{2}\) to 1.72 cm\({}^{2}\). Considering all the experiments, the error is less than 15%. Figure S2 represents the cyclic voltammetry experiment for copper-based electrode and the linear fitting to the Randles-Sevcik equation (Equation (9)). Ru(NH3)6 Cl3 was also used for the determination of the copper oxide active area. The experimental conditions were the same as in the determination of the copper active area. In this case, the active area was 1.50 cm\({}^{2}\). Figure S3 represents the cyclic voltammetry experiment for a copper oxide electrode. Considering other tested electrodes, the average active area was 1.6 \(\pm\) 0.2 cm\({}^{2}\).