## 4 Discussion  
The electrochemical behavior of DCF was studied on GCE and SPCE after the anodic activation. This approach allowed us to explore the analytical response of the two electrode types to DCF, highlight the differences between the two devices, and optimize the measurement conditions for both electrode types. On both kinds of electrodes, the electrochemical response of DCF is not reversible. DCF signals showed to be enhanced by the anodic activation of the electrode surface, even if to a lesser extent than other previously investigated drugs and acid pH. Moreover, DCF and its by-products tend to be adsorbed on the surfaces; in particular, the by-products can be easily adsorbed on the activated surface of the GCE. From this point of view, the behavior of DCF on activated GCE and SPCEs is different. The measures showed that by using SPCEs, we obtain a linear relationship between the concentration of DCF and its anodic current, while this is not possible for GCE on which the adsorption of the by-products is significant. With GCE, it was possible to work in linear conditions exploiting the pre-adsorption of the oxidation by-product on the electrode surface. Despite the fact that the SPEs were supplied as ready-for-use, we observed that the preliminary polishing process by alumina-water dough, commonly used for the solid electrodes, smooths the electrode surface and significantly stabilizes the electrochemical signals. The polishing process can be conducted only once, before the measurement, since the SPE is considered to be a disposable device and was discarded after the measurement. By both GCE and SPCE, it was possible to find conditions for which the current signals linearly increase with DCF concentration and to estimate the limit parameters. SPCEs from two different suppliers were tested, and the best performance was obtained with the SPCE1. The different performances can be accounted for by the smallest WE surface of SPCE1 or by the difference in the ink composition. The GCE showed lower limit parameters and better repeatability with respect to SPCEs. The limit parameters defined for both GCE and SPCEs are lower or similar to those reported in the scientific literature (see Table 1), except for the case studied by [PERSON] et al. [18; 19], who modified the electrode surface with carboxyl functionalized multi-walled carbon nanotubes. The quality of the signals and the concentration range obtained by aGCE are quite satisfactory but operating by adsorption presents some concerns. In fact, if we consider the use of real samples, the adsorption phase can also involve the matrix, with possible arising of interferences or reduction in the sensibility by the saturation of the active surface of the WE. Another fact to take into account is that DCF is well adsorbed on many surfaces, so the polishing step before every single measurement has to be very careful in order to avoid memory effect due to previous measures. The process is rather tedious and non-automatable and causes the analytical method to be unsuitable for screening analyses. These issues can be well avoided by the adoption of a different kind of electrode, such as the SPCE. The use of SPCE may have some other advantages. In fact, by SPCE, it is possible to directly measure the current due to DCF's oxidation without a preliminary adsorption step. Moreover, SPCEs are disposable devices; therefore, it is possible to avoid the risk of memory effects, polishing, and electrode treatment between successive measurements, and they allow the consumption of small sample volumes. Therefore, SPCEs may be particularly suitable for screening analysis in order to process many samples in a very short time. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12167983/s1](https://www.mdpi.com/article/10.3390/app12167983/s1)]([https://www.mdpi.com/article/10.3390/app12167983/s1](https://www.mdpi.com/article/10.3390/app12167983/s1)).