## 3 Results
### Characterization of DCF Electrochemical Behavior
DPV signals of 4.0 mg L\({}^{-1}\) DCF in 4.5\(\cdot\)10\({}^{-2}\) mol L\({}^{-1}\) PB, pH 2.0, \(E\) vs. Ag/AgCl; _E_pulse 55 mV; _t_pulse 25 ms; _E_step 5 mV; scan rate 10 mV/s. Figure 2: (a) Consecutives CV scans (aGCE) of 20 ug L\({}^{-1}\) DCF in 1 \(\times\)10\({}^{-2}\) mol L\({}^{-1}\) phosphoric buffer, pH = 2.0; (b) Redox electrochemical-chemical (E1C1E2) mechanism of DCF [8]. The results are in agreement with those obtained by [PERSON] et al. [8] with an edge plane pyrolytic graphite sensor. They also revealed a drop in the oxidation peak current along with an appearance of an anodic peak of the species S and explained this by supposing the strong adsorption of the reaction products of DCF, which blocks the electrode surface and reduces the effective reaction sites at the modified electrode. This last statement is in accordance with our observations: the peak current due to the by-product increases with increasing the scan rate, but the peak separation between the reversible couple does not remain constant, suggesting that the electrode process is adsorption controlled. Furthermore, the peak due to the oxidation of DCF does not increase proportionally with the concentration of DCF in solution, nor does it increase the scan rate, and its height changes by replicating the measurements in the same solution, suggesting the presence of a disturbance process of the measurement. Since the peaks due to the by-product show a more regular behavior, only the reduction peak of the by-product was considered for DCF quantification with aGCE, and the measurement conditions at pH 2.0 in 2.5 \(\times\) 10\({}^{-2}\) mol L\({}^{-1}\) phosphoric acid were deemed the optimal ones, despite the greater peak height observed with hydrochloric acid, because of the low background signal and the good shape of the detectable peaks (see Figure S1). The CV scans recorded with aSPCE are very similar to the ones obtained with aGCE. Indeed, the plots show three peaks: the first one is at 0.85 V, due to the oxidation of DCF, and then the other two peaks, an anodic one at about 0.55 V and a cathodic one at 0.50 V. These two peaks are related to the chemical reaction that occurs after DCF oxidation, with the production of a by-product, as seeable in Figure 2b. The difference between the aGCE and the aSPCE lies in the interaction of the oxidation by-product with the electrode surface. In the case of aSPCE, the adsorption process is less important, and this leads to a regular increase in the peak current intensity due to the oxidation of DCF with increasing concentration of the analyte. Therefore, with aSPCE, the peak considered for the quantification of DCF is the one at 0.85 V due to the direct oxidation of DCF. After these observations, we set the working procedure in order to obtain the best linear response: for the aGCE, we decided to oxidize DCF by an anodic potential (0.8 V for 30 s) and then measure the reduction peak of its by-product adsorbed on the electrode surface; on the other hand, for the aSPCEs, the determination is pursued by the direct analysis of the solution without a pre-oxidation/adsorption process.