## 3 Results
### Solution Analysis  
Analyses were performed using two aSPCEs purchased from different companies in order to evaluate whether there were any differences in the analytical response. The measurements performed with the aSPCE1 showed good linearity in the concentrations range between 20 and 146 \(\upmu\)g L\({}^{-1}\), as we can see in Figure 8a,c. Figure 8c shows three calibration lines obtained on three different days using three different electrodes, indicating the good repeatability of the measurement. The peak heights used for the linear fits were estimated by baseline subtraction. The signals obtained with the aSPCE2 show a higher current value with respect to the signals obtained with aSPCE1, as expected based on the size of the surface area of the WE, but the peaks are not well defined and repeatability is lower. The linear fit parameters are collected in Table 3.  
Figure 7: (a) Reduction peaks obtained by DPV on the DCF by-product adsorbed on the aGCE surface (3-22 \(\upmu\)g L\({}^{-1}\) DCF; PB 5 \(\times\) 10\({}^{-2}\) mol L\({}^{-1}\) pH 2.0; E vs. Ag/AgCl; three replicates for each concentration level); (b) current signals obtained for each concentration level (four independent replicates, one measurement for each concentration level); (c) linear fit of signals of DCF in the concentration range 3–16 \(\upmu\)g L\({}^{-1}\); (d) Linear fit parameters estimated on the DPV signals of DCF by-product adsorbed on the aGCE surface (3–16 \(\upmu\)g L\({}^{-1}\) DCF; PB 5 \(\times\) 10\({}^{-2}\) mol L\({}^{-1}\) pH 2). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Electrode**} & \multicolumn{4}{c}{**Linear Fit Parameters**} \\ \hline \hline  & \multicolumn{4}{c}{**Solution Analysis**} \\ \hline
**aSFCE-1** & **Slope** & **Std. err. ** & **Intercept** & **Std. err. ** & \(\mathbf{R^{2}}\) \\ \hline
1 & \(5.03\times 10^{-4}\) & \(1.0\times 10^{-5}\) & \(-2.13\times 10^{-5}\) & \(9.6\times 10^{-4}\) & 0.9983 \\
2 & \(4.75\times 10^{-4}\) & \(7.9\times 10^{-6}\) & \(4.48\times 10^{-4}\) & \(7.4\times 10^{-4}\) & 0.9989 \\
3 & \(4.56\times 10^{-4}\) & \(1.3\times 10^{-5}\) & \(1.09\times 10^{-3}\) & \(1.25\times 10^{-3}\) & 0.9965 \\ \hline
**aSFCE-2** & **Slope** & **Std. err. ** & **Intercept** & **Std. err. ** & \(\mathbf{R^{2}}\) \\ \hline
1 & \(1.41\times 10^{-2}\) & \(1.0\times 10^{-3}\) & \(-1.05\times 10^{-1}\) & \(3.5\times 10^{-2}\) & 0.9840 \\
2 & \(1.99\times 10^{-2}\) & \(1.4\times 10^{-3}\) & \(-1.46\times 10^{-1}\) & \(4.8\times 10^{-2}\) & 0.9851 \\
3 & \(1.54\times 10^{-2}\) & \(1.6\times 10^{-3}\) & \(-1.74\times 10^{-1}\) & \(5.4\times 10^{-2}\) & 0.9695 \\ \hline \hline \multicolumn{2}{c}{**Drop analysis**} \\ \hline
**aSFCE-1** & **Slope** & **Std. err. ** & **Intercept** & **Std. err. ** & \(\mathbf{R^{2}}\) \\ \hline
1 & \(3.68\times 10^{-4}\) & \(6.5\times 10^{-6}\) & \(-2.72\times 10^{-3}\) & \(5.4\times 10^{-4}\) & 0.9991 \\
2 & \(3.19\times 10^{-4}\) & \(1.1\times 10^{-5}\) & \(-8.45\times 10^{-4}\) & \(8.2\times 10^{-4}\) & 0.9967 \\
3 & \(2.68\times 10^{-4}\) & \(8.9\times 10^{-4}\) & \(-2.04\times 10^{-3}\) & \(6.9\times 10^{-4}\) & 0.9967 \\ \hline
**aSFCE-2** & **Slope** & **Std. err. ** & **Intercept** & **Std. err. ** & \(\mathbf{R^{2}}\) \\ \hline  & \(1.31\times 10^{-2}\) & \(7.4\times 10^{-4}\) & \(-3.17\times 10^{-1}\) & \(7.2\times 10^{-2}\) & 0.9975 \\  & \(7.83\times 10^{-3}\) & \(3.0\times 10^{-4}\) & \(-1.85\times 10^{-2}\) & \(2.9\times 10^{-2}\) & 0.9956 \\  & \(1.33\times 10^{-2}\) & \(6.1\From the plots shown in Figure 8c,d, the mean values of LoD and LoQ were estimated by the standard deviation of the intercepts defined by the ordinary linear regressions. The LoD and LoQ values correspond to the analyte amount for which the current signal is equal to 3.3 times or 10 times the standard deviation of the intercepts, respectively. The value of the LoD and LoQ estimated for aSPCE1 and aSPCE2 were, respectively, 7 ug L\({}^{-1}\) and 20 ug L\({}^{-1}\), and 9 ug L\({}^{-1}\) and 28 ug L\({}^{-1}\).