## 4 Discussion
### HMs Analysis Experiments Using BR Films  
**BR**-CMEs obtained by CPE have been used for HM ions analyses. The DPV curves (Figures 9-12) show stripping peaks for all studied ions, suggesting that these ions have been complexed in the electrogenerated films (which ensure the immobilization of **BR** complex units). Peaks for Cd, Pb, Cu, and Hg dissolution were observed at the potentials of \(-0.796\), \(-0.548\), \(-0.098\), and \(0.269\) V, respectively, for CMEs obtained at \(1.4\) V and \(1.5\) mC. These values are slightly different from those obtained in similar conditions of preparation for rhodamine-modified electrodes, **R**-CMEs, which are as follows: \(-0.766\), \(-0.547\), \(-0.118\), and \(+0.269\) V, respectively [13]. The difference between the potentials on **BR**-CMEs and **R**-CMEs shows that the change of the ligand by the introduction of a phenyl group selectively influences the complexation ability of CMEs (the signals potentials for the investigated ions). This influence is strong enough for Cd and Cu. No influence occurs in the case of Pb and Hg. From the tested potentials for CPE (1.1 V; 1.4 V; and 1.6 V) corresponding to the processes highlighted by the DPV and CV curves a1 and a2-a3 (Figure 9), the potential value of 1.4 V corresponds to an optimal response for Pb(II). This potential was further maintained for the synthesis of **BR**-CMEs during the process to optimize the preparation conditions of these CMEs. The increase in polymerization charge from 0.5 to 1 mC resulted in a two-times increase in Pb(II) response (Figure 10). A further increase to 1.5 mC did not add another significant increase. The peaks for all ions are influenced by the electropolymerization charge, which is connected to the film's thickness. Figure 10 inset curves show a selective complexation for Pb (which has the highest signal) and a specific influence in the electropolymerization charge in case of Pb analysis (the DPV peaks increase with charge) and Cd (the DPV peaks decrease with charge). Other parameters connected to the conditioning of the modified electrodes have influence on HM analyses. Larger accumulation times result in larger responses, as anticipated. Figure 11 shows a significant increase (over 10 times) in the signal for Pb when the accumulation time increased from 10 to 15 min for **BR**-CMEs prepared by using a charge of 1.5 mC. This is accompanied by an increase in selectivity in the analysis of Pb(II) compared to Cd(II) and Hg(II). To obtain the calibration curves on **BR**-CMEs plots of Cd(II), Pb(II), Cu(II), and Hg(II), stripping currents on **[BR]** have been performed for various accumulation solutions containing HMs (with concentrations ranging from 5 \(\times\) 10\({}^{-7}\) M to 10\({}^{-4}\) M). Their stripping peaks are shown in Figure 12 insets, as the difference between the peak current and the background current (\(\Delta\)i peak) in I1 inset of Figure 12 and the calculated area (\(A\)) of the DPV peak (versus the background current) in I2 inset, respectively. All plots indicate the greatest sensitivity of **BR**-CMEs for Pb(II) among the other complexed ions such as Cd(II), Cu(II), and Hg(II)). The complexation of Pb(II) ions with **BR** was also studied in solution, taking into account that the signal obtained for Pb(II)'s detection by using **BR**-CMEs is much higher than that for the other investigated HMs ions, following the changes of the UV-Vis spectrum of **BR** (Figure 13). Complexation was a fast process (the spectra recorded during a 15 min