## Results and Discussion
### Influence of the Cork Concentration  
The performance of the composite electrode is affected by the amount of cork that is mixed with the graphite. In a previous work, we showed that the quantity of cork influences the electroactive area of the sensor as well as the electron transfer during the oxidation of caffeine [13]. In order to evaluate the effect of the quantity of cork on the detection of Pb(II) by DPASV, 0.5 M H\({}_{2}\)SO\({}_{4}\) was used as the supporting electrolyte. As can be observed in Figure 3a-c, the peak current recorded on GrRAC depends on the amount of cork present in the composite sensor. In particular, a linear relationship between the peak current and the Pb(II) concentration was obtained in the Pb(II) concentration range from 1 to 25 uM in 0.5 M H\({}_{2}\)SO\({}_{4}\) (inserts in Figure 3a-c), considering at least 11 different concentrations of the analyte. Pb(II) calibration curves were obtained for each of the prepared sensors. From the analytical curves obtained using GrRAC-10%, GrRAC-70%, and GrRAC-90% by DPASV, it can be seen that the stripping peak currents (I\({}_{\text{p}}\)) increased linearly with the concentration of Pb(II) (inserts in Figure 3). The calculated correlation equations are  
\[\text{GrRAC-10\%: I}_{\text{p}}\ (\text{\mu A})=(0.05\ \pm\ 0.03)\times\text{C - (0.1\ \pm\ 0.05); R}^{2}=0.97\]  
\[\text{GrRAC-70\%: I}_{\text{p}}\ (\text{\mu A})=(0.08\ \pm\ 0.04)\times\text{C - (0.3\ \pm\ 0.1); R}^{2}=0.98\]  
\[\text{GrRAC-90\%: Ip}\ (\text{\mu A})=(0.11\ \pm\ 0.09)\times\text{C - (0.2\ \pm\ 0.1); R}^{2}=0.95\]  
According to Figure 3, the best performing GrRAC sensor in terms of sensitivity, capable of providing an LOD for Pb(II) of only about 0.8 uM, was GrRAC-70%. In contrast, the GrRAC-10% and GrRAC-90% sensors provided higher LODs of approximately 1.5 and 1.2 uM, respectively. This difference in the LODs is attributable to the different degrees of dispersion of the surface active sites, as reported in our previous study on the determination of caffeine [23]. The highest peaks were obtained with GrRAC-70% and this cork-graphite ratio was used for detecting Pb(II). The results show that Pb(II) stripping signals with GrRAC-70% are superior to those obtained with the GrRAC-10% and GrRAC-90% electrodes. This can be attributed to the honeycomb macroporous structure of the cork granules, which favor the presence of propagation paths through the cork cells, forming macroporosity with textural properties. Based on our previous results, the surface morphology of GrRAC-70% appears particularly homogeneous because the graphite sheets are arranged in close contact within the porosities of the cork [23].  
Figure 2: Effect of the preconcentration time on the quantification of Pb(II) ions by DPASV, using the GrRAC-70% composite sensor. Experimental conditions: 25 uM of Pb(II) (n = 10; R\({}^{2}\) = 0.98; \(p\) = 95%). Reduction potential: \(-\)1.2 V (vs. Ag/AgCl); scan rate: 50 mV s\({}^{-1}\); potential range: \(-\)1.0 to 0 V. Supporting electrolyte: 0.5 M H\({}_{2}\)SO\({}_{4}\).  
Figure 3: DPASV curves recorded for different concentrations of Pb(II) with (**a**) GrRAC-10%, (**b**) GrRAC-70%, and (**c**) GrRAC-90%. Lead concentrations: (a) 0, (b) 1.2, (c) 2.4, (d) 4.8, (e) 7.1, (f) 9.5, (g) 11.8, (h) 14.0, (i) 16.3, (j) 18.6, (k) 20.8, (l) 25 uM. Inserts: plots of the electrochemical response, in terms of current, as a function of the lead concentration.