## Results and Discussion  
### Effect of the Supporting Electrolyte  
In order to evaluate the voltammetric response of the proposed modified sensor, the quantification of Pb(II) was carried out in different supporting electrolytes. Figure 1a-c show the DPASV response for the determination of Pb(II) using a GrRAC-70% sensor (this composition was selected for preliminary analysis based on the results reported in the literature) from solutions of 0.1 M acetate buffer (pH 4.5), 0.5 M NaNO\({}_{3}\), and 0.5 M H\({}_{2}\)SO\({}_{4}\), respectively, using a preconcentration time of 30 s. The sulfuric acid solution proved to be the most suitable electrolytic solution because it provided a well-defined voltammetric signal and the response increased linearly without significant deviations (Figure 1). The limit of detection (LOD), for each of the supporting electrolytes used, was estimated by the equation \(LOD=3\times S_{y/y}/b\), where \(S_{y/x}\) is the residual standard deviation and \(b\) is the slope of the calibration plot, in accordance with IUPAC recommendations of the mean value for samples analyzed in triplicate. This approach allows the control of both false positive and negative errors (\(\alpha=\beta=0.05\)), as recommended by IUPAC [28; 29], and has been confirmed and recommended by experts in the field [30; 31]. For the 0.1 M acetate buffer solution, no significant current response was obtained, resulting in an LOD of 4.8 uM; a similar outcome was obtained in 0.01 M acetate buffer solution, where an LOD of approximately 3.2 uM confirmed the poor performance of the sensor in acetate buffer solutions. Conversely, the analytical approach significantly improved when 0.5 M NaNO\({}_{3}\) and 0.5 M H\({}_{2}\)SO\({}_{4}\) solutions were used, which resulted in LOD values of 2.8 (Figure 1b) and 1.6 uM (Figure 1c). Comparing the values obtained with an unmodified graphite electrode and with the modified cork-graphite electrodes in H\({}_{2}\)SO\({}_{4}\), the LOD is 3 times higher on graphite (\(\approx\)4.8 uM) than that obtained with the GrRAC-70% electrode. The best result obtained using the composite material shows that the cork-graphite mixture is able to influence the intensity of the current signals. Figure 1: DPASV curves recorded for different concentrations of Pb(II) in (**a**) 0.1 M acetate buffer (pH 4.5), (**b**) 0.5 M NaNO\({}_{3}\), and (**c**) 0.5 M H\({}_{2}\)SO\({}_{4}\). 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. According to the literature [32], cork has a great adsorption capacity, which is assumed to occur through a so-called biosorption mechanism consisting of an initial physical adsorption (rapid metal uptake) and then a slower chemisorption. The biosorption mechanism is the result of several kinds of interactions, such as complexation, coordination, chelation, ion exchange, inorganic microprecipitation, and hydrolysis products of metal ions; in the case of metal ion biosorption, ion exchange is usually the main mechanism. Hence, the type of cork, the pH conditions, and the contact time determine the interactions that can occur between the target compound and the cork surface. Depending on the cork used, specific active sites may predominate in its surface composition (phenolic, carboxylic, sulfonic, phosphate, and amino groups as well as coordination sites), in addition to the cork surface charge depending on pH conditions [33]. It is also important to consider that carbonaceous materials have micropores and mesopores, the accessibility of which will be increased following the inclusion of cork as a surface modifier. Thus, an improvement in voltammetric current signals can be achieved. Another important feature to consider is that the surface morphology of GrRAC-70% is more homogeneous, as evidenced by the SEM micrographs, which can positively influence its current response [34].