## Results and Discussion
### SWASV Analysis  
The GrRAC-70% composite sensor produced the best DPASV results for Pb(II) detection and was therefore chosen as the working electrode for evaluating Pb(II) traces by SWASV. Figure 4 shows the SWASV voltammetric responses of Pb(II) under pre-selected experimental conditions: 0.5 M H\({}_{2}\)SO\({}_{4}\) as the supporting electrolyte, \(-\)1.2 V as the preconcentration potential, 120 s of preconcentration time plus 10 s of resting time. The stripping voltammetric peaks of Pb(II) ions appeared at \(-\)0.44 V for the GrRAC-70% sensor. The peak current (I\({}_{\rm p}\)) increased linearly with the concentration of Pb(II) in the range from 1 to 25 uM; the linear regression equation (I\({}_{\rm p}\) vs. C) was obtained as  
\[\rm I_{p}\ (\mu A)=(0.4\ \pm\ 0.1)\ \times\ C\ -\ (0.8\ \pm\ 0.2);\ R^{2}=0.98\]  
The LOD was found to be 0.3 uM. Compared with DPASV, SWASV produced much better results: the regression residuals are randomly distributed around zero and the linearity is practically perfect. Another important point is that no noticeable alterations were noted in the calibration curves recorded on different days, confirming the stability of the GrRAC-70% composite sensor. The GrRAC-70% sensor used in this work remained stable for at least two months of intensive use.  
Table 1 collects the results available in the literature and relating to the analysis of Pb(II) with different electrodes and allows a comparison with the results obtained in this study. The ease of sensor preparation and the analytical protocol suggested here offer advantages over the other methods reported.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Electrodes** & **Method** & **Electrolyte** & **Linear Range uM** & **LOD/uM** & **Ref.** \\ \hline MTZ-PMO-S-S 1 & SWASV & 0.2 M HCl & 0.01–10 & 0.024 & [35] \\
5-Br-PADAP/MWCNT 2 & DPA & 0.1 M acetate buffer & 0.9–114 & 0.5 & [36] \\ Pry/CNFs/CPE 3 & SWASV & 0.1 M acetate buffer & 0.2–130 & 0.05 & [37] \\ SBE 4 & DPASV & 0.01 M HNO\({}_{3}\) + 0.01 M KCl & 0.01 –0.1 & 0.02 & [38] \\ Glassy carbon & ADSV & 0.5 M NaNO\({}_{3}\) & & 0.5 & [12] \\ ErGO-MWNTs–L\({}_{\rm cys}\) 5 & DPASV & 0.1 M acetate buffer & 0.2–40 & 0.05 & [39] \\ CPE modified with IP-MWCNTs 6 & DPSV & 0.1 M acetate buffer & 3–55 & 0.5 & [40] \\ IP-MW-CNT 7 & SWASV & 0.1 M acetate buffer & 5–20 & 0.05 & [41] \\
**GrRAC** & **SWASV** & **0.5 M H\({}_{2}\)SO\({}_{4}\)** & **1-25** & **0.3** & **This work** \\ \hline \hline \end{tabular}  
* Mercaptothazoline-disulfide-bridged periodic mesoporous organosilica. 2–(5-bromo-2-pyridyllambda)-5-diethylaminophenol modified multiwalled carbon nanotube electrode. 3 Nanocomposite of polyethylene (PPy) and carbon nanofibres (CNFs)-modified carbon paste electrode (CPE).
* Silver ring electrode. 5 Electrochromically reduced graphene oxide-multiwalled carbon nanotubes-Lcysteine. 6 Carbon paste electrode (CPE) modified with non-imprinted polymer nanoparticles and multivalued carbon nanotubes. 7 Inglet-printed multivalued carbon nanotubes.  
\end{table}
Table 1: Comparison of the analytical parameters of the sensors reported in the literature for the determination of Pb(II).  
Figure 4: SWASV curves of GrRAC-70% recorded for different concentrations of Pb(II) in 0.5 M H\({}_{2}\)SO\({}_{4}\): (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.