## 4 Conclusions  
Cork-graphite-based sensors offer a fast, reliable, cost-effective, and simple way to determine Pb(II) in real samples. The composite sensor exhibits higher sensitivity and reproducibility than conventional unmodified graphite sensors, and the low LOD allows for reduced matrix effects in dilute solutions. As for the materials tested, the affinity of the cork with the analyte allowed a substantial improvement in sensitivity. According to the results reported in this work, the sensor obtained by mixing 70% w/w of cork with 30% w/w of graphite allowed obtaining higher voltammetric responses and a rapid detection of Pb(II). The proposed approach is precise, with a limit of quantification of 0.3 uM, reproducible, and less expensive, both in terms of time and materials, compared to other analytical methods. The composite electrode can be applied effectively for the determination of Pb(II) in acidic media. As for the physical and chemical properties, which favor the interactions with the analytes to be detected or/and quantified, more experiments are needed to better understand the chemical and electrochemical processes that occur on the cork-graphite surface when the current is applied or when cork participates as a mediator.  
Finally, even if the LOD reported in this work (0.3 umol L\({}^{-1}\)) is slightly above the limit established by the WHO (Pb: 0.24 umol L\({}^{-1}\)), there is room for improvement; for example, the size of the electrochemical sensor could be reduced, in order to approximate a micro-electrode, while the use of other carbon-based modifiers could allow improving the sensitivity.  
Conceptualization, E.V.D.S. and C.A.M.-H.; methodology, M.V.; formal analysis, I.B.S., S.F., and C.A.M.-H.; investigation, I.B.S. and C.A.M.-H.; resources, C.A.M.-H.; data curation, I.B.S., E.V.D.S., and C.A.M.-H.; writing--original draft preparation, I.B.S. and D.M.d.A.; writing--review and editing, M.V., S.F., and C.A.M.-H.; funding acquisition, D.M.d.A. and C.A.M.-H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (Brazil), grant number 306323/2018-4, and Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (Brazil), grant numbers 2014/50945-4 and 2019/13113-4.  
[PERSON] acknowledges the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior for her fellowship. Financial supports from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq-306323/2018-4) and from Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (Brazil) (FAPESP 2014/50945-4 and 2019/13113-4) are gratefully acknowledged. The authors also thank [PERSON] from the University of Porto (Portugal) for providing the cork samples used in this study.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & **Present Method 1** & **Pb2+ Added (μM)** & **Pb2+ Found (μM) 1** & **Recovery (\%)** \\ \hline Groundwater & Not detected & 10 & 11.5 \(\pm\) 0.3 & 115 \\  & Not detected & 50 & 52.1 \(\pm\) 1.8 & 104 \\ Tap water & Not detected & 10 & 10.8 \(\pm\) 0.5 & 108 \\  & Not detected & 50 & 50.4 \(\pm\) 1.5 & 100 \\ Produced water & 12.0 \(\pm\) 0.4 μM & 10 & 19.6 \(\pm\) 0.5 & 89 \\  & 50 & 60.3 \(\pm\) 2.1 & 97 \\ \hline \end{tabular}
\end{table}
Table 2: Pb(II) content in real water samples, measured with SWASV using the GrRAC-70% composite sensor.