No treatment.	SPEs were used as received, after standard cleaning. No unwanted lead signals were observed in the stripping analysis.
Treatment A (Type A).	The screen-printed electrode is preoxidized at +1.50 V in 0.1 M acetate buffer at pH = 4.4 for 120 s, following what proposed by Amine et al. [12].
Treatment B (Type B).	The screen-printed electrode is preoxidized under basic conditions in a saturated solution of sodium carbonate at +1.20 V for 240 s.