*2.4. nCeO2 Extraction from Plant Tissues*

In our study, plants grew for the entire life cycle in a solid substrate enriched with *n*CeO2 at the beginning and with additional treatments during the experiment. From a subset of pots prepared for this purpose, 20 days after the appearance of the cotyledon leaves of *S. flos-cuculi*, the plants were harvested in order to verify the entry of *n*CeO2 in their tissues by enzymatic digestion and single particle inductively coupled plasma mass spectrometry (spICP-MS) analysis. The plant fractions (roots, stems and leaves) were separated and in turn sent for preparation. The digesting enzyme used was Macerozyme R-10 Pectinase from *Rhizopus* sp. (Sigma-Aldrich). Small sections (0.03 g) of fresh roots, stems, and leaves were harvested, rinsed three times with deionized water, and homogenized with 8 mL of 2 mM citrate buffer at pH 4.5, using an ultrasonic bath for 5 min. After the homogenization, for every sample 2 mL of the enzyme solution (0.05 g of enzyme dissolved in 2 mL of MilliQ water) were added [30]. The final supernatants were analyzed via spICP-MS (NexION 350 ICP-MS PerkinElmer, Waltham, MA, USA) to obtain the size distribution of *n*CeO2 present in roots and leaves.
