*2.1. nCeO2 Characterization*

Nanoparticle characterization was carried out at the laboratories of the National Research Council—Institute of Science and Technology for Ceramics (Faenza, Italy). The cerium oxide nanopowders with an average particle size of 25 nm were purchased from Sigma-Aldrich (St. Louis, MO, USA). The *n*CeO2 had a mass weight of 172.11 g mol−1, density of 7.13 g mL−<sup>1</sup> at 25 ◦C, and 99.95% purity. The size and average shape were measured with a transmission electron microscope (TEM, FEI Tecnai F20, FEI Company, Eindhoven, The Netherlands). The *n*CeO2 was suspended in deionized water and sonicated in a water bath for 30 min with a sonication intensity of 180 watts. The suspensions were characterized for Z—average size at pH 7 and hydrodynamic diameter (Hd), whose distributions were measured by dynamic light scattering (DLS) on a Zetasizer Nano ZS (Malvern Ltd., Worcestershire, UK).

### *2.2. Plant Material*

*Silene flos-cuculi* L. (synonym *Lychnis flos-cuculi*) is a diploid polycarpic herbaceous perennial wetland plant, belonging to the *Caryophyllaceae* family. This species is native and distributed throughout Europe, where it is found in moist open habitats, along roads and flood plain, in wet meadows and pastures, but it also grows in the northern United States and eastern Canada [27]. In arable landscapes, it has become scarce because of the loss of habitats, but is still found in secondary habitats such as ditches and stream verges. The species is predominantly outcrossing, but capable of self-fertilization [28]. *S. flos-cuculi* forms vegetative rosettes with numerous flower stems that could be from 30 to 90 cm tall. The stems have barbed hairs that make the plant rough to the touch; stems grow over the foliage and end with pink flowers, which open between April and June, and many types of insects are attracted by the flower's nectar. Another characteristic of these flowers is that they have five narrow petals divided into four lobes. The leaves are paired: the lower ones are stalked and the upper leaves present pointed apexes. The fruits consist of small capsules, containing many dark seeds, which can be dispersed mechanically [29].
