Atmospheric deposition of anthropogenic nanoparticle (NP) emissions (e.g. from automobiles and industrial operations) onto edible plant surfaces is a threat to food safety in urban areas. Nanoparticle internalization and translocation from roots to leaves has been investigated, but less is known about leaf to root translocation and leaf-NP interactions. In this study, two widely cultivated and consumed plant species with large leaves and differing foliar surface free energy (SFE; wettability): arugula (Eruca sativa Mill.; low SFE) and escarole (Cichorium endivia L.; high SFE), were exposed to Pt NPs dispersed in ultrapure water through roots (one application of 1 mg Pt NPs in 20 mL water) or leaves (5 day exposure to 2 L droplets [5, 50, or 500 mg/L] applied per 0.5 cm2 abaxial and adaxial leaf surfaces) upon reaching the 5-leaf growth stage. Platinum was ideal for this study because of its low environmental abundance (2.7 ppb) [1], resistance to dissolution, and direct connection to anthropogenic activity (e.g. catalytic converters) [2]. Throughout the exposure period, SFE of foliar-exposed leaves was evaluated by solving a variation of Young's equation based on contact angle measurements from ultrapure water, glycerol, and diiodomethane [3]. Analysis of foliar Pt NP adsorption patterns by scanning electron microscopy showed that Pt NPs appeared in larger amounts and in a more aggregated/agglomerated state on arugula leaves, consistent with the fact that at low SFE, liquid droplets are less likely to spread out and drip off or down the leaf. Platinum quantification of washed roots and leaves with inductively coupled plasma-mass spectrometry also revealed statistically higher leaf Pt concentrations for arugula, and statistically higher root Pt concentrations for escarole at all exposures. Uptake and translocation of Pt NPs from leaves to roots was statistically significant for both plants at a concentration of 500 mg/L (p  0.05). Since Pt is resistant to dissolution [2], the presence of Pt in unexposed plant segments was interpreted as having been in NP form. The results highlight the relevance of foliar SFE in discussions of NP retention by (edible) leaves and demonstrate the possibility for NP uptake and translocation from leaves to roots.

This work is being supported by the ISO-FOOD Project "ERA Chair for Isotope Techniques in Food Quality, Safety and Traceability" (grant No. 621329).