Cerium oxide nanoparticles (nanoceria) are widely used in living organisms, from basic research to bioengineering, due to their distinct catalytic reactive oxygen species (ROS) scavenging ability1,2,3. Nanoceria have ROS scavenging abilities due to a large number of surface oxygen vacancies that alternate between two oxidation states (Ce3+ and Ce4+) 4,5,6. The Ce3+ dangling bonds effectively scavenge ROS while the lattice strains at the nanoscale promote the regeneration of these defect sites via redox cycling reactions7. Nanoceria have also been recently used for studying and engineering plant function8,9. Plants under abiotic stress experience accumulation of ROS, causing oxidative damage to lipids, proteins, and DNA10. In A. thaliana plants, nanoceria catalytic scavenging of ROS in vivo leads to improved plant photosynthesis under high light, heat, and chilling stresses8. Applying nanoceria to soil also increases shoot biomass and grain yield of wheat (Triticum aestivum)11; canola (Brassica napus) plants treated with nanoceria have higher plant biomass under salt stress12.
Nanoceria offer bioengineers and plant biologists a nanotechnology-based tool to understand abiotic stress responses and enhance plant abiotic stress tolerance. Nanoceria's in vivo ROS scavenging capabilities are independent of plant species, and the facile delivery into plant tissues has the potential to enable broad application outside of model organisms. Unlike other genetically-based methods, nanoceria do not require generating plant lines with the overexpression of antioxidant enzymes for higher ROS scavenging ability13. Leaf lamina infiltration of nanoceria to plants is a practical approach for lab-based research.
The overall goal of this protocol is to describe 1) the synthesis and characterization of negatively charged poly (acrylic) acid nanoceria (PNC), 2) the delivery and tracking of PNC throughout leaf cells, and 3) the monitoring of PNC-enabled ROS scavenging in vivo. In this protocol, negatively charged poly (acrylic) acid nanoceria (PNC) are synthesized and characterized by their absorption spectrum, hydrodynamic diameter, and zeta potential. We describe a simple leaf lamina infiltration method to deliver PNC into plant leaf tissues. For in vivo imaging of nanoparticle distribution within mesophyll cells, a fluorescent dye (DiI) was used to label PNC (DiI-PNC) and observe the nanoparticles via confocal fluorescence microscopy. Finally, we explain how to monitor in vivo PNC ROS scavenging through confocal microscopy.