Adsorption of Nanoceria by Phosphocholine Liposomes

Nanoceria (CeO₂ nanoparticle) possesses a number of enzyme-like activities. In particular, it scavenges reactive oxygen species based on in vitro and in vivo antioxidation studies. An important aspect of fundamental physical understanding is its interaction with lipid membranes that are the main components of the cell membrane. In this work, adsorption of nanoceria onto phosphocholine (PC) liposomes was performed. PC lipids are the main constituents of the cell outer membrane. Using a fluorescence quenching assay, a nanoceria adsorption isotherm was determined at various pH values and ionic strengths. A non-Langmuir isotherm occurred at pH 4 because of lateral electrostatic repulsion among the adsorbed cationic nanoceria. The phosphate group in the PC lipid is mainly responsible for the interaction, and the adsorbed nanoceria can be displaced by free inorganic phosphate. The tendency of the system to form large aggregates is a function of pH and the concentration of nanoceria, attributable to nanoceria being positively charged at pH 4 and neutral at physiological pH. Calcein leakage tests indicate that nanoceria induces liposome leakage because of transient lipid phase transition, and cryo-transmission electron microscopy indicates that the overall shape of the liposome is retained although deformation is still observed. This study provides fundamental biointerfacial information at a molecular level regarding the interaction of nanoceria and model cell membranes.