An investigation of the interaction and pH-activity relationship of ceria nanoparticles with a model membrane

Using rapid cyclic voltammetry (RCV) and an established electrochemical sensor, we clarify the interactions of in-house synthesised and well characterised cerium (IV) oxide (ceria) nanoparticles and nanocubes (NPs) with phospholipid membranes. The RCV results reveal that the interaction of ceria NPs with the phospholipid depends not only on the dispersion media and the particle coating, but also on the pH. No interaction was observed with the phospholipid layer when ceria NPs were dispersed in phosphate buffered saline (PBS) at pH 7.4 or in citric acid/citrate buffers (CCB) at pH values ≥4. However, a clear interaction was observed when ceria NPs were dispersed in acidic media. Notably, CeO2 NPs dispersed in either a glycine buffer at pH 3.0 (GLY 3.0) or CCB at pH 3.0, when in contact with the phospholipid monolayer, exhibited similar interaction behaviours to soluble Ce (+III) ions dispersed in the same media. However, both RCV responses also showed an increase in the background current (a semiconductor effect) indicative of the direct interaction of the nanoparticles with the uncoated electrode. In contrast, PBS-incubated ceria NPs did not produce an interaction with the phospholipid monolayer when dispersed in CCB 3.0 or GLY 3.0. Together, these findings suggest that the dispersion medium plays an important role in the interaction of CeO2 NPs either impeding or promoting interaction by changing the characteristics of the NP surface. In particular, we conclude that the interaction of ceria NPs with the phospholipid layer at acidic pH is facilitated by Ce(+III) ions or their complexes on the surface of the nanoparticles.