Adsorption of citric acid on cerium oxide nanoparticles (nanoceria): effects of pH, surface charge and aggregation

Among a large number of engineered nanomaterials, CeO₂ nanoparticles (nanoceria) are of particular interest due to their unique oxidative, optical and electrical properties. With the increasing use of this important rare-earth metal oxide nanoparticle, there is the potential for it to be released in to the environment. Surface adsorbed ligands affect the surface chemistry of nanomaterials and thereby ultimately determining their fate and transformation in the natural environment. Citric acid is a naturally abundant organic acid, which can play an important role in determining the environmental fate of nanomaterials. This study focuses on citric acid adsorption onto nanoceria for three different particle diameters of 4, 9 and 39 nm. Both the details of surface adsorption of citric acid at different pH and its impact on nanoparticle behavior are explored.Speciation of adsorbed citric acid as a function of pH is probed using ATR–FTIR measurements, whereas HPLC and X–ray photoelectron spectroscopy are used to quantify the adsorption coverage. These results show that the surface speciation of citric acid differs from that of bulk solution in all pHs studied and the coordination to the surface as well as surface coverage is a function of particle size. Nanoparticle–nanoparticleinteractions and suspension stabilities are further probed through sedimentation and zeta potential measurements to better understand the behavior of ceria nanoparticles with and without the presence of citric acid.