Characterization of cerium oxide nanoparticles. Part 2: Nonsize measurements

Part 1 of the present study discussed the application of a multi-method approach in characterizing the size of cerium oxide nanoparticles (NPs) (see companion paper by Baalousha et al). However, other properties less routinely investigated such as shape and morphology, structure, chemical composition, and surface properties are likely to play an important role in determining the behavior, reactivity, and potential toxicity of these NPs. In the present study, the authors describe the measurement of the aforementioned physicochemical properties of NPs (applied also to nanomaterials [NMs]) compared to micron particles (MPs). They use a wide range of techniques, including high resolution-transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and electrophoresis. The authors compare these techniques, their advantages, and limitations along with recommendations about how best to approach NM characterization, using an application to commercial cerium oxide NPs and MPs. Results show that both cerium oxide NPs and MPs are formed of single polyhedron or truncated polyhedron crystals. Cerium oxide NPs contain a mixture of Ce³⁺ and Ce⁴⁺ cations, whereas the MPs contain mainly Ce⁴⁺ potentially important in understanding toxicity of cerium oxide NPs. The isoelectric point of cerium oxide NPs was approximately pH 8, which explains their propensity to aggregate in aqueous media (see compantion paper by Baalousha et al.).