Solubility of metal oxide nanomaterials: Cautionary notes on sample preparation

Abstract

Eight metal oxides were obtained to investigate the dissolution behaviour of engineered nanomaterials (ENMs) dispersed in biologically relevant media. Identities of the metal oxide compounds, and their crystal form and size were checked using powder X-ray diffraction (XRD). Methods for sonication of metal oxide nanoparticles were optimized to achieve stable stock dispersions, and methods for separation of dissolved metal ions from dispersed nanoparticles were evaluated. The results of the optimization experiments showed that each metal oxide ENM required a different combination of sonication time and power (% amplitude). Optimized values for delivered sonication energy (J/mL) ranged from 24 for CuO to 833 for -Al2O3. Centrifugation at 20000G was found to be more effective and less prone to artefacts than using commercially available ultrafiltration devices for separation of dissolved metal fraction, under these experimental conditions. XRD results indicated that the composition of two metal oxide nanopowders (Mn2O3 and Fe2O3) did not meet the manufacturers' claims, underscoring the importance of double-checking physical-chemical properties of commercial ENMs purchased for research.