Influence of Nanoparticle Morphology on Ion Release and Biological Impact of Nickel Manganese Cobalt Oxide (NMC) Complex Oxide Nanomaterials

Lithium intercalation compounds such as nickel manganese cobalt oxides (Li_xNi_yMn_zCo_1-y-zO₂, 0 < x, y, z < 1, or NMCs) are complex transition metal oxides of increasing interest in nanoscale form for applications in electrochemical energy storage and as tunable catalysts. These materials exhibit sheetlike structures that expose low-energy basal planes and higher-energy edge planes in relative amounts that vary with the nanoparticle morphology. Yet there is little understanding of how differences in nanoparticle morphology and exposed crystal planes affect the biological impact of this class of technologically relevant nanomaterials. We investigated how changing nanoparticle morphology from two-dimensional (001)-oriented nanosheets to three-dimensional nanoblocks affects the release of ions and the resulting biological impact using Shewanella oneidensis MR-1 as a model organism. NMC nanoparticles were synthesized in sheetlike morphology and then converted to block morphologies by heating, leading to two morphologies of identical chemical composition that were compared to a commercially available NMC. Ion dissolution studies reveal that NMC nanomaterials release transition metal ions incongruently (Ni > Co > Mn) in amounts that vary with nanoparticle morphology. However, when normalized by the specific surface areas, the rates of release of each transition metal from flakes, blocks, and commercial material were equivalent. Similarly, the impact on S. oneidensis MR-1 was different when using mass-based dosing but was nearly identical using surface-area-normalized exposure dosing. Our results show that even though nanosheets and nanoblocks expose different crystal faces with significantly different surface energies, the rate of ion release is independent of the distribution of crystal faces exposed and depends only on the total surface area exposed. These data suggest that the key protonation steps that control release of transition metals do not depend on the degree of coordination of the initially exposed surface, providing insights into the molecular-level factors that influence the environmental impact of complex metal oxide nanomaterials. Our results have significant implications for establishing methodologies to assess toxicity of reactive nanomaterials.