Cytotoxicity and physicochemical properties of hafnium oxide nanoparticles

Nano-sized hafnium oxide (HfO₂) particles are being considered for applications within the semiconductor industry. However, little is known about their cytotoxicity. The objective of this work was to assess several HfO₂ nanoparticles (NPs) samples for their acute cytotoxicity. Dynamic light scattering analysis of the samples indicated that the average particle size of the HfO₂ in aqueous dispersions was in the submicron range with a fraction of particles having nano-dimensions. The media used in the toxicity assays decreased or increased the average particle size of HfO₂ NPs due to dispersion or agglomeration. Static time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed numerous surface contaminants on the NPs. Only one HfO₂ sample caused moderate cytotoxicity to human cell lines. The inhibitory sample caused a 50% response in the Live/Dead assay with HaCaT skin cells at 2200mgL^-1; and a 50% response in the mitochondrial toxicity test at 300mgL^-1. A microbial inhibition assay based on methanogenic activity also revealed that another HFO₂ sample caused moderate inhibition. The difference in toxicity between samples could not be attributed to size. Instead the difference in toxicity was likely due to differences in the contaminants of the HfO₂. The ToF-SIMS analysis indicated unique signatures of Br and P in the sample toxic to human cell lines suggesting a distinct synthesis was used for that sample which may have been accompanied by inhibitory impurities. The results taken as a whole indicate that HfO₂ itself is relatively non-toxic.