Stability and microbial toxicity of HfO₂ and ZrO₂ nanoparticles for photolithography

Hafnium dioxide (HfO₂) and zirconium dioxide (ZrO₂) nanoparticles (NPs) have gained attention as components of photoresists for next-generation photolithography. Coating of these NPs with organic ligands has been shown to increase their efficiency in photolithography. This study evaluated the microbial toxicity of hafnium dioxide and zirconium dioxide NPs coated with benzoate, isobutyrate or methacrylate toward heterotrophic aerobes, methanogens and the bioluminescent bacterium Aliivibrio fischeri. The stability of NPs in the media was assessed as a function of zeta potential, particle size distribution and leaching analyses. NP dispersions were unstable in all bioassay media, resulting in particle aggregation and settling. Leaching tests showed that dissolution of the organic ligands from the NPs varied widely depending on the nanomaterial and medium considered. The NPs were harmless to aerobic heterotrophs and methanogens at high concentrations (800-1200mg/l). In contrast, they displayed low to moderate toxicity to A. fischeri (50% inhibition at 286-1372mg/l), with benzoate-coated NPs causing the highest inhibition. Further analyses confirmed that the inhibition observed in assays with A. fischeri should be attributed to the coated NPs rather than to the dissolved organic ligands. Overall, the NP photoresists tested in this study are unlikely to exert acute microbial toxicity at environmentally relevant concentrations (sub-parts-per-million levels).