Effect of secondary particle size of nickel oxide nanoparticles on cytotoxicity in A549 cells

The effect of nanoparticle type, shape, as well as primary and secondary particle size on toxicity remains poorly characterized. In this study, suspensions of nickel oxide (NiO) nanoparticles with the same primary particle size (< 50 nm) but different secondary particle sizes were prepared, and their cytotoxicity was investigated. A planetary ball mill wet nanopulverizer with zirconium milling balls of decreasing sizes (phi: 0.5, 0.1, and 0.05 nun) yielded NiO nanoparticles of decreasing mean particle size (310.4 +/- 6.7, 172.0 +/- 2.8, and 102.0 +/- 0.5 nm). Stock solutions were diluted to various concentrations in 10% heat-inactivated fetal bovine serum containing minimum essential medium, and shown to have the same primary particle size, but different secondary particle sizes. Tests with A549 cells revealed that cytotoxicity increased with increasing secondary particle size: milling ball diameter phi 0.05 mm (IC50: 148 mu g/mL) < phi 0.1 mm (IC50: 83.5 mu g/mL) < phi 0.5 mm (IC50: 33.4 mu g/mL). Uptake experiments indicated that the intracellular amount of Ni increased with increasing secondary particle size. In summary. the present findings show that differences in secondary particle size affected the cytotoxicity of NiO suspensions, which could be ascribed at least in part to differences in the amount of NiO taken up by the cells.