Effect of the surface texture and crystallinity of ZnO nanoparticles on their toxicity

We have investigated the correlation between the structural properties of ZnO nanoparticles (NPs) and their toxicity to mesenchymal stem cells (C2C12 cell line) and macrophage-derived cells (RAW 264.7 cell line). Nanopowders of grain size ranging between 5 nm and 50 nm were prepared by chemical route. Their structural properties were characterized extensively by X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM). The XRD spectra showed that 50 nm sized NPs are well crystallized and present a preferential orientation along the direction normal to the (001) plane while the HREM observations revealed that most of the large size (50 nm) crystallized nanoparticles have polygonal shape which is consistent with a texture of along [001] direction. The toxicity tests showed that [001] large textured NPs have higher toxicity to inflammatory cells than nanoparticles of low crystallinity and much smaller size (5 nm). In addition, NPs have cytotoxic effects on inflammatory cells at concentration as low as 0.05 mM while ten times higher concentrations did not have significant cytotoxic effects on cells representing mesenchymal tissues. These observations are explained by the enhanced generation of Reactive Oxygen Species (ROS) at the (0001) polar surface of ZnO NP. These results provide a direct evidence of the correlation between the toxicity and the surface texture of the oxide nanoparticles. Similar correlation has been reported for the photocatalytic properties of ZnO nanoparticles.