ZnO nanoparticles dissolution, penetration and toxicity in human epidermal cells. Influence of pH

Abstract

Zinc oxide (ZnO) nanoparticles are widely used in cosmetics, food packaging, drug delivery and biosensors. There is therefore a risk of toxicity for organisms, including humans, exposed to nanoparticles. ZnO nanoparticles have been suggested to be toxic in vitro and in vivo. However, the cause of toxicity induced by ZnO nanoparticles is poorly understood. Therefore, we studied here the dissolution of ZnO nanoparticles in water at pH 4 and 7 because the cellular cytoplasm has a neutral pH, while the cellular organelles such as lysosomes have an acidic pH. We also assessed nanoparticle penetration and dissolution in human epidermal cells, and their toxicity for those cells. Methods included atomic absorption spectrophotometer, cytotoxicity assay, fluorescent microscope and transmission electron microscopy (TEM). Results show a significant increase in nanoparticle dissolution in Dulbecco’s modified Eagles medium and Milli-Q water at pH 7 after 48 h, whereas at pH 4 the dissolution of nanoparticles in water did not increase. ZnO nanoparticles were observed in the cytoplasm after 72 h by transmission electron microscopy.