Effects of iron or manganese doping of ZnO nanoparticles on their dissolution, ROS generation and cytotoxicity

Abstract

ZnO nanoparticles (NPs) have found wide applications due to their unique optoelectronic and photocatalytic characteristics. However, their safety aspects remain a concern especially considering that they are common constituents in sunscreen formulations. Production of reactive oxygen species (ROS) and dissolution of particles to ionic zinc are identified as two key determinants for the toxicity of ZnO. Doping transitional metals into the ZnO lattice has been demonstrated to be effective in suppressing NP dissolution and lowering cytotoxicity. However, the possibility of triggering excessive ROS by these transition metals has not been discussed. In this study, the behaviour of particle dissolution and the ability for ROS generation of iron and manganese doped ZnO NPs were studied in detail and further correlated with their cytotoxicity. Although Fe doping significantly reduced the level of released Zn ions, the cytotoxicity did not decrease as expected compared with undoped ZnO because more ROS were activated and damaged cells through oxidative stress. For Mn-doped ZnO NPs, their dramatically elevated intracellular ROS level was associated with high cytotoxicity compared with undoped ZnO even though both of them released similar amounts of free Zn ions.