Titanium oxide shell coatings decrease the cytotoxicity of ZnO nanoparticles

Although nanozinc oxide (nano-ZnO) is applied widely in photocatalysts and gas sensors and in biological fields, it can cause serious oxidative stress and DNA damage to mammalian cells. Our aim in this study was to reduce the cytotoxicity of nano-ZnO by coating it with a TiO2 layer. We used a sol-gel method to synthesize core (nano-ZnO)/shell (TiO2) nanoparticles (NPs) with various degrees of coating. Transmission electron microscopy and Raman spectroscopy confirmed that TiO2 was coated on the nano-ZnO. Moreover, a decrease in the intensity of the pre-edge signal in Ti K-edge X-ray absorption near edge structure spectra revealed that the core/shell NPs had more Ti-O coordination than pure TiO2 particles; in addition, the Zn K-edge extended X-ray absorption fine structure spectra revealed that after the ZnO NPs had been coated with TiO2, the coordination number of the ZnO shell increased to 3.3 but that of the ZnZn shell decreased to 6.2, providing further evidence for the ZnO/TiO2 core/shell structure. To ensure that the core/shell structures did indeed decrease the toxicity of nano-ZnO, we tested the effects of equal amounts of physical mixtures of ZnO and TiO2 NPs for comparison, employing methyl tetrazolium (MTT), interleukin-8 (IL-8), lactate dehydrogenase (LDH), and 2′,7′-dichlorofluorescin diacetate (DCFH-DA) to assess the particle-induced cytotoxicity, inflammatory response, membrane damage, and intercellular reactive oxygen species (ROS). From X-ray diffraction patterns, we identified the TiO2 shell as having an amorphous phase, which, unfortunately, exhibited slight cytotoxicity toward the human lung epithelial cell line (A549). Nevertheless, our core/shell nanostructures exhibited less oxidative stress toward A549 cells than did their corresponding ZnO/TiO 2 physical mixtures. In addition, a greater coating of TiO 2 decreased the toxicity of the ZnO NPs. It appears that the ZnO/TiO2 core/shell structure moderated the toxicity of nano-ZnO by curtailing the release of zinc ions and decreasing the contact area of the ZnO cores.