Hydroxyapatite (HA) doped with trace elements has attracted much attention recently owing to its excellent biological functions. Herein, we use a facile co-precipitation method to incorporate selenium into HA by adding sodium selenite during synthesis. The obtained selenium-substituted HA products are needle-like nanoparticles which have size and crystallinity that are similar to those of the pure HA nanoparticles (HANs) when the selenium content is low. HANs are found to have the ability to induce the apoptosis of osteosarcoma cells, and the anti-tumour effects are enhanced after incorporation of selenium. Meanwhile, the nanoparticles can also support the growth of bone marrow stem cells. Furthermore, the flow cytometric results indicate that the apoptosis induction of osteosarcoma cells is caused by the increased reactive oxygen species and decreased mitochondrial membrane potential. These results show that the selenium-substituted HANs are potentially promising bone graft materials in osteosarcoma treatment due to their dual functions of supporting normal cell growth and inducing tumour cell apoptosis.

As one of the most widely investigated and used materials, TiO₂ is expected to play a leading role in helping solve environmental and energy issues. In this work, we developed a facile, greener and more controllable approach of preparing anatase TiO₂ mesocrystals with largely exposed {001} facets. This approach utilizes the solid–solid topotactic transformation of NH₄TiOF₃ to anatase TiO₂ and only needs a simple sintering process. In order to obtain the exposed {001} facets on the surfaces of final products, the sintering temperature must be between 700 °C and 900 °C. Temperatures below 700 °C lead to unexposed {001} facets. Temperatures above 900 °C lead to a mixed phase of anatase and rutile. The photocatalytic activity of the TiO₂ mesocrystals is much higher than that of the corresponding TiO₂ polycrystalline materials and comparable with that of commercially available P25. The exposed {001} facets are believed to play a very important role in the photocatalysis process.

A totally novel, extremely easy, much greener and low-cost method has been developed to synthesize TiO₂ mesocrystals. These materials are built of TiO₂ nanocrystals with similar crystallographic orientation and largely exposed {001} facets. Their unique structure optimizes the relationship between specific surface area, crystallite size and active facets, and therefore leads to superior photocatalytic activity.

Largely improved visible light photocatalytic activities of pristine TiO ₂ by constructing stepped structures on exposed {001} facets.