Formation of TiO₂ nanostructures by enzyme-mediated self-assembly for the destruction of macrophages

Conifer foliage-like rutile TiO₂ nanoparticles (CFR NPs), spherical anatase TiO2 NPs (SA NPs), and a mixture of CFR and SA NPs were synthesized in aqueous solution at low temperature. Furthermore, sea urchin-like nanostructures combining SA and CFR NPs, which were designated as anatase/rutile nanostructures (A/R NSs), were produced through the self-assembly of jack bean urease (JBU). The specific surface area of the A/R NSs was considerably larger than those of the CFR NPs, SA NPs, and commercial TiO ₂ (P25). In addition, the amount of reactive oxygen species (ROS) yielded from the A/R NSs was significantly higher than that yielded from CFR NPs, SA NPs, and P25 because of the large surface area of the A/R NSs and a synergistic effect caused by the integration of anatase and rutile phases. The A/R NSs showed no cytotoxicity at concentrations <100 μg/mL, although CFR NPs, SA NPs, and P25 were cytotoxic, probably because of their size and shape. Using the high surface area and the superior photocatalytic activity of the A/R NSs, macrophages were effectively destroyed by UV irradiation for the purpose of treating atherosclerosis. Macrophages were killed more effectively by the A/R NSs than P25. Furthermore, different mechanisms of cell destruction resulting from UV irradiation, A/R NSs, and a combination of both were investigated. The death of cells treated with A/R NSs and exposed to UV irradiation was induced primarily by apoptosis rather than necrosis; cells that were not treated with the NSs died mainly from necrosis.