Immobilization of Nanoscale Sunscreening Agents onto Natural Halloysite Micropowder

Titanium dioxide (TiO2) nanoparticles contained in sunscreen products are typically smaller than 30nm in order to enhance the efficacy of the sun protection. TiO2 nanoparticles of this size range raise toxicity concerns because of the generation of reactive oxygen species (ROS) and the ability to penetrate into the skin tissue. To solve these problems, we immobilized the nanoparticles on a natural tubular powder, halloysite. The nanoparticle-halloysite hybrid powders are prepared by two synthetic methods: an ex situ immobilization of TiO2 nanoparticles (after separate synthesis of a nanoparticle batch), and an in situ generation of TiO2 nanoparticles inside halloysite. Notably, TiO2 nanoparticles of pure rutile phase are synthesized so that the ROS generation is suppressed before immobilization. The ultraviolet-visible (UV-vis) light extinction results show that the sunscreening efficacy of the hybrid powder produced via the ex situ route appears to be 22% higher than that of the hybrid powder fabricated by the in situ method. An increase in the hybrid powder concentration results in a sunscreening efficiency equivalent to that of the bare TiO2 nanoparticles. Our results can be used to produce nanoparticle-halloysite hybrid powders suitable for UV light screening while reducing the potential toxicity concerns. [doi:10.2320/matertrans.M2015086]