Particle size determination of sunscreens formulated with various forms of titanium dioxide

Abstract

Background: There has been some apprehension expressed in the scientific literature that nanometer-sized titanium dioxide (TiO₂) and other nanoparticles, if able to penetrate the skin, may cause cytotoxicity. In light of a lack of data regarding dermal penetration of titanium dioxide from sunscreen formulations, the Food and Drug Administration Center for Drug Evaluation and Research initiated a study in collaboration with the National Center for Toxicology Research using minipigs to determine whether nanoscale TiO₂ in sunscreen products can penetrate intact skin. Four sunscreen products were manufactured. Method: The particle size distribution of three TiO₂ raw materials, a sunscreen blank (no TiO₂) and three sunscreen formulations containing uncoated nanometer-sized TiO₂, coated nanometer-sized TiO₂ or sub-micron TiO₂ were analyzed using scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), and X-ray diffraction (XRD) to determine whether the formulation process caused a change in the size distributions (e.g., agglomeration or deagglomeration) of the TiO₂. Results: SEM and XRD of the formulated sunscreens containing nanometer TiO₂ show the TiO₂ particles to have the same size as that observed for the raw materials. This suggests that the formulation process did not affect the size or shape of the TiO₂ particles. Conclusion: Because of the resolution limit of optical microscopy, nanoparticles could not be accurately sized using LSCM, which allows for detection but not sizing of the particles. LSCM allows observation of dispersion profiles throughout the sample; therefore, LSCM can be used to verify that results observed from SEM experiments are not solely surface effects.

Keywords:

• Laser scanning confocal microscopy
• nanoparticle
• scanning electron microscopy 
• titanium dioxide