Most nanoscale materials, whether engineered or natural, fall into one of four categories: 1) metal oxides such as zinc and titanium that are used in ceramics, chemical polishing agents, scratch-resistant coatings, cosmetics and sunscreens; 2) nanoclays, which are naturally occurring, platelike clay particles that strengthen or harden materials or make them flame retardant; 3) nanotubes, which are used in coatings to dissipate or minimize static electricity; and 4) quantum dots, used in exploratory medicine or in the self-assembly of nanoelectronic structures.
Generally, nanoparticles used in cosmetics such as titanium dioxide and zinc oxide belong to the first category and are employed as sun filters. Both types of nanoparticles are engineered and have a regular shape; it is this regular shape that makes them useful in cosmetics. At least two aspects here are important: the absolute size of the nanoparticles as well as their surface properties, which determine their efficacy as protective sun filters.
