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.
Particle size determines the relative surface area. The surface area of one cubic centimeter of a solid material is 6 cm2 whereas that of one cubic centimeter of 1-nm particles in an ultrafine powder is 6,000 m2—literally one-third larger than a football field. Surface area is important because most chemical reactions involving solids happen at their surfaces where the chemical bonds are incomplete. On the other hand, the surface properties of metal oxides such as titanium dioxide are also unique.
Titanium dioxide and zinc oxide are examples of so-called inorganic or physical sun filters that exist as solid particles in sunscreens. Organic or chemical sun filters, however, are either liquid at room temperature or solubilized in the oil phase of sunscreen formulations. Inorganic filters provide broad-spectrum protection against both UVA and UVB, whereas organic sun filters are usually classified as either UVB or UVA filters. The difference between titanium dioxide and zinc oxide is that titanium dioxide is optimized for UVB attenuation to give protection from sunburn, whereas zinc oxide is optimized for UVA attenuation to give protection from UV-induced aging. Since the sun protection factor (SPF) is related to UVB protection, titanium oxide is more effective in terms of SPF at a given concentration.
Figure 1 depicts the light attenuation properties for titanium dioxide of various mean particle sizes. Pigmentary titanium dioxide, with a crystal size of around 220 nm, gives UV protection but also attenuates visible light and hence appears white on the skin. Reducing the crystal size increases UV attenuation and reduces visible attenuation. The optimum particle size for UVB and UVA attenuation but good transparency in the visible region ranges from 40–60 nm. To achieve complete visible transparency, the particle size can be reduced further to ~20 nm; however, such a small particle size provides little UVA or UVB attenuation and is therefore ineffective as a sunscreen.
Formulating with Nanoparticles
Modern cosmetic sun protection products use both organic and inorganic sun filters but the choice depends on the final product and application. Whereas organic sun filters are easier to formulate with and provide high efficacy at low concentrations, cocktails of different sun filters are required to obtain high SPFs. High concentrations of multiple filters are necessary to achieve such levels, which with solid filters that are difficult to solubilize, may create formulation problems. In addition, some organic sun filters decay upon exposure to UV light but inorganic sun filters are photostable and can achieve high SPF levels using a single filter (titanium dioxide). Powders can be difficult to formulate with but dispersions also exist to make these filters much easier to incorporate into formulations without a loss of efficacy due to coagulation of the active.