Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness

Abstract

Sunscreens are used to provide protection against adverse effects of ultraviolet (UV)B (290-320 nm) and UVA (320-400 nm) radiation. According to the United States Food and Drug Administration, the protection factor against UVA should be at least one-third of the overall sun protection factor. Titanium dioxide (TiO2) and zinc oxide (ZnO) minerals are frequently employed in sunscreens as inorganic physical sun blockers. As TiO2 is more effective in UVB and ZnO in the UVA range, the combination of these particles assures a broad-band UV protection. However, to solve the cosmetic drawback of these opaque sunscreens, microsized TiO2 and ZnO have been increasingly replaced by TiO2 and ZnO nanoparticles (NPs) (<100 nm). This review focuses on significant effects on the UV attenuation of sunscreens when microsized TiO2 and ZnO particles are replaced by NPs and evaluates physicochemical aspects that affect effectiveness and safety of NP sunscreens. With the use of TiO2 and ZnO NPs, the undesired opaqueness disappears but the required balance between UVA and UVB protection can be altered. Utilization of mixtures of micro- and nanosized ZnO dispersions and nanosized TiO2 particles may improve this situation. Skin exposure to NP-containing sunscreens leads to incorporation of TiO2 and ZnO NPs in the stratum corneum, which can alter specific NP attenuation properties due to particle-particle, particle-skin, and skin-particle-light physicochemical interactions. Both sunscreen NPs induce (photo)cyto- and genotoxicity and have been sporadically observed in viable skin layers especially in case of long-term exposures and ZnO. Photocatalytic effects, the highest for anatase TiO2, cannot be completely prevented by coating of the particles, but silica-based coatings are most effective. Caution should still be exercised when new sunscreens are developed and research that includes sunscreen NP stabilization, chronic exposures, and reduction of NPs' free-radical production should receive full attention.

Keywords: (photo) toxicity; TiO2; UV-radiation; ZnO; blue shift; cancer; nanoparticles; physicochemical; scattering; skin barrier.

Figure 1

The viable epidermis, underlying the SC, contains three layers, the stratum basale, the stratum spinosum, and the stratum granulosum. The SC consists of approximately 15 layers of corneocytes. The main cell type in the viable epidermis is the keratinocyte. Pathways for cutaneous penetration include the paracellular (a), transcellular (b), and the transappendagael route, which includes the transport along hair follicles (c1), sweat pores (c2), and sebaceous glands (c3) Abbreviation: SC, stratum corneum.

Figure 2

Graphical representation of the band gap in a semiconducting material. The electronic structure of the semiconductor is characterized by bands that consist of orbitals. Bands are separated by gaps in the energy for which there are no orbitals. Upon light absorption of minimally the band gap energy, a valence band electron (e⁻) is excited to the conduction band leaving a hole in the valence band (h⁺).

Figure 3

Absorbance of bulk titanium dioxide and zinc oxide at room temperature. Adapted with permission of American Scientific Publishers, from Popov AP, Zvyagin AV, Lademann J, et al. Designing inorganic light-protective skin nanotechnology products. J Biomed Nanotechnol. 2010;6:432–451; permission conveyed through Copyright Clearance Center, Inc.

Figure 4

Superoxide anion radical (O₂^−•), equation (1) and hydroxyl radical (^•OH), equation (2) formation resulting from the photo-excitation of TiO₂. An electron transfer from photo-excited TiO₂ to molecular oxygen leads to production of the superoxide anion radical. Hydroxyl radicals can be formed by electron release from water catalyzed by photo-excited TiO₂. By reoxidation of the Ti³⁺ ions back to Ti⁴⁺ ions, the process can start again. Similar generation of superoxide anion and hydroxyl radicals occurs in the case of ZnO.