An international research team has leveraged nonlinear optical microscopy to study the effect of zinc oxide (ZnO) nanoparticles in sunscreen that have raised a debate over product safety. The nanoparticles’ high optical absorption in the UVA and UVB range, along with their visible-spectrum transparency, makes them appealing for inclusion in lotions—but the particles have been shown to be toxic to certain types of cells in the body. By characterizing the nanoparticles’ optical properties, the Australian and Swiss team found a way to quantitatively assess how far the nanoparticles might migrate into skin.
|A confocal/multiphoton overlay of excised human skin shows skin autofluorescence excited by 405 nm (yellow) and ZnO nanoparticle distribution in skin (stratum corneum) excited by 770 nm (purple), with collagen-induced faint SHG signals in the dermal layer. (Image courtesy of Biomedical Optics Express)|
To test the concentration of ZnO at different depths, the researchers used an optical nonlinear microscope to send short pulses of laser light to skin samples and measure the return signals. Initial results showed that ZnO nanoparticles—in a formulation that had been rubbed into skin patches for 5 minutes, incubated at body temperature for 8 hours, and then washed off—did not penetrate beneath the stratum corneum, or topmost layer of the skin. According to the researchers, the optical characterization approach should be a useful tool for future noninvasive in-vivo studies.
1. Z. Song et al., Biomed. Opt. Exp., 2, 12, 3321–3333 (2011).