In an attempt to answer safety questions about the use of nanoparticles in sunscreen, a new laser imaging technique has been developed to assess the risks associated with one of sunscreen’s ingredients — zinc oxide (ZnO). Overlay of the confocal/multiphoton image of the excised human skin. Yellow color represents skin autofluorescence excited by 405 nm; the purple color represents zinc oxide nanoparticle distribution in skin (stratum corneum) excited by 770 nm, with collagen-induced faint second-harmonic generation signals in the dermal layer. (Image: Biomedical Optics Express) ZnO nanoparticles have a high optical absorption in the UVA and UVB range, as well as transparency in the visible spectrum when mixed with lotions — making them ideal candidates for inclusion in sunscreens. The particles, however, have been shown to be toxic to certain types of cells within the body. By characterizing the optical properties of ZnO nanoparticles, scientists from Australia and Switzerland found a way to quantitatively assess how far the nanoparticles might be absorbed into the skin. Zinc oxide nanoparticle distribution in excised human skin. The black line represents the surface of the skin (top), blue represents zinc oxide nanoparticle distribution in the skin (stratum corneum), and pink represents skin. (Image: Timothy Kelf, Macquarie University) The team used a technique called nonlinear optical microscopy, which illuminates the sample with short pulses of laser light and measures a return signal. Initial results show that ZnO nanoparticles from 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. The researchers said the new optical characterization should be a useful tool for future noninvasive in vivo studies. The work was published this month in the Optical Society’s (OSA) Biomedical Optics Express. For more information, visit: www.opticsinfobase.org/boe