A new laser imaging technique could help answer safety questions surrounding nanoparticles by assessing the risks associated with zinc oxide (ZnO), a sunscreen ingredient. Although ZnO nanoparticles have a high optical absorption in the UVA and UVB ranges and are transparent in the visible spectrum when mixed with lotions – making them appealing candidates for inclusion in sunscreens – they have been shown to be toxic to certain types of cells. By characterizing the optical properties of ZnO nanoparticles, scientists from Australia and Switzerland have developed a way to quantitatively assess how far the nanoparticles might be absorbed into the skin. Using nonlinear optical microscopy, the team illuminated samples with short laser pulses and measured the return signals. Initial results showed that ZnO nanoparticles from a formulation rubbed into skin patches for 5 min, incubated at body temperature for 8 h and then washed off, did not penetrate beneath the stratum corneum, or topmost layer of skin. Overlay of a confocal/multiphoton image of excised human skin. The yellow color represents skin autofluorescence excited at 405 nm; the purple color represents zinc oxide nanoparticle distribution in skin (stratum corneum) excited at 770 nm, with collagen-induced faint second-harmonic-generation signals in the dermal layer. The arrows point to the zinc oxide nanoparticle high-density clusters. Courtesy of Andrei V. Zvyagin, Macquarie University. “Based on the existing body of evidence, it is reasonably well established that nanoparticles, including ZnO nanparticles, of the size range greater than 15 nm, cannot pass the intact top skin layer, under conditions close to leisure and occupational practice,” said Dr. Andrei V. Zvyagin, associate professor of physics and astronomy at Macquarie University. “It is now important to establish whether this holds if sunscreen is worn under broader range physiological conditions; for example, sweating or damaged skin.” To establish this, Zvyagin said the team would like to determine a “nanoparticle permeability threshold that depends on size, charge and surface functional groups.” This would create a knowledge base from which scientists could design nanoparticle vehicles for transdermal drug delivery or even design means of protection against environmental assualt at the nanoscale, he added. The researchers predict that the new optical characterization will be a useful tool for future noninvasive in vivo studies. Their work was published online in OSA’s open-access journal Biomedical Optics Express (doi: 10.1364/BOE. 2.003321).