Zinc oxide nanoparticles are among the ingredients list of some commercially available sunscreen products, raising concerns about whether the particles may be absorbed beneath the outer layer of skin. To help answer these safety questions, an international team of scientists around Zhen Song, Macquarie University, Sydney, Australia, used nonlinear optical microscopy to test the concentration of ZnO nanoparticles at different skin depths.

They found that the nanoparticles did not penetrate beneath the outermost layer of cells when applied to patches of excised skin.

^{Image: © Biomedical Optics Express}

• Characterization of optical properties of ZnO nanoparticles for quantitative imaging of transdermal transport,
  Zhen Song, Timothy A. Kelf, Washington H. Sanchez, Michael S. Roberts, Jaro Rička, Martin Frenz, Andrei V. Zvyagin,
  Biomed. Opt. Express 2011, 2 (12), 3321–3333.
  DOI: 10.1364/BOE.2.003321