Characterization of optical properties of ZnO nanoparticles for quantitative imaging of transdermal transport.

Widespread applications of ZnO nanoparticles (NP) in sun-blocking cosmetic products have raised safety concerns related to their potential transdermal penetration and resultant cytotoxicity. Nonlinear optical microscopy provides means for high-contrast imaging of ZnO NPs lending in vitro and in vivo assessment of the nanoparticle uptake in skin, provided their nonlinear optical properties are characterized. We report on this characterization using ZnO NP commercial product, Zinclear, mean-sized 21 nm. Two-photon action cross-section of this bandgap material (E-bg = 3.37 eV, lambda(bg) = 370 nm) measured by two techniques yielded consistent results of eta(ZnO)sigma((2ph))(ZnO) = 6.2 +/- 0.8 mu GM at 795 nm, and 32 +/- 6 mu GM at 770 nm per unit ZnO crystal cell, with the quantum efficiency of eta(ZnO) = (0.9 +/- 0.2) %. In order to demonstrate the quantitative imaging, nonlinear optical microscopy images of the excised human skin topically treated with Zinclear were acquired and processed using sigma((2ph))(ZnO) and eta(ZnO) values yielding nanoparticle concentration map in skin. Accumulations of Zinclear ZnO nanoparticles were detected only on the skin surface and in skin folds reaching concentrations of 800 NPs per mu m(3). (C) 2011 Optical Society of America