Abstract. Zinc oxide ͑ZnO-nano͒ and titanium dioxide nanoparticles ͑20 to 30 nm͒ are widely used in several topical skin care products, such as sunscreens. However, relatively few studies have addressed the subdermal absorption of these nanoparticles in vivo. We report on investigation of the distribution of topically applied ZnO in excised and in vivo human skin, using multiphoton microscopy ͑MPM͒ imaging with a combination of scanning electron microscopy ͑SEM͒ and an energy-dispersive x-ray ͑EDX͒ technique to determine the level of penetration of nanoparticles into the sub-dermal layers of the skin. The good visualization of ZnO in skin achieved appeared to result from two factors. First, the ZnO principal photoluminescence at 385 nm is in the "quiet" spectral band of skin autofluorescence dominated by the endogenous skin fluorophores, i.e., NAD͓P͔H and FAD. Second, the two-photon action cross section of ZnO-nano ͓ ZnO ͑TPEF͒ ϳ 0.26 GM; diameter, 18 nm͔ is high: ϳ500-fold of that inferred from its bulk third-order nonlinear susceptibility ͓Im ZnO ͑3͒ ͔, and is favorably compared to that of NAD͓P͔H and FAD. The overall outcome from MPM, SEM, and EDX studies was that, in humans in vivo, ZnO nanoparticles stayed in the stratum corneum ͑SC͒ and accumulated into skin folds and/or hair follicle roots of human skin. Given the lack of penetration of these nanoparticles past the SC and that the outermost layers of SC have a good turnover rate, these data suggest that the form of ZnO-nano studied here is unlikely to result in safety concerns.

Systematic studies probing the effects of nanoparticle surface modification and formulation pH are important in nanotoxicology and nanomedicine. In this study, we use laser-scanning fluorescence confocal microscopy to evaluate nanoparticle penetration in viable excised human skin that was intact or tape-stripped. Quantum dot (QD) fluorescent nanoparticles with three surface modifications: Polyethylene glycol (PEG), PEG-amine (PEG-NH₂) and PEG-carboxyl (PEG-COOH) were evaluated for human skin penetration from aqueous solutions at pH 7.0 and at pHs of solutions provided by the QD manufacturer: 8.3 (PEG, PEG-NH₂) and 9.0 (PEG-COOH). There was some penetration into intact viable epidermis of skin for the PEG-QD at pH 8.3, but not at pH 7.0 nor for any other QD at the pHs used. Upon tape stripping 30 strips of stratum corneum, all QDs penetrated through the viable epidermis and into the upper dermis within 24 h.