The penetration of spherical and rod-like gold nanoparticles into human skin is reported. Several skin preparation techniques are applied, including cryo techniques, such as plunge freezing and freeze drying, and the use of wet cells. Their advantages and drawbacks for observing nanoparticle uptake are discussed. Independent of the particle shape no uptake into intact skin is observed by a combination of imaging approaches, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and scanning X-ray microscopy (STXM). These results are discussed along with suitable skin preparation approaches. Experiments on barrier-disrupted skin, i.e. mechanical lesions made by pricking, indicate, however, that gold particles can be identified deep in the dermis, as follows from STXM studies on wet skin samples.
First results on single particle detection in human skin samples by x-ray microscopy are reported. 94±6 and 161±13 nm gold core particles with silica shells and 298±11 nm silica particles coated with a gold shell on ultramicrotome sections of human skin were determined. The particles were applied on fresh intact skin samples, which were sectioned prior to imaging. After screening the sections by conventional microscopy techniques, defined areas of interest were qualitatively investigated by scanning transmission x-ray microscopy at the Swiss Light Source. In studies on the percutaneous penetration of 161±13 nm particles on human skin samples, x-ray microscopy yielded high-resolution images of single particles spreading on the superficial layer of the stratum corneum and on the epithelium in superficial parts of hair follicles. No deeper penetration was observed. The present work using x-ray microscopy provides the unique opportunity to study qualitative penetration processes and membrane-particle interactions on the level of single particles. This goes beyond present approaches using optical microscopy. Further improvement of this approach will allow one to study particles with different physicochemical properties and surface modifications, including responses of the exposed tissue.