Measuring the diffusion of fluorophores in human skin by two-photon fluorescence correlation spectroscopy combined with measurements of point spread function

Two-photon excitation fluorescence correlation spectroscopy (TPFCS) has been used in combination with measurements of the point spread function (PSF), for quantitative analysis of fluorophores in excised human skin. Measurements have been performed at depths between 0 and 40 mu m. The PSF, measured as full width at half maximum, was found not to depend on the depth. Measurements revealed difference in diffusion coefficient depending on extra-or intracellular location of fluorophore. The number of molecules was accumulating close to the surface and then decreased by the depth. The results from our study show that TPFCS can be used for quantitative analyses of fluorescent compounds in human skin.