Near-field fluorescence correlation spectroscopy measurements of 20 and 40nm polystyrene nanoparticles were performed, using fiber probes with aperture diameters ranging from 100 to 250 nm. The experimental data were best fit by a two-dimensional diffusion model, implying that there was significant anisotropy associated with the movement of the particles, such that mobility normal to the wall was much lower than that parallel to the wall. Interestingly, the lateral diffusion coefficients were approximately ten times greater than predicted from the Stokes-Einstein relationship corrected by taking into account hydrodynamic drag forces due to particle-wall interactions. This discrepancy between experimental and theoretical results is possibly due to strong viscous forces as well as attractive forces at the interface.
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