Near-wall nanoparticles perpendicular distribution measured using evanescent illumination

Nanoparticle imaging using evanescent illumination is a very useful technique for elucidating physical phenomena near the walls of microchannels in microfluidic systems. Since the intensity of evanescent illumination decays exponentially with distance from the wall surface, particles closer to the wall appear brighter than those further from the wall. This enables the three-dimensional positions of nanoparticles in near-wall regions to be measured. In this study, the behavior of nanoparticles in a microfluid was observed experimentally. 20-nm-diameter fluorescent nanoparticles were employed rather than biological molecules for single-particle imaging. To overcome the poor measurement accuracy due to the small diameter of the nanoparticles used for imaging and the low fluorescence intensity of a single nanoparticle far from the wall, the image settings were optimized and a novel image processing algorithm was proposed. The experimental results reveal that the nanoparticle concentration varies in the direction normal to the wall. The nanoparticle concentration decreased with proximity to the wall surface; in particular, the concentration in the region from 0 to 170 nm, was much lower than that in more distant regions from the wall. Moreover, the nanoparticle concentration in the near-wall region decreased as the diffusion coefficient of the nanoparticles decreased.