Measurements of the minimum elevation of nano-particles by 3D nanoscale tracking using ratiometric evanescent wave imaging

Abstract

Effect of saline concentration on the minimum elevation of nanoparticles has been examined under the electric double layer interactions with the substrate glass surface. The use of ratiometric total internal reflection fluorescence microscopy (R-TIRFM) allows three-dimensional tracking of nanoparticles in the near-wall region within less than 1 μm from the surface. The measurements of minimum elevation were made for polystyrene fluorescent nanospheres of 100, 250, and 500 nm in radii (SG = 1.05) for the salinity ranging from 0.1 to 10 mM. Special care was taken to insure cleaned surface conditions by elaborate sonication and rinsing of the glass substrate. The laser illumination intensity and duration also had to be carefully examined to minimize photobleaching of the fluorescence emission from particles. It is reported that the minimum elevation decreases with increasing saline concentration and with increasing particle sizes, for the first time experimentally and quantitatively to the authors’ knowledge.