Investigating Particle - Particle and Molecule - Surface Interactions Using Three - Dimensional Super Localization and Tracking Method

ZHAO, YAQING. Investigating Particle-Particle and Molecule-Surface Interactions Using Three-Dimensional Super Localization and Tracking Method. (Under the direction of Gufeng Wang). Single particle/molecule tracking (SPT) is among the most useful tools for investigating dynamic processes at individual molecule level in chemical and biological systems. Conventional SPT is limited to two dimensional studies. In this dissertation, we developed three-dimensional (3D) super localization and tracking technique to better understand the interactions between colloidal particles. Long-range attractions that extend beyond the working range of van der Waals force were reported between samely charged particles, which cannot be explained with current dominant theory, namely Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, in colloidal and surface sciences. To date, this observed long-range attraction is not well explained, partly because of technical challenges in tracking the fast movement of small particles in the 3Dspace. The phenomenon of long-range attraction is still controversial and being questioned to be experimental artifacts. We conducted systematic study so that we are able to quantitatively describe how samely charged particles interact with each other in the long-range. The three-dimensional tracking method provides axial information, which is crucial in determining relative positions of colloids that was lost in previous studies. We also developed the two-particle super-localization method, which can be used to analyze two closely spaced particles simultaneously and provides molecular locations with nanometer precision. By tracking the 3D trajectories of pairs of interacting particles, we confirmed mutual attraction between paired particles near glass surfaces. The super localization result shows that interacting distance between two particles is ‘long’ compared to van der Waals attraction distance in DLVO theory. When ionic strength was changed in the aqueous solution, the inter-particle distance did not significantly change in the experiment, possibly indicating that the potential energy surface is not sensitive to ionic strength. In a second study, single molecule super localization method was also applied to investigate molecule-surface interaction in Reversed Phase Liquid Chromatography (RPLC). As solute-stationary phase interaction is central in RPLC, fundamental study on its mechanism contributes to improving the technique. Several past studies show that peak tailing is correlated to rare strong sorption events on C18 stationary phase. Here we studied sorption on/in C18 stationary phase at individual molecular level to provide further insight into the phenomenon. We found that there are two modes of “apparent adsorptions”: the strong adsorption that molecules are permanently immobilized within our observation time scale, and the weak adsorption that molecules adsorb and desorb during observation. Using single molecule super localization technique, we recovered the positions of molecules that are apparently adsorbed on C18 with a localization precision of ~ 20 nm. We found that in the second mode, the ‘apparently’ adsorbed molecules are in fact diffusing on the interface of or inside the stationary phase in a confined area of ~50 × 50 nm 2 . Varying the mobile phase composition changes the ratio of the two types of sorption sites. The more polar the mobile phase is, the more permanently adsorptions can be observed, indicating that the extent of the solvation of C18 has an impact on how stationary phase retains solute molecules. Investigating Particle-Particle and Molecule-Surface Interactions Using Three-Dimensional Super Localization and Tracking Method