Imaging and Kinetic Study of Single Polymer Particles

Donggeng Yu, Ph.D. Candidate Kevin D. Welsher Ph.D., Advisor Abstract: The ability to directly observe chemical reactions at the single-molecule and single-particle level has enabled the discovery of behaviors otherwise obscured by ensemble averaging in bulk measurements. However powerful, a common restriction of these studies to date has been the absolute requirement to surface tether or otherwise immobilize the chemical reagent/reaction of interest. This constraint arose from a fundamental limitation of conventional microscopy techniques, which could not track molecules or particles rapidly diffusing in three dimensions, as occurs in solution. However, many chemical processes occur entirely in the solution phase, leaving single-particle/-molecule analysis of this critical area of science beyond the scope of available technology. Here we employed the 3D single-molecule active real-time tracking (3D-SMART) to untether the study of polymerization and monitor the growth of polymer particles in the solution phase. Another common feature of previous single-molecule and single-particle study is the use of external tag such as fluorescent labels. Doping of fluorescently labeled molecules have been playing an important role in the study of chemical reactions since the fluorescence microscopy offers high sensitivity and specificity. However, the labeling of molecules can be very complicated and the effect of the labels on the reactions are unknown or not fully understood. Here, we applied the stimulated Raman scattering (SRS) microscopy to study the growth of precipitated polymer particles. SRS can probe the intrinsic vibrational frequencies of chemical bonds or groups and image the material without adding external fluorescent labels. We also incorporated the Interferometric scattering (iSCAT) microscopy in the study of polymerization. Interferometric scattering (iSCAT) allows for sensitive non-fluorescent detection of nano-scale samples. It is label-free, fast and has high resolution, allowing for study of early stage of polymerization while the polymer particles are still small.