Studying Diffusion of Particles in Solution by SEM and DLS

Microgels are polymer-based nanoparticles that exhibit a reversible volume phase transition in water. Dynamic light scattering (DLS) measures collective particle diffusion in a sample yielding microgel structure and dynamics. DLS is optimized for monodisperse and dilute samples. Scanning electron microscopy (SEM) can provide images of individual microgels but uses high vacuum conditions. There are two main drawbacks of microgel dry imaging: dehydrated particles deswell and their dynamics are not observable. To counter this, wet particle imaging in an ionic liquid was explored. Still images and movies of particles suspended in a thin film of ionic liquid on a copper grid were recorded to analyze particle size distribution and dynamics. Average microgel size from DLS and SEM agreed in ionic liquid and water at room temperature. Particle diffusion was studied by tracking its mean square displacement in ionic liquid. Silica spheres were used as a control due to their stable nature. For a large sample volume, average diffusion coefficient of tracked silica particles agreed with DLS results, but varied for individual particles. Microgels proved to be a more complicated system, exhibiting complex behavior such as clustering, drift, rotation, and motion quenching.