exploded view of liquid cell holder TEM
Image courtesy of Protochips


The dynamics of processes of nanoparticles such as diffusion, attraction and repulsion, and self-assembly of structures of nanoparticles at the solid-liquid interfaces differ significantly from those occurring for bulk conditions and their fundamental physical rules are still unknown. Here, we used liquid phase scanning transmission electron microscopy (LP-STEM) to study several aspects of nanoparticle dynamics of colloidal chitosan coated gold nanoparticle (TCHIT-AuNP) clusters in a liquid layer enclosed between two SiN membranes. We found that upon beam irradiation using an electron flux of 0.9 e/sÅ2, the AuNPs assembled in clusters that shifted and rotated with time. The newly formed clusters could join and form larger clusters via a mechanism of oriented attachment. By increasing the electron flux to 6.2 e/sÅ2, we observed the fragmentation of some of the clusters and TCHIT-AuNPs were exchanged between clusters. At the highest electron flux studied 25 e/sÅ2, we observed AuNPs moving at a very slow speed compared to Brownian motion in liquid even though they were not directly attached or pinned to the liquid-enclosing membrane. Experiments using branched polyethylenimine (BPEI) coated AuNPs were carried out for comparison.

Impact Statement

The clustering dynamics of gold nanoparticles as a function of surface chemistry and electron flux was studied using LC-TEM. Nanoparticles with chitosan and branched polytheylenimine surface chemistries were tracked and observed to cluster together in oriented assembly under electron beam irradiation. Clusters continued to rotate after formation and the characteristic suppressed Brownian motion reported in LC-TEM was observed.