posted on 2021-08-02, 13:34authored byThilini
U. Dissanayake, Mei Wang, Taylor J. Woehl
Liquid-phase
transmission electron microscopy (LP-TEM) enables
real-time imaging of nanoparticle self-assembly, formation, and etching
with single nanometer resolution. Despite the importance of organic
nanoparticle capping ligands in these processes, the effect of electron
beam irradiation on surface-bound and soluble capping ligands during
LP-TEM imaging has not been investigated. Here, we use correlative
LP-TEM and fluorescence microscopy (FM) to demonstrate that polymeric
nanoparticle ligands undergo competing crosslinking and chain scission
reactions that nonmonotonically modify ligand coverage over time.
Branched polyethylenimine (BPEI)-coated silver nanoparticles were
imaged with dose-controlled LP-TEM followed by labeling their primary
amine groups with fluorophores to visualize the local thickness of
adsorbed capping ligands. FM images showed that free ligands crosslinked
in the LP-TEM image area over imaging times of tens of seconds, enhancing
local capping ligand coverage on nanoparticles and silicon nitride
membranes. Nanoparticle surface ligands underwent chain scission over
irradiation times of minutes to tens of minutes, which depleted surface
ligands from the nanoparticle and silicon nitride surface. Conversely,
solutions of only soluble capping ligand underwent successive crosslinking
reactions with no chain scission, suggesting that nanoparticles enhanced
the chain scission reactions by acting as radiolysis hotspots. The
addition of a hydroxyl radical scavenger, tert-butanol,
eliminated chain scission reactions and slowed the progression of
crosslinking reactions. These experiments have important implications
for performing controlled and reproducible LP-TEM nanoparticle imaging
as they demonstrate that the electron beam can significantly alter
ligand coverage on nanoparticles in a nonintuitive manner. They emphasize
the need to understand and control the electron beam radiation chemistry
of a given sample to avoid significant perturbations to the nanoparticle
capping ligand chemistry, which are invisible in electron micrographs.