Revealing Reactions between the Electron Beam and Nanoparticle Capping Ligands with Correlative Fluorescence and Liquid-Phase Electron Microscopy

Abstract

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.

Visualizing formation of high entropy alloy nanoparticles with liquid phase transmission electron microscopy

“…48 Similar to the previous work, we utilized lower metal : ligand ratios than solution chemical synthesis to scavenge hydroxyl radicals that can oxidize metals and desorb ligands via oxidative elimination. 48,65,66 Solution pH can be affected by the electron beam due to production of hydronium ions; however, prior models have shown the pH is not significantly reduced for the low pH of the precursor solution ( pH ≈ 4). 67 Despite these differences, the qualitative processes that occur during reduction by NaBH 4 and aqueous electrons are expected to be similar.…”

Visualizing Formation of High Entropy Alloy Nanoparticles by Aggregation of Amorphous Metal Cluster Intermediates

“…46 Similarly, we utilized lower metal:ligand ratios than solution chemical synthesis because our previous work demonstrated that excess ligand was required to scavenge oxidizing hydroxyl radicals that can preferentially oxidize some metals and desorb ligands via oxidative elimination. 43,65,66 (diffusion limited) to 𝑑~𝑡 1/2 (reaction limited). 67 Prior in situ small angle x-ray scattering (SAXS) revealed similar linear increases in particle diameter with time during aggregative growth of gold nanoparticles by NaBH4 reduction of aqueous gold chloride.…”

The effect of flow on radiolysis in liquid phase-TEM flow cells

“…at the nano-meter scale in wet-chemical environments [1][2][3]. Besides the uncontested benefits, such as high spatial resolution of TEM [4], utilising electrons for imaging caused several challenges to arise [5][6][7][8]. The main issue of using highenergy electrons for imaging is the inevitable creation of radiation damage in the irradiated matter, with radiolysis of water molecules being the most relevant in LP-TEM [3].…”