Quantitative Imaging of Organic Ligand Density on Anisotropic Inorganic Nanocrystals

Abstract

A longstanding challenge in nanoparticle characterization is to understand anisotropic distributions of organic ligands at the surface of inorganic nanoparticles. Here, we show that using electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope we can directly visualize and quantify ligand distributions on gold nanorods (AuNRs). These experiments analyze dozens of particles on graphene substrates, providing insight into how ligand binding densities vary within and between individual nanoparticles. We demonstrate that the distribution of cetyltrimethylammonium bromide (CTAB) on AuNRs is anisotropic, with a 30% decrease in ligand density at the poles of the nanoparticles. In contrast, the distribution of (16-mercaptohexadecyl)­trimethylammonium bromide (MTAB) is more uniform. These results are consistent with literature reported higher reactivity at the ends of CTAB-coated AuNRs. Our results demonstrate the impact of electron spectroscopy to probe molecular distributions at soft–hard interfaces and how they produce spatially heterogeneous properties in colloidal nanoparticles.

Nanoscale cooperative adsorption for materials control

“…We chose to study Au nanoparticles for their wide applications as catalysts or probes and for their availability in variable sizes and shapes 18 – 20 . We first examined the adsorption of cetyltrimethylammonium bromide (CTAB), a ligand widely used in Au nanoparticle synthesis and surface modification 5 , 18 , on pseudospherical 5-nm Au nanoparticles using COMPEITS.…”

Recent Progress in Using Graphene as an Ultrathin Transparent Support for Transmission Electron Microscopy

“…Graphene serves as an excellent substrate by providing transparent and large surface ultraclean area to image them and to study their mobility and structures of their conformers. Janicek et al [ 106 ] showed that EELS can be used in STEM mode of imaging to directly visualize and quantify ligand distributions on gold nanorods, when deposited on graphene. The results showed ligand binding densities between individual nanoparticles and the impact of electron spectroscopy to understand the molecular distributions.…”

Understanding graphene's role as a protective substrate for atomic-resolution electron microscopy of small organic molecules

“…At one atom thick, graphene substrates are the ultimate low-background support membrane for (scanning) transmission electron microscopy. Graphene is also impermeable, electrically and thermally conductive; as a result, it increases the dose resistance of the materials it supports while minimizing background contrast [1], making it an ideal substrate for imaging beam-sensitive materials [2][3][4]. In particular, graphene can play a critical role in enabling high-resolution studies of soft materials and molecules, which are exceptionally beam sensitive with typical critical doses between 0.5-1000 e -/Å 2 [5].…”