High-Resolution Scanning Tunneling Microscopy Characterization of Mixed Monolayer Protected Gold Nanoparticles

Abstract

Gold nanoparticles protected by a binary mixture of thiolate molecules have a ligand shell that can spontaneously separate into nanoscale domains. Complex morphologies arise in such ligand shells, including striped, patchy, and Janus domains. Characterization of these morphologies remains a challenge. Scanning tunneling microscopy (STM) imaging has been one of the key approaches to determine these structures, yet the imaging of nanoparticles' surfaces faces difficulty stemming from steep surface curvature, complex molecular structures, and the possibility of imaging artifacts in the same size range. Images obtained to date have lacked molecular resolution, and only domains have been resolved. There is a clear need for images that resolve the molecular arrangement that leads to domain formation on the ligand shell of these particles. Herein we report an advance in the STM imaging of gold nanoparticles, revealing some of the molecules that constitute the domains in striped and Janus gold nanoparticles. We analyze the images to determine molecular arrangements on parts of the particles, highlight molecular "defects" present in the ligand shell, show persistence of the features across subsequent images, and observe the transition from quasi-molecular to domain resolution. The ability to resolve single molecules in the ligand shell of nanoparticles could lead to a more comprehensive understanding of the role of the ligand structure in determining the properties of mixed-monolayer-protected gold nanoparticles.

Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces

“…In both cases, the main difficulty is to control the final composition of the ligand shell. In order to understand, control, and predict the properties of such very common, yet complex, hybrid nanoparticles made of an inorganic core and a bicomponent organic corona, the composition of the shell must be assessed. Various methods can unveil the composition of biligand shells, such as mass spectrometries, electron paramagnetic resonance, fluorescence, and surface enhanced Raman spectroscopies .…”

“…First, quantified partition could be used to predict the composition of the mixed SAM for a given ligand ratio in the initial solution, and even to select the right initial ratio to reach a targeted surface composition. Second, and more fundamentally, quantifying the partition for a series of well‐chosen ligands should provide new insights into the role of the chain length and functionality, and of the end‐group on ligand exchange and the stability of mixed ligand shells. In brief, such quantified partitions may contribute to decipher the impact of intermolecular forces and entropic effects on the stability of the ligand shells and their role as driving forces for ligand exchange—a topical issue for the control of nanoparticles properties and self‐assembly …”

Contact angle and adsorption energies of nanoparticles at the air–liquid interface determined by neutron reflectivity and molecular dynamics

“…[36][37][38] Among these arrangements, Janus (two different sides), narrow nanodomains (stripes) and uniformly mixed morphologies have been reported in experimental and theoretical papers. [39][40][41] The size of NPs, chemical nature of the ligands and their arrangement at the surface affect their interaction with interfaces and therefore their use in different applications.…”

Microfluidic-based high-throughput optical trapping of nanoparticles

“…Many well-established microscopy techniques can be used for characterization of nanoparticles at single-particle level. These include confocal laser scanning microscopy (CLSM) 9 , near-field scanning optical microscopy (NSOM) 10 , scanning electron microscopy (SEM) 11 , transmission electron microscopy (TEM) 12 , atomic force microscopy (AFM) 13 and scanning tunneling microscopy (STM) 14 . New techniques such as nanoparticle tracking analysis (NTA) 15 , nanopore integrated with resistive pulse techniques 16 , electronic detection 17,18 and optical detection 19,20 have also been used for characterization of size, concentration and other parameters of nanoparticles.…”