Gold Nanoparticle Ligand Structure Investigated with Solution NMR: Effects of Ligand Length on Headgroup Dynamics and Ion Penetration

Abstract

Herein, we report the synthesis of a library of 16 gold nanoparticle (AuNP) types (2, 4, 9, and 12 nm in diameter and appended with mercapto-(X-alkyl)-N,N,N-trimethylammonium bromide (MxTAB) ligands (X = 11, 16, 18, or 20)) and detailed characterization of their ligand shell with solution ¹H NMR in deuterium oxide. The trimethylammonium headgroup is bulky, and the unique chemical shifts of its protons allow for systematic studies of ligand density and dynamics as a function of both nanoparticle size and ligand length for fully saturated surfaces. Chemical shift analysis of the solvent-exposed headgroup protons supports the notion that ligand headgroups pack closer together as the AuNP diameter increases for all ligands. Quantitative analysis shows that ligand density for the shorter ligands (MUTAB (X = 11) and MTAB (X = 16)) is dependent on nanoparticle size, ranging from ∼10 to ∼2 molecules/nm² as the nanoparticle size increases, while ligand density is independent of size (∼2 molecules/nm²) for longer ligands (MOTAB (X = 18) and MITAB (X = 20)). T₂ relaxation analysis shows less headgroup motion and therefore more ordering as both the NP diameter and the chain length increase. Gold etching experiments with potassium cyanide were performed to determine the ability of ions to penetrate the ligand layers; core protection and headgroup motion, as judged by T₂, were negatively correlated for the two shorter ligands but not correlated with the two longer ligands. Molecular dynamics simulations indicated that the longer ligands have a stronger tendency to form ligand islands on curved surfaces due to increased van der Waals interactions between the alkane portions of ligands, suggesting that the presence of patchy ligand islands displays hydrophobic character that prevents the cyanide ion from penetrating the AuNP cores. The relationship between ligand length and nanoparticle diameter/curvature leads to rudimentary predictions of ligand dynamics.