Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self -Assembled Monolayers on Gold Nanoparticles: The Effect of Surface Curvature and Ligand Structure

We report a solution NMR-based analysis of (16-mercaptohexadecyl)trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nano-spheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the nanoparticles increase beyond similar to 10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from similar to 3 molecules per nm(2) for 25 nm particles to up to 5-6 molecules per nm(2) in similar to 10 nm and smaller particles for in situ measurements of bound ligands; after I-2/I- treatment to etch away the gold cores, ligand density ranges from similar to 2 molecules per nm(2) for 25 nm particles to up to 4-5 molecules per nm(2) in similar to 10 nm and smaller particles. T-2 relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to similar to 13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)-trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for similar to 10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T-2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.