Enhanced Stability and Areal Density Tunability of Nanoparticle Monolayer Films from Gold Nanoparticles with Mixed Tetradecane-/PEG-thiol Ligand Shells

The preparation of gold nanoparticle (AuNP) monolayer films with a broad range of tunable particle spacings is demonstrated. A simple two-ligand approach employing poly(ethylene glycol)thiol (PEG-SH) and tetradecanethiol (C14SH) was used to prepare nanoparticles of varying hydrophobicity. Nanoparticle compositions were analyzed extensively by thermogravimetric analysis, ¹H nuclear magnetic resonance spectroscopy, transmission electron microscopy (TEM), zeta potential measurements, and infrared spectroscopy. Surface pressure vs area isotherms were obtained at the air/water interface, and the results were compared with changes to the film morphology using TEM and ultraviolet-visible spectroscopy of films transferred to solid supports. AuNP were found to have film morphologies ranging from close-packed islands to dispersed AuNP separated by more than 100× the coating thickness, which is an improvement from a maximum separation of 2× for other types of AuNP films. Unbound PEG-SH impurities were found to improve the stability of the morphology and separation of the AuNP in the assembled interfacial films. The simple addition of C14SH to the nanoparticle coating along with the bound PEG-SH improves the stability of the assembled AuNP films at the air/water interface compared to that of the PEG-thiol alone or specialized single ligand systems. The separated AuNP film morphologies are shown to be stabilized to an extent that enables tunable areal density control by adjusting the compression achieved in the Langmuir balance.