Ligand Tilt Increases with Gold Nanoparticle Diameters up to 35 nm: Implications for Stability, Sensing, and Self-Assembly

The properties of ligand self-assembled monolayers (SAMs) bound to nanoparticles are critical to controlling both nanoparticle stability and self-assembly of close-packed films and other superstructures for applications in plasmon-based sensing and metamaterials. The tilt of organized SAMs can have a significant effect on the overall SAM thickness, however measurements which can directly determine the tilt on organized nanoparticle SAMs have yet to be demonstrated, limiting our understanding of how the tilt may be related to the nanoparticle core size. In this study we employ a reparametrized extended DLVO model which included assessment of ligand tilt via the well-characterized response of the localized surface plasmon resonance (LSPR) of gold nanoparticle cores. Ligand tilt was determined for mercaptoundecanoic acid monolayers formed on three different nanoparticle core sizes (3, 15, 35 nm). We determine that the ligand tilt changes significantly with nanoparticle core size up to ∼35 nm, from 36 to 80° from the surface normal as core size increases, with data to suggest the tilt remains constant for larger nanoparticles. A maximal ligand density was determined by X-ray photoelectron spectroscopy. ATR-IR spectroscopy of the nanoparticles confirms ordered monolayers capping the nanoparticles, with attenuation of certain vibrational modes that progresses with increasing core size. The observed differences in stability of purified nanoparticles and the shift in the LSPR extinction of visible light by well-separated nanoparticles vs condensed phases of nanoparticles in dried films further supports the changes in tilt determined. The cause of such tilt angles is discussed and compared with self-assembled monolayers on planar gold. These findings provide new insight into the properties of SAMs for nanoparticle systems that is very different from that of SAMs on 2D gold surfaces and has important implications on nanoparticle-ligand design where stability and proximity to the core surface are critical.