Advanced 13C Nuclear Magnetic Resonance Investigation of Metal-Ligand Interactions in Monolayer-Protected Gold Nanoparticles: NMR Shifts and Relaxations

Abstract

ABSTRACTWhile self-assembled monolayer (SAM) systems have been intensively investigated since the seminal work of Nuzzo and others in the later 1980s, the detailed nature of the interactions between thiol and gold in terms of surface bonding and ligand motion, a central theme crucial for any further advancements in the field of SAMs, is still elusive and subject to controversy. Nuclear magnetic resonance (NMR) spectroscopy, a canonical technique of chemistry and physics, has the unique ability of providing insightful chemical bonding and ligand motional information. Recent advances in wet chemistry in terms of synthesizing alkanethiol-protected, very stable and almost monodispersed, gold nanoparticles, due to Schriffin and co-workers, enable this feature of NMR to be harnessed in the study of thiol-metal bonding and ligand motions on gold surfaces. Here, we report a detailed 13C NMR shift and relaxation investigation of monolayer-protected gold nanoparticles as a function of metal particle size, by using selectively 13C1-labelled octanethiol. Several interesting results are obtained for the first time: as the gold particle size increases from ca. 1.5 to 4.0 nm, the 13C1 NMR shift moves down-field from ca. 39 to 50 ppm (with respect to TMS), the spin-lattice relaxation rate 1/T1 decreases, and the spin-spin relaxation rate 1/T2 increases. For a given particle size, the 1/T2 varies across the heterogeneously broadened line shape, being higher at the lower field, while there is no clear trend shown in 1/T1. All these results suggest that while particle tumbling may be the dominant spin-lattice relaxation mechanism, the physical properties of the metal core, i.e., more metallic vs. less metallic, are most probably the dominant factors influencing the 13C1 NMR shift and spin-spin relaxation. Now new doors are opened up for more detailed NMR investigations of metal-ligand interactions in SAM systems.