Structure and chain dynamics of alkanethiol-capped gold colloids

The structure and dynamical behavior of short and long chain alkanethiols, CH3(CH2)(7)SH and CH3(CH)(17)SH, and of a hydroxyalkanethiol, HO(CH2)(16)SH, adsorbed onto gold nanoparticles were studied by variable temperature solid-state C-13 NMR spectroscopy. In both the solution and solid state, the resonances of the first three carbons next to the sulfur headgroup disappear upon binding to the gold, indicating a strong interaction with the surface. A C-13-enriched sample, CH3(CH2)(12)*CH2SH/gold, displays a broad resonance centered at 42 ppm for the carbon next to the sulfur headgroup. Whereas the solid-state C-13 shifts of CH3(CH2)(7)SH/gold are essentially the same as in solution, the methylene carbons of CH3(CH2)(17)SH and HO(CH2)(16)SH/gold shift, downfield by 4.5 ppm in the solid state, indicating that the chains crystallize into an extended all-trans conformation. The high conformational order, along with reduced methylene proton line widths in the CH3(CH)(17)SH/gold sample, indicates that the chains are undergoing large-amplitude motions about their long axes. Molecular mobility increases toward the unbound ends which have a higher population of gauche conformers. Relaxation measurements show the coexistence of motionally restricted all-trans chains and a smaller population of liquid-like conformationally disordered chains in CH3(CH2)(17)SH/gold at room temperature. The two types of chains are proposed to arise from close packing of the gold colloidal spheres, resulting in interstitial spaces and regions where chains of neighboring colloids can interdigitate to produce ordered domains. Phase transitions of the thiol-capped gold nanocrystals, which are detected by differential scanning calorimetry, are shown to involve a reversible disordering of the alkyl chains.