Electrochemical and NMR characterization of octanethiol-protected Au nanoparticles

Three octanethiol-protected Au nanoparticle samples having average particle sizes of 1.7, 2.1 and 2.8 nm, respectively are synthesized and characterized by electrochemical differential pulse voltammetry (DPV) and 1H-/13C-NMR. The potentials of zero charge, Epzc, of the Au nanoparticles are found to be particle size dependent. They are −0.654, −0.449 and −0.203 V (w.r.t the Ag ∣ AgCl ∣ 3 M Cl− reference electrode) as the particle size increases from 1.7 to 2.1 and to 2.8 nm, reflecting a change in the work function of the systems. However, based on the particle double-layer capacitances measured by DPV and the particle sizes by transmission electronic microscopy, the converging value of the dielectric constant, ε, of the protecting octanethiol layer is found to be 3.2±0.3. On the other hand, the 1H-/13C-NMR characterization shows that the significant line broadening observed for the nuclear magnetic resonance (NMR) lines of the C3–C8 carbons and the associated protons, once the octanethiols are bound to the metal surface, is mainly due to the reduction in motion of the species. In addition, the large particle size dependent down-field shift observed for the NMR signal from the 13C-labeled α-carbon indicates an underlying electronic metal-ligand bonding mechanism for the shift and the width of the corresponding peak, in accordance with the Epzc versus particle size relationship.