Hydrophobic Organization of Monolayer-Protected Au Clusters on Thiol-Functionalized Au(111) Surfaces

Abstract

The present study explains a novel hydrophobic organization of 4.8 ± 0.5 nm core diameter Au clusters in contrast to the organization using covalent and electrostatic interactions where specific bifunctional molecules are used. This unique method of organization is demonstrated using quartz crystal microbalance (QCM), UV−vis spectroscopy, cyclic voltammetry, X-ray photoelectron spectroscopy, X-ray diffraction, and I−V measurements. QCM results show a slow attainment of saturation coverage (1010 clusters/cm2) of Au nanoclusters on the self-assembled monolayer (SAM) functionalized substrate, and the equilibrium constant (Keq) is three times less compared to that for the monolayer formation using dodecanethiol. The electronic and optical properties (e.g., surface plasmon band ∼525 nm) of these films show that the Au colloids maintain their individual character without fusion to larger units, and the current−voltage behavior shows nonlinearity. X-ray photoelectron spectra of the functionalized gold surface treated with monolayer-protected Au clusters (MPCs) reveal that S 2p shows a 0.2 eV shift compared to that of a dodecanethiol SAM. Cyclic voltammetric studies confirm the redox accessibility of these MPCs with an E° value of 0.65 V (ΔE ≈ 60 mV, Ipa/Ipc ≈ 1) and a surface coverage of 2.15 × 10-9 mol/cm2 on the SAM-functionalized surface. The hydrophobic organization of MPCs on the functionalized gold substrate forms an ideal platform for examining the existing theoretical models associated with the adsorption of colloids and proteins, as well as cellular attachment and adhesion at solid surfaces.