Hydrophobic organization of monolayer-protected Au clusters on thiol-functionalized Au(111) surfaces

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 u sed. This unique method of organization is demonstrated using quartz crysta l microbalance (QCM), W-vis spectroscopy, cyclic voltammetry; X-ray photoel ectron spectroscopy, X-ray diffraction, and I-V measurements. QCM results s how a slow attainment of saturation coverage (10(10) clusters/cm(2)) of Au nanoclusters on the self-assembled monolayer (SAM) functionalized substrate and the equilibrium constant (K-eq) is three times less compared to that f or the monolayer formation using dodecanethiol. The electronic and optical properties (e.g., surface plasmon band similar to 525 nm) of these films sh ow 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 mono layer-protected Au clusters (MPCs) reveal that S 2p shows a 0.2 eV shift co mpared to that of a dodecanethiol SAM. Cyclic voltammetric studies confirm the redox accessibility of these MPCs with an E value of 0.65 V (DeltaE app roximate to 60 mV, I-pa/I-pc approximate to 1) and a surface coverage of 2. 15 x 10(-9) mol/cm(2) on the SAM-functionalized surface. The hydrophobic or ganization of MPCs on the functionalized gold substrate forms an ideal plat form for examining the existing theoretical models associated with the adso rption of colloids and proteins, as well as cellular attachment and adhesio n at solid surfaces.