Self-assembly of short and long-chain n-alkyl thiols onto gold surfaces: A real-time study using surface plasmon resonance techniques

Gold surfaces coated with monolayers of alkyl thiols are significant in the field of biosensors and chromatography. There is a general concern about the quality of coatings in terms of surface density and voids. The self-assembly of the short-chain hexane (C 6 ) thiol and long-chain dodecane (C 12 ) and hexadecane (C 16 ) thiols to polycrystalline gold surfaces has been investigated in situ and in real time using surface plasmon resonance (SPR) spectroscopy and surface plasmon microscopy (SPM). The self-assembly was followed based on observed changes in reflectivity at a fixed angle of incidence as a function of time. Our results indicate that the data for hexane, dodecane, and hexadecane thiol adsorption to gold surfaces were best fit by a two-step process rather than a one-step process. The mechanism may involve fast adsorption to the surface to give 80% (C 12 , C 16 ) or 50% (C 6 ) coverage followed by a slow (100-fold slower) "rearrangement" of the adsorbed thiol. SPM shows these surfaces to be smooth and homogenous at a 4 μm scale. An understanding of the process of rearrangement could lead to control of the quality of coatings for analytical applications. Key words: surface plasmon, alkyl thiols, gold, monolayer, kinetics, ellipsometry, microscopy.