Alkanethiol, SAMs on gold

Figure 3. Structure elucidation of n-alkanethiol SAMs on gold by techniques excluding diffrac-tometry, STM and AFM.

Thiol monolayers are not removed by solvents, but by sulfur-active chemicals which pass through the surface monolayers. Laser desorption mass spectrometry has shown that thiolate molecules are intact on the gold surface, but through air oxidation, some sulfonates develop. The relative stability of alkanethiol SAMs on gold to air oxidation is to be expected due to the covalent nature of the S—Au bond. Photooxidation via UV excitation of electrons in the metal surface is, however, possible and leads to sulfonate salts which have again been characterized by mass spectrometry as well as by XPS . Alkene-thiolate monolayers can best be desorbed from gold by a one-electron reductive path. Stable monolayers on gold were also obtained with benzenesulfinate. 

There is considerable interest in understanding the factors that control long-range (>10 A) electron transfer, especially in proteins [230] and DNA [231]. The alkanethiol SAMs offer an excellent means of controlling the spacing between a redox molecule and the metal electrode with a spatial resolution of approximately 1 A (corresponding to the insertion of one methylene in the alkyl chain). Standard rate constants versus number of methylenes m have been measured for alkanethiol SAMs on gold... 

In addition to Grignard chemistry and hydrogermylation, for which analogs also exist for silicon, there is another type of functionalization chemistry that has been carried out on germanium surfaces that of alkanethiol attachment to H-terminated Ge. Alkanethiols are well known for their use in self-assembled monolayers (SAMs) on gold surfaces. 

Gold is known not having a stable oxide at its surface,20 which makes it a good candidate substrate to study self-assembled monolayers. The deposition of an alkanethiol SAM on a gold surface is done by immersion of a clean gold substrate into the solution, and a spontaneous chemisorption at the interface produces gold thiolate species (See Figure 3.1) 11,12... 

We compare these monolayers with SAMs on gold. Features of the alkanethiol monolayers on gold which make them particularly attractive include their relatively uncomplicated preparation, their thermodynamic and chemical stability and mechanical strength. The stability transpires through the formation of a chemical Au—S bond which gives rise to two-dimensional lattice structure in which the head groups form an overlayer with a lattice constant of... 

The influence of pH and other factors on the reductive desorption and oxidative reformation of a hexadecanethiol SAM on a gold electrode was studied by reflection-absorption infrared spectroscopy (RAIRS) and cyclic voltammetry. At low pH the lack of solubility of desorbed molecules was problematic however, after three adsorption-desorption cycles the process was reproducible301. The reductive desorption in alkaline solution of an alkanethiol SAM on a single-crystal Au(l 11) electrode involves a one-electron process to yield a thiolate. Oxidative removal is a complicated process, involving up to eleven electrons per alkanethiol molecule302. 

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