Gold-thiol monolayers stability

In view of the limited stability of gold-thiol monolayers to laser light and X-ray, electron and metastable atom beams reported recently (see Section VI), one has to exercise caution to avoid damage of the monolayer in such experiments as SERS, XPS, GIXD, LEED etc. 

Polymerized diacetylenes show remarkably improved stability as compared to monomeric gold-thiol monolayers (see Section VI). 

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. 

The C-C bond formed through this process is very stable compared to say a gold-thiol bond. It can resist temperatures up to 700 K without being lost from the surface [42] and is stable in a wide potential window. A further advantage over other common surface modification techniques (e.g., self-assembly of silane monolayers on oxide) is that the process is quite fast, deposition times are usually on the order of 10 s. In contrast to the enhanced stability, formation of these layers on electrode surfaces is less controlled than for the alkanethiol system. In addition, not all the radicals generated participate in the formation of C-C bonds. It varies based on the surface condition and the substituent in the para position of... 

In 1994, thiols were firstly used as stabilizers of gold nanoparticles [6a]. Thiols form monolayer on gold surface [18] and highly stable nanoparticles could be obtained. Purification of nanoparticles can be carried out, which makes chemical method of metal nanoparticles a real process for nanomaterial preparation. Various thiol derivatives have been used to functionalize metal nanoparticles [6b, 19]. Cationic and anionic thiol compounds were used to obtain hydrosols of metal nanoparticles. Quaternary ammonium-thiol compounds make the nanoparticle surface highly positively charged [20]. In such cases, cationic nanoparticles were densely adsorbed onto oppositely charged surfaces. DNA or other biomolecule-attached gold nanoparticles have been proposed for biosensors [21]. 

Numerous bisthiols have been observed to form spontaneously multilayers on gold and silver on the basis of the oxidative formation of disulfides.15-27 Nonetheless, most of these compounds lack electroactive character, with few notable exceptions.23,27 In principle, the introduction of redox centers at the core of these molecules and their subsequent assembly into multilayers can be exploited to generate electroactive films. The concentration of redox centers within the resulting electrode coatings, as well as their thickness, can be significantly larger than those possible with electroactive thiols such as 1-4 (Fig. 7.1).11 14 In addition, the transition from electroactive monolayers to electroactive multilayers can translate into a significant enhancement in stability and a much more effective protection of the electrode surface. 

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