Alkanethiol adsorption

Kinetic studies of alkanethiol adsorption from ethanol solutions onto Au(lll) surfaces have shown that at relatively low concentrations (10 M) two distinct adsorption kinetic steps can be observed a very fast step, which takes a few minutes and leads to about 80-90% of maximal coverage and a slow step, which lasts several hours and at the end of which the thickness and contact angles reach their final values [110]. The initial step, described by the diffusion-controlled Langmuir adsorption, was found to be strongly dependent on the thiol concentration. At 1 mM solution the first step was over after 1 min, while it required over 100 min at 1 xM concentration. The second step can be described as a surface crystallisation process where the alkyl chains move from disordered states into unit cells, thus forming a two-dimensional crystal [110]. A more detailed study of Buck et al. involved different types of adsorption sites and showed that the rate-determining step strongly depends on the type of solvent [111-113]. 

DiMilla P A, Folkers J P, Biebuyck H A, Harter R, Lopez G P and Whitesides G M 1994 Wetting and protein adsorption of the self-assembled monolayers of alkanethiolates which are supported on transparent films of gold J. Am. Chem. Soc. 116 2225-6... 

The above stm study also discovered a facile transport of surface gold atoms in the presence of the Hquid phase, suggesting that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/desorption processes may have an important role in the formation of the final equiHbrium stmcture of the monolayer. Support for the importance of a desorption process comes from atomic absorption studies showing the existence of gold in the alkanethiol solution. The stm studies suggest that this gold comes from terraces, where single-a tomic deep pits are formed (281—283). 

Covalent attachment of enzymes to surfaces is often intuitively perceived as being more reliable than direct adsorption, but multisite physical interactions can in fact yield a comparably strong and stable union, as demonstrated by several biological examples. The biotin/streptavidin interaction requires a force of about 0.3 nN to be severed [Lee et al., 2007], and protein/protein interactions typically require 0.1 nN to break, but values over 1 nN have also been reported [Weisel et al., 2003]. These forces are comparable to those required to mpture weaker chemical bonds such as the gold-thiolate bond (1 nN for an alkanethiol, and even only 0.3 nN for a 1,3-aUcanedithiol [Langry et al., 2005]) and the poly(His)-Ni(NTA) bond (0.24 nN, [Levy and Maaloum, 2005]). 

When cells are suspended in a biological fluid or culture medium, both serum proteins and cells interact with the surface substrate. Serum protein adsorption behavior on SAMs has been examined with various analytical methods, including SPR [58-61], ellipsometry [13, 62, 63], and quartz QCM [64—66]. These methods allow in situ, highly sensitive detection of protein adsorption without any fluorescence or radioisotope labeling. SPR and QCM are compatible with SAMs that comprise alkanethiols. In our laboratory, we employed SPR to monitor protein adsorption on SAMs. 

To suppress cell adhesion on a material surface, one approach is to inhibit the adsorption of proteins. SAMs of alkanethiols that carry oligo(ethylene glycol) (OEG) [69] and phosphorylcholine [46, 70, 71] have been shown to prevent... 

Tidwell CD, Ertel SI, Ratner BD (1997) Endothelial cell growth and protein adsorption on terminally functionalized, self-assembled monolayers of alkanethiolates on gold. Langmuir 13 3404-3413... 

Lestelius M, Liedberg B, Tengvall P (1997) In vitro plasma protein adsorption on co-functionalized alkanethiolate self-assembled monolayers. Langmuir 13 5900-5908... 

Examples of the various methods of DNA immobilization are shown in Fig. 5. Amongst these are physical adsorption and covalent immobilization. The covalent immobihzation is best exemplified by the use of organo-silane chemistries and alkanethiol chemistries. 

Adsorption of Thiols Self-assembly of alkanethiols (RSH) on gold has especially been studied intensively. 

This le electrode reaction implies the loss of the thiol hydrogen and the oxidation of the S to a formal oxidation state of —1 upon adsorption of -alkanethiol (RSH) at Au. 

Gold clusters are stabilized by the adsorption of surfactants. Alkanethiols play an important role in this stabilization. 

Widrig et al. [196] have studied voltam-metrically, the SAMs of several -alkanethiols formed on pc-Ag electrodes. Analysis of data showed that during adsorption, the hydrogen of thiol group is lost and the sulfur is oxidized by one electron. Based on the charge required for the reductive desorption of the mono-layer, the surface coverage was found to be 7.0 X 10-10 j -2... 

Self assembly of monolayers on gold electrodes provided a viable approach to sensor construction (see below), as well as permitted electrochemical characterization of the mechanism involved in chemisorption [233,236-239]. Electrochemical investigation of the spontaneous adsorption of n-alkanethiols onto a gold electrode and the subsequent desorption of the monolayer were rationalized in terms of oxidation and reduction of the sulfur atom [233]. More intimate details concerning the chemical fate of all species involved in the chemisorption process have not yet been elucidated. 

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