Thiols on gold

Bryant and Pemberton [379, 380] have employed surface Raman scattering to study thiol films on silver and gold and their results confirm the postulated difference in bonding. 

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Bain C D and Whitesides G M 1989 Formation of monolayers by the ooadsorption of thiols on gold—variation in the length of the alkyl ohain J. Am. Chem. Soc. Ill 7164-75... 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Bain CD, Evall J, Whitesides GM (1989) Formation of monolayers by the coadsorption of thiols on gold variation in the head group, tail group, and solvent. J Am Chem Soc 111 7155-7164... 

F. Frederix, K. Bonroy, W. Laureyn, G. Reekmans, A. Campitelli, W. Dehaen, and G. Maes, Enhanced performance of an affinity biosensor interface based on mixed self-assembled monolayers of thiols on gold. Langmuir 19, 4351-4357 (2003). 

M. Buck reviews in great depth the literature on self-assembled monolayers (SAMs) of thiols on gold, a classic means of surface modification. The wide variety of functional groups that is provided by synthetic chemists makes thiol-SAMs an exciting playground for applications where the gap between two worlds, the inorganic and the organic, needs to be closed. Examples are molecular electronics and biochemistry. 

The preparation and application of SAM systems patterned by STM and their use in catalysis was demonstrated by Wittstock and Schuhmann [123]. The patterning (local desorption) of SAMs from alkane thiols on gold was performed by scanning electrochemical microscopy (SECM), followed by the assembly of an amino-deriva-tized disulfide and coupling of glucose oxidase to form a catalytically active pattern of the enzyme. The enzymatic activity could be monitored/imaged by SECM. 

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

Although thiols on gold and silanes on silicon are the most prominent examples, they are not the only self-assembling systems. Various substrates beside gold and silicon oxide are capable of binding SAMt s such as silver, copper, palladium, platinum and metal oxides as zirconium oxides and indium tin oxide (ITO). As organic molecules alcohols ( OH), amines ( NH2), organic acids ( COOH), or isocyanides ( NCO) have been used. 

Adsorption from the liquid phase is experimentally much easier (cheaper), but requires, even for highest-purity solvents, chemical inert substrates, e.g., hopg, or selective adsorbate systems, like thiols on gold. 

Pool et al studied the adsorption of alkyl thiols on gold surfaces, both without and with a solvent. As solvent they considered w-hexane. For the simulations they used a force field with a so called united-atom model. This means that the thiol heads, the CH2 and CH3 groups, and the gold atoms are treated as individual united atoms . The interactions between these united pseudoatoms are described in terms of the force fields. Subsequently, molecular dynamics simulations were carried through for the different systems. 

SPR detection of hybridization and denaturation kinetics for tethered ssDNA thiol on gold was achieved by monitoring the gain or loss of DNA at the interface in the presence of an applied electrostatic field. Redox reactions were avoided and the current measured was limited to the capacitive, non-faradaic charging current, at selected potentials applied to the gold electrode interface, as described by Georgiadis and co-workers [47], The specific DNA thiol monolayer films were robust and could be reused. 

Choi and co-workers [579] studied the reactivity of vinyl-terminated self-assembled monolayers (SAMs) of undec-lO-ene-1-thiol on gold (see Fig. 6.32) toward olefin cross-metathesis (CM). Vinyl groups on SAMs were successfully converted into a, P-unsaturated carbonyl groups by CM with acrylic acid, methyl acrylate, and acrylamide. Result shows that various useful functional groups can be introduced to SAMs on gold (and other solid surfaces) by olefin CM and suggests an alternative to the S3mthesis of desired molecules in solution [579]. 

Szafianski, C.A., Tanner, W., Laibinis, P.E., and Garrell, R.L. (1998) Surface-enhanced Raman spectroscopy of halogenated aromatic thiols on gold electrodes. Langmuir, 14, 3580-3589. 

The above advantages of the gold-thiol system have resulted in the extensive use of gold-thiol monolayers by the whole scientific community. A brief research of the literature shows that more than 2000 papers have been published since 1983 studying self-assembled monolayers of thiols on gold. We have therefore made no attempts to make the present review comprehensive however, we have endeavoured to include all recent seminal work in the area. The subject is highly interdisciplinary, and we put perhaps more emphasis on the chemical aspects of the monolayer science than the authors of some other reviews. 

The fact that self-assembly of thiols on gold makes it possible to create well-defined surfaces with controlled properties was used extensively to understand the forces controlling orientation of liquid crystals adsorbed at surfaces545 - 549. 

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