Stability and Self-Exchange in Alkanethiol Monolayers

Abstract

Alkanethiols bearing radiolabeled (35S) head groups have been incorporated into self-assembled monolayers (SAMs) on a variety of substrates to determine coverage, thermaland photostability, and surface roughness. Spontaneous desorption of octadecanethiol SAMs immersed in solvent under ambient conditions is observed for all substrate/solvent combinations, including gold, silver, platinum, and copper and water, ethanol, tetrahydrofuran and hexane. A quantitative analysis of desorption suggests pseudo-first-order kinetics with rate constants in the range of 10-5 s-1 for THF. Evidence for multilayering on copper is found. Molecular hydrogen is not involved in the rate-determining step for desorption. Self-exchange of surface-labeled thiol with solution unlabeled thiol is also described by first-order kinetics. Both desorption and self-exchange experiments yield residual thiols at the surface which cannot be exchanged, and which are presumed to result from stronger binding at defect sites. Similarities in kinetics for exchange and desorption point toward a common mechanism for surface detachment, postulated to be a rate-limiting desorption step as a disulfide. Estimates for free energies of adsorption of a thiol generating molecular hydrogen and adsorption of a disulfide yield -5.5 and -24 kcal mol-1, respectively. In conjuction with recent evidence that thiols are actually adsorbed as disulfides, RSSRAu2(S), the desorption is represented by RSSRAua® -* RSSR + 2Au(S) (slow). For exchange this is followed by 2RSH + 2Au(S) -RSSRAu2(S> + H2 (fast), with some contribution from direct thiol/disulfide interchange for high concentrations of solution thiol, viz: RS*SRAu2(S) + RSH -* RSSRAu2(S) + R*SH. On clean gold surfaces, adsorption is shown to be diffusion limited. Finally, strategies for enhancing the stability of SAM monolayers made from thiols are discussed.

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

“…In the case of molecular selfexchange (e.g., the exchange between a 1-dodecanethiolate monolayer and 1-dodecanethiol, C12), most molecules are kinetically trapped in domains, and exchange occurs primarily at defect sites. 20 The constituent molecules can be tailored to engineer molecular-exchange reactions. For instance, a combination of weak intermolecular interactions and low surface density makes 1-adamantanethiolate SAMs susceptible to molecular exchange by n-alkanethiols.…”

Nanopatterned self-assembled monolayers

“…Second, thiols desorb upon heating even to moderate temperatures [33,34], which is a significant problem if an annealing step has to be applied at some time during processing. Third, thiols bleed along gold grain boundaries, which tends to blur the nanopatterns.…”

Antibody Oriented Immobilization on Gold using the Reaction between Carbon Disulfide and Amine Groups and Its Application in Immunosensing

“…9,10 For immunosensors prepared on gold, alkanethiol-based self-assembled monolayers (SAMs) 11−17 are among the most popular surface modification strategies, because they can be easily prepared to form a highly organized and ultrastable organic thin film in nanometer scale with good reproducibility. 15,18 If carboxylic functions are the terminal groups of SAMs, they can be used to covalently immobilize compounds with amine groups after an activation process, such as with N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). 19,20 Dithiocarbamates (DTCs) which have been commonly used as chelating ligands in coordination chemistry monolayers 21 provide an alternative to alkanethiol SAMs for gold surface modification.…”