Electrochemically Assisted Self-Assembly of Alkylthiosulfates and Alkanethiols on Gold: The Role of Gold Oxide Formation and Corrosion

Abstract

Electrochemically directed self-assembly of alkylthiosulfates enables the selective formation of monolayers on gold surfaces. These monolayers are identical to those formed from the corresponding alkanethiols. However, the mechanistic details of monolayer formation under electrochemical conditions as well as the role of other variables and residual water in the solvent have not been extensively studied. A systematic investigation shows that self-assembly is not a result of an outer-sphere one-electron oxidation of alkylthiosulfate. Voltammetry and electrochemical quartz crystal microbalance techniques reveal that self-assembly involving alkylthiosulfates as well as alkanethiols under oxidative conditions proceed through direct reaction with gold oxide and in some cases is accompanied by corrosion. X-ray photoelectron spectroscopy indicates that monolayers can undergo rapid exchange with molecules in solution under electrochemically directed self-assembly conditions.

Enhancement of structural and charge-transfer barrier properties of n-alkanethiol layers on a polycrystalline copper surface by electrochemical potentiodynamic polarization

“…Namely, although the self-assembly would also enable the formation of films on copper that could offer equally good barrier properties to electrochemically-modified films, this method would require a considerably longer formation time, while the electrochemical method is very rapid. 24,25,26 Results presented in this manuscript consistently demonstrate that the n-alkanethiol chain length and the electrochemical modification of n-alkanethiol self-assembled films on the copper surface significantly influence their physical properties. Independent measurements performed using three different experimental techniques (EIS, PM-IRRAS, CA) clearly show superior structural properties of electrochemicallymodified over chemically-formed thiol films, e.g., the higher surface packing density/concentration, the higher structure order, the higher hydrophobicity and better protecting properties in corrosive environments containing aggressive ions.…”

Compatibility of ω-Functionality in the Electrochemically Directed Self-Assembly of Monolayers on Gold from Alkyl Thiosulfates

“…We have previously demonstrated that regioeselective self-assembly can be achieved by applying anodic potentials to selected gold electrodes in solutions of alkyl thiosulfates (Bunte salts). − In the presence of tetrafluoroborate anion in THF, monolayer formation can be activated by application of a sufficiently oxidative potential (∼1 V) and occurs only at the active electrode. Oxidation appears to lead to S−S bond cleavage, which liberates SO 3 and produces an alkyl sulfide radical that can bond to the gold surface. , Application of the anodic potential at a particular electrode, even when others are present, thus allows for control of where the monolayer forms. Methyl-terminated SAMs on gold surfaces, created electrochemically in this way, are nearly indistinguishable from those produced by the chemisorption of thiols …”

Self-Assembly of Alkylthiosulfates on Gold: Role of Electrolyte and Trace Water in the Solvent

“…Inhibition of alkylthiosulfate self-assembly by the electrolyte, TBATFB, has been previously reported. − , However, other tetrabutylammonium salts including tetrabutylammonium hexafluorophosphate and TBAP do not exhibit this behavior. , For example, a comparison of the effect of TBATFB and TBAP electrolyte on alkylthiosulfate self-assembly has been explored by electron transfer blocking experiments using ferricyanide/ferrocyanide redox couple, as shown in Figure . Spontaneous adsorption of dodecylthiosulfate on gold electrode from THF in the presence of TBATFB for 30 min yields only partial monolayer formation (Figure a).…”

“…A novel method for the selective formation of SAMs on gold electrodes was reported using electrochemical oxidation of alkylthiosulfates under solution conditions where spontaneous self-assembly is inhibited . In this particular system the monolayer formation depends on many factors including the applied potential, solvent, substrate, and electrolyte. , Spontaneous chemisorption of alkylthiosulfates on gold was found to be inhibited by the presence of an electrolyte, tetrabutylammonim tetrafluoroborate (TBATFB), enabling electrochemically directed self-assembly . This behavior was attributed to the tetrafluoroborate anion and was not observed with other tetrabutylammonium salts including tetrabutylammonium hexafluorophosphate and tetrabutylammonium perchlorate (TBAP). , …”