Atomic Sulfur Formation Mechanism on 3‑Mercaptopropanoic Acid Derivative Self-Assembled Monolayers: Understanding the C–SBond Cleavage

Abstract

Self-assembled monolayers (SAMs) of ω-carboxylic acid thiols are very important in the surface modification of metals, especially on gold surfaces. Indeed, the 3-mercaptopropanoic acid (MPA) and its ester or amide derivatives are widely used for SAM-based sensor design. It was already shown that MPA does not suffer C−S bond scission when adsorbed on Au. On the other hand, in this work we demonstrate that its simplest derivative, methyl 3-mercapto propionate (Me-MPA), is prone to form significant amounts of atomic sulfur when adsorbed on Au. The MPA derivatives are more sensible than MPA itself to alkaline solutions, and its SAM-based sensors will rapidly degrade given atomic sulfur. In this work, we study the simplest MPA derivative Me-MPA SAMs on preferentially oriented Au(111) surfaces by XPS and electrochemical measurements. It was found that the desulfuration of Me-MPA depends on its preparation conditions (grown from ethanol or toluene solution) and on its post-treatment with alkaline solution. In order to explain the S−C bond scission on Me-MPA SAMs, we discuss different reaction mechanisms. We concluded that the reaction mechanism involves an E1cB elimination pathway (β-elimination). This reaction mechanism also explains the desulfuration behavior of other important related molecules like L-cysteine and glutathione.