Atomic Sulfur Formation Mechanism on 3-Mercaptopropanoic Acid Derivative Self-Assembled Monolayers: Understanding the C-S Bond Cleavage

AZCÁRATE, JULIO C.; ZELAYA, EUGENIA; AAGAARD, NATALIA D.; FONTICELLI, MARIANO H.; ZAMPIERI, GUILLERMO

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Año: 2019 vol. 123 p. 24156 - 24164

1932-7447

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 SAMs-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 adsorbs 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 mechanism. 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.