Chemisorption of $\text{l}$-cysteine and 3-mercaptopropionic acid on gold and copper surfaces: An infrared reflection-absorption study

Abstract

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the chemisorption of $\text{l}$-cysteine ($\text{l}$-Cys) and 3-mercaptopropionic acid (MPA) on evaporated gold and copper surfaces. The infrared data show that monomolecular films are formed for both $\text{l}$-Cys and MPA on gold, and that the bonding to the surface occurs primarily via the SH group. More complex reactions occur on the copper surface. MPA forms a multilayer structure on copper when adsorbed from solution at pH 3.5. In the very first layer the SH group is coordinated to the copper surface and the CO2− group to dissolved copper ions. The subsequent layers consist of MPA molecules in the dimeric form. Dissolution of copper is not very pronounced at high pH values as evidenced by the formation of a sodium salt instead of a cuprous (cupric) complex when MPA is adsorbed from a NaOH solution at pH 11.5. $\text{l}$-Cys also forms a complex ionic structure on copper where the SH, NH2 and CO2− groups appear to be involved in the bonding to the copper surface and/or dissolved copper ions.