Electrochemical evidence of two-dimensional surface compounds of heterocyclic molecules at sulphur-covered gold and platinum. Part I. Methylene blue

Abstract

Two-dimensional surface compounds are formed by irreversible adsorption from an aqueous solution of electroactive heterocyclic molecules at sulphur-covered platinum and gold surfaces. Purely electrochemical methodology was used to investigate the surface compounds formed by exposing sulphur-pre-covered gold and platinum to an aqueous solution of methylene blue (MB+ ). The surface compounds were studied by cyclic voltammetry at four different surfaces: flame-cleaned gold and platinum, free and modified by a monolayer of sulphur adatoms. Each of the four surfaces was investigated in two ways:. (1) in situ: The electrode was immersed directly into a solution of MB+, 0.05 M phosphate buffer, pH 7.9, in the electrochemical cell and the surface compound was formed under potential control;. (2) ex situ: the electrode was put in contact with a solution of MB+ for 60 s, rinsed with water and transferred to the electrochemical cell without MB+ in the solution. Phosphate buffer was used as the test solution. The electroactive surface compounds were formed spontaneously only on platinum and gold pre-covered by sulphur. Arguments have been presented for the formation of the surface compound through S-S interaction. The electrochemical analysis shows that the surface compounds of MB+ at the two surfaces are different: the reduction peak potential is -0.460 V on Au-S and -0.380 V vs. SCE on Pt-S; the number of electrons exchanged in the reduction of a molecule of the surface compound, as deduced from the width of the voltammetric peak, is 2 on Au-S while it could be 1 on Pt-S. The methylene blue molecule is proposed as a specific redox probe for characterizing sulphur adatoms at metallic substrates. © 1991.