ELECTROCHEMICAL EVIDENCE OF 2-DIMENSIONAL SURFACE-COMPOUNDS OF HETEROCYCLIC MOLECULES AT SULFUR-COVERED GOLD AND PLATINUM .1. METHYLENE-BLUE

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-precovered 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 precovered 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.