THIONINE SELF-ASSEMBLY ON POLYORIENTED GOLD AND SULFUR-MODIFIED GOLD ELECTRODES

A new type of self-assembled monolayer (SAM) on gold has been reported . Electroactive SAMs of thionine formed on polyoriented gold and sulph ur-modified polyoriented gold from aqueous thionine solutions of pH 7. 9 have been characterized by voltammetry. In thionine SAMs formed on g old modified by a monolayer of sulphur adatoms the attachment takes pl ace through S-S bonding between the S adatom and S heteroatom of thion ine with edge-on orientation. A fast 2e(-) reduction of thionine molec ules in the SAM, at potentials 150 mV more negative than the solution couple, leads to a complete detachment of the reduced molecules leavin g the sulphur adlayer intact. In the thionine SAM formed on a flame-tr eated surface of polyoriented gold, the surface concentration of 4.59 x 10(-10) mol cm(-2) corresponds to the most efficient packing of thio nine molecules in a monolayer (end-on orientation). On 2e(-) reduction of thionine molecules, the initial thionine SAM transforms irreversib ly to a new surface state with one-half of the initial surface concent ration.