Synthesis and self-assembly of a novel cobalt(II) porphyrin lipoic acid derivative on gold

A novel Co(II) porphyrin lipoic acid derivative was synthesized starting from the commerically available red blood pigment hemin. The disulfide functionalities of the lipoic acid moieties allowed its immobilization on gold by a self-assembly method. The Co(II) porphyrin self-assembled monolayer (SAMs) on gold (111) surfaces were characterized electrochemically through monolayer reductive desorption and evaluation of the redox properties of the immobilized molecules in organic medium, and by scanning tunneling microscopy (STM). It was found that after assembly the Co(II) porphyrin is electroactive exhibiting the typical redox processes observed for its precursor without the appended lipoic acid in solution. A coverage of 2.7 x 10(-10) mol. cm(-2) has been estimated assuming that four electrons (one per each sulfur atom) are involved in the process. The porphyrin-modified gold electrodes exhibit catalytic acitivity demonstrated towards the reduction of molecular oxygen in acidic solution.