Self-assembly and heterogeneous electron transfer properties of metallo-octacarboxyphthalocyanine complexes on gold electrode

Abstract

Electrochemical properties of redox-active self-assembled molecular films of novel metallo-octacarboxyphthalocyanine (MOCPc, M = Fe, Co and Mn) complexes integrated with cysteamine (Cys) monolayer on gold electrodesviaamide bonds were investigated. X-Ray photoelectron spectroscopy confirmed the appearance of the various elements in their expected chemical environment upon immobilization of these species. The heterogeneous electron transfer properties of the Au–Cys–MOCPc molecular films using an outer-sphere ([Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻) redox probe were studied using cyclic voltammetry and electrochemical impedance spectroscopy. The electron transfer rate constant (k_app) depends markedly on the central metal of the metallophthalocyanine cores (k_app: Co > Mn > Fe). A strong pH dependence of the electron transport of the Au–Cys–MOCPc molecular films was found. The surface pK_a values of the MOCPc complexes were essentially the same (ca. 7.5). The differences in the electron transports and ionization constants are discussed. The electrodes are sensitive to the electrooxidation of epinephrine in physiological pH conditions, peak potential (E_p/V vs. Ag|AgCl, saturated KCl) decreasing as FeOCPc (0.20 V) < MnOCPc (0.26 V) < CoOCPc (0.34 V).