The dual role of self-assembled monolayers of a tetraazamacrocyclic copper(II) complex in ascorbate oxidation catalysis

Abstract

The copper(II) complex with 1,4,8-trimethyl-11-(2-thioethyl)-1,4,8,11-tetraazacyclotetradecane was adsorbed on the surface of glassy carbon (GCE) and gold electrodes using the self-assembly method. More stable coverage of the electrode was obtained using a gold substrate when the complex is anchored to the electrode through its thioethyl substituent. The amount of the complex on the electrode surface based on the charge of the Cu 2+/Cu + peak points to the nearly monolayer coverage of the electrode surface. The catalytic effect of the monolayer towards ascorbic acid oxidation was observed in neutral and alkaline solutions, and ascribed to the electrostatic interaction of ascorbate anion with the positively charged copper complex bound to the electrode surface. In dioxygenated solutions, the copper complex adsorbed on the electrode catalyses both the electrooxidation and auto-oxidation of ascorbate. The copper-catalyzed autoxidation of ascorbic acid leads to a decrease of the ascorbate electrooxidation peak with time. The progress of this reaction was monitored based on the changes of the electrooxidation current. Copper catalyzed autoxidation of ascorbate is known to require turnover between Cu 2+ and Cu + ions. This indicates that the molecules of the copper complex anchored through the thiol groups to the gold electrode, and tightly packed on the surface play a dual role in the ascorbate oxidation process. The role of the monolayer in enhancing the autoxidation of ascorbic acid resembles the function of copper in some biological complexes catalyzing in vivo ascorbate oxidation.