ELECTROCHEMICAL OXIDATION OF AMINE-CONTAINING COMPOUNDS - A ROUTE TO THE SURFACE MODIFICATION OF GLASSY-CARBON ELECTRODES

A method for the modification of glassy carbon electrodes (GCEs) with amine-containing compounds for electrocatalytic and biosensor purposes is investigated. The method utilizes the electrooxidation of amines to their analogous cation radicals to form a chemically stable covalent linkage between the nitrogen atom of the amine and edge plane sites at the GCE surface. By use of X-ray photoelectron spectroscopy (XPS) for coverage assessment, the capability of this route is demonstrated by the immobilization of a simple primary amine at the GCE surface. An investigation of the influence of substituents on the nitrogen atom (e.g., primary, secondary, tertiary amines) revealed that the surface coverage of primary amines was approximately 3 times higher than that of secondary amines, whereas tertiary amines were not immobilized at a detectable level. This behavior is attributed to a strong steric effect whereby bulky substituents on the nitrogen atom hinder accessibility of the reactive amine cation radical to surface binding sites. Amine salts and amides also showed no detectable coverage by XPS. The utility of the method for creation of a GCE with electrocatalytic activity is demonstrated by the immobilization of dopamine (DA) at the GCE surface. The DA-modified GCE is used to facilitate oxidation of beta-NADH via a surface EC mechanism. The applicability of this method for the construction of biosensors that are based on biotin-avidin complexation is also demonstrated. Both of these examples illustrate the facility of this route for simplifying and shortening dramatically the processing required for immobilization using other synthetic methods. A mechanism for the immobilization process is also briefly discussed.