Electrochemical study of pyrroloquinoline quinone covalently immobilized as a monolayer onto a cystamine-modified gold electrode

Abstract

The electrochemistry of pyrroloquinoline quinone (PQQ) was studied in solubilized and immobilized states on a Au electrode modified with a chemisorbed cystamine monolayer. An electrochemically reversible diffusion-controlled reaction ( k ≈ 1.7 × 10 −3 cm s − at pH 7.0) is observed for solubilized PQQ on the cystamine-modified electrode under acidic and neutral conditions (pH ⩽ 7) when the surface amino groups are positively charged. However, the electrochemical reduction of PQQ is completely irreversible on a non-modified Au electrode as well as on an electrode surface modified with neutral or negatively charged groups. The cystamine monolayer on the Au electrode surface was used as a basis for the covalent immobilization of PQQ via carbodiimide coupling of the PQQ carboxylic groups with the surface amino groups. The electrochemical reaction of the immobilized PQQ was reversible over a wide pH range (pH 2–11). The PQQ modified electrodes exhibited very high stability. A surface concentration of ca. 1 × 10 −10 mol cm −2, corresponding to a monolayer, and an electron transfer rate constant k s of ca. 3.3 s −1 (pH 7.0) were evaluated for the PQQ-modified electrode. The total amount of immobilized PQQ can be increased dramatically if an Au electrode with a very high surface roughness is used. The redox potential E° of −0.125 ± 0.003 V/SCE (pH 7.0) was obtained for both solubilized and immobilized PQQ. It is suggested that the PQQ-modified electrodes developed in this work can be used to prepare biosensors based on PQQ enzymes and to facilitate chemical reactions characteristic of PQQ itself directly on the electrode surface.