Electrochemistry of ABTS at Glassy Carbon Electrodes

The electrochemical and the mass transport behavior of ABTS2−/ABTS.− (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)) redox couple at glassy carbon electrode (GCE) in phosphate buffer solution (PBS, pH=4.4) is studied in detail by cyclic voltammetry combined with rotating disk electrode system. From the i-E curves recorded at different electrode rotating rate, rate constant, and transfer coefficient for ABTS 2− ⇌ ABTS.−+e reaction at GCE electrode and the diffusion coefficient of ABTS2− in PBS are estimated to be 4.6 × 10−3 cm/s, 0.28, and 4.4 × 10−6 cm2/s, respectively. The transfer coefficient with a value of ca. 0.28 differs largely from the value of 0.5 that is always assumed in the literature. The origins for the difference of the rate constant determined and the challenges for estimating the standard rate constant are discussed. The performance for such ABTS2− mediated bio-cathode toward oxygen reduction reaction is discussed according to the over-potential drop as well as current output limit associated with the charge transfer kinetics of ABTS2− ⇌ ABTS.−+e redox reaction and/or the mass transport effect.