Simultaneous determination of droxidopa and carbidopa using a carbon nanotubes paste electrode

A novel carbon paste electrode modified with carbon nanotube and 5-amino-2’-ethyl -biphenyl-2-ol (5AEB) was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of droxidopa and carbidopa, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, ks, and transfer coefficient, α, for electron transfer between 5AEB and carbon nanotubes paste electrode were calculated as 17.3 ± 0.1 s−1 and 0.5, respectively. The electrode was also employed to study the electrocatalytic oxidation of droxidopa, using cyclic voltammetry, chronoamperometry, square wave voltammetry and electrochemical impedance spectroscopy as diagnostic techniques. It has been found that the oxidation of droxidopa at the surface of modified electrode occurs at a potential of about 355 mV less positive than that of an unmodified CPE. The diffusion coefficient, electron transfer coefficient, and heterogeneous rate constant, for oxidation of droxidopa at the modified electrode surface were also determined. Square wave voltammetry exhibits a linear dynamic range from 1.2 × 10−7 to 2.25 × 10−4 M and a detection limit of 50.0 nM for droxidopa. Finally this modified electrode was used for simultaneous determination of droxidopa and carbidopa for the first time.