Electrochemistry of Azure C Adsorbed on Glassy Carbon and Screen-Printed Graphite Electrodes from Reline and Phosphate Buffer

The electrochemical activity of the phenothiazine dye Azure C adsorbed in a single potential cycle from either a phosphate buffer or a deep eutectic solvent (reline) is studied on the glassy-carbon and screen-printed graphite electrodes. A stable voltammetric signal from the dye can be obtained and remains unchanged in the subsequent multiple potential scans. The Azure C adsorption from the phosphate buffer produces a multilayer coating in which the electron exchange formally corresponds to the diffusion–adsorption limiting stage. The super-Nernstian slope of the dependence of the equilibrium potential of adsorbed Azure C on the pH points to significant contributions by the protonation of various adsorbed dye forms and this process’s multistep nature. The adsorption of Azure C from reline does not affect significantly the morphology of voltammograms. The less efficient adsorption is manifested by the lower current peak of the dye and the adsorption control over the limiting stage. The differences in the behavior of Azure C adsorbed from the phosphate buffer and reline can be attributed to the changes in the degrees of agglomeration and hydration of the dye. Electrodes modified with Azure C can electrostatically accumulate native and thermally denatured DNA that suppresses the redox peaks of Azure C in voltammograms. The results of this study can be used in the further development of the methods of electropolymerization of phenothiazine dyes and also in the elaboration of electrochemical sensors and biosensors based on the quantitative evaluation of the redox activity of dyes on electrodes.