Electrochemical study of the redox dyes Nile Blue and Toluidine Blue adsorbed on graphite and zirconium phosphate modified graphite

Solid graphite electrodes were modified with the redox dyes Nile Blue (NB) and Toluidine Blue (TB), following irreversible adsorption from ethanol solution, and a droplet evaporation procedure. The shift of redox potential for both dyes to the negative direction upon adsorption on graphite was estimated from cyclic voltammetry, and from extrapolation of the dependence of Ep versus Ip to Ip=0, to vary between 60 and 110 mV. For electrodes containing a controlled amount of NB or TB, heterogeneous charge transfer rate constants of 31 and 11.6 s−1, respectively, were estimated by extrapolation of the data to zero coverage of the electrode surface. The rate constants obtained are several times higher than those obtained for multilayer coatings with the dyes. The modification of solid graphite electrodes with zirconium phosphate (ZrP), followed by the adsorption of NB and TB was performed. For NB, two forms of the dye were found to be present, one of them presenting an usual adsorbate of NB on graphite, and another one having Epc shifted by ca. 0.15–0.30 V to more positive potential values. The latter form is ascribed to NB, included in the ZrP host matrix, as follows by comparing its electrochemical behavior with that observed for NB, included in carbon paste containing ZrP.