Spectroelectrochemical characterization of ultra-thin films formed by electropolymerization of phenothiazine derivatives on transparent gold electrodes

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Transparent gold electrodes covered with an ultra-thin polyphenothiazine film layer have been obtained by electropolymerization of the monomeric parent compounds, Methylene Blue, Methylene Green and Azur A, under alkaline pH conditions. The electrochemical behaviour of the polymers immobilized on the electrode surface has been characterized by UV-visible difference spectroelectrochemistry. A study of the dependence of the oxidation state of the surface redox species with the applied potential of any of the different polymer layers immobilized on the electrode surface showed one rather irreversible redox process, at potentials shifted towards 100–120 mV more positive values than those obtained for the monomer in solution. The spectroelectrochemical curves, obtained from the absorbance changes observed at different applied potentials, showed a hysteresis in the oxidation process, as well as the loss of some electroactive material from the electrode surface during the reduction process for all polymers studied. For poly-Methylene Blue, a shift of the redox potential towards more negative values coupled with an increase in the irreversibility of the redox process at increasingly higher pH values has been observed. The simultaneously obtained UV-visible difference spectra of the different films resembled those of the solubilized monomers. However, the spectra of the polymers showed a shift of the main visible maximum towards 50–80 nm shorter wavelengths, depending on the monomer studied.