Cobalt(II) hematoporphyrin IX and protoporphyrin IX complexes immobilized on highly dispersed titanium(IV) oxide on a cellulose microfiber surface: electrochemical properties and dissolved oxygen reduction study

Hematoporphyrin IX (8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H-23H-porphine-2,18-dipropionic acid) and protoporphyrin IX (8,13-divinyl-3,7,12,17-tetramethyl-21H-23H-porphine-2,18-dipropionic acid) were efficiently immobilized on a cellulose/titanium (IV) oxide composite fiber surface by the reaction of the porphyrin COOH groups with TiO2, presumably by forming the COOTi chemical bond. Furthermore, Co(II) was incorporated into the porphyrin ring, with this reaction being followed by UV–vis spectra in the solid state and confirmed by the change of the absorption bands due to a local symmetry change from D2h to D4h upon metallation of the porphyrin ring. Electrochemical studies by using cyclic voltammetry and chronoamperometry techniques, showed that the immobilized complexes catalyzed O2 reduction at −0.18 V for hematoporphyrin and −0.20 V for protoporphyrin in 1 mol l−1 KCl solution at pH 6. The cathodic current peak intensities plotted against O2 concentrations in the range from 0.5 to 13 mg l−1, showed a linear correlation. Rotating disk experiments were carried out in order to estimate the number of electrons involved on the process. It was observed that for both modified electrodes, dissolved O2 was reduced to H2O2 in a two-electron process.