Electrochemical property of methylene blue redox dye immobilized on porous silica-zirconia-antimonia mixed oxide

The mixed oxide SiO₂/ZrO₂ obtained by the sol-gel processing method adsorbs Sb(V) from acid solution, forming a new phase on the matrix surface designated as SiO₂/ZrO₂/Sb₂O₅. The amount of Zr(IV) in the matrix was 8.1 wt.%, corresponding to 0.89 mmol g⁻¹, and the amount of Sb(V) incorporated in the matrix was 6.3 wt.% (0.52 mmol g⁻¹). This material presented a specific surface area, S_BET=590 m² g⁻¹. The immobilized Sb₂O₅ is a good cation exchanger and adsorbs methylene blue (MB) cationic dye with an average surface density δ=1.1×10⁻¹¹ mol cm⁻² (about 0.07 molecule nm⁻²). The dye is strongly entrapped in the pores of the matrix and it is not leached off in KCl solutions as concentrated as 1.0 M. A carbon paste of the SiO₂/ZrO₂/Sb₂O₅/MB material showed a reversible redox pair with midpoint potential E_m=−0.1 V versus SCE. This midpoint potential is not affected by solution pH change between 2 and 7. The immobilized dye mediated NADH catalytic oxidation at ca. 50 mV.