Redox-active silica nanoparticles. Part 7. Redox behavior of core/shell structured viologen modified silica particles immobilized at paraffin impregnated graphite electrodes

The redox behavior of viologen modified spherical and nonporous Stِber silica particles with diameters of 125 nm immobilized at paraffin impregnated graphite electrodes in 0.1 M KCl/H2O is described. A permanent particle coverage was obtained by mechanical attachment to the graphite electrodes. The particles preferentially form clusters on the electrode surface as indicated by SEM. The presence of the viologen salt was shown by EDS of a modified electrode. Cyclic and differential pulse voltammetry were used to study the redox behavior in the immobilized state. Cyclic voltammograms of the modified particles exhibit two chemically reversible reduction waves corresponding to the first and second single electron reduction of the viologen dication. The mid-point potentials of the viologen units in the immobilized state are close to those of the reference compound methyl viologen. However, values of the semi-quinone formation constant Ksem of the modified silica material are increased compared to those of dissolved methyl viologen. This indicates a stabilizing effect of the solid supporting matrix/electrode with respect to disproportionation of the anchored viologen mono radical cations compared to methyl viologen.