Photoinduced electron transfer from zinc cytochrome c to cationic ruthenium(III) complexes within silica glass prepared by the sol-gel method

Abstract

We tested two methods for introducing cationic complexes [Ru(NH3)6]3+ and [Ru(NH3)5Cl]2+ (as chloride salts) into silica hydrogel (referred to simply as glass) prepared from Si(OCH3)4 by the sol-gel method. We also tested the ability of these complexes to quench oxidatively the triplet state of the protein zinc cytochrome c, encapsulated in the sol-gel glass. When preformed glass is soaked in solutions of these cations in phosphate buffer having pH 7.0 and low ionic strength, the cations diffuse into the pores very slowly, over days. Their concentration becomes higher in the glass pores than in the external solution. The complex [Ru(NH3)6]3+ quenches the triplet state in a multiphasic reaction. When the complexes [Ru(NH3)6]3+ and [Ru(NH3)5Cl]2+ (as chloride salts) are added to the sol at the onset of polymerization, a fraction of them is lost by the rinsings that are required for the glass formation. The fraction remaining in the glass pores apparently does not quench the triplet state. Because the silica surface may bear various charges, depending on the pH, even ions that are smaller than the glass pores have different concentrations at equilibrium in the glass and in the external solution. The slowness of the diffusion of even small analytes, microscopic heterogeneity of the pore environments, and multiphasic kinetics of reactions occurring in the pores should be considered when doped sol-gel glasses are proposed as sensors.