Layer-by-layer assembly of Ru ³⁺ and Si ₈O ^8- ₂₀ into electrochemically active silicate films

A porous silicate is obtained from octa-anionic Si ₈O ^8- ₂₀ cage-like poly-silicate (PS) and Ru ³⁺ cations in an ethanol-based layer-by-layer assembly process. Electrochemical experiments (voltammetry and impedance spectroscopy) confirm the formation of redox-active ruthenium centers in the form of hydrous ruthenium oxide throughout the film deposit. Oxidation of Ru(III) to Ru(IV) at a potential below 0.5 V vs saturated Calomel electrode (SCE) is reversible, but a potential positive of 0.5 V vs SCE is associated with an irreversible change in reactivity, which is characteristic for very small hydrous ruthenium oxide nanoparticles. Further voltammetric experiments are performed in aqueous phosphate buffer solutions, and the effects of number of layers, scan rate, and pH are investigated. Three aqueous redox systems are studied in contact with the PS-Ru ³⁺ films. The reduction of cationic methylene blue adsorbed onto the negative surface of the nanocomposite silicate is shown to occur, although most of the bound methylene blue appears to be electrochemically inactive either bound to silicate or buried into small pores. The PS-Ru ³⁺-catalyzed oxidations of hydroquinone and arsenite(III) are investigated. Scanning electron microscopy images show that a macroscopically uniform porous surface is formed after deposition of 50 layers of the PS-Ru ³⁺ nanocomposite. However, atomic force microscopy images demonstrate that in the initial deposition stages, irregular island growth occurs. The average rate of thickness increase for PS-Ru ³⁺ nanocomposite films is 6 nm per deposition cycle.