Electrochemically assisted fabrication of size-exclusion films of organically modified silica and application to the voltammetry of phospholipids

Modification of electrodes with nanometer-scale organically modified silica films with pore diameters controlled at 10- and 50-nm is described. An oxidation catalyst, mixed-valence ruthenium oxide with cyano cross-links or gold nanoparticles protected by dirhodium-substituted phosphomolybdate (AuNP-Rh₂PMo₁₁), was immobilized in the pores. These systems comprise size-exclusion films at which the biological compounds, phosphatidylcholine and cardiolipin, were electrocatalytically oxidized without interference from surface-active concomitants such as bovine serum albumin. Ten-nanometer pores were obtained by adding generation-4 poly(amidoamine) dendrimer, G4-PAMAM, to a (CH₃)₃SiOCH₃ sol. Fifty-nanometer pores were obtained by modifying a glassy carbon electrode (GC) with a sub-monolayer film of aminopropyltriethoxylsilane, attaching 50-nm diameter poly(styrene sulfonate), PSS, spheres to the protonated amine, transferring this electrode to a (CH₃)₃SiOCH₃ sol, and electrochemically generating hydronium at uncoated GC sites, which catalyzed ormosil growth around the PSS. Voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ demonstrated the absence of residual charge after removal of the templating agents. With the 50-nm system, the pore structure was sufficiently defined to use layer-by-layer electrostatic assembly of AuNP-Rh₂PMo₁₁ therein. Flow injection amperometry of phosphatidylcholine and cardiolipin demonstrated analytical utility of these electrodes.