Electroassisted Fabrication of Free-Standing Silica Structures of Micrometer Size

Free-standing porous silica microstructures have been made via the electroassisted deposition of silica in an appropriately patterned array of recessed electrodes consisting of hydrophilic and hydrophobic domains. The 100 nm deep recessed indium tin oxide (ITO) electrodes were prepared by a photolithographic/chemical etching process on Glass/ITO/Au substrates. Hydrophobic areas were formed by passivation of unetched gold with a self-assembled monolayer of 1-octadecanethiol. Application of sufficiently negative potentials produced thick layers of silica that extended across the whole substrate; however, because of adhesion differences of silica on hydrophilic (ITO) and hydrophobic (thiol-modified gold) surfaces, selective removal of silica from the more hydrophobic areas of the substrate was achieved. The surface morphology, porosity, and thickness of resultant microstructures depended on the concentration of tetramethoxysilane in the sol, the electrolysis time, and the applied potential, all of which have been varied. Free-standing silica features of different geometries including bands, squares, and circles, ranging in width from 60 to 500 μm and heights >1 μm, have been prepared using this approach. Scanning electron microscopy (SEM) images showed the materials to consist of aggregates of colloidal particles that extend tens to thousands of nanometers above the surface. Such film-like materials have important characteristics that make them ideally suited as a platform for chemical sensors; most notably, an open framework and the presence of interconnected pores within individual microstructures.