Ultrathin silica films immobilized on gold supports: Fabrication, characterization, and modification

A novel method for covalent attachment of ultrathin silica films (thickness < 10 nm) to gold substrates is reported. Silica layers were prepared using spin-coating of sol-gel precursor solutions onto gold substrates that were cleaned and oxidized using UV photo-oxidation in an ozone atmosphere. The gold oxide layer resulting from this process acts as a wetting control and adhesive agent for the ultrathin silica layer. Control of silica layer thickness between ~6 and 60 nm through modification of precursor solution composition or by repetitive deposition is demonstrated. Films were characterized using infrared spectroscopy, ellipsometry, atomic force microscopy, and cyclic voltammetry. For the standard deposition parameters developed here, films were determined to be 5.5 ± 0.75 nm thick, and were stable in aqueous solutions ranging in pH from 2 to 10 for at least 30 min. Films contained nanoscopic defects with radii of ≤ 2 μm in all cases, at a density of 1.6 defects per 10 μm × 10 μm region, with a weighted average defect radius of 200 nm. This array of defects was shown to exhibit a voltammetric response consistent with an ultramicroelectrode array. Finally, ultrathin silica films were also modified with self-assembled monolayers of octadecylsilane, demonstrating the surface chemistry of bulk silica.