Electrode modification with spin-coated films of mesoporous molecular sieve silicas

Spin coating of mixtures of cetyltrimethylammonium chloride (CTAC1) and prehydrolyzed tetramethyl orthosilicate (TMOS) on tin-doped indium oxide (ITO) conductive substrates gave transparent uniform coatings. Electron microscopy, X-ray diffraction and N-2-adsorption showed the coatings to be continuous thin films of mesoporous MCM-41. Upon calcination, the thickness of the films decreased from 1.2 to 1.0 mum, and cracks appeared in the films. Spin coating of more diluted CTACl/TMOS mixtures gave thinner films that did not crack during calcination. For example, spin coating of a solution that had been diluted by 50% gave 0.25 mum thick films that remained intact after calcination. The spin-coated films were not stable in neutral or basic salt solutions. They were only stable in pure water, or in acidic electrolytes. The spin-coated MCM-41 films adsorbed [Ru(bpy)(3),](2+) [Fe(bpy)(3)](2+) and [Os(bpy)(3)](2-) ions, resulting in large increase in the sizes of the cations voltammetric waves. No voltammetric waves were obtained when the spin coated films were placed in solutions of [Fe(CN)(6)](3-), [Ru(CN)(6)](3-) or [Mo(CN)(8)](4-). The spin-coated films completely blocked the electrochemical activity of these anions. This was not the case for electrodes modified with MCM-41 powder over coated with polystyrene. The voltammetric response of these MCM-41/polystyrene composite electrodes to solution anions was almost identical to that of the bare ITO substrates. (C) 2001 Elsevier Science B.V. All rights reserved.