(331w) Self-Assembled Opal Films & Membranes

Opal films were deposited on Pt microdisc electrodes. The films were modified with 3-aminopropyltriethoxysilane and showed permselectivity for neutral and negative molecules over positive molecules at pH 4, where the films are fully protonated. This behavior was reversible when the pH was changed from pH 4 to pH 7. The diffusion of Ru(NH₃)₆³⁺ at pH 4 was studied as a function of ionic strength. The current of Ru(NH₃)₆³⁺ increased with increasing ionic strength, thus further supporting the permselectivity of the amine modified opal films is due to the positive charge on the opal surface. Opal membranes suspended inside frustum-shaped openings in 0.3-mm-thick silicon wafer or inside cylindrical holes in 0.15-mm-thick glass cover slips have been prepared by self-assembly of silica spheres of 170 - 440 nm diameter. Opal membrane formation was achieved by placing the substrates vertically in a colloidal solution of silica spheres and allowing the solvent to evaporate at room temperature. Opal membranes were prepared in openings ranging between 0.0016 mm² to 1 mm² and thicknesses ranging from 0.03 to 0.3 mm. Diffusion rates of charged dyes were studied as a function of opal membrane size and thickness, and were found to be in good agreement with the calculated rates, confirming that no major defects are present in the membranes. Opal membranes were modified with 3-aminopropyltriethoxysilane and diffusion rates for charged dyes were studied as a function of pH to evaluate the permselectivity of the surface-modified opal membranes to charged species.