Studies of diffusion of molecules and ions through nanoporous opal films and membranes

Surface-modified opals with controlled diffusion of molecules and ions were studied. Opal films deposited on platinum electrodes and opal membranes, supported and free standing, were fabricated and modified to achieve permselectivity. The selectivity of opal films was tested using electrochemistry and the selectivity of the opal membranes was examined spectrophotometrically.; Opal films were modified with 3-aminopropyltriethoxysilane and showed blocking of transport for positive species at low pH, where the films are fully protonated. The diffusion of positively charge species at pH 4 was studied as a function of ionic strength and transport rate increased as ionic strength increased, suggesting that the permselectivity of the amine modified opal films is due to the positive charge on the opal surface.; Opal films were modified to introduce a light responsive spiropyran molecule on the surface. While in the presence of acid, the limiting current of positively charge species decreased after irradiation with UV light, but was restored after the films were irradiated with visible light.; Opal membranes, 0.03 to 0.3 mm in thickness, suspended inside frustum-shaped openings in silicon wafer or inside cylindrical holes in glass were prepared by the self-assembly of silica spheres (170 to 440 nm in diameter) in openings ranging between 0.0016 mm2 to 1 mm2. Free standing opal membranes 200--550 mum in thickness, were fabricated from silica spheres (250--340 nm in diameter). 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 the membranes contain no major defects. Opal membranes were modified with amines and the diffusion rates for dye molecules were studied as a function of acid concentration. The diffusion rate of a positively charged species decreased when acid was present, while the diffusion rate of a neutral molecule did not change. The diffusion rate of the positively charged species was restored by deprotonating the surface. Both types of opal membranes have been shown to electrostatically repel the positively charged dye.