Nanochannel design by molecular imprinting on a free-standing ultrathin titania membrane

A free-standing ultrathin film of a poly(vinyl alcohol) (PVA)/titania (TiO₂) composite was prepared by spin coating. The thickness of the film was adjusted to 30-50 nm by changing the spin-coating speed and the concentrations of PVA and the TiO₂ precursor. A template molecule, (4-phenylazo)benzoic acid (4PABA), was introduced into the film as a mixture in the TiO₂ precursor and was removed after film formation by dipping the film in aq NH₃ (1%). Aqueous solutions of tetraphenylporphyrin tetrasulfonic acid (TPPS), 4PABA, and sodium benzoate (SB) were filtered through this film, and the concentrations of these compounds in the filtered solution were monitored by UV-vis absorption measurements. The filtered TPPS solution was colorless, and its absorbance at 413 nm was 8% that of the original solution. In contrast, almost 100% of SB and 28.4% of 4PABA were passed through the film. The ultrathin TiO2/PVA film obtained without imprinting with 4PABA had no cavities, and aqueous solutions did not pass through this film. Therefore, it was concluded that the nonimprinted film was defect free and that imprinting of the template molecule in the film resulted in the formation of a size-selective channel across a 40 nm thickness.