Enantioselective anion exchange on a positively charged poly(l-lysine) layer assembled on thin TiO2-gel films

Abstract

Ion exchange between negatively charged fluorescent dyes and chiral glutamic acids was examined on a poly(L-lysine) (PLL) layer assembled on ultrathin TiO₂-gel films. First, a TiO₂-gel film with a thickness of 6–7 nm was prepared on a quartz substrate by means of the surface sol–gel process. A PLL layer of about 2.4 nm was then immobilized by immersing the substrate in the polymer solution at pH 10.7. Subsequently, fluorescent dye (sulforhodamin B or carboxyfluorescein) was electrostatically adsorbed on the PLL layer, and desorption of the dye molecules into dilute D- (or L-) glutamic acid solution was monitored by UV-Vis absorption and fluorescence spectroscopies. The speed and amount of desorption depended highly on the chirality of glutamic acids as well as the molecular structure of fluorescent dyes.