Model investigation of a consecutive dye uptake and release process by using a bilayered chitosan-mesoporous silica nanocoating system

Two-layer model systems were designed by covering a 130 nm thick mesoporous silica coating (30% porosity, 1.8 nm average pore radius) with a 470 nm thick chitosan coating (degree of acetylation 18-19%) to study the accumulation and release of a cationic model molecule (rhodamine 6G dye). The coatings were impregnated in the aqueous solution of the dye (pH 6.0, 7.4 and 8.8, 12 °C, 22 °C and 32 °C) to study the effect of electrostatic interactions and morphology on the dye uptake. The accumulation of dye molecules was examined in terms of the pseudo-first-order and pseudo-second-order kinetic model, while the release (at pH 7.4, 25 °C) in terms of the Korsmeyer-Peppas and Higuchi model. The release exponents (0.43-0.71) of the Korsmeyer-Peppas model revealed the significance of an interaction of the impregnation solution with the silica pore wall during dye uptake. According to the Higuchi model, a straight line with a higher and a lower slope could be fitted to the experimental points in the first ("fast rate") and second ("slow rate") stages of the process. Interestingly, the slope values and their differences significantly depended on the conditions of dye uptake: impregnation at alkaline pH not only increased the uptake but also resulted in faster release.