A Hierarchical Study on Load/Release Kinetics of Guest Molecules into/from Mesoporous Silica Thin Films

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Abstract

The load/release behaviors of a fluorescent molecule, fluorescein isothiocyanate (FITC), using mesoporous silica thin films as hosts are investigated through a hierarchical study of three parameters (i.e., loading methods, porous structures, and morphologies) from microscopic to macroscopic levels. Three different methods, physical adsorption (AD), entrapment (EN), and cleavable binding (CB), were used to load FITC into three-dimensional hexagonal (3DH) mesoporous silica thin films. On the basis of the results of fluorescence spectroscopies, we found a diffusion-controlled release (first-order release) for the AD-3DH case, a dissolution-controlled release (zero-order release) for the EN-3DH case, and a stimuli-responsive release for the CB-3DH case. Mesoporous silica thin films with two kinds of mesostructures, two-dimensional hexagonal (2DH) and 3DH, were successfully synthesized. The adsorption and release kinetics, including the dissociation constant (Kd) and half-life value (t1/2), for the 2DH and 3DH films are similar due to the existence of micropores in both cases, although the 2DH film exhibited a larger maximum adsorption amount (Nads∞) than the 3DH film. Linear and circular patterns with a 3DH mesostructure were fabricated. Compared with the 3DH film, the patterns exhibit more external pore entrances, resulting in easier adsorption and desorption of FITC molecules. Unlike conventional mesoporous solids, we demonstrated that mesoporous thin films with the desired loading method, mesostructure, and morphology could serve as a new substrate for controlled release systems.

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