A mesoporous silica thin film as uptake host for guest molecules with retarded release kinetics

Uptake and release processes of various fluorescent rhodamine dyes and antitumor drugs to/from an ordered mesoporous silica film are investigated by means of UV/Vis absorption and fluorescence spectroscopies. The pores in the 160 nm-thick silica film strongly withdraw the dyes from water, thus allowing the storage of several micrograms of guest molecules per square centimeter of film. The binding equilibrium of the dyes follows a Langmuir-type adsorption. The dissociation constant, K_d, and the maximum binding amount to the film, N_ads∞ are determined by fitting the binding curves. The release kinetics of the guests from the film to a simulated body fluid (SBF) solution follows a bimodal first-order exponential behavior. The release kinetics from the mesoporous thin film is remarkably retarded relative to that from mesoporous powders. Among all the studied dyes, rhodamine 101 is released most slowly, which implies that the release rate depends not only on the interactions between the guests and the silica surface, but also on intermolecular interactions between the guest molecules. Comparison of the release kinetics of different antitumor drugs, such as actinomycin D and mitoxantrone, into an SBF solution shows that mitoxantrone is released much slowly. This slower release is attributed to the positive molecular charge and the formation of dimers in the pores.