Spectroscopic study of fluorescein immobilized on anodic porous alumina in aqueous solutions of different pH

Abstract

Fluorescein, a popular fluorescent dye, can exist in various prototropic forms, depending on the pH of the solution. We studied the spectroscopic properties of fluorescein immobilized on anodic porous alumina (APA) dipped in aqueous fluorescein solutions with various pH values. Fluorescein was successfully immobilized on the APA surface to obtain a golden color between pH 5 and 6. From the absorption and fluorescence studies, we concluded that fluorescein is dominantly immobilized on APA in a monoanionic form with minor contribution from the dianionic form. A surface analysis showed that fluorescein molecules can penetrate approximately 500 nm in depth from the APA surface, which is almost the surface of the pores, considering an oxide film thickness of APA (10 μm). These results confirmed that adsorption occurs because of the electrostatic interaction between the carboxylate moiety in monoanionic/dianionic fluorescein molecules and the positively charged APA surface.