Photochemistry on surfaces: solvent–matrix effect on the swelling of cellulose. An emission and absorption study of adsorbed auramine O

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Abstract
Auramine O exhibits a significant fluorescence emission at room temperature when adsorbed on microcrystalline cellulose. This emission is about two to four orders of magnitude higher than the previously reported emission in different solvents, also at room temperature. The fluorescence quantum yield, ϕF, varies with the residual degree of humidity of the sample, as alterations in the rigidity of the environment greatly affect the non-radiative de-excitation of auramine O. A limiting upper value of ϕF≈ 0.35 was obtained after prolonged evacuation of the samples. A matrix-isolation mechanism was evaluated by using room-temperature fluorescence measurements of powdered samples of auramine O adsorbed on microcrystalline cellulose. Using polar protic and aprotic solvents, different degrees of swelling of the microcrystalline cellulose were obtained, resulting in changes of the ground-state reflectance spectra of auramine O trapped in the natural polymer chains, as shown by remission function data. The amount of dye adsorbed or entrapped is reflected in both the fluorescence quantum yields and lifetimes. When auramine O was adsorbed on porous silica, the fluorescence quantum yield was much lower, indicating a higher mobility of dye molecules on the silica surface. Oxygen does not affect the room-temperature fluorescence emission of auramine O in silica or in cellulose.

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