The extent of leaching of indicator dyes of varying molecular mass, following encapsulation into hydrated sol-gel monoliths prepared from tetramethylorthosilicate (TMOS), was investigated. Dye modification of a water-soluble phthalocyanine with poly(ethylene glycol) side chains (2000 MW) reduced leaching relative to the unmodified dye, but significant losses were still observed. Monoliths doped with fluorescein modified with dextrans of higher molecular weight were investigated in an attempt to determine the minimum size necessary for dye retention. In comparison to monoliths doped with unmodified fluorescein, attachment to a dextran carrier significantly decreased the extent of dye leaching. However, even a 70000 MW carrier was not sufficient to prevent some loss of dye. In contrast, no leaching of myoglobin (18000 MW) occurred, which shows that when the entrapped molecule has a organized tertiary structure, diffusion within the pores is eliminated. Malachite green and its dextran conjugate were used to examine the effect of electrostatic interactions on leaching. Minimal loss of both the dye and the conjugate was observed, which indicates that when a favorable interaction between the dye and sol-gel matrix exists, attachment to a macromolecular carrier may not be necessary to prevent leaching.
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