Reduction of indicator leaching from doped sol-gels by attachment of macromolecular carriers

The extent of leaching of indicator dyes of varying molecular mass, followi ng encapsulation into hydrated sol-gel monoliths prepared from tetramethylo rthosilicate (TMOS), was investigated. Dye modification of a water-soluble phthalocyanine with poly(ethylene glycol) side chains (2000 MW) reduced lea ching relative to the unmodified dye, but significant losses were still obs erved. Monoliths doped with fluorescein modified with dextrans of higher mo lecular weight were investigated in an attempt to determine the minimum siz e necessary for dye retention. In comparison to monoliths doped with unmodi fied fluorescein, attachment to a dextran carrier significantly decreased t he extent of dye leaching. However, even a 70 000 MW carrier was not suffic ient to prevent some loss of dye. In contrast, no leaching of myoglobin (18 000 MW) occurred, which shows that when the entrapped molecule has a organ ized tertiary structure, diffusion within the pores is eliminated. Malachit e green and its dextran conjugate were used to examine the effect of electr ostatic interactions on leaching. Minimal loss of both the dye and the conj ugate was observed, which indicates that when a favorable interaction betwe en the dye and sol-gel matrix exists, attachment to a macromolecular carrie r may not be necessary to prevent leaching.