Self-assembled monolayers formed on electroless cold deposited on silica gel: A potential stationary phase for biological assays

This paper reports the electroless deposition of gold onto the surface of micrometer-sized particles of silica (silica gel) and the self-assembly of monolayers formed from omega-substituted alkanethiols on the surface of the gold-coated silica gel. Whereas the proteolytic enzyme subtilisin BPN' adsorbs irreversibly from aqueous solution (100 mM Tris buffer, 10 mM CaCl2, pH 8.6) onto the surface of untreated silica gel, reversible adsorption of subtilisin BPN' is measured on silica gel coated with gold and derivatized with self-assembled monolayers (SAMs) formed from HO(CH2CH2O)(2)(CH2)(11)SH. We demonstrate the usefulness of gold-coated silica gel for the preparation of stationary phases with controlled surface properties by forming a variety of mixed SAMs through coadsorption of pNA-FPAA-suc-(CH2)(11)SH and X(CH2)(11)SH (where X = CH3, OH, or COOH), These stationary phases were used to demonstrate that the extent of enzymatic hydrolysis of pNA from the surface-immobilized tetrapeptide depends on the functional group X as well as on the dilution of the substrate within the mixed SAM. We also demonstrate that SAMs presenting biotin (X = biotin) can be used to form multilayer structures of biomolecules on the surface of the gold-coated silica gel. These results, when combined, demonstrate the usefulness of gold-coated silica gel for the preparation of well-defined, surface-functionalized supports for biological assays.