Sol-gel preparation of apatite-coated silica macrospheres from water glass and their adsorption of bovine serum albumin and lysozyme

Silica gel macrospheres were prepared from water glass as the silica source. The water glass solution was mixed with sodium alginate solution and added dropwisely to 1M CaCl₂ solution to form macrospheres with ³4 and 2.6mm in diameter. Those SiO₂ macrospheres were soaked in 1:1 (volume) mixture of ethanol and 0.1M Na₂HPO₄ to deposit hydroxyapatite layer (HAp-SiO₂ macrospheres). Bovine serum albumin (BSA) and egg lysozyme (LYZ) were adsorbed those SiO₂ and HAp-SiO₂ macrospheres. The amount of proteins adsorbed was well correlated to the Langmuir-type adsorption equation. The electrostatic interaction was predominant between the macrosphere surface and the protein molecules. Protein molecular entrapment in the mesopores (5-25 nm) was also probable for the SiO₂ macrospheres but not applicable to the HAp-SiO₂ ones without the mesopores. From consideration that protein molecule has a few groups with negative or positive partial charges, a multi-layer adsorption model was proposed.