Adsorption of bovine serum albumin onto metal oxide surfaces

The adsorption of bovine serum albumin (BSA) on surfaces of silicon dioxide (silica), titanium dioxide (titania), zirconium oxide (zirconia), and aluminium oxide (alumina) was studied as a function of pH. The adsorption isotherms for these metal oxides showed well-defined saturation plateaus in the pH range of 3.5 to 9.0. On positively charged metal oxides the isotherms showed a steeper initial slope, i.e., a high affinity character. The saturation values of BSA adsorption varied with pH, showing maxima around the isoelectric point (I.E.P.) of BSA for all of the metal oxides. The maximum amounts of adsorbed BSA (mg/m(2)) were 2.2 on silica, 3.4 on titania, 4.2 on zirconia, and 4.7 on alumina. The extent of maximum adsorption depended on the surface charge density of the metal oxides. Potentiometric proton titration showed that all of the profiles of net charge equivalent of BSA molecules adsorbed on positively or negatively charged metal oxides at the I.E.P. of BSA shifted to the positive side compared with that of the solute BSA in the pH range of 3.5 to 9.0, giving more alkaline I.E.P.'s. The degree of shifts in the charge profiles was more pronounced for titania, zirconia, and alumina which have positive surface charges than for negatively charged silica. It was suggested that negatively charged groups, mostly carboxyl groups, on BSA molecules were oriented toward both the positively and negatively charged metal oxide surfaces upon adsorption.