Characterization of immobilized antibodies on silica surfaces

-There is considerable interest in the immobilization of an­ tibodies on silica surface for fiber optic biosensor applications. The physical and chemical properties of antibodies immobilized on silica surfaces were investigated in this study. Two antibody (Ab)-antigen (Ag) model systems, goat anti-human IgG Ab (polyclonal Ab)/human IgG (multivalent Ag) and mouse anti-digoxin IgG (monoclonal Ab)1 digoxin (monovalent Ag), were used. Both physical and covalent im­ mobUizatlon of antibody on silica surfaces were investigated. The co­ valently immobilized antibody shows better stability and Ag binding capacity. In the case of large Ag molecules, such as human IgG, the maximum antigen binding capacity is probably restricted by steric fac­ tors. The results of an antibody specificity study reveal that the posi­ tively charged surface shows high nonspecific binding. The binding constants of anti-digoxin IgG and digoxin are similar in solution and on different surfaces, suggesting that the original conformation of Ag binding sites is preserved. However, the binding constants of goat anti­ human IgG and human IgG on surfaces are one order higher than that in solution. This suggests that the enhancement in binding constant is a function of both local Ab concentration and size of Ag molecules.