Oriented immobilization of antibodies on a silicon wafer using Si-tagged protein A

We previously reported a silica-binding protein, designated Si-tag, which can be used to immobilize proteins on silica surfaces. Here, we constructed a fusion protein of Si-tag and immunoglobulin-binding staphylococcal protein A for oriented immobilization of antibodies on a silicon wafer whose surface is oxidized to silicon dioxide (silica). The fusion protein, Si-tagged protein A, strongly bound to the silica surface with a dissociation constant of 0.31 nM. Time-of-flight secondary ion mass spectrometry analysis of the silicon wafer coated with Si-tagged protein A, combined with principal component analysis and mutual information, demonstrated that protein A is localized on the outermost surface of the bound protein layer. Immunoglobulin G (IgG) was immobilized both on the silicon wafer coated with Si-tagged protein A and, as a control, directly on the intact silicon wafer via physical adsorption. The silicon wafer coated with Si-tagged protein A bound 30–70% more IgG than the uncoated silicon wafer, whereas the antigen-binding activity was 4- to 5-fold higher for the former, indicating that IgG was functionally immobilized on the silicon wafer via Si-tagged protein A in an oriented manner.