A porous silicon optical biosensor: Detection of reversible binding of IgGto a protein A-modified surface

The reversibility, specificity, stability, and scaling of signal response t o analyte mass were quantified for a porous silicon-based optical interfero metric biosensor. The sensor system studied consisted of a thin layer (5 mu m) of porous silicon modified with Protein A. The system was probed with v arious fragments of an aqueous Human IgG analyte. The sensor operates by me asurement of the Fabry-Perot fringes in the white light reflection spectrum fi om the porous silicon layer. Molecular binding is detected as a shift i n wavelength of these fringes. IgG was added to and removed from the protei n A-modified surface by changing solution pH in a flow cell, and the system was found to be reversible through several on-off cycles. The molecule use d to link protein A to the porous Si surface incorporated bovine serum albu min (BSA). This approach was found to completely eliminate signal due to no nspecific binding, tested by exposure of the sensor to the F(ab')(2) fragme nt of IgG (which does not bind to protein A). The linker/protein A-modified surface was also found to be stable toward oxidation in the aqueous buffer solutions used. The shift in the Fabry-Perot fringes was found to scale wi th the mass of analyte bound in the porous Si layer.