A Porous Silicon Optical Biosensor:  Detection of Reversible Binding of IgG to a Protein A-Modified Surface

The reversibility, specificity, stability, and scaling of signal response to analyte mass were quantified for a porous silicon-based optical interferometric biosensor. The sensor system studied consisted of a thin layer (5μm) of porous silicon modified with Protein A. The system was probed with various fragments of an aqueous Human IgG analyte. The sensor operates by measurement of the Fabry−Perot fringes in the white light reflection spectrum from the porous silicon layer. Molecular binding is detected as a shift in wavelength of these fringes. IgG was added to and removed from the protein 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 used to link protein A to the porous Si surface incorporated bovine serum albumin (BSA). This approach was found to completely eliminate signal due to nonspecific binding, tested by exposure of the sensor to the F(ab')2 fragment 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 with the mass of analyte bound in the porous Si layer.