An interferometric biosensor comprised of two layers of porous Si, stacked one on top of the other, is described. A fast Fourier transform (FFT) of the reflectivity spectrum reveals three peaks that correspond to the optical thickness of the top layer, the bottom layer, and both layers together. Binding of immunoglobulin G to a protein A capture probe adsorbed to the surface of the top layer induces changes in reflectivity at the top layer/solution interface. The FFT method allows discrimination of target analyte binding from matrix effects due to nonspecific changes in the analyte solution. The sensor response is shown to be insensitive to the addition of 4000-fold excess sucrose or 80-fold excess bovine serum albumin interferents.