Porous silicon is an attractive material for label-free optical biosensors because of its biocompatibility, its large internal surface area, its open pore network, and its widely tunable refractive index. Many structures using this material and exploring reflectometry can be used for biosensing. The sensor performances and sensitivity depends on the parameters of the porous silicon layers and its thermal treatment such as porosity, pore size, oxidation degree, and used wavelength. A theoretical framework to model the reflectance spectra of three optical nanostructures (monolayer, Bragg mirror, and microcavity based on porous silicon layers) before and after the functionalization step is used to study the merit parameters for each device. Based on this theoretical work, optimized conditions to fabricate glucagon biosensors are proposed. A microcavity formed by a period constituted of two porous layers of porosities equal to 95% and 65% with a pore size of 60 and 51 nm, respectively, and with 40% oxidation degree allows a significant redshift to be obtained. The value of minimum detectable coating thickness for a detection system capable of resolving a wavelength shift of 0.1 nm is about 5×10−3 nm.