Porous silicon based optical biosensors offer several advantages in label-free detection and identification of organic molecules due to the well known properties of this material [1. 2], PSi has a spongy structure with a large specific surface of the order of 100-500 m2 cm" 3 [3], so that a very effective interaction with several adsorbates is assured. When a biological solution penetrates into the PSi pores, it substitutes the air, so that the average dielectric properties of the heterogeneous silicou-air-liquid system change inducing a red-shift in the reflectivity spectrum of the device. The effect depends on the refractive index value of the solution but also on how it penetrates into the pores. Due to this sensing mechanism, PSi optical devices cannot identify the