A photochemical functionalization process which passivates the porous silicon surface of optical biosensors has been optimized as a function of the thickness and the porosity of the devices. The surface modification has been characterized by contact angle measurements. Fluorescence measurements have been used to investigate the stability of the DNA single strands bound to the nanostructured material. A dose-response curve for an optical label-free biosensor in the 6-80 mM range has been realized.