In this article we present an optical biosensor for label-free detection of trace levels of protease activity. The scheme is based on surface functionalized porous silicon optical structures which supports optical Bloch surface modes. The optical structure provides a resonant optical mode for high sensitivity detection and open access of the sensing layer to the target enzyme. Protease detection is based on the digestion of gelatin, covalently attached inside the pore space, resulting in a spectral blue-shift of the optical mode. Monitoring of spatially separated resonant optical modes is used to eliminate optical response from nonspecific adsorption.