An optical biosensor for protease activity, capable of detecting picomole quantities of protease, is constructed from a 1D photonic crystal of porous Si. The porous Si layer is coated with a thin film of zein, a natural hydrophobic protein. Proteolytic cleavage produces small fragments that infiltrate the pores of the photonic crystal, producing a distinct color change (see figure). The amount of protease pepsin used on each of the spots is indicated.