Optical label-free porous Si-based biosensors for rapid bacteria detection are introduced. The biosensors are designed to directly capture the target bacteria cells onto their surface with no prior sample processing (such as cell lysis). Two types of nanostructured optical transducers based on oxidized porous Si (PSiO(2)) Fabry-Pérot thin films are synthesized and used to construct the biosensors. In the first system, we graft specific monoclonal antibodies (immunoglobulin G's) onto a neat electrochemically-machined PSiO(2) surface, based on well-established silanization chemistry. The second biosensor class consists of a PSiO(2)/hydrogel hybrid. The hydrogel, polyacrylamide, is synthesized in situ within the nanostructured PSiO(2) host and conjugated with specific monoclonal antibodies to provide the active component of the biosensor. Exposure of these modified-surfaces to the target bacteria results in "direct-cell-capture" onto the biosensor surface. These specific binding events induce predictable changes in the thin-film optical interference spectrum of the biosensor. Our studies demonstrate the applicability of these biosensors for the detection of low bacterial concentrations, in the range of 10(3)-10(5) cell/ml, within minutes. The sensing performance of the two different platforms, in terms of their stability in aqueous media and sensitivity, are compared and discussed. This preliminary study suggests that biosensors based on PSiO(2)/hydrogel hybrid outperform the neat PSiO(2) system.