Provided herein is an optical biosensor for detecting a target bioanalyte in a sample. The biosensor includes: a porous silicon or alumina substrate having a surface and a detection agent immobilised on the surface. The detection agent includes a sensing domain and a signaling domain, the sensing domain having a linker capable of interacting with the target bioanalyte and the signaling domain having a luminescence donor and a luminescence acceptor wherein the luminescence donor and the luminescence acceptor are connected by the linker and are optically coupled in the absence of the target bioanalyte. Emission of light from the luminescence donor is substantially quenched by the luminescence acceptor, and interaction of the target bioanalyte with the linker results in optical un-coupling of the luminescence donor and the luminescence acceptor to thereby result in light emission from the luminescence donor.