An optical biosensor for detecting a target bioanalyte in a sample, the biosensor comprising: - a porous silicon or alumina substrate comprising a surface and a detection agent immobilised on the surface, the detection agent comprising a sensing domain and a signaling domain, the sensing domain comprising a linker capable of interacting with the target bioanalyte and the signaling domain comprising 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 such that 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; and - a plurality of light interacting pores on the surface of the substrate, wherein the pores are configured to interact with the light emission from the luminescence donor to provide a measurable light emission which is indicative of the presence of the target bioanalyte; the biosensor being characterized by : - a bioanalyte specific capture agent for selectively binding to the target bioanalyte; wherein the bioanalyte specific capture agent is deposited on or near the surface of the porous silicon or alumina substrate so that at least some of the target bioanalyte captured by the capture agent is capable of interacting with the sensing domain of the detection agent for the target bioanalyte to be detected by the optical biosensor. - a porous silicon or alumina substrate comprising a surface and a detection agent immobilised on the surface, the detection agent comprising a sensing domain and a signaling domain, the sensing domain comprising a linker capable of interacting with the target bioanalyte and the signaling domain comprising 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 such that 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; and - a plurality of light interacting pores on the surface of the substrate, wherein the pores are configured to interact with the light emission from the luminescence donor to provide a measurable light emission which is indicative of the presence of the target bioanalyte; - a bioanalyte specific capture agent for selectively binding to the target bioanalyte; wherein the bioanalyte specific capture agent is deposited on or near the surface of the porous silicon or alumina substrate so that at least some of the target bioanalyte captured by the capture agent is capable of interacting with the sensing domain of the detection agent for the target bioanalyte to be detected by the optical biosensor. <EOS>