Disclosed are biosensors useful in the detection of potentially harmful or undesirable agents, particularly chemicals and microorganisms in food and water. The biosensors operate under the principle of time-resolved luminescence resonance energy transfer. In a preferred embodiment, the biosensor comprises at antibodies that recognized different but proximal epitopes on a particular agent. One antibody contains a luminescence donor that emits energy over time, such as a lanthanide series-based luminophor. Another antibody contains a luminescence acceptor that is excited by the emission spectrum of the donor and emits at a particular wavelength, such as for example the fluorophor Cy3. In the presence of the agent, the donor and acceptor are brought into close proximity, such that the energy transfer can occur. The donor is excited by a transient burst of light and the emitted wavelength is received by a photodiode, quantified and correlated to amount of agent in a sample.