Abstract: The present invention concerns a method of detection of an analyte in which a protein capable of binding the analyte and comprising a fluorescent energy label and an energy acceptor moiety capable of accepting energy emitted by the label or protein by Forster energy transfer (FRET), is exposed to incident electromagnetic energy to excite the protein or label, and the fluorescent emission of the label is measured; characterised in that the protein is encapsulated in a biocompatible, optically transparent matrix which is permeable to the analyte, and in that the protein undergoes no substantial conformational change during the method; further characterised in that the energy acceptor moiety has a more active and less active state, which is determined by the presence of analyte, and the emission from the label is indicative of the presence of analyte. A biocompatible optically transparent matrix in which a protein capable of binding an analyte is also provided.