1. A method of detecting an analyte in a sample, comprising the steps of: (a) providing a sensor construct comprising: (i) a molecular recognition domain which specifically binds to the analyte; (ii) a first label comprising an RET donor; and (iii) a second label comprising an RET acceptor for the RET donor of the first label, the RET acceptor being separated from the RET donor by a distance that allows Förster resonance energy transfer to occur, wherein proximity of the analyte to the RET donor and/or the RET acceptor interferes with a FRET interaction between the RET donor and RET acceptor when the molecular recognition domain binds the analyte; (b) contacting the sample with the sensor construct; and then (c) measuring an optical signal from the sensor construct, wherein the presence of the analyte in the sample is determined when interference with a Förster resonance energy transfer interaction between the RET donor and the RET acceptor is detected.