This invention relates to a device for detecting an analyte in a sample comprising: a radiation source adapted to generate a series of pulses of electromagnetic radiation; a transducer having a pyroelectric or piezoelectric element and electrodes which is capable of transducing energy generated by non-radiative decay into an electrical signal; a detector which is capable of detecting the electrical signal generated by the transducer; a first reagent proximal to the transducer, the first reagent having a binding site which is capable of binding a labelled reagent proportionally to the concentration of the analyte in the sample, which labelled reagent being capable of absorbing the electromagnetic radiation generated by the radiation source to generate energy by non-radiative decay; a second reagent proximal to the transducer, the second reagent having a lower affinity for the labelled reagent under the conditions of the assay than the first reagent; and a third reagent proximal to the transducer, the third reagent having a binding site which is capable of binding the labelled reagent, wherein the third reagent has an affinity for the labelled reagent which is less influenced than the first reagent by the concentration of the analyte or the complex or derivative of the analyte.