A chemical composition including a fluorescent polymer and a receptor that is specific for both a target biological agent and a chemical moiety including (a) a recognition element, (b) a tethering element, and (c) a property-altering element is disclosed. Both the fluorescent polymer and the receptor are co-located on a support. When the chemical moiety is bound to the receptor, the property-altering element is sufficiently close to the fluorescent polymer to alter the fluorescence emitted by the polymer. When an analyte sample is introduced, the target biological agent, if present, binds to the receptor, thereby displacing the chemical moiety from the receptor, resulting in an increase of detected fluorescence. Assays for detecting the presence of a target biological agent are also disclosed.