A method and kit for determining the quantity of an analyte include providing a functionalized substrate and a reagent. The functionalized substrate includes metallic nanoparticles and a plurality of substantively identical bioactive target molecules affixed to a substrate. The bioactive target molecule binds to a particular analyte. The reagent includes identical detection molecules. Each detection molecule includes a fluorophore, and binds to a particular analyte or competes with a particular analyte for binding to the target molecule. The functionalized substrate is contacted to a test sample and the reagent. The functionalized substrate and a covering solution are exposed to polarized electromagnetic waves that excite the fluorophore. A quantity of the particular analyte in the test sample is determined based on measuring polarization anisotropy of fluorescent emissions from the substrate and the covering solution.