LAUSR

Bright‐field imaging of nanoscale bioparticles is a challenging task for optical microscopy because the light–matter interactions of bioparticles are weak on conventional surfaces due to their low refractive index and small size. Alternatively, advanced imaging techniques, including near‐field microscopy and phase microscopy, have enabled visualization and quantification of the bioparticles, but they require assistance of sophisticated/customized systems and post‐processing with complex established algorithms. Here, a simple and fast immunoassay device, Gires–Tournois immunoassay platform (GTIP) is presented, which provides unique color dynamics in response to optical environment changes and thus enables the label‐free bright‐field imaging and facile quantification of bioparticles using conventional optical microscopy. Bioparticles on GTIP slow down the velocity of reflected light, leading to vivid color change according to the local particle density and maximizing chromatic contrast for high spatial distinguishability. The particle distribution and density on the surface of the resonator are readily analyzed through 2D raster‐scanning‐based chromaticity analysis. GTIP offers multiscale sensing capability for target analytes that possess different refractive indices and sizes.