This image shows sensitive optical biosensing over a “rainbow” on a plasmonic surface. The so-called “trapped-rainbow” storage of light is an intriguing on-chip slow light phenomenon, which has been demonstrated using negative refractive index metamaterials and graded plasmonic surface gratings. However, applications of this rainbow-trapping effect are still limited. In this issue, Gan et al. fill this gap by reporting a new and powerful trapped-rainbow imager for super-resolution displacement spectroscopic analysis and surface biosensing. In their work, a wavelength shift resolution of 0.032 nm is obtained on an extremely small 30 μm × 64 μm grating. A 2 × 2 array of the rainbow sensor unit on the same chip is used to perform high-throughput sensing of A549-derived exosomes and distinguish patient samples from healthy controls using exosomal EGFR expression values. Importantly, this remarkable spectroscopic sensing performance is observed through a simple 4× microscope system that can be easily integrated with a smart phone, opening up new opportunities for future personalized and accurate bio/chemical sensing and point-of-care diagnostics.