Researchers have turned to M13 bacteriophages as a sensing layer for colorimetric sensors. M13 is a nanoscale, benign virus with a shape that closely resembles collagen fibers. It can change its structure by shrinking or expanding in response to a changing surrounding environment. By using a highly lossy resonant promoter as the substrate, the spin-coated M-13 virus layer exhibits strong resonance even with ultrathin thickness variations, resulting in colorimetric behavior with enhanced chromaticity.