LAUSR

It is highly desired to develop a visual sensing system for ultrasensitive detection of colorless diclofenac (DCF), yet with a significant challenge. Herein, a novel dye‐based photosensitization sensing system has been successfully developed for detecting DCF for the first time, in which the used dye eosin Y (DeY) can strongly absorb visible light and then be decolorized obviously by transferring photogenerated electrons to g‐C3N4 nanosheets (CN), while the built single‐atomic Co─N2O2 sites on CN by boron‐oxygen connection can competitively adsorb DCF to impede the photosensitization decoloration of DeY. This system exhibits a broad detection range from 8 ng L−1 to 2 mg L−1 with 535 nm light, an exceptionally low detection limit (3.5 ng L−1), and remarkable selectivity. Through the time‐resolved, in situ technologies, and theoretical calculations, the decolorization of DeY is attributed to the disruption of DeY's conjugated structure caused by the triplet excited state electron transfer from DeY to CN, meanwhile, the adsorbed oxygen facilitates the charge transfer process. The preferential adsorption of DCF mainly depends on the strong interactions between the as‐constructed single‐atom Co and Cl in DCF. This study opens an innovative light‐driven sensing system by combining dye and single‐atom metal/nanomaterial for visually intuitive detection of environmental pollutants.