Developing an economical and robust sensor using an earth-abundant metal-based metal-organic framework (MOF) to detect trace water content rapidly is crucial for laboratories and chemical industries. Herein, a mixed-valent chiral [{Cu+Cu32+(2,4-Imdb)3(H2O)3}·Cl]∞ MOF manifests atypical quantitative detection of a trace amount of water [0.016% (v/v)] in organic solvents. Single-crystal X-ray diffraction and time-correlated single-photon counting supported the mechanism for turn-on fluorescence.