Under physiological conditions, DQCd1 can be protonated and result in an essential resonance process. Upon binding of Cd2+, the resonance process of the protonated DQCd1 was turned off, and this led to a ratiometric signal output for sensing Cd2+. The laser scanning confocal microscopy experiments reveal that cell-permeable DQCd1 can visualize the changes of intracellular Cd2+ via dual-emission ratiometry.