the millimolar level. Therefore, DQCd1 can act as a ratiometric fluorescent sensor for Cd2+ through inhibition of the resonance process. Confocal microscopy and cytotoxicity experiments indicate that DQCd1 is cell-permeable and noncytotoxic under our experimental conditions. It can indeed visualize the changes of intracellular Cd2+ in livingcells using dual-emission ratiometry.