Luminescent Metal–Organic Framework Sensor: Exceptional Cd²⁺ Turn-On Detection and First In Situ Visualization of Cd²⁺ Ion Diffusion into a Crystal

With regard to fluorescence quenching commonly observed during metal-ion detection, “turn-on” chemical sensing has been rarely reported, but could be extremely important because it facilitates the selective recognition of target objects of interest against a dark background. A metal–organic framework (MOF) chemosensor has been prepared that serves as an efficient platform for the selective detection of Cu²⁺ and Cd²⁺ ions over other metal ions. In particular, this framework shows the highest fluorescence enhancement (≈60-fold relative to Cd-free MOF) for the hazardous metal ion Cd²⁺ among luminescent MOFs and displays excellent reusability in repeated cycles. The direct diffusion of Cd²⁺ into the crystal pores has also been visualized for the first time.