Highly sensitive and fast-responsive fluorescent chemosensor for palladium: reversible sensing and visible recovery.

The well-known rhodamine spiro-lactam framework offers an ideal model for the development of fluorescence-enhanced chemosensors through simple and convenient syntheses. Herein, we report a new tridentate PNO receptor, which was introduced into a rhodamine spiro-lactam system to develop Pd2+-chemosensor RPd4, that displayed significantly improved sensing properties for palladium. Compound RPd4 shows a very fast response time (about 5 s), high sensitivity (5 nM), and excellent specificity for Pd2+ ions over other PGE ions (Pt2+, Rh3+, and Ru3+). In addition, RPd4 displays quite different responses to different valence states of the Pd ions, that is, very fast response towards Pd2+ ions but slow response towards Pd0, which may provide us with a convenient method for the selective discrimination of Pd species in different valence states. According to proof-of-concept experiments, RPd4 has potential applications in Pd2+-analysis in drug compounds, water, soil, and leaf samples. Owing to its good reversibility, RPd4 can also be used as a sensor material for the selective detection and visual recovery of trace Pd2+ ions in environmental samples.