A Piperazine Linked Rhodamine-BODIPY FRET-based Fluorescent Sensor for Highly Selective Pd2+ and Biothiol Detection

A class of rhodamine-based fluorescent sensors for the selective and sensitive detection of Pd²⁺ metal ions in aqueous media has been developed. A rhodamine-based sensor PMS and a rhodamine-BODIPY Förster resonance energy transfer (FRET)-pair sensor PRS have been incorporated with a piperazine linker and an O−N−S−N podand ligand for specific recognition of Pd²⁺ ion. Both probes displayed colorimetric and fluorescent ratiometric changes when exposed to Pd²⁺, due to their spirolactam rings opening and restoring rhodamine conjugation. PRS is highly selective to Pd²⁺ over 22 other metal ions, showing a 0.6-fold ratiometric difference at I_600nm/I_515nm. Additionally, the lactam ring in Pd²⁺ coordinated PRS-Pd could be switched back to the closed form in the presence of various thiols, providing a “red-green traffic light” detection mechanism between red and green emission. Furthermore, PRS showed excellent cell viability and was successfully employed to image Pd²⁺ and the PRS-Pd complex ensemble could interchangeably detect biothiols including glutathione (GSH) in A549 human lung cancer cells.