Turn-off-and-on: chemosensing ensembles for sensing chloride in water by fluorescence spectroscopy.

The recognition and sensing of aqueous chloride by synthetic receptors is a challenging task. Herein we apply the chemosensing ensemble methodology to optically detect chloride in water at near physiological pH. Variants based on two closely related receptors have been explored. The sensors can be obtained in situ by mixing a rhodium complex, a bidentate N,N-chelate ligand, and a fluorescent dye in buffered aqueous solution. Upon mixing the sensor components, the rhodium complex binds to the N,N-chelate ligand to yield a metal-based receptor. This latter associates with the fluorophore to give a non-emissive ground-state complex. The chemosensing ensembles respond to chloride via a turn-on fluorescence signal and can be used for optical detection of chloride down to mid-micromolar concentrations.