Selective anion sensing in high salt water via a remote indicator displacement assay.

Water-soluble deep cavitands with cationic functions at the lower rim can selectively bind iodide anions in purely aqueous solution. By pairing this lower rim recognition with an indicator dye that is bound in the host cavity, optical sensing of anions is possible. The selectivity for iodide is high enough that micromolar concentrations of iodide can be detected in the presence of molar chloride. Iodide binding at the "remote" lower rim causes a conformational change in the host, displacing the bound dye from the cavity and effecting a fluorescence response. The sensing is sensitive, selective, and works in complex environments, so will be important for optical anion detection in biorelevant media.