Demonstration of Selective Single-Barium Ion Detection with Dry Diazacrown Ether Naphthalimide Turn-on Chemosensors.

Single-molecule fluorescence imaging (SMFI) of gas-phase ions has been proposed for "barium tagging," a burgeoning area of research in particle physics to detect individual barium daughter ions. This has potential to significantly enhance the sensitivity of searches for neutrinoless double-beta decay (0νββ) that is obscured by background radiation events. The chemistry required to make such sensitive detection of Ba by SMFI in dry Xe gas at solid interfaces has implications for solid-phase detection methods but has not been demonstrated. Here, we synthesized simple, robust, and effective Ba-selective chemosensors capable of function within ultrapure high-pressure Xe gas. Turn-on fluorescent naphthalimide-(di)azacrown ether chemosensors were Ba-selective and achieved SMFI in a polyacrylamide matrix. Fluorescence and NMR experiments supported a photoinduced electron transfer mechanism for turn-on sensing. Ba selectivity was achieved with computational calculations correctly predicting the fluorescence responses of sensors to barium, mercury, and potassium ions. With these molecules, dry-phase single-Ba ion imaging with turn-on fluorescence was realized using an oil-free microscopy technique for the first time-a significant advance toward single-Ba ion detection within large volumes of Xe, plausibly enabling a background-independent technique to search for the hypothetical process of 0νββ.