Fluorescent Lower Rim 1,3-Dibenzooxadiazole Conjugate of Calix[4]arene in Selective Sensing of Fluoride in Solution and in Biological Cells Using Confocal Microscopy.

p- tert-Butyl-calix[4]arene was derivatized by integrating a benzooxadiazole fluorescent tag into its 1,3-arms at the lower rim to result in L and was characterized. L was titrated with 17 anions in THF and found selective for F ions with lowest detection limit of 109 ppb. L and F form a 1:1 complex. L self-assembles in THF to result in sheet like structures which converts into smaller spherical particles upon addition of F. The site of interaction of F was deduced based on H NMR spectroscopy and the coordination features by density functional theory (DFT) computations wherein six noncovalent interactions of the type X-H···F (where X = O, N, or C) were noticed. The sensing of F is reversible when titrated with Ca, and the reversibility was demonstrated for 10 cycles without losing sensitivity. The study has been extended to the biological cells using fluorescence and confocal microscopy. While L shows strong fluorescence in HeLa cells, increasing concentrations of F exhibited greater fluorescence quenching. Thus, L acts as a good sensor for F in solution as well as in biological cells, a rare and unique combination for a calixarene conjugate to exhibit such sensing behavior in dual media.