Abstract
Optical sensors with distinct colorimetric output upon binding with fluoride offer a convenient, fast and reliable methodology for its on-site detection. Most of such sensors lose their affinity towards fluoride in aqueous medium, limiting their applicability in real time analysis. In this report, we have been successful in selective, ratiometric and naked eye sensing of F− ion in aqueous medium by a pyrrole-benzimidazole conjugate based receptor R, in presence of Ni2+ salt. It is noteworthy that the receptor R interacts with neither F− nor Ni2+ ions alone in aqueous medium. As studied by UV–Vis spectroscopy and EPR analysis, the sensing mechanism involves F− induced deprotonation of the receptor R, followed by the formation of an in-situ Ni(II) complex, [Ni(II)R2]2−. This complex is responsible for the colorimetric change from dark yellow to tangerine orange. The structure of the complex is further elucidated by DFT calculations. The limit of detection (LOD) is found to be 0.98 ppm.
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