Selective Sensing of the Fluoride Ion in Water and B16F10 Cells by Naphthoimidazolium Salts

The present study relates to discovering pyridine-substituted naphthoimidazolium-based fluorescent probes for fluoride and hydroxide ions. The synthesis of naphthoimidazolium based ionic salts, 3-isopropyl-1-methyl-1H-naphtho[2,3-d]imidazol-3-ium hexafluorophosphate (NIMIP) and 3-isopropyl-1-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazol-3-ium hexafluorophosphate (NIMIPyP), and their detailed characterizations are reported. NIMIPyP exists in the ionic form and was observed to have a solubility of 40 mg/mL in water at 25 °C and provides a greenish cyan color emission of 471 nm wavelength on excitation at 340 nm. NIMIPyP was titrated with different tetrabutyl ammonium salts with varying anions (OH^-, F^-, Cl^-, Br^-, I^-, HSO₄^-, ClO₄^-, H₂PO₄^-, HCO₃^-, and NO₃^-) and metal ions (Ca²⁺, Mg²⁺, Cu⁺, Fe³⁺, Zn²⁺, Cd²⁺, and Pb²⁺) to observe their fluorescence quenching behavior toward it. It was observed that among the studied salts only the TBAF and TBAOH salts quench the fluorescent spectra of NIMIPyP at 471 nm with a concomitant shift in the emission peak to 388 nm. The limit of detection (LOD) of probe NIMIPyP for the F^- (4.01 × 10^-8 M) and OH^- (1.29 × 10^-8 M) ions was observed to be 14-fold and 38-fold lower than NIMIP, respectively. The fluoride-sensing ability of NIMIPyP was also demonstrated by the LED bulb. The LED bulb (3 V, 410 nm emission) was fabricated with the CH₃CN solution of poly(methyl methacrylate) (PMMA) and NIMIPyP. The fabricated LED bulb on connecting with an external power supply showed two distinguished peaks at 410 nm (blue color) and 504 nm (greenish color), corresponding to the emission peaks of the LED and NIMIPyP, respectively. The predominantly observed greenish color of the LED (coated with NIMIPyP) emitted a light blue color when dipped in the TBAF solution in CH₃CN and connected with an external power supply. Further, the practical utility of NIMIPyP acting as a fluorescent probe for OH^- and F^- was established with real water samples. The applicability of NIMIPyP toward fluoride-sensing is also shown in the living B16F10 cells.