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.