Amide-imidol tautomerism based fluorescence turn-on probe for selective detection of fluoride ion through restricted intramolecular rotation.

Fluoride ions play a crucial role in many biological functions such as bone disease treatment, water fluoridation, and caries many other treatments. Excessive intake of fluoride ions causes chronic diseases such as Arthritis, oxidative stress, brittle bones, brain damage, and cancer. With these reasons, a cost-effective and highly specific probe 3PDA have been developed for real-time monitoring of fluoride ions. Probe 3PDA is a pyridine-based "Turn-On" sensor, easily synthesized through the Schotten-Baumann reaction. This pyridine based probe demonstrated interesting optical characteristics such as large Stoke's shift (109 nm), solvatochromism, pH sensing, and ACQ behavior. Furthermore, probe 3PDA was effectively utilized for nanomolar (nM) sensing of fluoride ions through covalent interactions. UV-visible, fluorescence, and 1H NMR titration analyses were executed to verify amide-imidol tautomerism as a sensing mechanism. Amide-imidol tautomerism become highly favorable when fluoride interacts with probe 3PDA molecule. Fluoride ion as a strong hydrogen bond acceptor facilitates the conversion of amide form into imidol tautomer and causes restriction of intramolecular rotation (RIR) of bonds. Overall, presence of fluoride ions makes probe 3PDA molecule more rigid and less flexible. Moreover, DLS analysis was executed to verify the formation of aggregation and excimer disaggregation. DFT studies were employed to validate the sensitivity of probe 3PDA toward fluoride ions. The calculated LOD was 150 nM and LOQ was 502 nM for fluoride ion detection. Probe 3PDA was employed for naked eye solid state sensing of fluoride ions under daylight and UV irradiation at 365 nm. Furthermore, 3PDA was practically applied to estimate fluoride concentration in oral care products, mineral water, and spiked lake water. 3PDA-coated TLC strips were also designed as tiny portable tools for fluoridesensing.