Intramolecular Charge Transfer-Based Rapid Colorimetric In-Field Fluoride Ion Sensors

Although fluoride has several crucial roles in both the biological and medicinal fields, it also suffers from various harmful effects. By this virtue, tracking fluoride is very much essential. For this purpose, a norbornene-coupled 4-aminophenol derivative has been converted to a 2,4-dinitrophenyl hydrazine derivative to detect fluoride ions in both aqueous and dimethylsulfoxide solutions (1:9, v/v). Both the monomer (NDNP) and polymer (Poly-NDNP) can effectively detect fluoride ions in solution and toothpaste samples. Large redshift and a drastic distinguishable color change from yellow to reddish-brown established the usefulness of both NDNP and Poly-NDNP toward detecting fluoride ions. An UV–vis spectroscopic study and ¹H NMR spectroscopic titration confirmed the stepwise fluoride-induced strong H-bond interaction followed by deprotection of the −NH proton, which activates the intramolecular charge-transfer process for spectral and color changes. The sensors’ ratiometric response selectively toward fluoride ions encouraged us to detect and estimate the fluoride ion concentration accurately up to 22 and 17 nM, respectively.