Ratiometric colorimetric detection of fluoride ions using a schiff base sensor: enhancing selectivity and sensitivity for naked-eye analysis

Abstract

A Schiff base receptor with an active –NH group was designed and synthesized for the selective and sensitive colorimetric detection of inorganic fluoride (F⁻) ions in an aqueous medium. The sensitivity of the receptor for F⁻ ions was enhanced by the influence of two electron-withdrawing –NO₂ groups at ortho and para positions which result in a vivid color change. The receptor underwent a remarkable color change from light yellow to violet, enabling naked-eye detection of F⁻ ions without the need for spectroscopic equipment. To ensure the structural integrity of the synthesized receptors, prominent spectroscopic techniques such as ¹H NMR, FTIR, and GCMS analysis were used for characterization. With a limit of detection (LoD) of 0.0996 ppm, a 1 : 2 stoichiometric binding ratio was observed for receptor and F⁻ ions. The binding mechanism confirmed the deprotonation of the –NH group followed by the formation of –HF₂, resulting in an intramolecular charge transfer (ICT) transition, which correlates with UV-vis and ¹H NMR titration results. In addition, the proposed binding mechanism of F⁻ ion interaction with the receptor was theoretically validated using DFT and TDDFT calculations. Furthermore, as a real-life implementation of the receptor, quantification of the F⁻ ions present in a commercially available mouthwash was demonstrated. To assess the sensitivity performance, a paper-based dip sensor and a solid substrate sensor by functionalizing the receptor on diatomaceous earth were demonstrated. Finally, sensors were built into smartphones that could recognize the red, green, and blue percentages (RGB%) where each parameter defines the intensity of the color, which could also be used as a supplement to the colorimetric investigations.