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

作者：Mahishi, Anusha A.; Shet, Sachin M.; Mane, Padmaja V.; Yu, Jingxian; Sowriraajan, A. Ve.; Kigga, Madhupra***^*; Bhat, Mahesh P.^*; Lee, Kyeong-Hwan^*; Kurkuri, Mahaveer D.^*

来源：Analytical Methods, 2023, 15(26): 3259-3267.

DOI：10.1039/d3ay00541k

• CHEMOSENSOR
• FLUORESCENCE

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 H-1 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 H-1 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.

• 单位
  桂林理工大学