Unusual redshift due to selective hydrogen bonding between F− ion and sensor motif: a naked eye colorimetric sensor for F− ions in an aqueous environment

Abstract

Naked-eye detection of anions is a challenging but useful technique. In this study, two novel colorimetric monomeric sensors based on highly selective 2,4-dinitrophenylhydrazine were designed, synthesized, and employed as a selective ratiometric optical sensor for F⁻ ion detection via the naked eye. The anion recognition properties were investigated via a pattern of color change as well as changes in absorbance signaling maxima. Sensor molecules were characterized by ¹H NMR, ¹³CNMR, HRMS, and FTIR spectroscopy. Selectivity towards F⁻ ions was attributed to the capability of H-bonding interaction of N–H bonds, which was determined by ¹H NMR titration, ¹⁹F NMR titration, and UV-vis spectroscopic titration. The association constants of 0.263 × 10⁶ M⁻¹ (compound 4) and 6.4988 × 10⁶ M⁻¹ were calculated using the Benesi–Hildebrand equation from UV-vis spectroscopic analysis. The limit of detection (LOD) for both monomeric sensors was 68.182 nM (compound 4) and 72.341 nM (compound 9) and for the polymeric sensor, it was 18.98157 nM (compound 11). Job's plot analysis revealed the existence of 1 : 1 complex formation of F⁻ ions with both monomeric sensor molecules. The same colorimetric response and selectivity properties were exhibited by the norbornene-based homopolymer of the polymeric sensor molecule towards F⁻ ions.