Oxazoline-appended benzimidazolium salts for selective micromolar detection of Fe3+ ions in water and an experimental and theoretical insight into Fe3+ ion sensing mechanism

Abstract

Four novel water-soluble benzimidazolium and bis-benzimidazolium salts appended with oxazolines through methylene linker were synthesized and characterized. These compounds with a “fluorophore-spacer-receptor” design displayed large Stokes shifts, which was attributed to the TICT phenomenon. These salts were able to selectively detect Fe3+ ions in water despite other competing metal ions being present. A quenching of fluorescence intensity was observed with quenching constants (Ksv) ranging from 8.18 × 104 to 2.51 × 105 M−1 upon complexation of Fe3+ ions with the probes in a 1:1 stoichiometry. A visible color change from colorless to orange-brown was observed during complex formation. The selective sensing of Fe3+ ions and its binding to the benzimidazolium salts was explored with the help of theoretical and experimental studies. The sensing mechanism traces the fluorescence quenching of ligands by ferric ions to a redox reaction. The feasibility of this redox reaction between the ligand and ferric ions was established by cyclic voltammetry experiments The binding constants were found to be as high as 2.29 × 105 M−1 and the detection limit was found to be as low as 0.697 µM, which is appreciably lower than the WHO guidelines (5 µM) for Fe3+ ions in drinking water. The probes were stable and sensitive to Fe3+ over a wide pH range, making them potentially useful in biological and environmental applications. A hands-on experimental kit was also explored for colorimetric quantification using a smartphone camera.