A fast, convenient and stable fluorescent probe for detecting Fe3+/2+ and its applications

A turn-on benzimidazole-based fluorescent probe (E)-2-((4-(5-bromo-1H-benzo[d]imidazol-2-yl)benzylidene)amino)phenol (BRAP) for the detection of Fe3+/2+ was developed. The probe BRAP was designed and synthesized using 2-hydroxyaniline as the recognition site and benzimidazole as the fluorophore. It was highly selective and sensitive with a detection limit as low as 2.1×10-8 mol/L and didn’t interfere with other analytes. Using EtOH: H2O (v:v=9:1) as the detection system, BRAP had non-fluorescence at 466 nm, which was mainly due to isomerization and rotation of the C=N bond. The dihedral angle rotation hindered the charge transfer between benzimidazole and 2-hydroxyaniline. However, upon the addition of Fe3+/2+, the C=N bond in probe BRAP was cleaved to form compound 3, whose electrons were transferred from the benzimidazole ring to the benzaldehyde portion upon excitation and showed blue fluorescence. In addition, the optimized structure and orbital energies of probe BRAP and its products were calculated by comparing three functional combinations with time-dependent density functional theory (TD-DFT) and the optimal functional and basis set [B3LYP /6-311+G(d,p)]. In addition, the probe BRAP combined with a smartphone was explored for the rapid detection of Fe3+/2+ in wine.