A ratiometric fluorescent sensor for selective recognition of Al³⁺ ions based on a simple benzimidazole platform

An efficient new ratiometric chemosensor, 6-imidazo-2-yl-5,6-dihydro-benzo[4,5]imidazo[1,2-c]quinazoline (PB), which enables selective sensing of Al³⁺ in aqueous DMSO was synthesized by a facile one step condensation reaction of imidazole-2-carboxaldehyde with 2-(2-aminophenyl)-1H-benzimidazole. The probe PB was characterized by elemental analysis, ¹H and ¹³C NMR, ESI-MS and IR spectral techniques. Upon addition of Al³⁺, the fluorescent probe PB induces ratiometric responses in both absorption and fluorescence spectra on the basis of a charge transfer mechanism. The receptor PB serves for highly selective, sensitive and ratiometric detection of Al³⁺ without the interference of biologically and environmentally relevant cations. The detection limit is found to be 5.3 × 10⁻⁷ M. The Job's plot analysis indicates the binding stoichiometry of the complex to be 1 : 1 (host/guest).