A quinoline derived Schiff base as highly selective 'turn-on' probe for fluorogenic recognition of Al³⁺ ion

A quinoline-derived Schiff base QnSb has been synthesized for fluorescent and colorimetric recognition of Al³⁺ ions in a semi-aqueous medium. The compound QnSb has been characterized by elemental analysis, FT-IR, ¹H/¹³C NMR, UV-Vis and fluorescence spectral techniques. The crystal structure of the QnSb was confirmed by single crystal X-ray diffraction (SC-XRD) analysis. Notably, almost non-fluorescent QnSb served as a 'turn on' responsive probe for Al³⁺ by inducing a remarkable fluorescence enhancement at 422 nm when excited at 310 nm. The probe QnSb exhibited high selectivity for Al³⁺ in CH₃CN/H₂O (4:1, v/v) solution over several competing metal ions (e.g., Mg²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Cu²⁺, Ca²⁺, Ni²⁺, Fe^3+/2+, Cr³⁺, Mn²⁺, Sn²⁺, and Hg²⁺). The limit of detection (LoD) was computed as low as 15.8 nM which is significantly lower than the permissible limit set by WHO for Al³⁺ ions in drinking water. A 1:1 binding stoichiometry of complex QnSb-Al³⁺ was established with the help of Job's plot, ESI-MS, NMR and DFT analyses. Based on its remarkable sensing ability, the probe QnSb was utilized to establish molecular logic gates, and the fluorescence detection of Al³⁺ could clearly be demonstrated on the filter paper test strips.