Turn-on detection of Al3+ ions using quinoline-based tripodal probe: mechanistic investigation and live cell imaging applications†

In this study, an easily synthesizable Schiff base probe TQSB having a quinoline fluorophore is demonstrated as a fluorescent and colorimetric turn-on sensor for Al³⁺ ions in a semi-aqueous medium (CH₃CN/water; 4 : 1; v/v). Absorption, emission and colorimetric studies clearly indicated that TQSB exhibited a high selectivity toward Al³⁺, as observed from its excellent binding constant (K_b = 3.8 × 10⁶ M⁻¹) and detection limit (7.0 nM) values. TQSB alone was almost non-fluorescent in nature; however, addition of Al³⁺ induced intense fluorescence at 414 nm most probably due to combined CHEF (chelation-enhanced fluorescence) and restricted PET effects. The sensing mechanism was established via Job's plot, NMR spectroscopy, ESI-mass spectrometry, and density functional theory (DFT) analyses. Furthermore, to evaluate the applied potential of probe TQSB, its sensing ability was studied in real samples such as soil samples and Al³⁺-containing Digene gastric tablets as well as on low-cost filter paper strips. Fluorescence microscopy imaging experiments further revealed that TQSB can be used as an effective probe to detect intracellular Al³⁺ in live cells with no cytotoxicity.