Spectral and DFT/TD-DFT studies on turn-on fluorescent detection of Al(III) by a quinolin-8-ol-based Schiff base and its bioimaging

Abstract

A new Schiff base (AK2) is synthesized, characterized and used to detect Al(III) ions as the fluorometric probe in an aqueous solution. AK2 exhibits high selectivity towards Al(III) by forming a 1:1 octahedral complex wherein the probe acts as a NNN-tridentate ligand. The sensing mechanism is verified by 1H NMR, 27Al NMR and mass spectral techniques in addition to Job's plot analysis. In an aqueous solution, AK2 is scarcely emissive due to the photoinduced electron transfer (PET) process and upon the addition of Al(III) ions the formed AK2+Al(III) complex results in instantaneous intense bluish-green emission (at 475 nm) by chelation-enhanced fluorescence (CHEF) effect. AK2 was found to have high selectivity, high binding constant (Kb = 3.9 × 104 M−1) and an acceptable limit of detection (LOD = 0.9 µM). The experimentally observed optical properties of AK2 and AK2+Al(III) complex have been correlated with computational results using B3LYP/6–311G** and B3LYP/LACV3P** basis sets by Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TD-DFT) methods. The probe has proved its real-time application as a reagent in the quantitative determination of Al(III) in natural matrices such as bore water, drinking water, tap water and BSA solution. Fluorescence microscopic studies showed that AK2 could be used as an effective probe for detecting intracellular Al(III) in HeLa cells without toxicity.