Highly selective and sensitive turn-on fluorescent sensor for detection of Al³⁺ based on quinoline-base Schiff base

A new aluminum ion fluorescent probe (4-(diethylamino)-2-hydroxybenzylidene)isoquinoline-1-carbohydrazide (HL¹) has been conveniently synthesized and characterized. HL¹ exhibited a highly selective and pronounced enhancement for Al³⁺ in the fluorescence emission over other common cations by forming a 2:1 complex, with a recognition mechanism based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). The strong fluorescent emission can be observed even at ppm level concentration of the probe in the presence of Al³⁺ with 41 fold intensity enhancement at 545 nm. HL¹ displays good linear relationship with Al³⁺ in the low concentration and the limit of detection is 8.08 × 10^-8 mol/L. Similar molecules with different substituents on salicylaldehyde phenyl ring were synthesized for studying the structure-activity relationship. Density-functional theory (DFT) calculations are in agreement with the proposed mechanism. It is confirmed that HL¹ could be used to detect Al³⁺ ions in real sample by fluorescence spectrometry and Al³⁺ ions in cells by bioimaging.