Green-Emitting AIEgen for Fluorescent Switch-Off Detection of Multi-Cations and Applications

An easy-to-synthesize aggregation-induced emission (AIE) active Schiff base HNSA was obtained by condensing equimolar amount of 3-hydroxy-2-naphthohydrazide and salicylaldehyde. In pure DMSO, HNSA is non-fluorescent, but increasing the HEPES (H₂O, 10 mM, pH 7.4) fraction (f_HEPES) ≥ 90% showed an intense green fluorescence with maximum fluorescence intensity at 515 nm. The SEM analysis of HNSA at f_HEPES 95% supported the formation of sheet-like self-aggregates of HNSA. The HNSA aggregates were found to be emissive over a wide pH range from pH 2.20 to 7.50. Further, metal ions sensing ability by AIE luminogen (AIEgen) HNSA was performed in f_HEPES 95%. The fluorescence intensity of AIEgen HNSA was quenched by Co²⁺, Ni²⁺, Cu²⁺ and Fe²⁺ ions with an estimated detection limit of 0.43 µM, 0.48 µM, 6.26 µM, and 6.74 µM, respectively. AIEgen HNSA coated paper strips were developed for the visual detection of these metal ions. Also, AIEgen HNSA was successfully utilized to qualitatively detect metal ions in chickpea and lima bean sprouts. In addition, AIEgen HNSA was used to prepare green fluorescent gelatin gel and fluorescent invisible ink to write on filter paper to transfer confidential information. The probe AIEgen HNSA was employed to quantify metal ions in real environmental water samples.