Chelation-enhanced fluorescence-enabled coumarin-hydrazone Schiff bases for the detection of Al3+ ions and its real-time applications

We designed a simple coumarin-hydrazone-based Schiff base fluorescent probe, namely 7-(diethylamino)-N'-(2,4-dihydroxybenzylidene)-2-oxo-2H-chromene-3-carbohydrazide (L), which was successfully synthesized and utilized for selective detection of Al3+ ion via fluorescence 'turn-on' method. The Probe L could detect Al3+ by a sharp increment in fluorescence intensity (∼15 times) in a semi-aqueous DMSO medium. The detection limit of L towards Al3+ was calculated to be 50 nM, and the former undergoes a 1:1 stoichiometry binding with a higher binding constant of ∼ 2.14 × 104 M−1. Probe L detects Al3+ by the well-known chelation-enhanced fluorescence (CHEF) mechanism and inhibits C = N isomerization by fluorescence time-resolved measurement. Experimental results from FT-IR, NMR, and ESI mass spectrometry confirm the interaction mechanism between probe L and Al3+ ions, further supported by DFT calculations. In addition, the probe L fabricated paper strips could detect Al3+ in the naked eye under UV light.