A reversible eugenol derived colorimetric and fluorescence "turn-off" probe for Fe²⁺ and Pb²⁺ ions detection

A simple and robust colorimetric and fluorescent eugenol-based chemical sensor, namely, (E)-N'-(5-allyl-2-hydroxy-3-methoxybenzylidene)-2-hydroxybenzohydrazide (EABH) was synthesized and characterized using spectroscopic techniques such as, NMR (¹H and ¹³C) and mass spectra. The chemosensor shows dual behavior for the colorimetric detection of Fe²⁺ and fluorometric detection of Pb²⁺ ions with high sensitivity and selectivity towards both the ions. The EABH detects Fe²⁺ by "naked eye" color change from lime yellow to brown and displayed fluorescence "Turn-off" response to Pb²⁺ ion. The binding stoichiometry of EABH based on absorption (for Fe²⁺) and fluorescence (for Pb²⁺) data along with mass spectral studies indicated 1:2 binding of metal to EABH ratio, which is further verified by Job's plot measurement. Furthermore, the detection limits for Fe²⁺ and Pb²⁺ ions were found to be 0.166 µM and 0.095 µM, respectively proving its promising sensitivity. On the other hand, live cell imaging experiments showed that EABH has capability of penetrating through the cell membrane and exhibit significant response to the intracellular interactions when Fe²⁺ and Pb²⁺ are introduced in living cells, which reveals its effectiveness in the bioimaging applications. Furthermore, the fabricated chemical sensor EABH was successfully employed for cotton swab and test-paper applications. It is noteworthy that the developed sensor for the detection of Fe²⁺ and Pb²⁺ using colorimetric and fluorimetric assays acts as a reliable, cost-effective method compared to other conventional methods. In addition, computational studies such as Frontier Molecular Orbitals (FMOs) and Molecular Electrostatic Potential (MEP) provided a significant information on the interaction sites within EABH, giving profound insights about the chemical reactivity and various physical parameters.