A facile synthesis of a novel 4-hydroxyl-3-azo coumarin based colorimetric probes for detecting Hg2+ and a fluorescence turn-off response of 3CBD to Fe3+ in aqueous environment

Abstract

Two azo dyes, (E)-3-(benzo[d]thiazol-2-yldiazenyl)-4-hydroxy-2H-chromen-2-one (3CBD) and (E)-4-hydroxy-3-(quinolin-2-yldiazenyl)-2H-chromen-2-one (3CQD), were designed and synthesized using facile methods. The structures were validated through FTIR and NMR spectroscopy. The photophysical property analyses were further studied using UV-Vis and fluorescence spectrophotometers. Consequently, the absorption and emission spectra of 3CBD confirmed its selectivity of Hg²⁺ and turn-off response to Fe³⁺. On the other hand, the absorption spectra analysis of 3CQD demonstrated selectivity in the presence of Hg²⁺. The colorimetric investigations demonstrated a significant visual response specifically for Hg²⁺, enabling real-time analysis in the corresponding solutions. The presence of other coexisting metal ions does not interfere with the detection of the target metal ion. The fluorescence studies of the two probes revealed that 3CBD was highly fluorescent, which was significantly quenched by Fe³⁺, upon excitation at 340 nm. Utilizing Job plot analyses, it was determined that the complexes 3CBD–Hg²⁺ and 3CQD–Hg²⁺ exhibit a binding stoichiometry of 1 : 1. The association constants for these complexes were measured to be 7.48 × 10⁵ and 9.12 × 10⁵ M⁻¹, respectively, indicating a strong association between both probes and their respective metal ions. Both chemosensors exhibited comparable limits of detection (LOD) and limits of quantification (LOQ) of 0.03 μM and 0.10 μM, respectively. Reversible studies confirmed that only chemosensor 3CQD could serve as a secondary sensor for EDTA. The theoretical studies calculated using Density Functional Theory (DFT) program at B3LYP/6-31G** (Spartan '10 package) level.