Mechanistic Insight of Sensing Hydrogen Phosphate in Aqueous Medium by Using Lanthanide(III)-Based Luminescent Probes

The development of synthetic lanthanide luminescent probes for selective sensing or binding anions in aqueous medium requires an understanding of how these anions interact with synthetic lanthanide probes. Synthetic lanthanide probes designed to differentiate anions in aqueous medium could underpin exciting new sensing tools for biomedical research and drug discovery. In this direction, we present three mononuclear lanthanide-based complexes, EuLCl(3) (1), SmLCl(3) (2), and TbLCl(3) (3), incorporating a hexadentate aminomethylpiperidine-based nitrogen-rich heterocyclic ligand L for sensing anion and establishing mechanistic insight on their binding activities in aqueous medium. All these complexes are meticulously studied for their preferential selectivities towards different anions such as HPO(4)(2−), SO(4)(2−), CH(3)COO(−), I(−), Br(−), Cl(−), F(−), NO(3)(−), CO(3)(2−)/HCO(3)(−), and HSO(4)(−) at pH 7.4 in aqueous HEPES (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid) buffer. Among the anions scanned, HPO(4)(2−) showed an excellent luminescence change with all three complexes. Job’s plot and ESI-MS support the 1:2 association between the receptors and HPO(4)(2−). Systematic spectrophotometric titrations of 1–3 against HPO(4)(2−) demonstrates that the emission intensities of 1 and 2 were enhanced slightly upon the addition of HPO(4)(2−) in the range 0.01–1 equiv and 0.01–2 equiv., respectively. Among the three complexes, complex 3 showed a steady quenching of luminescence throughout the titration of hydrogen phosphate. The lower and higher detection limits of HPO(4)(2−) by complexes 1 and 2 were determined as 0.1–4 mM and 0.4–3.2 mM, respectively, while complex 3 covered 0.2–100 μM. This concludes that all complexes demonstrated a high degree of sensitivity and selectivity towards HPO(4)(2−).