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, EuLCl3 (1), SmLCl3 (2), and TbLCl3 (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 HPO42-, SO42-, CH3COO-, I-, Br-, Cl-, F-, NO3-, CO32-/HCO3-, and HSO4- at pH 7.4 in aqueous HEPES (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid) buffer. Among the anions scanned, HPO42- showed an excellent luminescence change with all three complexes. Job's plot and ESI-MS support the 1:2 association between the receptors and HPO42-. Systematic spectrophotometric titrations of 1-3 against HPO42- demonstrates that the emission intensities of 1 and 2 were enhanced slightly upon the addition of HPO42- 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 HPO42- 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 mu M. This concludes that all complexes demonstrated a high degree of sensitivity and selectivity towards HPO42-.

Automated summary

The study focused on the selective interactions between three mononuclear lanthanide-based complexes and various anions in aqueous media, highlighting their excellent luminescence changes specifically with HPO42-. The complexes demonstrated high sensitivity and selectivity towards HPO42-, with complex 3 showing the most steady quenching of luminescence throughout the titration.