Selective Luminescence Turn-On-Based Sensing of Phosphate in the Presence of Other Interfering Anions Using a Heterobimetallic (3d–4d) MOF with an Acidic Pocket

Abstract

A luminescent metal–organic framework with the molecular formula [YMn1.5(C7N1H3O5)3(H2O)6]·11H2O, 1 {where C7N1H3O5 = chelidamate}, was synthesized by a hydrothermal method by employing chelidamic acid as an organic ligand and Y(III) and Mn(II) as metal ions. A two-dimensional heterobimetallic structure with phenolic hydroxyl-functionalized pockets was revealed by single-crystal X-ray diffraction analysis of compound 1. PXRD, TGA, IR, BET analysis, and UV–vis spectroscopy were used for the thorough characterization of compound 1. Upon excitation at 280 nm, compound 1 shows bright blue emission, which was utilized for the selective and sensitive turn-on detection of the PO4 3– ion. Based on Bronsted–Lowry acid–base interactions, the photoluminescence of compound 1 was enhanced in the presence of very low concentrations of the aforementioned anion. The mechanism behind the detection of the phosphate ion has been explored in detail. It was seen that the PO4 3– anion entered the hydroxyl-functionalized pockets of compound 1 and stabilized the aromatic portion of compound 1 via molecular-level interactions through acid–base interactions. These molecular-level interactions are responsible for the enhancement of the photoluminescence intensity of compound 1 after the incorporation of phosphate ions by reducing the nonradiative transitions. These phenomena were also confirmed by time-correlated single photon counting (TCSPC) measurement, which shows that the excited-state lifetime increased with the increase in addition of phosphate anions. The calculated limit of detection (LOD) of 1 was 19.55 ppb for phosphate (PO4 3–), which was significantly lesser than the recommended level for the PO4 3–anion toward the human body. The luminescence enhancement coefficient, K SV, value was also much higher than those of other reported metal–organic frameworks.

Fluorescence sensing and device fabrication with luminescent metal–organic frameworks

“…Mahata and co-workers synthesized a 3d–4d bimetallic MOF with the molecular formula [YMn 1.5 (C 7 N 1 H 3 O 5 ) 3 (H 2 O) 6 ]·11H 2 O, which was able to detect PO 4 3− by fluorescence enhancement. 66 The MOF presents a two-dimensional heterobimetallic structure, with free phenolic hydroxyl groups decorating the inner wall of the channel. PO 4 3− enters the pores of the MOF through having a suitable molecular size, while strong acid–base interactions fix the aromatic part of the MOF.…”

A Multifunctional CaII–EuIII Heterometallic Organic Framework with Sensing and Selective Adsorption in Water

“…Attributed to the featured emissions of Ln III , luminescent Ln-MOFs with characteristic emission, large Stokes shifts, long luminescence lifetimes, and high quantum yields, on the one hand, have become efficient sensing platforms for heavy metal ions and nitroaromatics with a high signal-to-noise ratio, high selectivity, sensitivity, and fast response in recent years. − Upon thorough literature search, several Ln-MOFs are obtained for Fe 3+ and 3-NP detection, however, their detection limits mostly stay at the micromolar level, and highly efficient sensors with low nanomolar levels are rare . On the other hand, Ln-MOFs could be good adsorbing materials for organic dyes due to their large specific surface area, tunable pore sizes, designable channels, active sites on the organic linker, and charge characteristics. ,, Notably, heterometallic Ln III -containing systems with amazing structural features and multifunctional performance are highly desirable due to the complementary and synergistic effects on the electronic and space of different metal centers. − Compared to numerous documented 3d-4f heterometallic MOFs, Ln-MOFs incorporated with alkaline earth metals are still in their infancy. − Ln-MOFs merged with inexpensive, readily available, and environmentally friendly alkaline earth metal ions are particularly promising . These metal ions have a high charge density to favor the formation of strong interactions, and their larger ionic radius is helpful to offer a large coordination number for enhancing the chemical stability.…”