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.

A luminescent metal-organic framework with the molecular formula [YMn 1.5 (C 7 N 1 H 3 O 5 ) 3 (H 2 O) 6 ]·11H 2 O, 1 {where C 7 N 1 H 3 O 5 = 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 PO 4 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 PO 4 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 (PO 4 3- ), which was significantly lesser than the recommended level for the PO 4 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.