A europium metal–organic framework for dual Fe3+ ion and pH sensing

• Published in: Scientific reports Vol. 12; no. 1; p. 11982
• Main Authors: Rozenberga, Linda, Skinner, William, Lancaster, David G., Bloch, Witold M., Blencowe, Anton, Krasowska, M., Beattie, David A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 14.07.2022; Nature Publishing Group; Nature Portfolio
• Subjects: 639/638/11; 639/638/298; Environmental effects; Europium; Humanities and Social Sciences; Iron; multidisciplinary; NMR; Nuclear magnetic resonance; pH effects; Science; Science (multidisciplinary); Spectroscopy; X-ray diffraction
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-022-15663-z

Metal–organic frameworks (MOFs) with ratiometric sensing properties are desirable for many applications due to their intrinsic self-calibration. We report the re-assessment of the sensing properties of a MOF, originally reported as containing europium(III) and 2-hydroxyterephtalic acid, and having fluorescent ratiometric iron(III) sensing properties. Synchrotron single-crystal X-ray diffraction and proton nuclear magnetic resonance ( 1 H NMR) spectroscopy revealed that the MOF is composed of 2-methoxyterephthalate, not 2-hydroxyterephthalate as originally reported. We found that the MOF exhibits a sensor turn-off response towards Fe 3+ ion concentrations in the range 0.5–3.7 ppm (band 425 nm), and a turn-on response towards a decrease of pH from 5.4 to 3.0 (band 375 nm), both resulting from the addition of acidic Fe 3+ salt solution to a MOF suspension. Thus, the ratiometric sensing properties and the originally proposed mechanism no longer apply; our work reveals a dynamic quenching mechanism for the fluorescence turn-off response due to the presence of Fe 3+ ions, and a ligand protonation mechanism for the turn-on response to a decrease in pH. Our work highlights the importance of a thorough investigation of the structure of any newly synthesized MOF, and, in the case of potential sensors, their selectivity and any environmental effects on their sensing behavior.