Stable Lanthanide–Organic Frameworks: Crystal Structure, Photoluminescence, and Chemical Sensing of Vanillylmandelic Acid as a Biomarker of Pheochromocytoma

Abstract

Several isostructural lanthanide metal-organic frameworks, viz. <b>[Ln(DCHB)</b>_<b>1.5</b><b>phen]</b>_<b>n</b> (<b>Ln-MOFs</b>, where Ln = Eu for <b>1</b>, Tb for <b>2</b>, Sm for <b>3</b> and Dy for <b>4</b>), are successfully synthesized through the hydrothermal reactions of 4'-di(4-carboxylphenoxy)hydroxyl-2, 2'-bipyridyl (H₂DCHB) and lanthanide nitrates as well as chelator 1,10-phenantroline (phen). These structures are characterized by single-crystal X-ray diffraction, and the representative <b>Ln-MOF 1</b> is a fivefold interpenetrated framework with the uncoordinated Lewis base N sites form DCHB^2- ligands. The photoluminescence research studies reveal that <b>Ln-MOFs 1</b>-<b>4</b> exhibit characteristic fluorescent emissions from ligand-induced lanthanide Ln(III) ions, while the single-component emission spectra of <b>Ln-MOF 4</b> are all located in a white region under different excitations. The absence of coordinated water and the interpenetration property of the structures are conducive to the structure rigidity, and the results display that <b>Ln-MOF 1</b> has high thermal/chemical stabilities in common solvents and a wide pH range as well as the boiling water. Notably, luminescent sensing studies reveal that <b>Ln-MOF 1</b> with prominent fluorescence properties can perform in highly sensitive and selective sensing of vanillylmandelic acid (VMA) in aqueous systems (<i>K</i>_SV = 562.8 L·mol^-1; LOD = 4.6 × 10^-4 M), which can potentially establish a detection platform for the diagnosis of pheochromocytoma via multiquenching mechanisms. Moreover, the <b>1</b>@MMMs sensing membranes comprised of <b>Ln-MOF 1</b> and a poly(vinylidene fluoride) (PVDF) polymer can also be facilely developed for VMA detection in aqueous media, suggesting the enhanced convenience and efficiency of practical sensing applications.