This work presents three series of Eu/Tb metal–organic frameworks (MOFs) containing benzophenone-4,4′-dicarboxylic acid (H2BPNDC), 4,4′-dicarboxydiphenyl ether (H2OBA), and terephthalic acid (H2BDC) as the ligands. Eu/Tb MOFs have the same structural features in that their 3D frameworks are simplified as 2,3,10-connected {42.6}2{46.618.819.102}{4}2 topological networks. The solid-state fluorescence spectra of three Eu/Tb MOF series are attributed to the combined emissions of 5D0 → 7F J (J = 1–4) transitions in Eu3+ and 5D4 → 7F J (J = 6–5) transitions in Tb3+. The n Eu:n Tb of Eu/Tb MOFs is optimized as 1:69 based on the relationships between I Tb(545)/I Eu(614) and n Eu:n Tb; that is, Eu0.0143Tb0.9857-L (L = BPNDC2–, OBA2–, and BDC2–) were selected to carry out the following temperature (T)-sensing tests. The fluorescence mechanism of Eu0.0143Tb0.9857-L can be explained by a ligand-to-metal charge transfer combined with an intermetallic Tb3+ → Eu3+ energy transfer. The T-dependent fluorescence indicates linear relationships with sensitivities of 1.85% K–1 for Eu0.0143Tb0.9857-BPNDC, 6.49% K–1 for Eu0.0143Tb0.9857-OBA, and 0.28% K–1 for Eu0.0143Tb0.9857-BDC. The influence of T on the lowest excited triplet energy levels (T1 values) of the ligands reveals that the ligand energy regulation impacts their fluorescence properties, including the sensitivity, fluorescence quenching rate, quantum yield, and fluorescence lifetime. This shows that Eu0.0143Tb0.9857-BPNDC is sufficiently sensitive to T, making it applicable in noncontact T measurements.
Quinolinic acid (QA) is an index for some diseases, whose detection is of importance. This work presents a samarium metal–organic framework (Sm-MOF) containing 5-sulfoisophthalate ligand (SIP3–). The fluorescence of Sm-MOF integrates the emission at 339 nm from the SIP3– ligand and four characteristic 4G5/2 → 4H j (j = 5/2, 7/2, 9/2, and 11/2) transitions at 559, 596, 642, and 701 nm from Sm(III). Sm-MOF as a turn-off fluorescence sensor to QA exhibits high sensitivity, selectivity, and durability. The fluorescence quenching response to QA shows a linear relationship of I 0/I = 0.00496·C QA + 1.12474 in the QA concentration of 0–500 μM and a limit of detection calculated as 4.11 μM. Sm-MOF shows the structural and fluorescent stabilities in five quenching–recovery cycles. The recoveries of close to 100% in human urine and serum indicate high reliability. The paper-based Sm-MOF sensor displays a rough QA quantitative analysis by recognizing red values in the on-site QA detection.
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