Construction of Ln3+-Substituted Arsenotungstates Modified by 2,5-Thiophenedicarboxylic Acid and Application in Selective Fluorescence Detection of Ba2+ in Aqueous Solution

Developing organic-inorganic hybrid lanthanide(Ln)-functionalized polyoxometalates (POMs) has long been a main research topic. In this article, two novel hexameric Ln-containing arsenotungstates modified by 2,5-thiophenedicarboxylic acid (H2tpdc) [H2N(CH3)2]21Na13{Ln6W14O40(H2O)18(Htpdc)2}[B-α-AsW9O33]6·nH2O [Ln = Eu3+, n = 94 (1Eu); Ln = Gd3+, n = 114 (2Gd)] were successfully obtained, and the sulfur-heterocyclic carboxylic acid ligand was for the first time introduced to the POM system. Both polyoxoanion skeleton structures can be regarded as an aggregation of six trivacant Keggin [B-α-AsW9O33]9- entities linked by a S-shaped [Ln6W14O40(H2O)18(Htpdc)2]20- cluster. More surprisingly, two carboxyl O atoms in Htpdc- groups directly coordinate with two W atoms other than Ln atoms, which is rarely seen in tungsten-oxo clusters. Fluorescence properties of 1Eu in solid state and aqueous solution have been systematically investigated. Also, 1Eu as a fluorescence sensor in aqueous solution toward the detection of the Ba2+ ion has been explored. As expected, 1Eu may be a promising fluorescence probe for the Ba2+ detection in the absence of Ca2+ or Sr2+ ions in aqueous environment with high sensitivity, good selectivity, and low detection limit (1.19 × 10-3 mM). This work not only first introduces the S-heterocyclic carboxylic acid ligand into the POM system and enriches the types of organic ligands which can be used in POM chemistry but also broadens the POMs' application in fluorescence detection.