Highly stable lanthanide(III) metal-organic frameworks as ratiometric fluorescence sensors for vitamin B₆

Three lanthanide(III)-based metal-organic frameworks, formulated as [(CH₃)₂NH₂]₂[Ln₆(μ₃-OH)₈(EBTC)₃(H₂O)₆]·4H₂O·2DMF (Ln = Eu (1), Tb (2) and Ce (3)), were synthesized using a rigid tetracarboxylate organic ligand (1,1'-ethynebenzene-3,3',5,5'-tetracarboxylic acid, H4EBTC). Complexes 1-3 possess 12-connected hexanuclear [Ln₆(μ₃-OH)₈(OOC-)₁₂(H₂O)₆] clusters with the ftw topology, which were stable in water and acid/alkaline aqueous solution. Due to the antenna effect, complexes 1 and 2 presented double fluorescence emission peaks, which are the characteristic emission peaks of Ln³⁺ ions and the ligand H4EBTC, respectively. The doped bimetallic EuTb_1---MOFs were obtained by tuning the Eu(III)/Tb(III) ratio during the reaction, which exhibited a colour change from red, orange, and yellow to green. Furthermore, complexes 1, 2 and Eu2Tb8-MOF as ratiometric fluorescence sensors exhibited excellent sensing ability for vitamin B₆ (VB₆) in phosphate buffer solution (pH = 7.35) and real samples with high selectivity and reusability. The low detection limit (LOD) values were calculated to be 1.03 μM for complex 1, 0.25 μM for complex 2 and 0.11 μM for Eu2Tb8-MOF in aqueous solution. Finally, a visual film based on Ln-MOF@SA was prepared to detect VB₆ with high reusability.