Pyrazine functionalization to boost the antenna effect in rare-earth metal–organic frameworks for tetracycline detection

Abstract

Here we report a generalizable strategy for the synthesis of rare-earth metal–organic frameworks (RE-MOFs) with 12-connected RE₉ clusters and shp topology. A total of 26 isostructural RE-MOFs (JNU-205-RE and JNU-206-RE) were obtained from two X-shaped tetracarboxylate linkers, 4,4′,4′′,4′′′-benzene-2,3,5,6-tetrayl-tetrabenzoate (BTEB) and 4,4′,4′′,4′′′-pyrazine-2,3,5,6-tetrayl-tetrabenzoate (BTTB). Among them, the two europium-based RE-MOFs (JNU-205-Eu and JNU-206-Eu) were observed to show characteristic red luminescence of Eu³⁺, indicating the antenna effect of both linkers for Eu³⁺ sensitization. Interestingly, the one constructed with a pyrazine-centered BTTB linker (JNU-206-Eu) exhibited a luminescence quantum yield 120 times that of the one constructed with a benzene-centered BTEB linker (JNU-205-Eu). We suspect pyrazine functionalization renders the antenna linker not only more electron rich but also with better conjugation, facilitating energy transfer from the linker to the metal and therefore leading to efficient Eu³⁺ sensitization. As a result, the luminescence quenching sensitivity of JNU-206-Eu for tetracyclines (TCs) was improved by a factor of 3 with a detection limit comparable to those of the best-performing MOF materials reported so far. Experimental data and theoretical calculations suggest the quenching mechanism be mainly ascribed to the internal filtration effect (IFE). Furthermore, a mixed matrix film by dispersion of JNU-206-Eu crystalline powders on a polycaprolactone sheet was applied to demonstrate rapid and visualizable luminescence response towards TCs with good reusability in aqueous solutions.