Ratiometric fluorescence detection of anthrax biomarker 2,6-dipicolinic acid using hetero MOF sensors through ligand regulation

Abstract

2,6-Dipicolinic acid (DPA) is an anthrax biomarker, whose detection is in great need. This work presented nine MOFs of three series containing three ligands of 2,6-naphthalene-dicarboxylic acid (H₂NDC), 4,4′-biphenyldicarboxylic acid (H₂BPDC), and 1,4-benzenedicarboxylate acid (H₂BDC): six single Ln-MOFs (Ln = Eu³⁺ (1, 4 and 7) and Tb³⁺ (2, 5 and 8)) and three hetero Eu/Tb-MOFs (3, 6, and 9). This work aimed at ligand regulation of the fluorescence properties and DPA detection. The solid-state emissions of 3, 6, and 9 can be assigned to the characteristic transitions of Eu³⁺ (⁵D₀ → ⁷F_J (J = 1–4)) and/or Tb³⁺ (⁵D₄ → ⁷F_J (J = 6–3)). The hetero MOFs were applied as fluorescence sensors in DPA detection. The fluorescence and DPA detection performances of 3, 6 and 9 are related to the energy differences between the ligands/DPA and Ln³⁺, showing the effect of ligand energy regulation and the competition between the three ligands and DPA. The detection performances of 3, 6 and 9 show linear responses of I_Tb/I_Euvs. DPA concentration with the limit of detections of 0.248 μM (3), 0.874 μM (6), and 2.277 μM (9). Seven interferents did not affect the DPA detection using 3, 6 and 9. Time-dependent fluorescence measurement suggested that 3, 6 and 9 responded very fast to DPA. High recoveries in the DPA detection of 3, 6 and 9 in human serum (93.96 to 108.84%) indicated their high reliabilities. The paper-based MOF sensors of 3, 6 and 9 could display emission color changes depending on C_DPA, which offers a fast field detection method for DPA.