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

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+ (D-5(0) -> F-7(J) (J = 1-4)) and/or Tb3+ (D-5(4) -> F-7(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 Ln(3+), 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(Eu)vs. DPA concentration with the limit of detections of 0.248 mu M (3), 0.874 mu M (6), and 2.277 mu M (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.