A dual-emissive Ln-MOF as a visual dipicolinic acid sensor of high sensitivity and selectivity

Dipicolinic acid (DPA) is a valuable biomarker for the detection of Bacillus anthracis, but its detection still faces challenges such as enhancing sensitivity and selectivity. Herein, biphenyl-3,3′,5,5′-tetracarboxylic acid (H₄BPTC), possessing several carboxyl groups, and large conjugated system, was selected as ligand to synthesize a mixed-lanthanide metal-organic framework (MOF), Tb_xEu_1−x-BPTC, for an effective DPA sensor. The optimal sample, Eu_0.549Tb_0.451-BPTC, demonstrated excellent water and thermal stability. Luminescence investigations revealed Tb³⁺ emissions intensified while Eu³⁺ emissions dimmed towards addition of DPA. The detection mechanism was due to inner filter effects, inhibition of energy transfer from Tb³⁺ to Eu³⁺ and chelation by DPA. Eu_0.549Tb_0.451-BPTC showed remarkable sensitivity (LOD = 1.68 μM) and excellent selectivity when detecting DPA. To expand the application prospect, Eu_0.549Tb_0.451-BPTC was mixed with agarose to form a MOF-based mixed matrix membrane. This flexible and portable membrane can achieve visual detection of DPA by analyzing RGB values using smartphone. This work offers an effective method to ascertain the presence of DPA in practical application scenario.