A Co-based mixed-ligand metal-organic framework (MOF), ZJU-109, is reported by utilizing 6-(4-pyridyl)-terephthalic acid (H2pta) and 4,4'-bis(imidazolyl)biphenyl (4,4'-bimbp) as double linkers. By tuning the d-PET process between ligands 4,4'-bimbp and H2pta, a fluorescent turn-on sensor for Fe3+ with a detection limit as low as 0.053 μM has been demonstrated.