Flexible Hexacarboxylate-Based Cobalt Metal–Organic Framework for the Turn-On/Off Detection of Cr³⁺, Al³⁺, Fe³⁺, and Phenylglyoxylic Acid

Abstract

Two novel cobalt(II)-based metal–organic frameworks (MOFs), namely, {[Co3(TTHA)(bibp)2(H2O)]}n (1) and {[Co2(H2TTHA)(bidpe)2]}n (2) (H6TTHA = 1,3,5-triazine-2,4,6-triamine hexaacetic acid, bibp = 4,4′-bis(imidazolyl)biphenyl, bidpe = 4,4-bis(imidazoly)diphenyl ether), were hydrothermally prepared based on the mixed ligands strategy and fully characterized. Due to the rich coordination modes of the carboxylic acid ligand, 1 possesses a three-dimensional (3D) structure with two secondary building units ([Co3N3(CO2)4(CO)4] and [Co2N2(CO2)4]), whereas 2 exhibits a tangled one-dimensional (1D) chain structure and further forms a 3D structure assembled by weak interactions. Interestingly, 1 can serve as an unusual turn-on sensor for the detection of trivalent metal ions (Al3+, Cr3+, and Fe3+) in an aqueous solution with a low detection limit, and 2 shows high-sensitivity fluorescence turn-off behavior toward polystyrene and styrene biomarker phenylglyoxylic acid (PGA). Significantly, 2 can effectively sense the PGA in complicated biological environments. The mechanism study indicated that the absorbance-caused enhancement mechanism is responsible for the fluorescence turn-on phenomena of 1 induced by trivalent metal ions, while the occurrence of the structural collapse of 2 leads to a turn-off effect toward PGA.