Ammonium molybdate-assisted shape-controlled synthesis of fluorescent Co(II)-based MOFs nanoflakes as highly-sensitive probes for selective detection of vanillin in milk powders

Abstract Fluorescent Co(II)-based MOFs, [Co2(Bpy)(Oba)2], nanoflakes were successfully synthesized via a (NH4)6Mo7O24∙4H2O assisted hydrothermal route at 180 °C for 12 h, employing Co(NO3)2∙6H2O, 4,4′-Oxybis(benzoic acid) (Oba) and 4,4′-Bipyridine (Bpy) as the initial reactants. The as-obtained Co-MOFs nanoflakes were characterized by XRD, FESEM, and FTIR. It was found that the presence of (NH4)6Mo7O24∙4H2O could markedly promote the formation of Co-MOFs nanoflakes. The PL studies showed that as-obtained Co-MOFs nanoflakes emit a violet light at 440 nm under the UV light excitation of 297 nm; and that the morphology of Co-MOFs did not affect the PL intensity. Interestingly, the fluorescence of as-obtained Co-MOFs nanoflakes was highly sensitive to vanillin, an important food additive, and could be selectively quenched by vanillin in aqueous systems. Simultaneously, the fluorescence quenching mechanism was discussed.