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

Fluorescent Co(II)-based MOFs, [Co-2(Bpy)(Oba)(2)], nanoflakes were successfully synthesized via a (NH4)(6)Mo7O24 center dot 4H(2)O assisted hydrothermal route at 180 degrees C for 12 h, employing Co(NO3)(2)center dot 6H(2)O, 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)(6)Mo7O24 center dot 4H(2)O 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.