Fabrication of ultrathin 2D MOF nanosheets for folic acid detection

Metal organic frameworks (MOFs) {[Zn2(L1)2(SDB)2]·H2O}n (MOF1), [Zn2(L2)(SDB)2]n (MOF2) and [Zn2(L3)(hfipbb)2]n (MOF3) (L1 = 4,4′-(2,5-dimethyl-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine), L2 = (E,E,E)-1,6-bis(4-pyridyl)-l,3,5-hexatriene, L3 = 4,4′-((2-methyl-1,4-phenylene)bis(ethene-2,1-diyl))bipyridine, H2SDB = 4,4′-sulfonyldibenzoic acid, H2hfipbb = 2,2-bis(4-carboxyphenyl) hexafluoropropane) were prepared by hydrothermal synthesis. Weak van der Waals forces were observed between 2D layers in MOF1 and MOF2. The weak interlayer interactions could be cleaved by external mechanical force and bulk samples were exfoliated into single-layer two-dimensional nanosheets by ultrasound in ethanol. Single-layer MOF nanosheets which had a lateral size of approximately 2 µm were generated after ultrasound treatment of MOF1 and MOF2. However, 3D coordination network of MOF3 could not be successfully exfoliated because of its different network structure. MOF1 and MOF2 nanosheets dispersed in ethanol displayed strong photoluminescence (PL), which could be quenched specifically by folic acid (FA) through a concentration dependent manner. This hypersensitive and fast luminescence sensing benefited from numerous active sites and broad specific surface area of these ultrathin nanosheets.