Turn-on fluorescent probe toward glyphosate and Cr3+ based on Cd(II)-metal organic framework with Lewis basic sites

A microcrystalline Cd(II)-MOF, [Cd(NH2-bdc)(azp)]·DMF (NH2-H2bdc = 2-amino-1,4-benzenedicarboxylic acid and azp = 4,4′-azopyridine), namely 1-NH2 was rationally designed and synthesized as a fluorescent sensor in the off state utilizing the photo-induced electron transfer (PET) process. The crystal structure of 1-NH2 features a porous 2-fold interpenetrated 3D framework with 1D channels decorated with open Lewis basic amino and azo groups for recognizing the target species. Time-dependent density functional theory (TD-DFT) calculations were performed to gain insight into the PET process that induces the fluorescent quenching of 1-NH2. The proposed 1-NH2 is the first MOF that presents highly selective and sensitive visual turn-on fluorescence detection of glyphosate and Cr3+ with very low detection limits of 25 nM and 0.6 μM, respectively. The amino moiety in 1-NH2 plays a key role for the selective binding of Cr3+ to facilitate the sensitivity and selectivity of the detection. The fluorescent enhancement mechanism is associated with the inhibition of the PET process stimulated by the dissociation of the framework of 1-NH2 in the presence of glyphosate and Cr3+.