Double Solvent Sensing Method for Improving Sensitivity and Accuracy of Hg(II) Detection Based on Different Signal Transduction of a Tetrazine-Functionalized Pillared Metal-Organic Framework.

To design a robust, π-conjugated, low-cost, and easy to synthesize metal-organic framework (MOF) for cation sensing by the photoluminescence (PL) method, 4,4'-oxybis(benzoic acid) (HOBA) has been used in combination with 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (DPT) as a tetrazine-functionalized spacer to construct [Zn(OBA)(DPT)]·DMF (TMU-34(-2H)). The tetrazine motif is a π-conjugated, water-soluble/stable fluorophore with relatively weak σ-donating Lewis basic sites. These characteristics of tetrazine make TMU-34(-2H) a good candidate for cation sensing. Because of hydrogen bonding between tetrazine moieties and water molecules, TMU-34(-2H) shows different PL emissions in water and acetonitrile. Cation sensing in these two solvents revealed that TMU-34(-2H) can selectively detect Hg in water (by 243% enhancement) and in acetonitrile (by 90% quenching). The contribution of electron-donating/accepting characteristics along with solvation effects on secondary interactions of the tetrazine motifs inside the TMU-34(-2H) framework results in different signal transductions. Improved sensitivity and accuracy of detection were obtained using the double solvent sensing method (DSSM), in which different signal transductions of TMU-34(-2H) in water and acetonitrile were combined simultaneously to construct a double solvent sensing curve and formulate a sensitivity factor. Calculation of sensitivity factors for all of the tested cations demonstrated that it is possible to detect Hg by DSSM with ultrahigh sensitivity. Such a tremendous distinction in the Hg sensitivity factor is visualizable in the double solvent sensing curve. Thus, by application of DSSM instead of one-dimensional sensing, the interfering effects of other cations are completely eliminated and the sensitivity toward Hg(II) is highly improved. Strong interactions between Hg and the nitrogen atoms of the tetrazine groups along with easy accessibility of Hg to the tetrazine groups lead to a shorter response time (15 s) in comparison with other MOF-based Hg sensors.