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) (H2OBA) 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)0.5]·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 Hg2+ 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 Hg2+ by DSSM with ultrahigh sensitivity. Such a tremendous distinction in the Hg2+ 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 Hg2+ and the nitrogen atoms of the tetrazine groups along with easy accessibility of Hg2+ to the tetrazine groups lead to a shorter response time (15 s) in comparison with other MOF-based Hg2+ sensors.