To design a robust, π-conjugate, low cost and easy to synthesize MOF for cation sensing by photoluminescence (PL) method, H2OBA (4,4'-oxybis(benzoic acid)) has been used in combination with DPT, (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine) as a tetrazine functionalized spacer to construct [Zn(OBA)(DPT)0.5]•DMF (TMU-34(-2H)). Tetrazine motif is a relatively weak sigma-donor with Lewis basic sites, π-conjugated, and water-soluble/stable fluorophore. 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 both solvents reveal that TMU-34(-2H) can selectively detect Hg2+ in water (by 243% enhancement) and in acetonitrile (by 90% quenching). Contribution of electron donating/accepting characteristics along with solvation effects at secondary interactions of tetrazine motif inside the TMU-34(-2H) framework resulted in different signal transductions. Improved sensitivity and accuracy of detection was obtained using double solvent sensing method (DSSM) in which different signal transductions of TMU-34(-2H) in water and acetonitrile combined simultaneously to construct double solvent sensing curve and formulate sensitivity factor. Calculation of sensitivity factors for all cations has demonstrated that it is possible to detect Hg2+ by DSSM with ultrahigh sensitivity. Such a tremendous distinction in Hg2+ sensitivity factor is visualizable in double solvent sensing curve. So, by applying double solvent sensing method instead of 1D sensing, the interfering effects of others cations are completely eliminated and sensitivity toward Hg(II) is highly improved. Strong interactions between Hg2+ and nitrogen atoms of tetrazine groups along with easy accessibility of Hg2+ to tetrazine groups lead to a shorter response time (15 s) in comparison with other Hg2+ MOF based sensors.
