Aqueous Phase Sensing of Fe3+ and Ascorbic Acid by a Metal-Organic Framework and Its Implication in the Construction of Multiple Logic Gates

A new Hf-IV-based metal-organic framework with UiO-66 topology was synthesized via a one-step solvothermal method by using 3-methyl-4-phenylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2MPTDC) as a ligand. The MOF material showed a high stability in a broad pH range (from pH 2 to pH 12) in an aqueous medium. The presence of hydrophobic methyl and phenyl substituents in the carboxylic acid ligand and strong Hf-O bond play crucial roles in its stability. The new MOF material was systematically characterized by various techniques such as XRPD, N-2 sorption, thermogravimetric analyses and FT-IR spectroscopy. The photophysical properties of the MOF material were also examined by steady-state and time-resolved fluorescence studies. It was observed that the blue fluorescence of the MOF material was selectively quenched in the presence of Fe3+ ion in pure aqueous medium. A mechanistic study disclosed that quenching occurs via a strong inner filter effect (IFE) arising from Fe3+ ion in aqueous medium. Interestingly, the fluorescence of the MOF material can be recovered by elimination of the IFE of Fe3+ ion via reduction of Fe3+ ion by ascorbic acid (AA). Based on the fluorescence recovery by AA, a MOF based on-off-on probe was developed for the sensing of Fe3+ ion and AA in aqueous medium. Inspired by this reversible sensing event, we demonstrate basic (NOT, OR, YES, INHIBIT and IMP) and higher integrated logic operations utilizing this fluorescent MOF. This MOF-based logic systems could be potentially used for next-generation logic-gate based analytical applications as well as for the detection and discrimination of targeted molecules in various complex domains.