A Biomimetic Nose by Microcrystals and Oriented Films of Luminescent Porous Metal–Organic Frameworks

Abstract

Guest-free and porous [In(OH)(bdc)]n (bdc =1,4-benzenedicarboxylate) microcrystals were produced by heating indium(III) nitrate hydrate (In(NO3)3·xH2O) and terephthalic acid (H2bdc) in N,N-dimethylformamide (DMF) with the addition of ethyl acetate as a crystal habit modifier at 100 °C for 10 h and then vacuum drying them in an oven at 250 °C for 48 h to remove lattice DMF. These microcrystals were monodispersed and hexagonal rod-like in shape with a length of 30 μm, an aspect ratio of 7.5, a BET surface area of 1148.74 m2/g, and the total pore volume of 0.57 cm3/g. The oriented films of those hexagonal rods were fabricated first through the self-assembly of as-synthesized [In(OH)(bdc)·2DMF]n microcrystals at the water/n-heptane interface and then followed by the removal of DMF upon vacuum drying in an oven at 250 °C for 48 h. The stereochemical sensing capability of both guest-free and porous [In(OH)(bdc)]n microcrystals and oriented films toward odorants emanated from the analytes such as cumin, cinnamon, vanillin, p-xylene, m-xylene, o-xylene, water, and ethanol, was successfully transduced to the first set of photoluminescence (PL) emission responses. The original emission of the guest-free, porous [In(OH)(bdc)]n framework might be attributed to the ligand-to-metal charge transfer (LMCT). The inclusions of guest odorant molecules in the guess-free, porous [In(OH)(bdc)]n microcrystals could have quenched the excitons through: (1) delocalization over the conjugated polymer backbone, (2) interchain energy migration in the solid state, and (3) a highly organized molecular stacking structure, attributed to the pore confinement of the analyte inside the molecular-sized cavities of [In(OH)(bdc)]n framework which facilitated strong interactions between the analyte (or the odorant) and the host framework. Therefore, specific guest–host stereochemical interactions would dictate the characteristic quenching response of a given analyte. Red shifts were observed for cumin-, cinnamon-, vanillin-, p-xylene-, m-xylene-, o-xylene-, water-, and ethanol-adsorbed samples of [In(OH)(bdc)]n microcrystals, with emission λmax at 390, 425, 343, 428, 422, 400, 390, and 389 nm respectively, upon excitation at 270 nm in solid state. A second set of PL emission responses of the same analytes produced from the analyte-adsorbed Zn4O(bdc)3 (MOF-5) microcrystals for a demonstration purpose was also determined and coplotted with the first set of PL emission values to construct a 2D map of PL emission responses for our MOF-based “biomimetic nose”. The biomimetic nose could distinguish the odors of the analytes based on a pattern recognition method (i.e., principal component analysis) because on the 2D map of PL emission responses, ethanol, m-xylene, o-xylene, vanillin, cumin, p-xylene, cinnamon, and water, were represented by a line, a point, a point, a point, a line, a point, a rectangle and a point respectively.