Properties of metal–organic frameworks (MOFs) are determined by metal centers, organic ligands, and applied synthesis methods. For electrosynthesis of MOFs, the applied potential is expected to play a key role in determining the morphology, thickness, electrochemical properties, and applications of MOFs. Herein, Cu-BTC (H3BTC: 1,3,5-benzenetricarboxylic acid) films are electrosynthesized at different cathodic potentials. They feature different morphologies, thicknesses, and amounts of active copper centers, although they do show similar bonding properties, chemical compositions, phase purity, crystallinity, and surface electronic states of copper centers. Using nicotine amide adenine dinucleotide, the sensing application of these Cu-BTC films is explored, showing potential-dependent catalytic ability. Further monitoring of six other organic compounds (herein xanthine, hypoxanthine, diethylstilbestrol, estradiol, sunset yellow, and tartrazine) reveals the morphology and thickness of Cu-BTC films and the amount of copper centers inside these Cu-BTC films determines the accumulation or sensing ability of Cu-BTC films. Highly sensitive detection of these molecules individually and simultaneously is achieved with Cu-BTC electrosynthesized at −1.30 V. Electrosynthesized Cu-BTC films are thus excellent electrode materials for sensitive sensing of various analytes.
The properties and applications of metal-organic frameworks (MOFs) can be tuned by their metal centers and organic ligands. To reveal experimentally and theoretically the influence of metal centers and ligands on electrochemical performance of MOFs, three MOFs with copper or zinc centers and organic ligands of 2-methylimidazole (2MI) or 1,3,5-benzenetricarboxylic acid (H 3 BTC) are synthesized and characterized in this study. 2D and porous Cu-2MI exhibits a larger active area, faster electron transfer capability, and stronger adsorption capacity than bulk Cu-BTC and dodecahedron Zn-2MI. Density functional theory calculations of adsorption ability of three MOFs toward xanthine (XA), hypoxanthine (HXA), and malachite green (MG) prove that 2D Cu-2MI has the strongest adsorption energies to three targets. Rotating disk electrode measurements reveal that 2D Cu-2MI features the biggest intrinsic heterogeneous rate constant toward three analytes. On 2D Cu-2MI sensitive and selective monitoring of XA, HXA, and MG is then achieved using differential pulse voltammetry. Their monitoring in real samples on 2D Cu-2MI is accurate and comparable with that using high-performance liquid chromatography. In summary, regulation of electrochemical sensing features of MOFs is realized through defining selected metal centers and organic ligands.
Zhiqun Xie,Jiali Zhou,Jinnan Wang,Corvini Philippe François-Xavier,Thomas Wintgens
Novel Fenton-like catalyst γ-Cu-Al 2 O 3 -Bi 12 O 15 Cl 6 with electron-poor Cu centre and electron-rich Bi centre was synthesized for selective degradation of phenolic compounds. Different from electron-rich Cu centre catalysts, the electron-poor Cu centre, which was induced by high electronegativity of Bi, could facilitate the formation of σ-Cu-ligand with phenolic compounds. On one hand, H 2 O 2 could directly oxidize σ-Cu-ligand to HO-adduct radicals with the generation of OH. Meanwhile, Cu(II) in the σ-Cu(II) complexes could be reduced to Cu(I) by oxidization of the HO-adduct radicals to hydroxylation products. Such reaction not only prevented Cu(II) from oxidizing H 2 O 2 to HO 2 /O 2 − but also enhanced the redox cycle of Cu(II)/Cu(I), which improved the catalytic activity and H 2 O 2 utilization. On the other hand, although the σ-Cu-ligand was gradually decreased with the decrease of phenolic compounds, dual-reaction centre played the dominant role in catalytic reaction . In addition, large amounts of oxygen vacancies (OVs) are formed in Bi 12 O 15 Cl 6 during the calcining process (BiOCl→Bi 12 O 15 Cl 6 ) , which can also increase the generation of OH. Thus, both electron-rich Bi center and OVs could still reduce H 2 O 2 to · OH for organics degradation even without the formation of σ-Cu-ligand. Being attributed to the synergistic effect of σ-Cu-ligand, dual-reaction centre and OVs mentioned above, γ-Cu-Al 2 O 3 -Bi 12 O 15 Cl 6 achieved high catalytic activity for phenolic compounds degradation and utilization efficiency of H 2 O 2 (η). In addition, various analytic methods (XPS, TPR, FT-IR, Raman, EPR and solid-state EPR) were used to prove the synergistic effect of the electron-poor/rich centre, σ-Cu-ligand and OVs of catalyst.
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