Single-Site Metal-Organic Framework and Copper Foil Tandem Catalyst for Highly Selective CO2 Electroreduction to C2 H4

Tandem catalysis is a promising way to break the limitation of linear scaling relationship for enhancing efficiency, and the desired tandem catalysts for electrochemical CO₂ reduction reaction (CO₂ RR) are urgent to be developed. Here, a tandem electrocatalyst created by combining Cu foil (CF) with a single-site Cu(II) metal-organic framework (MOF), named as Cu-MOF-CF, to realize improved electrochemical CO₂ RR performance, is reported. The Cu-MOF-CF shows suppression of CH₄ , great increase in C₂ H₄ selectivity (48.6%), and partial current density of C₂ H₄ at -1.11 V versus reversible hydrogen electrode. The outstanding performance of Cu-MOF-CF for CO₂ RR results from the improved microenvironment of the Cu active sites that inhibits CH₄ production, more CO intermediate produced by single-site Cu-MOF in situ for CF, and the enlarged active surface area by porous Cu-MOF. This work provides a strategy to combine MOFs with copper-based electrocatalysts to establish high-efficiency electrocatalytic CO₂ RR.

Copper(II) Frameworks with Varied Active Site Distribution for Modulating Selectivity of Carbon Dioxide Electroreduction

Metal-organic frameworks (MOFs) can be utilized as electrocatalysts for CO₂ reduction reaction (CO₂RR) due to their well dispersed metal centers. However, the influence of metal node distribution on electrochemical CO₂RR was rarely explored. Here, three Cu-MOFs with different copper(II) site distribution were employed for CO₂ electroreduction. The Cu-MOFs [Cu(L)SO₄]·H₂O (Cu1), [Cu(L)₂(H₂O)₂](CH₃COO)₂·H₂O (Cu2), and [Cu(L)₂(H₂O)₂](ClO₄)₂ (Cu3) were achieved by using the same ligand 1,3,5-tris(1-imidazolyl)benzene (L) but different Cu(II) salts. The results show that the Faraday efficiency of CO (FE_CO) for Cu1 is 4 times that of the FE_H2, while the FE_CO of Cu2 is twice that of the FE_H2. As for Cu3, there is not much difference between FE_CO and FE_H2. Such difference may arise from the distinct electrochemical active surface area and charge transfer kinetics caused by different copper site distribution. Furthermore, the different framework structures also affect the activity of the copper sites, which was supported by the theoretically calculated Gibbs free energy and electron density, contributing to the selectivity of CO₂RR. This study provides a strategy for modulating the selectivity of CO₂RR by tuning the distribution of the active centers in MOFs.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.

Restructuring of Cu2O to Cu2O@Cu-Metal-Organic Frameworks for Selective Electrochemical Reduction of CO2

Electrochemical reduction of carbon dioxide to hydrocarbons, driven by renewable power sources, is a fascinating and clean way to remedy greenhouse gas emission as a result of overdependence on fossil fuels and produce value-added fine chemicals. The Cu-based catalysts feature unique superiorities; nevertheless, achieving high hydrocarbon selectivity is still inhibited and remains a great challenge. In this study, we report on a tailor-made multifunction-coupled Cu-metal-organic framework (Cu-MOF) electrocatalyst by time-resolved controllable restructuration from Cu₂O to Cu₂O@Cu-MOF. The restructured electrocatalyst features a time-responsive behavior and is equipped with high specific surface area for strong adsorption capacity of CO₂ and abundant active sites for high electrocatalysis activity based on the as-produced MOF on the surface of Cu₂O, as well as the accelerated charge transfer derived from the Cu₂O core in comparison with the Cu-MOF. These intriguing characteristics finally lead to a prominent performance towards hydrocarbons, with a high hydrocarbon Faradaic efficiency (FE) of 79.4%, particularly, the CH₄ FE as high as 63.2% (at -1.71 V). This work presents a novel and efficient strategy to configure MOF-based materials in energy and catalysis fields, with a focus on big surface area, high adsorption ability, and much more exposed active sites.

A multifunctional two-photon fluorescent probe for detecting H2S in wastewater and GSH in vivo

A novel turn-on fluorescent probe, which coumarin-fused coumarin was used as fluorophore to link with 2, 4- dinitrodiphenyl ether moiety that has high selectivity and sensitivity for H₂S and thiols, was designed and synthesized. Our probe YB can sensitively react with H₂S and GSH to release free fluorophore with strong fluorescence signals. The probe YB could use as a significant molecular imaging tool to study the endogenous GSH in living cell and a practical detector for H₂S in the papermaking wastewater.

Rapid and highly sensitive detection of extracellular and intracellular H2S by an azide-functionalized Al(iii)-based metal–organic framework

Fast and highly sensitive sensing of both extracellular and intracellular H₂S by an Al(iii)-based CAU-10-N₃MOF is presented.

Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide Based on Metal-Organic Frameworks

The sensing of hydrogen sulfide (H₂S) has become a long-time challenging task. In this work, we developed a general strategy for sensing of H₂S utilizing postsynthetic modification of a nano metal-organic frameworks (MOF) UiO-66-(COOH)₂ with Eu³⁺ and Cu²⁺ ions. The nano MOF Eu³⁺/Cu²⁺@UiO-66-(COOH)₂ displays the characteristic Eu³⁺ sharp emissions and the broad ligand-centered (LC) emission simultaneously. Because H₂S can strongly increase the fluorescence of Eu³⁺ and quench the broad LC emission through its superior affinity for Cu²⁺ ions, the MOF Eu³⁺/Cu²⁺@UiO-66-(COOH)₂ exhibits highly sensitive turn-on sensing of H₂S over other environmentally and biologically relevant species under physiological conditions. Furthermore, this approach for fluorescent turn-on sensing of H₂S is expected to extend to other water-stable MOFs containing uncoordinated -COOH.

Photoluminescent Metal-Organic Frameworks for Gas Sensing

Luminescence of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) is sensitive to the type and concentration of chemical species in the surrounding environment, because these materials combine the advantages of the highly regular porous structures and various luminescence mechanisms, as well as diversified host-guest interactions. In the past few years, luminescent MOFs have attracted more and more attention for chemical sensing of gas-phase analytes, including common gases and vapors of solids/liquids. While liquid-phase and gas-phase luminescence sensing by MOFs share similar mechanisms such as host-guest electron and/or energy transfer, exiplex formation, and guest-perturbing of excited-state energy level and radiation pathways, via various types of host-guest interactions, gas-phase sensing has its unique advantages and challenges, such as easy utilization of encapsulated guest luminophores and difficulty for accurate measurement of the intensity change. This review summarizes recent progresses by using luminescent MOFs as reusable sensing materials for detection of gases and vapors of solids/liquids especially for O₂, highlighting various strategies for improving the sensitivity, selectivity, stability, and accuracy, reducing the materials cost, and developing related devices.

Cyclodextrin-Based Metal-Organic Nanotube as Fluorescent Probe for Selective Turn-On Detection of Hydrogen Sulfide in Living Cells Based on H2S-Involved Coordination Mechanism

Hydrogen sulfide (H₂S) has been considered as the third biologically gaseous messenger (gasotransmitter) after nitric oxide (NO) and carbon monoxide (CO). Fluorescent detection of H₂S in living cells is very important to human health because it has been found that the abnormal levels of H₂S in human body can cause Alzheimer’s disease, cancers and diabetes. Herein, we develop a cyclodextrin-based metal-organic nanotube, CD-MONT-2, possessing a {Pb₁₄} metallamacrocycle for efficient detection of H₂S. CD-MONT-2′ (the guest-free form of CD-MONT-2) exhibits turn-on detection of H₂S with high selectivity and moderate sensitivity when the material was dissolved in DMSO solution. Significantly, CD-MONT-2′ can act as a fluorescent turn-on probe for highly selective detection of H₂S in living cells. The sensing mechanism in the present work is based on the coordination of H₂S as the auxochromic group to the central Pb(II) ion to enhance the fluorescence intensity, which is studied for the first time.

Gas sensing using porous materials for automotive applications

The use of zeolites and metal–organic frameworks in the sensing of gases emitted from automobile exhausts is reviewed.

Multifunctional water-soluble luminescent carbon dots for imaging and Hg2+ sensing

We propose an ingenious method for large-scale fabrication of water-soluble photoluminescent carbon dots (CDs) by a one-step microwave route in the presence of citric acid and ethylenediamine. In contrast to other CDs-based nanomaterials, the CDs prepared exhibit a highly fluorescent quantum yield (QY) and excellent stability in both organic and inorganic phases. After simple post-treatment, the CDs can be used as a new type of fluorescent ink for information storage and nanofiber electrospinning. It should be noted that the CDs are a superior fluorescent bioimaging agent in cells, plants and animals according to their excellent solubility and ultra-low toxicity. In addition, the CDs could be utilized as a modification-free biosensor reagent capable of detecting Hg²⁺ in complex environments. More significantly, environmental friendly "skillful pens" were fabricated that provided an effective platform for portable qualitative-detection of Hg²⁺.

Direct printing synthesis of self-organized copper oxide hollow spheres on a substrate using copper(II) complex ink: gas sensing and photoelectrochemical properties

The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu(II) complex ink with controllable high viscosity based on formate-amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 μm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.