Here we reveal the effects of hydrogen bonds and alkyl groups on the structure and emission of covalent organic frameworks (COFs). Hydrogen bonds improve molecular rigidity leading to high ...crystallinity and restrict intramolecular rotation to enhance the emission of COFs. An excited-state intramolecular proton transfer (ESIPT) effect for dual emission is achieved
via
the intramolecular hydrogen bonds between hydroxyl groups and imine bonds. Alkyl groups increase interlayer spacing as a natural "scaffold" and achieve a staggered AB stacking mode to decrease aggregation-caused quenching. Based on the above guidance, COF-4-OH with strong emission is prepared with 2,4,6-triformylphloroglucinol (TFP) and 9,9-dibutyl-2,7-diaminofluorene (DDAF). Strong dual emission is observed and used to differentiate organic solvents with different polarities, to determine the water content in organic solvents, and to detect different pH levels. Our work serves as a guide for the rational design of functional monomers for the preparation of emissive COFs.
The introduction of hydroxyl groups and
n
-butyl groups into COF-4-OH for the construction of COFs with strong dual emission was demonstrated.
Multi‐emission materials have come to prominent attention ascribed to their extended applications other than single‐emission ones. General and robust design strategies of a single matrix with ...multi‐emission under single excitation are urgently required. Metal–organic frameworks (MOFs) are porous materials prepared with organic ligands and metal nodes. The variety of metal nodes and ligands makes MOFs with great superiority as multi‐emission matrices. Guest species encapsulated into the channels or pores of MOFs are the additional emission sites for multi‐emission. In this review, multi‐emission MOFs according to the different excitation sites are summarized and classified. The emission mechanisms are discussed, such as antenna effect, excited‐state intramolecular proton transfer (ESIPT) and tautomerism for dual‐emission. The factors that affect the emissions are revealed, including ligand–metal energy transfer and host–guest interaction, etc. Multi‐emission MOFs could be predictably designed and prepared, once the emissive factors are controlled rationally in combination with the different multi‐emission mechanisms. Correspondingly, new and practical applications are realized, including but not limited to ratiometric/multi‐target sensing and bioimaging, white light–emitting diodes, and anti‐counterfeiting. The design strategies of multi‐emission MOFs and their extensive applications are reviewed. The results will shed light on other multi‐emission systems to develop the structure‐derived functionality and applications.
The variety of metal nodes, ligands, and guest molecules provide the potential emission centers to endow metal–organic frameworks (MOFs) with great potential as a multi‐emission matrix. In this review, the multi‐emission MOFs excited under a single wavelength are summarized and classified according to the different excitation sites. Furthermore, the design concept, and application of multi‐emission MOFs are summarized.
Carbon dots (C‐dots) are generally separated into graphene quantum dots (GQDs) and carbon nanodots (CNDs) based on their respective top‐down and bottom‐up preparation processes. However, GQDs can be ...prepared by carbonization of small‐molecule precursors as revealed with unconventional preparation strategies. Thus, it is their structures rather than their precursors and preparation strategy that govern whether C‐dots are GQDs or CNDs. Here, the composites, structure, and electronic properties of C‐dots are discussed. C‐dots generally consist of a graphite‐like core and amorphous oxygen‐containing shell. When graphite becomes C‐dots, its conduction and valence bands are separated, and the quantum confinement effect appears. Combined with the light‐harvesting ability inherited from graphite, electrons in the core of C‐dots are transferred from conduction to valence bands, leading to electron–hole pair formation upon light excitation. The photoexcitation activities, such as photovoltaic conversion, photocatalysis, and photodynamic therapy, are influenced by the electronic properties of the core. Different to the semiconductor properties of core, the C‐dot shell is electrochemically active, leading to electrochemiluminescence (ECL). The oxygen‐containing groups in shell can conjugate to functional species for use in imaging and therapy. The applications of C‐dots beyond photoluminescence, including ECL, solar photovoltaics, photocatalysis, and theranostics, are reviewed.
Carbon dots are reviewed in terms of their unconventional preparation strategies and applications beyond photoluminescence, including electrochemiluminescence, solar photovoltaics, photocatalysis, and theranostics. With the discussions, the dualities of carbon dots are revealed.
•Metal–organic framework (MOF)-based electrochemical applications are reviewed.•The active MOFs are designed with active ligands, metals, or complexes.•Redox-active guest molecules, enzymes, and ...nanoparticles facilitate the electrochemical activity of MOFs.•Conductive ligands and metal nodes are chosen to improve the conductivity.•Integrating guest molecules or mixing with electrical conductors is the alternative.
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Electrochemical devices are fast, sensitive, accurate, and convenient as sensing and mass or energy conversion platforms. Metal–organic frameworks (MOFs) attract much attention because of their intriguing properties. MOFs have great potential for electrochemical applications, but two challenges are confronted, including the design of redox-active MOFs (ra-MOFs) and the improvement of MOF conductivity. Redox or catalytic active sites can be introduced using active ligands or metal ions, and active complexes as building blocks. MOFs can also hold active guest molecules, enzymes, bacteria, and nanoparticles and promote electrochemical activity. In order to improve MOF conductivity, conductive ligands and metal nodes are rational choices to form long-range delocalized electrons for charge mobility. Integrating guest molecules or mixing with electrical conductors is the alternative. The MOF film attached on the substrate surface facilitates direct electron transfer and device building. High sensitivity and selectivity of MOF-based electrochemical sensors and improved mass or energy conversion are anticipated.
Here an excellent trimodality imaging‐guided synergistic photothermal therapy (PTT)/photodynamic therapy (PDT)/chemodynamic therapy (CDT) is proposed. To this end, a mixed‐metal Cu/Zn‐metal‐organic ...framework (MOF) is first assembled at room temperature on a nano‐scale. Interestingly, heating the MOF results in a Cu+/2+‐coexisting hollow porous structure. Subsequent heating treatment is used to integrate Mn2+ and MnO2 in the presence of manganese(II) acetylacetonate. The hollow composite achieves efficient loading of a photosensitizer, indocyanine green (ICG). Under laser irradiation, the aggregated ICG achieves photothermal imaging and PTT. Once released in the tumor site, ICG exhibits fluorescence imaging and PDT capacity. Cu+/Mn2+ ions perform Fenton‐like reaction with H2O2 to produce cytotoxic •OH for the enhanced CDT. Cu2+/MnO2 scavenge glutathione to improve the reactive oxygen species‐based therapy, while the formed Mn2+ ions enable “turn on” magnetic resonance imaging. Significantly, O2 is produced from the catalytic decomposition of endogenous H2O2 to improve ICG‐mediated PDT. Moreover, photothermal‐induced local hyperthermia accelerates •OH generation to enhance CDT. This synergistic drug‐free antitumor strategy realizes high treatment efficacy and low side effects on normal tissues. Thus, this mixed‐metal MOF is an efficient strategy to realize hollow structures for multi‐function integration to improve therapeutic capacity.
A mixed‐metal metal‐organic framework strategy is proposed to form a hollow structure, integrating mixed‐metals with mixed‐valences, and loading indocyanine green. The composite enables photothermal/magnetic resonance/fluorescence imaging‐guided photothermal therapy, photodynamic therapy, and chemodynamic therapy. High therapeutic efficiency and negligible side effects are realized as non‐drug treatment.
Integrating thermodynamically favorable ethanol reforming reactions with hybrid water electrolysis will allow room‐temperature production of high‐value organic products and decoupled hydrogen ...evolution. However, electrochemical reforming of ethanol has not received adequate attention due to its low catalytic efficiency and poor selectivity, which are caused by the multiple groups and chemical bonds of ethanol. In addition to the thermodynamic properties affected by the electronic structure of the catalyst, the dynamics of molecule/ion dynamics in electrolytes also play a significant role in the efficiency of a catalyst. The relatively large size and viscosity of the ethanol molecule necessitates large channels for molecule/ion transport through catalysts. Perforated CoNi hydroxide nanosheets are proposed as a model catalyst to synergistically regulate the dynamics of molecules and electronic structures. Molecular dynamics simulations directly reveal that these nanosheets can act as a “dam” to enrich ethanol molecules and facilitate permeation through the nanopores. Additionally, the charge transfer behavior of heteroatoms modifies the local charge density to promote molecular chemisorption. As expected, the perforated nanosheets exhibit a small potential (1.39 V) and high Faradaic efficiency for the conversion of ethanol into acetic acid. Moreover, the concept in this work provides new perspectives for exploring other molecular catalysts.
Nanoporous ultrathin bimetallic compound sheets are used as a model catalyst to realize synergistic optimization of ethanol molecular spatial distribution and chemisorption. They exhibit a small potential (1.39 V) and high Faradaic efficiency for acetic acid.
Superelastic carbon aerogels have been widely explored by graphitic carbons and soft carbons. These soft aerogels usually have delicate microstructures with good fatigue resistance but ultralow ...strength. Hard carbon aerogels show great advantages in mechanical strength and structural stability due to the sp3‐C‐induced turbostratic “house‐of‐cards” structure. However, it is still a challenge to fabricate superelastic hard carbon‐based aerogels. Through rational nanofibrous structural design, the traditional rigid phenolic resin can be converted into superelastic hard carbon aerogels. The hard carbon nanofibers and abundant welded junctions endow the hard carbon aerogels with robust and stable mechanical performance, including superelasticity, high strength, extremely fast recovery speed (860 mm s−1), low energy‐loss coefficient (<0.16), long cycle lifespan, and heat/cold‐endurance. These emerging hard carbon nanofiber aerogels hold a great promise in the application of piezoresistive stress sensors with high stability and wide detection range (50 kPa), as well as stretchable or bendable conductors.
A family of hard carbon aerogels with nanofibrous structure templated by various nanofibers is fabricated, displaying robust and stable mechanical performances, including high strength, extremely fast recovery speed (860 mm s−1), and ultralow energy loss coefficient (<0.16). After being compressed for 104 cycles (50% strain), they show only ≈2% plastic deformation and retain ≈93% stress.
As the typical unconventional reservoir, shale gas is believed to be the most promising alternative for the conventional resources in future energy patterns, attracting more and more attention ...throughout the world. Generally, the majority of shale gas is trapped within the tight shale rock with ultralow porosity (<10%) and ultrasmall pore size (as less as several nanometers). Thus, the accurate understanding of gas transport characteristic and its underlying mechanism through these microporous/nanoporous media is critical for the effective exploitation of shale reservoir. In this context, we present a comprehensive review on the current advances of multiscale transport simulations of shale gas in microporous/nanoporous media from molecular to pore-scale. For the gas transport in shale nanopores using molecular dynamics (MD) simulations, the structure and force parameters of various nanopore models, including organic models (graphene, carbon nanotubes, and kerogen) and inorganic models (clays, carbonate, and quartz), and flow simulation strategies (such as nonequilibrium molecular dynamics (NEMD) and Grand Canonical Monte Carlo simulations) are systematically introduced and clarified. The significant MD simulation results about gas transport characteristic in shale nanopores then are elaborated respectively for different factors, including pore size, ambient pressure, nanopore type, atomistic roughness, and pore structure, as well as multicomponent. Besides, the two-phase transport characteristic of gas and water is also discussed, considering the ubiquity of water in shale formation. For the lattice Boltzmann method (LBM) and pore network model (PNM) approaches to conduct pore-scale simulations, we briefly review its origins, modifications, and applications for gas transport simulations in a microporous/nanoporous shale matrix. Particularly, the upscaling methods to incorporate MD simulation into LBM and PNM frameworks are emphatically expounded in the light of recent attempts of MD-based pore-scale simulations. It is hoped that this Review would be helpful for the readers to build a systematical overview on the transport characteristic of shale gas in microporous/nanoporous media and subsequently accelerate the development of the shale industry.
The construction of biological proton channel analogues has attracted substantial interest owing to their wide potential in separation of ions, sensing, and energy conversion. Here, metal–organic ...framework (MOF)/polymer heterogeneous nanochannels are presented, in which water molecules are confined to disordered clusters in the nanometer‐sized polymer regions and to ordered chains with unique molecular configurations in the 1D sub‐1‐nm porous MOF regions, to realize unidirectional, fast, and selective proton transport properties, analogous to natural proton channels. Given the nano‐to‐subnano confined water junctions, experimental proton conductivities in the polymer‐to‐MOF direction of the channels are much higher than those in the opposite direction, showing a high rectification up to 500 and one to two orders of magnitude enhancement compared to the conductivity of proton transport in bulk water. The channels also show a good proton selectivity over other cations. Theoretical simulations further reveal that the preferential and fast proton conduction in the nano‐to‐subnano channel direction is attributed to extremely low energy barriers for proton transport from disordered to ordered water clusters. This study opens a novel approach to regulate ion permeability and selectivity of artificial ion channels.
Heterogeneous metal–organic framework (MOF)/polymer nanochannels are constructed, where water molecules are confined to ordered chains with unique molecular configurations in the 1D sub‐1‐nm porous MOF regions and to disordered clusters in the nanometer‐sized polymer regions, to realize unidirectional, fast, and selective proton transport properties, analogous to natural voltage‐gated proton channels.
The high fracture toughness of mollusk nacre is predominantly attributed to the structure‐associated extrinsic mechanisms such as platelet sliding and crack deflection. While the nacre‐mimetic ...structures are widely adopted in artificial ceramics, the extrinsic mechanisms are often weakened by the relatively low tensile strength of the platelets with a large aspect ratio, which makes the fracture toughness of these materials much lower than expected. Here, it is demonstrated that the fracture toughness of artificial nacre materials with high inorganic contents can be improved by residual stress‐induced platelet strengthening, which can catalyze more effective extrinsic toughening mechanisms that are specific to the nacre‐mimetic structures. Thereby, while the absolute fracture toughness of the materials is not comparable with advanced ceramic‐based composites, the toughness amplification factor of the material reaches 16.1 ± 1.1, outperforming the state‐of‐the‐art biomimetic ceramics. The results reveal that, with the merit of nacre‐mimetic structural designs, the overall fracture toughness of the artificial nacre can be improved by the platelet strengthening through extrinsic toughening mechanisms, although the intrinsic fracture toughness may decrease at platelet level due to the strengthening. It is anticipated that advanced structural ceramics with exceeding performance can be fabricated through these unconventional strategies.
This work illustrates an anti‐intuitive strategy that, with the merit of biomimetic designs, residual stress that is conventionally harmful to ceramics can inversely help improve the fracture toughness of biomimetic ceramics through nanoscale residual stress‐induced platelet strengthening. This provides new insights into the design principles of nacre‐like materials at the bottom level.
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