Metal–organic frameworks (MOFs) as chemical sensors have developed rapidly in recent years. There have been many papers concerning this field and interest is still growing. The reason is that the specific merits of MOFs can be utilized to enhance sensitivity and selectivity by various energy/charge transfers occurring among different ligands, ligand, and metal centers, such as from ligands to metal centers or metal centers to ligands, as well as from MOF skeletons to guest species. This review intends to provide an update on recent progress in various applications of different MOF‐based sensors on the basis of their luminescent and electrochemical responses towards small molecules, gas molecules, ions (cations and anions), pH, humidity, temperature, and biomolecules. MOF‐based sensors function by utilizing different mechanisms, including luminescent responses of “turn‐on” and “turn‐off”, as well as electrochemical responses. Come to your sensors! This review provides an update on various metal–organic framework (MOF)‐based chemical sensors and their classification on the basis of different mechanisms of luminescent (“turn‐on” and “turn‐off”) and optoelectronic responses (see figure). There are many examples of turn‐off detection, but far fewer of turn‐on detection. MOF‐based electrochemical sensors, in particular, are in their infancy.
Arrays of NiCo2S4 nanotubes on nickel foam were prepared by means of a two‐step method, and were directly applied as a binder‐free supercapacitor electrode. Such a binder‐free method enables intimate contact between the current collector and the active materials, and can effectively improve ion and charge transportation. As a result, the electrochemical performances of supercapacitors can be improved. The as‐prepared NiCo2S4 nanotubes/Ni foam electrode shows a high specific capacitance (738 F g−1 at 4 A g−1), excellent rate capability (78 % capacitance retention at 32 A g−1), and good cycling stability (retention capacity of 93.4 % after 4000 cycles), which suggests its promising application for electrochemical capacitors. Solid becomes hollow: Uniform NiCo2S4 nanotube arrays on conductive 3D nickel foam were obtained through use of a two‐step hydrothermal method (see figure) and were directly applied as the binder‐free electrode for electrochemical capacitors. The electrode of NiCo2S4 nanotube arrays exhibits enhanced specific capacitance and a superior stability.
Luminescent metal–organic frameworks (LMOFs) containing fluorescent probes for the detection of pollutants such as organic solvents and heavy metals are becoming increasingly important, with lanthanide‐MOF (Ln‐MOF) materials receiving greater attention owing to the possibility of achieving fine‐tuned luminescent properties. Herein, two unusual isostructural nanocage‐based three‐dimensional Ln‐MOFs, 1‐Ln (Ln=Tb, Eu), are constructed, using a new diisophthalate ligand with active Lewis basic triazole sites. Selective gas adsorption, especially the removal of CO2 from CH4, a primary component of natural gas and biogas, is desirable in terms of both economic and environmental considerations. 1‐Eu is found to exhibit highly efficient luminescent sensing for Fe3+ cations and Cr2O72− anions, as well as selective CO2 capture over CH4. Luminescent sensing MOFs: Two unusual isostructural nanocage‐based three‐dimensional Ln‐MOFs, 1‐Ln (Ln=Tb and Eu; see figure), are constructed using a new diisophthalate ligand with active Lewis basic triazole sites. 1‐Eu exhibits highly efficient luminescent sensing for Fe3+ cations and Cr2O72− anions, and selective capture of CO2 over CH4.
Azulene, as an isomer of naphthalene, has received increasing interest due to its unique chemical structure and unusual photophysical properties, including a large dipole moment of 1.08 D, a narrow energy gap between the HOMO and LUMO, and abnormal fluorescence (anti‐Kasha's rule) from the second excited state to the ground state. In this Minireview, the general strategies and representative synthetic methods for the preparation and functionalization of azulene and its derivatives are presented, and then the application of azulene‐based optoelectronic materials in organic field‐effect transistors and solar cells is discussed. Finally, the challenges and outlook on developing azulene‐based optoelectronic materials are discussed, together with several key points on molecular design and synthesis. Aromatic notes: The general strategies and representative synthetic methods for the preparation and functionalization of azulene and its derivatives are presented in this Minireview to give valuable insights into the development of azulene‐based optoelectronic materials (see figure). The application of azulene‐based conjugated molecules/polymers in organic field‐effect transistors and solar cells is also discussed.
Metal‐mediated base pairs represent an elegant way for the site‐specific functionalisation of nucleic acids. In this type of base pair, the hydrogen bonds between complementary nucleobases are replaced formally by coordinative bonds to one or more transition‐metal ions. This Review presents an overview of metal‐mediated base pairs reported so far. It gives an insight into the characterisation of metal‐mediated base pairs and into the structures of the resulting metal‐modified nucleic acids. In addition, a summary of applications exploiting the formation of metal‐mediated base pairs is presented. The applications include, for example, metal‐ion sensors of various types, a protein sensor, and DNA‐based molecular machines. Initial reports on the potential use of metal‐mediated base pairs in the context of charge transfer through nucleic acid double helices are summarised as well. Rise of the machines: This Review presents an overview of metal‐mediated base pairs. It also gives an insight into the structures of the resulting metal‐modified nucleic acids (see figure). Moreover, applications exploiting the formation of metal‐mediated base pairs are summarized. The applications include metal‐ion sensors of various types, a protein sensor, and DNA‐based molecular machines.
The individual and competitive adsorption of PbII, NiII, and SrII on graphene oxides (GOs) was investigated by experimental and density functional theory (DFT) studies. Experimental results indicate that 1) in all the single, binary, and ternary metal‐ion adsorption systems, the sequence of maximum adsorption capacities is PbII>NiII>SrII on GOs; 2) the desorption hysteresis of metal ions from GOs shows the adsorption affinity in the same sequence: PbII>NiII>SrII. For the first time, DFT calculations indicate that 1) PbII and NiII prefer to interact with the COH group, whereas SrII interacts with COH and COC comparably, and 2) PbII can easily the OH group from the GOs to form the much more stable Pb(OH)–GO complex. These findings are very important and useful for understanding the mechanisms of heavy‐metal‐ion adsorption on GOs and assessing the adsorption of coexisting heavy‐metal ions on GOs. Ready, set, GO! The adsorption of PbII, NiII, and SrII on graphene oxides (GOs) has been investigated by experimental and density functional theory studies. The results indicate that PbII and NiII prefer to interact with COH, whereas SrII interacts with COH and COC comparably and PbII can OH from GOs to form the more stable Pb(OH)–GO complex (see figure).
The individual and competitive adsorption of Pb-II, Ni-II, and Sr-II on graphene oxides (GOs) was investigated by experimental and density functional theory (DFT) studies. Experimental results indicate that 1)in all the single, binary, and ternary metal-ion adsorption systems, the sequence of maximum adsorption capacities is Pb-II>Ni-II>Sr-II on GOs; 2)the desorption hysteresis of metal ions from GOs shows the adsorption affinity in the same sequence: Pb-II>Ni-II>Sr-II. For the first time, DFT calculations indicate that 1)Pb-II and Ni-II prefer to interact with the COH group, whereas Sr-II interacts with COH and COC comparably, and 2)Pb-II can easily abstract the OH group from the GOs to form the much more stable Pb(OH)-GO complex. These findings are very important and useful for understanding the mechanisms of heavy-metal-ion adsorption on GOs and assessing the adsorption of coexisting heavy-metal ions on GOs.
A rhodamine‐based copper complex as a selective, sensitive turn‐on fluorescent chemosensor for NO/histidine has been developed. A conspicuous fluorescence enhancement was observed in the presence of nitric oxide/histidine. The probe was specific towards NO over other reactive oxygen species and reactive nitrogen species. The probe showed selective fluorescence enhancement with histidine; the other naturally occurring amino acids did not result in fluorescence enhancement. EPR and ESIMS studies clearly showed that NO‐induced reduction of copper ions leads to the fluorescence enhancement. The viability of the probe for fluorescent imaging of nitric oxide and histidine in living cells has been demonstrated by means of confocal laser scanning microscopy experiments. Let there be light: A rhodamine‐based copper complex as a selective, sensitive turn‐on fluorescent chemosensor for nitric oxide/histidine has been developed (see scheme). The probe displayed a fast and selective response to NO over other biologically active oxygen and nitrogen species. The mechanism was studied and the viability of the probe for fluorescent imaging of nitric oxide and histidine in living cells was demonstrated.
Recent interest in the iron–air flow battery, known since the 1970s, has been driven by incentives to develop low‐cost, environmentally friendly and robust rechargeable batteries. With a predicted open‐circuit potential of 1.28 V, specific charge capacity of <300 A h kg−1 and reported efficiencies of 96, 40 and 35 % for charge, voltage and energy, respectively, the iron–air system could be well suited for a range of applications, including automotive. A number of challenges still need to be resolved, including: efficient and moderate‐cost bifunctional oxygen electrodes, low‐cost iron electrodes able to decrease corrosion and hydrogen evolution, new cell designs using additive manufacturing technologies and mathematical models to improve battery performance. This Minireview considers the thermodynamics and kinetics aspects of the iron–air battery, the operational variables and cell components, thereby highlighting current challenges and assessing recent developments. Breathing space: The figure shows a unit iron–air cell with the structure of the bifunctional air‐breathing cathode for the reduction and evolution of oxygen, the electrolyte, and the iron anode. This Minireview analyzes the history and recent developments of this system and highlights the challenges and opportunities that the low‐cost iron–air cell provides.
A simple method has been developed to substantially improve the high‐rate capability of electrochemically anodized TiO2 nanotube arrays targeted for use as anode material in lithium‐ion microbatteries by annealing in a reducing atmosphere (5 % H2 and 95 % Ar). A series of complementary techniques including X‐ray diffraction (XRD) with Rietveld refining, scanning electron microscopy (SEM), high‐resolution transmission electron microscopy (HRTEM), X‐ray photoelectron spectroscopy (XPS), Raman spectrometry (Raman), Fourier‐transform infrared spectroscopy (FTIR), galvanostatic measurements, and electrochemical impedance spectroscopy (EIS) have been employed to investigate the structural and morphological changes as well as the electrochemical performance enhancement resulting from hydrogenation treatment of the TiO2 nanotube arrays. The results reveal that improvement of the rate capability is mainly attributed to the electronic conductivity increase of the bulk TiO2 nanotubes rather than conductive characteristics of the surface coating because hydrogenation treatment produces a high number of oxygen vacancies inside the crystal lattices that makes the TiO2 nanotube arrays favor a bulk n‐type conductor. Furthermore, the high‐rate capability of other kinds of TiO2 nanomaterials, including rutile TiO2 nanowire arrays and anatase TiO2 nanoparticles, can also be considerably improved by similar H2 treatment. Therefore, the current H2 treatment method is proved to be a general and facile technique to improve the power density of TiO2 anode materials for next‐generation, high‐power lithium‐ion batteries. Battery powered: Annealing under a reducing atmosphere (5 % H2 and 95 % Ar) has considerably improved the high‐rate capability of TiO2 nanotube arrays, which have been applied as anodes for lithium‐ion microbatteries. This improvement is attributed to the increased bulk electronic conductivity, making the TiO2 nanotubes favor a bulk n‐type conductor (see figure).
Organic fluorescent materials have been an integral part of recently emerged optoelectronic device technologies owing to their good photophysical properties such as high quantum yields and significant photostability. In particular, switchable and tunable solid‐state fluorescence has attracted increasing attention in recent years both in the field of fundamental research and industrial applications. Unlike in solution, fluorescence in the solid state is a collective phenomenon of molecules that are commonly modulated through controlling molecular packing and the electronic conjugation of fluorophores. Several strategies, including chemical modification, have been developed to alter the fluorophore molecular arrangement in the solid state. This Review article describes the various strategies that have been effectively utilised to achieve switchable and tunable fluorescence in the organic solid state. Give us a tune! Switching and tuning photophysical properties, particularly the solid‐state fluorescence of small organic molecules, has attracted great attention owing to the recent emergence of optoelectronic technologies. The strategies (see figure) that have been used effectively to modify the solid‐state fluorescence properties are discussed in this Review.
A new ZnII metal–organic framework, [Zn6(L)3(DMA)4]⋅5 DMA (H4L=[1,1′:3′,1′′‐terphenyl]‐3,3′′,5,5′′‐tetracarboxylic acid, DMA=dimethylacetamide), has been synthesized and characterized. The structure contains a three‐dimensional 3,4,4,6‐connected net with (4.62)2(66)(66)(42.610.83) topology and displays selective detection of nitrobenzene, CrO42− and Fe3+ ions. The present work thus indicates that this metal–organic framework could be a prospective candidate for developing novel luminescence sensors for the selective sensing of nitrobenzene, which can be used as a precursor for explosives. Furthermore, the photoluminescent properties of this material in different solvents and with various analytes were investigated and corroborated by theoretical calculations. The results were in good agreement with the experimental solvent‐dependent luminescence behavior. Explosive‐controlled afterglow: A new Zn metal–organic framework contains two distinct types of pores (A & B) and displays selective detection, through photoluminescence quenching, of nitrobenzene, a precursor for explosives, as well as CrO42− and Fe3+ ions.
Redox flow batteries are experiencing rapid growth for stationary energy‐storage applications. To satisfy the demand for wider applications, however, improved energy density of redox flow batteries is desperately required. Past and present efforts to increase the energy density are briefly surveyed herein and several strategies are explored. Go with the flow: The latest advances in redox flow battery (RFB) related technologies are highlighted in the context of a few decades of development. The technology has mainly advanced through a process of enlarging the electrochemical window by using organic solvents and replacing liquid electrolytes with solid or semisolid materials (see picture).
The novel, thermally stable explosive 5,5′‐bis(2,4,6‐trinitro‐phenyl)‐2,2′‐bi(1,3,4‐oxadiazole) (TKX‐55) is reported. This compound can be prepared by means of a facile synthetic procedure and shows outstanding properties (detonation velocity, detonation pressure, sensitivity toward mechanical stimuli, and temperature of decomposition). TKX‐55 was isolated and characterized by means of mass spectrometry, multinuclear (1H, 13C) NMR spectroscopy, and vibrational spectroscopy (IR and Raman). The structure in the crystalline state was determined by low‐temperature single‐crystal X‐ray diffraction. From the calculated standard molar enthalpy of formation (CBS‐4M) and the densities, the Chapman–Jouguet detonation properties were predicted by using the EXPLO5 V6.01 thermochemical computer code. The sensitivity of TKX‐55 towards impact, friction, and electrostatic discharge was determined. The shock reactivity (explosiveness) of TKX‐55 was measured by applying the small‐scale shock reactivity test. Controlled explosion: The synthesis of the new heat‐resistant explosive, 5,5′‐bis(2,4,6‐trinitrophenyl)‐2,2′‐bi(1,3,4‐oxadiazole) (TKX‐55), is reported. TKX‐55 shows extraordinary properties: high decomposition temperature, density, enthalpy of formation, detonation pressure, and detonation velocity, but low sensitivities toward friction, impact, and electrostatic discharge.
The synthesis of fine chemicals from the platform molecules obtained through the degradation of the cellulosic and lignocellulosic biomass is a most widely envisioned approach toward the implementation of renewable feedstocks for fuels and chemicals. Significant advances have been made in the synthesis of furan‐based polyester building block 2,5‐furandicarboxylic acid (FDCA) and related compounds such as 2,5‐bis(hydroxymethyl)furan and 2,5‐bis(hydroxymethyl)tetrahydrofuran from biomass‐derived 5‐hydroxymethylfurfural (HMF) by using homogeneous and nanoparticulate catalysts. This review provides a survey of selective aerobic oxidation of HMF to give FDCA as the end‐product. The article highlights the fundamental aspects of preferring nanoparticulate catalysts over the conventional supported metal catalysts for the synthesis of FDCA with high selectivity. Another objective of the review is to discuss how efficiently the HMF‐platform produces biofuels, including gasoline blendstock 2,5‐dimethylfuran (DMF), 5‐ethoxymethylfurfural (EMF), and ethyl levulinate (EL), which are competitive of existing liquid fuels. Out of the woods: Considerable advances have been made for the synthesis of furan‐based polyester building blocks, especially 2,5‐furandicarboxylic acid (FDCA), by using nanoparticulate gold catalysts. This Minireview discuss the fundamental aspects of using nanoparticulate catalysts for the aerobic oxidation of biomass‐derived 5‐hydroxymethylfurfural (HMF) into FDCA and selective transformation of HMF into potential biofuels (DMF=2,5‐dimethylfuran).
Recent investigations into proton conduction in metal–organic frameworks (MOFs) indicate that MOFs are promising materials as a new class of proton conductors. Hydrated proton‐conductive MOFs show not only high proton conductivity of approximately 10−2 S cm−1, which is comparable to that of a practical organic polymer, but also structural visibility of proton‐conducting pathways inside the materials owing to their high crystallinity. Herein, studies on the design, synthesis, and proton‐conductive properties of MOFs with hydrated proton‐conductive systems are introduced. Finding the way through: Metal–organic frameworks (MOFs) have emerged as a new class of proton conductors. Hydrated proton‐conductive MOFs show not only high proton conductivity, but also structural visibility of proton‐conducting pathways inside the materials owing to their high crystallinity (see figure). Studies on the design, synthesis, and proton‐conductive properties of MOFs with hydrated proton‐conductive systems are introduced.
A simple electrospinning technique is employed for the preparation of high‐performance V2O5 nanofibers. The fibers thus prepared are subjected to heat treatment under the optimized conditions at 400 °C in air to achieve a single phase. The powder X‐ray diffraction pattern confirms the formation of an orthorhombic structure with Pmmn space group. Morphological studies conducted by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), clearly reveal the presence of a highly interconnected network of fibers with the diameter ranging from approximately 500–800 nm. After the heat treatment, translation of smooth fibrous morphology into porous fibers with embedded nanocrystals of V2O5 is noticed from the SEM measurements. The sintered V2O5 nanofibers are used to fabricate a hybrid electrochemical capacitor (HEC) and it is coupled with a substrate‐free single‐walled carbon nanotube (SWCNT) network (called “Bucky paper”) in a conventional organic electrolyte solution. Supercapacitive behavior of HEC is studied in both potentiostatic and galvanostatic modes at room temperature. The HEC demonstrated very stable and excellent cycling behavior during 3500 cycles of galvanostatic charge and discharge tests. This hybrid system is also well established during the rate capability studies from the applied current density of 30 to 210 mA g−1 and delivered maximum energy and power densities of 18 Wh kg−1 and 315 W kg−1, respectively. A hybrid electrochemical capacitor (HEC) was fabricated using electrospun V2O5 coupled with single‐walled carbon nanotubes. Cyclic voltammetry analysis reveals the supercapacitive behavior and galvanostatic charge‐discharge studies show very stable cycle ability (given in figure and background shows SEM image of as‐spun V2O5 nanofibers). Applied currents of 30–210 mA g−1 delivers maximum energy and power densities of 18 Wh kg−1 and 315 W kg−1, respectively.