A dual emission metal–organic framework for rapid ratiometric fluorescence detection of CO32− in seawater

A dual emission metal–organic framework (IRMOF-10-Eu) was prepared and used as a ratiometric fluorescent sensor for CO₃²⁻ detection. IRMOF-10-Eu had good stability and excellent luminescence in aqueous solution. IRMOF-10-Eu showed dual fluorescence emission from the ligand and Eu³⁺ with single excitation. Upon treatment with CO₃²⁻, the fluorescence ratio (I₆₂₄/I₃₅₈) of the probe displayed significant change. The relative fluorescence intensity ratio (I₆₂₄/I₃₅₈) and CO₃²⁻ concentration had a linear relationship in 50–300 μM range with a low detection limit of 9.58 μM. And the luminescence probe of CO₃²⁻ showed a fast detection time. The possible mechanism was investigated. CO₃²⁻ changed the structure of IRMOF-10-Eu and interrupted the energy transfer process. Thus, the fluorescence emission intensity of the ligand was increased and Eu³⁺ was decreased with the addition of CO₃²⁻. IRMOF-10-Eu was used to detect CO₃²⁻ in seawater, which showed good prospect in practical application. Subsequently, a highly selective and sensitive probe, IRMOF-10-Eu, may pave an efficient way for CO₃²⁻ detection in seawater.

A dual emission metal–organic framework (IRMOF-10-Eu) was prepared and used as a ratiometric fluorescent sensor for CO₃²⁻ detection.     

Zirconium-based metal–organic framework gels for selective luminescence sensing

Metal–organic gelation represents a promising approach to fabricate functional nanomaterials. Herein a series of Zr-carboxylate gels are synthesized from rigid pyrene, porphyrin and tetraphenyl ethylene-derived tetracarboxylate linkers, namely Zr-TBAPy (H₄TBAPy = 1,3,6,8-tetrakis(4-carboxylphenyl)pyrene), Zr-TCPE (H₄TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene), and Zr-TCPP (H₄TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin). The gels are aggregated from metal–organic framework (MOF) nanoparticles. Zr-TBAPy gel consists of NU-901 nanoparticles, and Zr-TCPP gel consists of PCN-224 nanoparticles. The xerogels show high surface areas up to 1203 m² g⁻¹. MOF gel films are also anchored on the butterfly wing template to yield Zr-MOF/B composites. Zr-TBAPy and Zr-TCPE gels are luminescent for solution-phase sensing and vapour-phase sensing of volatile organic compounds, and exhibit a significant luminescence quenching effect for electron-deficient analytes. Arising from the high porosity and good dispersion of luminescent MOF gels, rapid and effective vapour-sensing of nitrobenzene and 2-nitrotoluene within 30 s has been achieved via Zr-TBAPy film or Zr-TBAPy/B.

Zr-based MOF nanomaterials are developed via a metal–organic gelation method for rapid and effective luminescence vapour-sensing.     

A channel-structured Eu-based metal–organic framework with a zwitterionic ligand for selectively sensing Fe3+ ions

A novel Eu-based MOF [Eu(IMS1)₂]Cl·4H₂O (1) was successfully constructed based on a semi-rigid zwitterionic 1,3-bis(4-carboxylbenzyl)-imidazolium (IMS1) ligand, featuring a 3-fold interpenetrating dia net structure with a point symbol of 6⁶ and charged permanent micropores. Considering its excellent luminescent property as well as thermal and chemical stability, complex 1 was explored as a potential sensor for detecting Fe³⁺ ions. The results show that complex 1 has a high sensitivity and selectivity for Fe³⁺ based on a ‘turn-off’ effect, for which the electrostatic interaction between Fe³⁺ ions and the inner surface of the micropores may play a critical role. The fluorescence quenching mechanism reveals that dynamic quenching and competitive adsorption between Fe³⁺ and 1 lead to the quenching effect of 1.

A channel-structured Eu-based metal–organic framework with a zwitterionic ligand may serve as a sensor for selectively detecting Fe³⁺ ions.     

Metal–organic framework bearing new viologen ligand for ammonia and Cr2O72− sensing

Three anionic metal–organic frameworks (MOFs) {[Zn₃(BTEC)₂(H₂O)(4-BCBPY)]·(H₂O)}_n (1–3) (BTEC⁴⁻ = 1,2,4,5-benzenetetracarboxylic acid anion, 4-BCBPY²⁺ = 1,1′-bis(4-cyanobenzyl)-4,4′-bipyridinium dication) were synthesized in the reaction of 1,2,4,5-benzenetetracarboxylic acid with different metal salts such as ZnNO₃, ZnCl₂, and ZnSO₄, under solvothermal conditions in the presence of 1,1′-bis(4-cyanobenzyl)-4,4′-bipyridinium chloride. Single crystal X-ray diffraction analysis shows that compounds 1, 2 and 3 have MOF structures based on binuclear metal building units, which are connected by two protonated BTEC⁴⁻ ligands and three zinc ions, and the viologen cation 4-BCBPY²⁺ is located in the channel to achieve charge balance. Compounds 1, 2 and 3 have good photosensitivity, respond to sunlight, UV light and blue ray, and turn blue. The D–A distance and π–π stacking distance of the discolored samples (1P, 2P and 3P) changed. In addition, the three compounds showed visible color changes to ammonia vapor, rapidly changing from white to blue. At the same time, the three compounds exhibited fluorescence quenching to ammonia vapor and Cr₂O₇²⁻. It is further proved that compounds 1, 2 and 3 are fluorescent sensors with a low detection limit (for Cr₂O₇²⁻: 10⁻⁵ M) and high sensitivity for ammonia vapor and Cr₂O₇²⁻. It was found that photochromic behavior, ammonia sensing properties can be tuned by the nature of metal salts.

Three MOFs based on different metal salts were synthesized, and metal salts were found to play a key role in regulating the performance of MOFs.     

Validation of ‘lock-and-key’ mechanism of a metal–organic framework in selective sensing of triethylamine

To develop the metal–organic framework (MOF)-based sensing of triethylamine (TEA) in an aqueous phase, Al-MIL-101-NH₂ (MIL: Material Institute Lavoisier) with a tripod-like cavity was utilized based on a lock-and-key model. Al-MIL-101-NH₂ (Al-MOF) was found to be an excellent fluorescent sensor for the TEA molecules in the range of 0.05–0.99 mM. The limit of detection (LOD) and linear calibration range of this probe towards TEA were found to be 3 μM and 0.05–0.40 mM, respectively. The mechanism of the sensing process indicates the dominant role of physical processes (e.g., non-covalent bond interactions). In addition, the exact fit of the TEA molecule (6.5 Å) in the tripod-like cavity (6.78 Å) supported the strong interaction between three ethyl groups (TEA) and aromatic rings (MOF). This kind of specific suitability between size/shape of the TEA and tripod-like cavity of MOF (ΔG: −46.7 kJ mol⁻¹) was not found in other molecules such as ethylamine (ΔG: −2.20 kJ mol⁻¹ and size: 3.7 Å), formaldehyde (ΔG: +1.50 kJ mol⁻¹ and size: 2.8 Å), and ammonia (ΔG: +0.71 kJ mol⁻¹ and size: 1.6 Å). As such, Al-MOF was found to be a selective and stable sensor for TEA.

To develop the metal–organic framework (MOF)-based sensing of triethylamine (TEA) in an aqueous phase, Al-MIL-101-NH₂ (MIL: Material Institute Lavoisier) with a tripod-like cavity was utilized based on a lock-and-key model.     

A 2D metal–organic framework/reduced graphene oxide heterostructure for supercapacitor application

Metal organic frameworks (MOFs) with two dimensional (2D) nanosheets have attracted special attention for supercapacitor application due to their exceptional large surface area and high surface-to-volume atom ratios. However, their electrochemical performance is greatly hindered by their poor electrical conductivity. Herein, we report a 2D nanosheet nickel cobalt based MOF (NiCo-MOF)/reduced graphene oxide heterostructure as an electrode material for supercapacitors. The NiCo-MOF 2D nanosheets are in situ grown on rGO surfaces by simple room temperature precipitation. In such hybrid structure the MOF ultrathin nanosheets provide large surface area with abundant channels for fast mass transport of ions while the rGO conductive and physical support provides rapid electron transport. Thus, using the synergistic advantage of rGO and NiCo-MOF nanosheets an excellent specific capacitance of 1553 F g⁻¹ at a current density of 1 A g⁻¹ is obtained. Additionally, the as synthesized hybrid material showed excellent cycling capacity of 83.6% after 5000 cycles of charge–discharge. Interestingly, the assembled asymmetric device showed an excellent energy density of 44 W h kg⁻¹ at a power density of 3168 W kg⁻¹. The electrochemical performance obtained in this report illustrates hybridization of MOF nanosheets with carbon materials is promising for next generation supercapacitors.

In this 2D NiCo-MOF/rGO hybrid, the MOF nanosheets provide abundant active sites while the conductive rGO provide rapid electron transport.     

A novel 3D terbium metal–organic framework as a heterogeneous Lewis acid catalyst for the cyanosilylation of aldehyde

A novel 3D lanthanide(iii) metal–organic framework (MOF) (namely Tb-MOF), was synthesized by self-assembly from Tb(iii) ion nitrate and the rigid organic ligand H₂sbdc (H₂sbdc = 5,5-dioxo-5H-dibenzo[b,d]thiophene-3,7-dicarboxylic acid), and could work as an efficient heterogeneous catalyst for the cyanosilylation of aromatic aldehydes at room temperature. The obtained Tb-MOF has been characterized and analysed in detail by single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis and so on. The pores of Tb-MOF provided a microenvironment that was beneficial for the substrates to be close to the Lewis acid catalytic sites. The IR spectrogram and the fluorescence titration proved that the substrates could be activated inside the channel of Tb-MOF. The heterogeneous Tb-MOF catalyst with fine catalytic efficiency exhibited a high TON (TON = 460), and could be recycled at least three times without significantly reducing its activity.

A novel 3D lanthanide metal–organic framework synthesized from Tb ions and the rigid organic ligand H₂sbdc could work as an efficient heterogeneous catalyst for the cyanosilylation of aromatic aldehydes.     

Selective sensing and visualization of pesticides by ABW-type metal–organic framework based luminescent sensors

A new ABW-type luminescent metal–organic framework (MOF) namely (H₃O)[Zn₂L(H₂O)]·3NMP·6H₂O (1), constructed with eco-friendly Zn²⁺ and the multicarboxylate intraligand (LH₅) was designed, synthesized and fully characterized by X-ray single-crystal diffraction, steady-state absorption and emission spectroscopy, and SEM observations. The MOF-based suspension sensor 1 (NMP) demonstrated high sensitivity to low-concentration pesticides of chlorothalonil (CTL), nitrofen (NF), trifluralin (TFL), and 2,6-dichloro-4-nitroaniline (DCN), which was assigned to the synergistic effect of the photoinduced electron transfer and the fluorescence resonance energy transfer. With the highest luminescent detection efficiency (K_SV up to 11.194 μmol⁻¹ and LOD down to 2.93 ppm) to DCN, 1 (NMP) was successfully applied for the selective sensing of DCN. The MOF-based film sensor 1 (film) illustrated the selective visualization sensing of trace amounts of DCN. In addition, based on the high saturated vapor pressure of TFL and the unique bathochromic shift effect to the emission maxima of 1, the MOF-based luminescent vapor sensing device 1 (LED) successfully exhibited operability for sensing of TFL vapor. The results illustrated a feasible approach to construct new MOF-based luminescent sensors for selective sensing and visualization of pesticides.

A novel ABW-type luminescent metal–organic framework was applied for selective visualization sensing of trace amounts of 2,6-dichloro-4-nitroaniline and vapor sensing of trifluralin.     

A three-dimensional metal–organic framework for a guest-free ultra-low dielectric material

A three-dimensional metal–organic framework compound [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) shows two step dielectric relaxation and its guest-free framework (1′) possesses an ultra-low κ value of 1.80 (at 100 kHz, it is the lowest value for MOFs reported to date) over a wide temperature range and high thermal stability.

A MOFs compound [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) shows two step dielectric relaxation and its guest-free framework (1′) possesses an ultra-low κ value of 1.80 (at 100 kHz) over a wide temperature range and high thermal stability.

The design and synthesis of low dielectric constant (low-κ) materials has been a subject of interest in terms of their potential for use in high performance electronic devices. Materials with extremely low-dielectric constants have been targeted as interlayer dielectrics (ILD) because they decrease the cross-talk noise, propagation delay, and power dissipation in most electronic components. ^1–5 Indeed, the search for new low-κ materials replacing silicon dioxide (SiO₂) as an ILD has always been dictated by industrial needs, resulting in a strong connection between fundamental research and technology.⁶ Many materials have been proposed and studied as potential candidates; two major classes are dense organic polymers and porous inorganic-based materials. Some dense organic polymers could have κ below 2.2, but they suffer from concerns of low thermal stability and thermal conductivity. For porous inorganic-based low-κ materials, sol–gel silica, doped oxides and mesoporous silica have been extensively studied.⁷ but its low mechanical strength, wide pore size distribution, and hydrophilicity have been cited as concerns.As air or vacuums have the lowest dielectric constant (κ = 1.01), the partial replacement of solid network with air or a vacuum appears to be the more intuitive and direct option to the development of new low-κ ILD materials. Thus, as per the International Technology Roadmap for Semiconductors (ITRS), robust porous materials and air gap structures will become target low-κ materials in the near future.⁸ Metal–organic frameworks (MOFs) with a well-defined monodisperse porosity, large surface area, ultra-low densities, high stability and easy tunability of the surface and structural properties have potential for meet the demands for use as stable low-κ materials.⁹ MOFs have been extensively studied over the past decade for their applications in gas storage, sensors, chemical separation, catalysis, drug delivery and biomedical imaging.^10–12 However, their electrical properties and applications in microelectronics remain under researched.¹³ MOFs should be stiffer and harder than other low-density amorphous inorganic or organic polymers because of their ordered framework and rigid organic linkers. With tunable structural properties, high porosity, and thermal/mechanical stability, MOFs represent an ideal replacement as an ILD material. Hermann and coworkers presented a brief theoretical model for using MOFs as low-κ materials in microelectronics applications.¹⁴ However, these theoretical calculations did not take into account the orientational and ionic contributions to the molecular polarizability, which drastically contribute to the dielectric constant. These theoretical results encouraged us to search for new MOFs materials with experimentally ultra-low κ values.In this work, we report on the preparation of a MOFs, [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) (H₂bpdc = 4,4′-biphenyldicarboxylic acid), which have 3D frameworks with high thermal stability (Fig. S1^†) and a ultra-low κ values of its guest-free sample 1′. The 1′ possesses a very low κ values of 1.80 (at 100 kHz) and high thermal stability at temperatures up to 360 °C (Fig. S2^†), making it a potential candidate for use as an ILD. To the best of our knowledge, to date, the κ values of 1′ is the lowest value for MOFs reported. Furthermore, compound 1 shows dielectric relaxation and anomalies in the temperature range of 35–140 °C. Dielectric relaxation and anomalies of 1 is related to reorientation of the dipole moment of surface absorbed water and guest DMF molecules, respectively.The compound 1 was obtained from the solvothermal reaction of Zn(NO₃)₂·6H₂O, H₂bpdc and [NH₂(CH₃)₂]Cl in DMF. X-ray crystallographic analysis reveals that it crystallizes in the space group Pna2₁.^§ The asymmetric unit contains of three crystallographically distinct Zn²⁺ ions, four deprotonated bpdc²⁻ ligands, two [NH₂(CH₃)₂]⁺ ions and three free DMF molecules. Three crystallographically independent Zn²⁺ ions have two coordination modes (Fig. 1a). The Zn(1) and Zn(3) adopts a four-coordinated and formed slightly distorted tetrahedral geometry, and the Zn(2) adopts a six-coordinated geometry. O13 and O16 atoms originated from monodentate coordination of the bpdc²⁻ ligands and the other oxygen atoms are coordinated by μ2-modes bpdc²⁻ ligands to Zn²⁺ ions. The shortest and longest Zn–O distance is 1.881(10) and 2.088(9) Å, respectively. Each Zn(2) atom is connected Zn(1) atom and Zn(3) atom by three bridged bidentate bpdc²⁻ ligands to form trinuclear building blocks. As shown in Fig. 1b, trinuclear building blocks are further linked together by bpdc²⁻ ligands make up the 3D anionic framework with two different channels, and the channels is occupied by [NH₂(CH₃)₂]⁺ ions and disordered DMF guest molecules. Parallel to the ac plane, the monodentate bpdc²⁻ ligands bridge trinuclear building blocks to afford layers stacking, and the layers are pillared by bidentate bpdc²⁻ ligands to give rise to a regular 3D network (Fig. 1c), and channel dimensions is about 13 × 18 Å along the b-axis direction. The triangle cage was formed along the c-axis direction with small channel (Fig. 1d). Overall, two individual triangle cage are independent interpenetrated to form the entire framework of 1 (Fig. 1e). It should be noted that although the framework of 1 is interpenetration networks, it is still highly porous. After the removal of solvent molecules in the channels, the accessible volume of 1 is 40.2%.Open in a separate windowFig. 1Structure of 1 (a) trinuclear metal cluster building blocks; (b) 3D anionic framework with two different channels; (c) regular channel along the b-axis direction; (d) the triangle cage along the c-axis direction; (e) two independent interpenetrated triangle cage.The temperature dependent dielectric properties were investigated in the temperature rang of 30–135 °C, and two-step dielectric relaxation were observed. As show in Fig. 2a, it is clear that compound 1 shows the first step dielectric relaxation in the temperature range of 30–80 °C. When 1 was heated from 30 to 37 °C, the dielectric constant progressive increased and reaches a maximum of 174.4 at 10³ Hz. Further increase in temperature results in the dielectric constant of 1 slowly decreasing and dielectric peak disappears. The first step dielectric relaxation is due to the relaxation of absorbed water molecules in the sample surface. The dielectric relaxation signal was not observed in the cooling process (from 95 °C to 30 °C) for losing surface water (Fig. 2b). The κ value is directly related to the polarization phenomena. The higher the polarization, the greater the increase in κ value will be. Usually, the MOF materials with low κ value feature the reorientational motions of polar guest molecules being restricted at low temperature or frameworks solvent-free. However, for 1, the thermally assisted dynamical dipole motion due to polar DMF molecules is appeared. The guest molecules get enough excitation thermal energy to be able to obey the change under the external electric field more easily in the high temperature regime, and the reorientational dynamics of guest molecules is activated above 105 °C. This in return enhances their contribution to the polarization leading to an sharp increase of dielectric permittivity value. At f = 10⁵ Hz, the dielectric constant reaches a maximum of 237, and then sharply decreased when the temperature increased. In the following cooling process, a very low κ value was observed and no dielectric relaxation was occurred (Fig. 2c). The second step dielectric relaxation at different frequency are shown in Fig. 2d, which can be ascribed to the guest polar DMF reorientational motions. In addition, the dielectric loss values shows similar features in the selected frequency range (Fig. S3^†).Open in a separate windowFig. 2(a) Temperature-dependent real part dielectric constant (ε′) in the temperature range of 30–95 °C at selected frequency of 1; (b) plots of ε′ vs. T in the 30–95 °C range at 5 × 10³ Hz with the heating (black dot) and cooling models (red dot); (c) plots of ε′ vs. T in the 100–135 °C range at 10⁵ Hz with the heating (black line) and cooling models (red line; (d) temperature-dependent ε′ in the temperature range of 30–135 °C.Removing polar guest molecules from the framework may be decreases the polarization and the possibility of any type of hydrogen bonding or ionic interactions between the framework and guest molecular, hence, the κ value will also decreases. The guest-free sample 1′ were obtained by simply heating method. The PXRD patterns of the 1 match well with the 1′ (Fig. S4^†), thus demonstrating the phase were unaltered. The dielectric properties of guest-free sample 1′ were investigated. As shown in Fig. 3, after removing the polar DMF molecules, a very low κ value of 1.78 at 100 kHz at 40 °C with a low dielectric loss (0.005) was observed (Fig. S5^†). It is very interesting as the temperature increase from 40 to 130 °C, κ value increases very slowly. κ value is 1.99 at 130 °C. With the ac electric field frequency increasing, the κ value slightly decrease (Fig. 3a). It is noteworthy that, to date, dielectric investigations of MOFs have received relatively little attention, although a few exciting examples have been reported. Only a limited number of MOFs have been reported to possess ultra-low κ value.^15,16 The ultra-low κ values for a few MOFs are shown in Table S2.^†17 To the best of our knowledge, the κ values of 1′ is the lowest value for MOFs reported. From the published paper and our results, to obtained the ultra-low κ MOFs, ligands should have high symmetry and small polarity and polar guest molecular should be avoid. Furthermore, some small counter ions could be decrease κ values. As the vacuum has the lowest dielectric constant, thus, κ values can be reduced significantly by increasing porosity of MOFs. High thermal stability and MOFs thin-film growth are required for the practical applications of ultra-low κ MOFs in microelectronics. ZIF-8 films with κ value of 2.4 were deposited on silicon wafers and characterized in order to assess their potential as future insulators (low-κ dielectrics) in microelectronics.¹⁸ We recently reported a hydrogen bonding MOFs [Zn(H₂EIDA)₂(H₂O)]·2DMF, which exhibited low-κ behaviour, but its thermal stability was not perfect.Open in a separate windowFig. 3Temperature-dependent real part dielectric constant (ε′) at selected frequency of 1′.     

Ternary dual Z-scheme graphitic carbon nitride/ultrathin metal–organic framework nanosheet/Ag3PO4 photocatalysts for boosted photocatalytic performance under visible light

Ternary graphitic carbon nitride/ultrathin metal–organic framework nanosheet/Ag₃PO₄ (CNUA) composite photocatalysts were prepared under ultrasonic irradiation in tetrahydrofuran. The aim was to use them as photocatalysts for the degradation of organic pollutants in water. The crystal structure, surface morphology, optical properties, and chemical composition of the photocatalytic materials were investigated using X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XPS analysis revealed the formation of Ag nanoparticles, which play an important role as an electronic mediator and photosensitizer in the composite during the synthesis. The photocatalytic activity of the composites in the degradation of 2-chlorophenol (2-CP) under visible light (>420 nm) was evaluated. Among the synthesized photocatalysts, the optimized CNUA with 10 wt% of g-C₃N₄ with respect to Ag₃PO₄ (CN10UA), exhibited the best photocatalytic performance in the degradation of 2-CP, which was almost decomposed completely upon ∼5 min of visible-light irradiation. Furthermore, the stability of the CN10UA photocatalyst could be maintained at a high level even after four cycling experiments, while that of pure Ag₃PO₄ declined significantly. The enhanced photocatalytic performance results from efficient charge separation through the dual Z-scheme mechanism involving Ag(0) bridges in the g-C₃N₄/Ag/Ag₃PO₄ and Ag₃PO₄/Ag/UMOFN pathways. The analysis of the photoluminescence of the catalysts also provided evidence for charge transport via the dual Z-scheme mechanism. In addition, radical scavenging tests confirmed that h⁺ and O₂˙⁻ are the main radical reactive species responsible for the photodegradation of 2-CP. The findings of this study enhance our understanding of the construction and mechanism of dual Z-scheme-type photocatalysts.

The enhanced photocatalytic activity of CN10UA results from fast charge transport through dual Z-scheme channels.     

Surface-tension-confined assembly of a metal–organic framework in femtoliter droplet arrays

Metal–organic frameworks (MOFs), produced by metal ions coordinated to organic linkers, have attracted increasing attention in recent years. For the utilization in MOFs in numerous applications, achieving positioned MOF growth on surfaces is essential to fabricate multiple-functional devices. We develop a novel miniaturized method to realize surface-tension-confined assembly of HKUST-1 in femtoliter droplet arrays. HKUST-1 crystal arrays grown by evaporation-induced crystallization are observed, and five typical crystal morphologies (i.e., hexagonal, irregular hexagonal, triangular, arc-like and ribbon-like crystals) are found in the large area on the patterned substrate during crystallization. Our research provides a better understanding of the formation mechanism of MOF crystals in confined sessile droplets. The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions. Understanding the formation of different morphologies of HKUST-1 crystals is useful to guide the production of crystals with desired shapes for various applications.

The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions.     

From metal–organic framework to morphology- and size-controlled 3D mesoporous Cr2O3 toward a high surface area and enhanced volatile organic compound oxidation catalyst

Morphology- and size-controlled 3D mesoporous Cr₂O₃ have always been a research hotspot due to their wide applications. Herein, we for the first time report that the carbonized Cr-MOFs can ignite spontaneously at room temperature and form the corresponding 3D mesoporous Cr₂O₃ with high specific surface areas (219.25 to 303.44 cm² g⁻¹). More importantly, the shape and size of 3D mesoporous Cr₂O₃ can be well controlled by a facile adjustment of the Cr-MOF synthesis conditions. Furthermore, these materials showed an exceptionally high catalytic performance in formaldehyde oxidation. These results are predicted to offer a novel method in the design and synthesis of 3D porous Cr₂O₃.

Morphology- and size-controlled 3D mesoporous Cr₂O₃ have always been a research hotspot due to their wide applications.     

An efficient access to β-ketosulfones via β-sulfonylvinylamines: metal–organic framework catalysis for the direct C–S coupling of sodium sulfinates with oxime acetates

A copper-based framework Cu₂(OBA)₂(BPY) was synthesized and used as a recyclable heterogeneous catalyst for the synthesis of β-sulfonylvinylamines from sodium sulfinates and oxime acetates via direct C–S coupling reaction. The transformation was remarkably affected by the solvent, and chlorobenzene emerged as the best option. This Cu-MOF displayed higher activity than numerous conventional homogeneous and MOF-based catalysts. The catalyst was reutilized many times in the synthesis of β-sulfonylvinylamines without considerably deteriorating in catalytic efficiency. These β-sulfonylvinylamines were readily converted to the corresponding β-ketosulfones via a hydrolysis step with aqueous HCl solution. To the best of our knowledge, this direct C–S coupling reaction to achieve β-sulfonylvinylamines was not previously conducted with a heterogeneous catalyst.

Cu₂(OBA)₂(BPY) was used as catalyst for the synthesis of β-sulfonylvinylamines from sodium sulfinates and oxime acetates. These β-sulfonylvinylamines were readily converted to corresponding β-ketosulfones via a hydrolysis step.     

Self-assembled membrane manufactured by metal–organic framework (UiO-66) coated γ-Al2O3 for cleaning oily seawater

In order to effectively clean oily seawater with anionic polyacrylamide (APAM), UiO-66 coated γ-Al₂O₃ (UA) composites were firstly synthesized using γ-Al₂O₃ as a template to induce the growth of high hydrophilic UiO-66 on its surface to form a uniform UA self-assembled membrane. The UA composites and self-assembled membrane were characterized and analyzed. Also, the membrane performance was investigated. The results show that the hydrophilicity of particles is enhanced with the water contact angle decreasing from 39.8° (γ-Al₂O₃ particles) to 26.2° (UA composites) by introducing the UiO-66 coating. Moreover, the UA self-assembled membrane performs attractive water yield and separation performance. The oil concentration in the permeate treated by the first class of UA self-assembled membrane declines apparently from 91.22 to 18.90 mg L⁻¹, while the water yield is as high as 657.89 L m⁻² h⁻¹. The reclaimed separation experiments show that the membrane materials could be recycled by calcination at 200 °C and hydraulic cleaning, which gives the material potential application in cleaning oily seawater.

The self-assembled membrane manufactured by UA composites exhibits excellent separation performance and water yield in the treatment of oily seawater.     

Graphene inclusion controlling conductivity and gas sorption of metal–organic framework

A general approach to prepare composite films of metal–organic frameworks and graphene has been developed. Films of copper(ii)-based HKUST-1 and HKUST-1/graphene composites were grown solvothermally on glassy carbon electrodes. The films were chemically tethered to the substrate by diazonium electrografting resulting in a large electrode coverage and good stability in solution for electrochemical studies. HKUST-1 has poor electrical conductivity, but we demonstrate that the addition of graphene to HKUST-1 partially restores the electrochemical activity of the electrodes. The enhanced activity, however, does not result in copper(ii) to copper(i) reduction in HKUST-1 at negative potentials. The materials were characterised in-depth: microscopy and grazing incidence X-ray diffraction demonstrate uniform films of crystalline HKUST-1, and Raman spectroscopy reveals that graphene is homogeneously distributed in the films. Gas sorption studies show that both HKUST-1 and HKUST-1/graphene have a large CO₂/N₂ selectivity, but the composite has a lower surface area and CO₂ adsorption capacity in comparison with HKUST-1, while CO₂ binds stronger to the composite at low pressures. Electron paramagnetic resonance spectroscopy reveals that both monomeric and dimeric copper units are present in the materials, and that the two materials behave differently upon hydration, i.e. HKUST-1/graphene reacts slower by interaction with water. The changed gas/vapour sorption properties and the improved electrochemical activity are two independent consequences of combining graphene with HKUST-1.

Changed electrochemical activity and CO₂/H₂O adsorption by graphene inclusion in Cu₃(1,3,5-benzenetricarboxylate)₂ and covalent tethering to glassy carbon electrodes.     

Self-template construction of nanoporous carbon nanorods from a metal–organic framework for supercapacitor electrodes

The morphologies and structures of nanostructured carbons generally influence their catalysis, electrochemical performance and adsorption properties. Metal–organic framework (MOF) nanocrystals usually have various morphologies, and can be considered as a template to construct nanostructured carbons with shaped nanocubes, nanorods, and hollow particles by thermal transformation. However, thermal carbonization of MOFs usually leads to collapse of MOF structures. Here, we report shape-preserved carbons (termed as CNRods) by thermal transformation of nickel catecholate framework (Ni-CAT) nanorods. Supercapacitors of CNRods treated at 800 °C were demonstrated to have enhanced performance due to their structural features that facilitate electron conduction and ion transport as well as abundant O content benefiting the wettability of the carbon materials. This may provide a potential way to explore novel carbon materials for supercapacitors with controllable morphologies and high capacitive performance.

The Ni-CAT-derived porous carbon materials at 800 °C remain regular with a rod-like morphology and exhibit enhanced capacitive performance.     

Synthesis,structure, and fluorescence properties of a calcium-based metal–organic framework

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O. This MOF possesses a new structure composed of calcium clusters and H₂NDC linker anions and shows a unique fluorescence property; it exhibits a fluorescence peak at 395 nm (λ_ex = 350 nm) at room temperature, which is blue-shifted compared with that exhibited by the free H₂NDC ligand. One of the possible mechanisms for this fluorescence is likely attributable to a ligand-to-metal charge transfer (LMCT) transition and is the first example of a calcium-based MOF exhibiting blue-shifted fluorescence due to LMCT.

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O.     

A dual-functional luminescent Tb(iii) metal–organic framework for the selective sensing of acetone and TNP in water

Herein, a novel water-stable luminescent terbium metal–organic framework, {[Tb(L₁)(L₂)_0.5(NO₃)(DMF)]·DMF}_n (TPA-MOF), with 1,10-phenanthroline (phen) and 3,3′,5,5′-azobenzene-tetracarboxylic acid (H₄abtc) ligands was solvothermally synthesized and structurally characterized. TPA-MOF possesses a two-dimensional (2D) extended framework featuring an 8-connected uninodal SP2-periodic net topology with the Schläfli point symbol of {3^12;4^14;5^2}. The π-electron rich luminescent TPA-MOF exhibits four characteristic emission bands of Tb³⁺ ion and acts as a selective and sensitive probe for acetone as well as the electron deficient 2,4,6-trinitrophenol (TNP). Moreover, gas sorption studies confirm that TPA-MOF displays ultra-micropores and adsorbs moderate amounts of N₂ and CO₂.

A novel 2D luminescent terbium metal–organic framework demonstrating highly efficient and selective sensing for acetone and 2,4,6-trinitrophenol (TNP) in an aqueous solution was solvothermally synthesized and structurally characterized.     

A simple fluorescent probe for detection of Ag+ and Cd2+ and its Cd2+ complex for sequential recognition of S2−

In this study, we designed and synthesized a simple probe 2-(8-((8-methoxyquinolin-2-yl)methoxy)quinolin-2-yl)benzo[d]thiazole (DQT) for detection of Ag⁺ and Cd²⁺ in a CH₃OH/HEPES (9 : 1 v/v, pH = 7.30) buffer system. Its structure was characterized by NMR, ESI-HR-MS and DFT calculations, and its fluorescence performance was also investigated. Probe DQT showed fluorescence quenching in response to Ag⁺ and Cd²⁺ with low detection limits of 0.42 μM and 0.26 μM, respectively. Importantly, the complexation of the probe with Cd²⁺ resulted in a red shift from blue to green, making it possible to detect Ag⁺ and Cd²⁺ by the naked eye under an ultraviolet lamp. The DQT-Cd²⁺ complex could be used for sequential recognition of S²⁻. The recovery response could be repeated 3 times by alternate addition of Cd²⁺ and S²⁻. A filter paper strip test further demonstrated the potential of probe DQT as a convenient and rapid assay.

A fluorescent probe for detection of Ag⁺ and Cd²⁺ and its Cd²⁺ complex for sequential recognition of S²⁻.     

Controlled dye release from a metal–organic framework: a new luminescent sensor for water

By introducing the dye Rhodamine 6G (R6G) into a metal–organic framework (MOF), Mn-sdc-2 (H₂sdc = 4,4′-stilbenedicarboxylic acid), with a pore size of 20 × 9.8 Ų, the composite R6G@Mn-sdc-2 was obtained. Subsequently, the MOF Mn-sdc-1 with a smaller pore size of 7.5 × 7.5 Ų can be formed through a single-crystal to single-crystal transformation from Mn-sdc-2, thus tightly locking the dye R6G within the pores. Compared with R6G@Mn-sdc-2, R6G@Mn-sdc-1 exhibits a stronger fluorescence emission of R6G. Because the MOF Mn-sdc-1 can reversibly transform back to Mn-sdc-2 in the presence of trace water, the dye R6G can be released. This enables R6G@Mn-sdc-1 to be used as a new luminescent sensor for trace water in organic solvents by monitoring the fluorescence intensity of released R6G. The limit of detection can reach 0.035% in ethanol (v : v), which is among the most sensitive fluorescent water probes.

A turn-on trace water sensor was obtained via the single-crystal to single-crystal transformation process of a metal–organic framework.