A novel metal–organic framework of Ba–hemin with enhanced cascade activity for sensitive glucose detection

Early glucose detection is important in both healthy people and diabetic patients. Glucose biosensing based on glucose oxidase (GOX) is a common method. However, native proteins are mostly membrane impermeable and are prone to degradation in complex sample environments. Herein, we report a facile one-step biomineralization method by simply mixing aqueous solutions of hemin and barium nitrate with glucose oxidase (GOX) to form Ba–hemin@GOX composites. Glucose (Glu) is introduced through self-driven sampling to trigger the GOX-catalysed production of hydrogen peroxide, which could help the subsequent 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation reaction catalysed by Ba–hemin to yield the blue-coloured product. The sensor exhibited a detection limit as low as 3.08 μM. The operability and accuracy of the Ba–hemin@GOX biosensor were confirmed by the quantitative determination of glucose in real samples, such as tap water, serum and drinks. Moreover, the Ba–hemin@GOX-based colorimetric biosensor showed good selectivity, storage stability and recoverability. The experimental results reveal that a GOX activity of more than 90% was still maintained even after being incubated at 60 °C for 30 minutes, and Ba–hemin@GOX could be reused for glucose detection at least six times. Even after 30 days of storage, the relative activity was still more than 90%. Overall, the developed Ba–hemin@GOX biosensor provides a valuable and general platform for applications in colorimetric biosensing and medical diagnostics.

The Ba–hemin@GOX composite is used for sensitive glucose detection.     

Preparation and characterization of an edible metal–organic framework/rice wine residue composite

In this communication, using rice wine residue (RWR) as the support, an edible γ-cyclodextrin-metal–organic framework/RWR (γ-CD-MOF/RWR) composite with a macroscopic morphology was synthesized. The obtained edible composite is promising for applications in drug delivery, adsorption, food processing, and others.

An edible metal–organic framework/rice wine residue composite was made with large surface area for potential applications in drug delivery, adsorption, food processing, and others.

As a typical class of porous materials, metal–organic frameworks (MOFs) have attracted increasing attention since being first proposed by Yaghi and co-workers.¹ Over the past two decades, owing to their large surface area, ultrahigh porosity and tunable pore size,² MOFs have exhibited great prospects for gas storage and separation,^3,4 catalysis,^5–8 sensors,⁹ drug delivery,^10–12etc. Among numerous reported MOFs, γ-cyclodextrin-MOF (γ-CD-MOF), which is connected by the (γ-CD)₆ units of alkaline earth metal ions, was initially synthesized and reported by Stoddart et al.^13,14 in the 2010s. Owing to the –OCCO– groups derived from γ-CD, this kind of MOF is edible and therefore opens a new path for preparing green, biocompatible and edible MOF materials.^13,15,16 For example, Stoddart et al.¹¹ reported a co-crystallization approach to trap ibuprofen and lansoprazole inside γ-CD-MOF, and the resultant composite microspheres can be used for sustained drug delivery. Zhang et al.¹⁷ proposed a strategy to graft cholesterol over the surface of γ-CD-MOF to form a protective hydrophobic layer to improve its water stability. Many researchers succeeded in preparing oral delivery medicine with high drug loading and an enhanced therapeutic effect by combining the drug molecules with γ-CD-MOF.^16,18–20 These works present the excellent application prospects of γ-CD-MOF in the medical field.Since MOFs possess so many attractive advantages, extensive studies have focused on combining MOFs with many other functional materials (metal nanoparticles, quantum dots, carbon matrices and polyoxometalates, etc.) by means of the synergistic effect, leading to the formation of novel composites designed for targeted applications.^21–28 However, these reported composites were still presented as loose powders, which may not be convenient for the applications. Therefore, the question of how to prepare MOFs-based composites for larger particles at low cost is of great significance. On the other hand, as a traditional alcoholic beverage, rice wine has been popular in southern China and some other Asian nations for thousands of years.²⁹ The rice wine lees or rice wine residue (RWR) is a by-product of the fermentation process of rice wine. It is a mixture of proteins, amino acids and polysaccharides. It is traditionally a health food in some Asian nations.³⁰ The edibility, extensive source, low cost and specific macroscopic shape make RWR a potential functional material for further use of MOFs.Herein, a facile and environmental-friendly strategy has been developed to realize the growth of γ-CD-MOF on rice wine residue, resulting in the formation of an edible MOF/RWR composite in the shape of rice grains. The material characterization confirmed the obtained composite possesses the characteristics of MOF. Except for the edible γ-CD-MOF/RWR, other MOF/RWR composites (HKUST-1, ZIF-67 and MIL-100(Fe)/RWR composites; shown in Fig. S1^†) were prepared to demonstrate the universality of this synthesis strategy.The synthesis procedure of the γ-CD-MOF/RWR composite is schematically illustrated in Fig. 1. The rice wine residue was soaked in deionized water for 12 h and then washed with deionized water three times before vacuum freeze-drying. Similar to the synthesis of γ-CD-MOF powder,¹⁵ KOH was dissolved into water. Then certain amounts of the aforementioned dry rice wine residue were soaked into the K⁺-containing solution for 2 h in order to absorb the sufficient potassium ions. K⁺ was then linked by the coordination of –OCCO– units in γ-CD and RWR with the three-dimensional interconnected network. After vapor diffusion of MeOH and some other procedures described in the synthesis of γ-CD-MOF powder (seen in ESI), the γ-CD-MOF/RWR composite (Fig. 2) was obtained. This method is convenient as no extra binders are needed during the whole process. The same procedure was employed to prepare the RWR composites with other MOFs (HKUST-1, ZIF-67 and MIL-100(Fe)). And the syntheses are briefly described in the ESI. The images of the obtained composites are shown in Fig. S1.^†Open in a separate windowFig. 1Schematic illustration of the synthesis procedure of γ-CD-MOF/RWR composite.Open in a separate windowFig. 2Digital photo of the γ-CD-MOF/RWR composite.The rice wine residue, of which the elemental analysis is shown in Table S1,^† is mainly composed of polysaccharides and proteins. Thus, a broad peak at around 22.2° in the XRD patterns of rice wine residue can be observed (Fig. S2^†), which is due to its poor crystallinity.³¹ The XRD patterns of γ-CD-MOF and γ-CD-MOF/RWR composite samples are shown in Fig. 3a. The characteristic peaks at 5.6°, 6.9°, 13.3°, 16.6°, 20.6° and 23.2°, observed from the XRD patterns of γ-CD-MOF, agree with the previously reported works.^32,33 Meanwhile, compared with γ-CD-MOF, the γ-CD-MOF/RWR composite shows similar characteristic peaks with lower intensity, indicating a lower crystallinity of the MOF within the composite. Fig. 3b shows the FT-IR spectra of different samples. Compared with the rice wine residue, the peaks in regions 1 and 2 of γ-CD-MOF and γ-CD-MOF/RWR can be ascribed to the stretching vibration of –CH₂ and –C–O–C– of the MOF, respectively.^15,34 These results further confirm the formation of the γ-CD-MOF in the γ-CD-MOF/RWR composite.Open in a separate windowFig. 3XRD patterns (a) and FT-IR spectra (b) of γ-CD-MOF/RWR composite, γ-CD-MOF and RWR.The SEM images were collected to further investigate the micromorphology of the as-prepared samples. As shown in Fig. 4a, a three-dimensional layered network structure and rich macropores of the rice wine residue rough surface can be seen. γ-CD-MOF (Fig. 4b) exhibits a uniform body-centered cubic shape with an average size of 4.27 μm, which is in accordance with the reported works.^15,35,36 Meanwhile, the images of the γ-CD-MOF/RWR composite (Fig. 4c and d) show that the cubic γ-CD-MOF crystals are well dispersed on the surface of the rice wine residue and even partially integrated into the framework of the rice wine residue. Compared with the pristine γ-CD-MOF, some γ-CD-MOF in γ-CD-MOF/RWR is not an intact cubic structure, exhibiting a significantly different morphology. This suggests a synergistic effect between the MOF crystals and the rice wine residue during the growth of MOF crystals, rather than a simple physical mixture of the two materials. The thermal stability of the γ-CD-MOF/RWR composite was investigated via TGA analysis. As shown in Fig. S3,^† the decomposition temperature of γ-CD-MOF/RWR composite slightly increased compared with those of pristine γ-CD-MOF and rice wine residue. Moreover, the γ-CD-MOF/RWR composite was stable in water, methanol and ethanol (shown in Fig. S4^†) even under mild stirring. These results indicate an improved physiochemical stability of γ-CD-MOF after the incorporation of rice wine residue. This finding further confirms the synergistic effect between them.Open in a separate windowFig. 4SEM images of rice wine residue (a), γ-CD-MOF (b) and γ-CD-MOF/RWR composite (c and d). Fig. 5a shows the nitrogen sorption isotherms of the γ-CD-MOF and γ-CD-MOF/RWR composite. Both pristine γ-CD-MOF and γ-CD-MOF/RWR exhibit typical type-I isotherms, demonstrating their microporous structures. The pore size distributions of pure γ-CD-MOF and γ-CD-MOF/RWR (Fig. 5b) confirm the existence of micropores (between 1 and 2 nm). The calculated Brunauer–Emmett–Teller (BET) surface areas, micropore volume and total pore volume are listed in 35,37 The specific surface area of the γ-CD-MOF/RWR composite is 651 m² g⁻¹, which is significantly higher than that of the pure rice wine residue (10.8 m² g⁻¹). Thus, the increase in the specific surface area of γ-CD-MOF/RWR composite can be attributed to the growth of γ-CD-MOF on the RWR support. Therefore, γ-CD-MOF/RWR composite inherits both the high porosity of γ-CD-MOF and the macroscopic morphology of rice wine residue, which should contribute to its practical applications.Open in a separate windowFig. 5N₂ adsorption and desorption isotherms (a) and pore size distributions (b) of γ-CD-MOF/RWR composite and corresponding comparative samples.Summary of the BET areas (S_BET), micropore volume (V_micro) and total pore volume (V_tot) of γ-CD-MOF, γ-CD-MOF/RWR composite and pure rice wine residue

A green metal–organic framework to monitor water contaminants

The CIM-80 material (aluminum(iii)-mesaconate) has been synthetized in high yield through a novel green procedure involving water and urea as co-reactants. The CIM-80 material exhibits good thermal stability with a working range from RT to 350 °C with a small contraction upon desolvation. Moreover, this material is stable in water at different pH values (1–10) for at least one week, and shows a LC₅₀ value higher than 2 mg mL⁻¹. The new material has been tested in a microextraction methodology for the monitoring of up to 22 water pollutants while presenting little environmental impact: only 20 mg of CIM-80 and 500 μL of acetonitrile are needed per analysis. The analytical performance of the CIM-80 in the microextraction strategy is similar to or even better for several pollutants than that of MIL-53(Al). The average extraction efficiencies range from ∼20% for heavy polycyclic aromatic hydrocarbons to ∼70–100% for the lighter ones. In the case of the emerging contaminants, the average extraction efficiency can reach values up to 70% for triclosan and carbamazepine.

A low cytotoxic MOF prepared with an environmental-friendly approach, as a novel extractant of water pollutants using a microextraction method.     

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.     

Improved NO reduction by using metal–organic framework derived MnOx–ZnO

Derivatives based on metal frameworks (MOFs) are attracting more and more attention in various research fields. MOF-based derivatives x% MnO_x–ZnO are easily synthesized by the thermal decomposition of Mn/MOF-5 precursors. Multiple technological characterizations have been conducted to ascertain the strengthening interaction between Mn species (Mn²⁺ or Mn³⁺) and Zn²⁺ (e.g., XRD, FTIR, TG, XPS, SEM, H₂-TPR and Py-FTIR). The 5% MnO_x–ZnO exhibits the highest NO conversion of 75.5% under C₃H₆-SCR. In situ FTIR and NO-TPD analysis showed that monodentate nitrates, bidentate nitrates, bridged bidentate nitrates, nitrosyl groups and C_xH_yO_z species were formed on the surface, and further hydrocarbonates or carbonates were formed as intermediates, directly generating N₂, CO₂ and H₂O.

Derivatives based on metal frameworks (MOFs) are attracting more and more attention in various research fields.     

Enhanced electrochemical properties of cerium metal–organic framework based composite electrodes for high-performance supercapacitor application

Cerium metal–organic framework based composites (Ce-MOF/GO and Ce-MOF/CNT) were synthesized by a wet chemical route and characterized with different techniques to characterize their crystal nature, morphology, functional groups, and porosity. The obtained Ce-MOF in the composites exhibit a nanorod structure with a size of ∼150 nm. The electrochemical performance of the composites was investigated in 3 M KOH and 3 M KOH + 0.2 M K₃Fe(CN)₆ electrolytes. Enhanced electrochemical behavior was obtained for the Ce-MOF/GO composite in both electrolytes and exhibited a maximum specific capacitance of 2221.2 F g⁻¹ with an energy density of 111.05 W h kg⁻¹ at a current density of 1 A g⁻¹. The large mesoporous structure and the presence of oxygen functional groups in Ce-MOF/GO could facilitate ion transport in the electrode/electrolyte interface, and the results suggested that the Ce-MOF/GO composite could be used as a high-performance supercapacitor electrode material.

The presence of oxygen functional groups in GO enhances the charge storage behavior of Ce-MOF/GO composites for use as supercapacitor electrode materials.     

Novel porous iron phthalocyanine based metal–organic framework electrochemical sensor for sensitive vanillin detection

Vanillin is widely used as a flavor enhancer and is known to have numerous other interesting properties, including antidepressant, anticancer, anti-inflammatory, and antioxidant effects. However, as excess vanillin consumption can affect liver and kidney function, simple and rapid detection methods for vanillin are required. Herein, a novel electrochemical sensor for the sensitive determination of vanillin was fabricated using an iron phthalocyanine (FePc)-based metal–organic framework (MOF). Scanning electron microscopy and transmission electron microscopy showed that the FePc MOF has a hollow porous structure and a large surface area, which impart this material with high adsorption performance. A glassy carbon electrode modified with the FePc MOF exhibited good electrocatalytic performance for the detection of vanillin. In particular, this vanillin sensor had a wide linear range of 0.22–29.14 μM with a low detection limit of 0.05 μM (S/N = 3). Moreover, the proposed sensor was successfully applied to the determination of vanillin in real samples such as vanillin tablets and human serum.

A novel electrochemical sensor based on an iron phthalocyanine (FePc) MOF for the sensitive detection of vanillin.     

Polyvinylpyrrolidone-assisted synthesis of highly water-stable cadmium-based metal–organic framework nanosheets for the detection of metronidazole

Recently, much effort has been dedicated to ultra-thin two-dimensional metal–organic framework (2D MOF) nanosheets due to their outstanding properties, such as ultra-thin morphology, large specific surface area, abundant modifiable active sites, etc. However, the preparation of high-quality 2D MOF nanosheets in good yields still remains a huge challenge. Herein, we report 2D cadmium-based metal–organic framework (Cd-MOF) nanosheets prepared in a one-pot polyvinylpyrrolidone (PVP)-assisted synthesis method with high yield. The Cd-MOF nanosheets were characterized with good stability and dispersion in aqueous systems, and were highly selective and sensitive to the antibiotic metronidazole (MNZ) with low limit of detection (LOD: 0.10 μM), thus providing a new and promising fluorescent sensor for rapid detection of MNZ in aqueous solution.

Except PVP was added for Cd-MOF nanosheets, the preparation process of bulk and Cd-MOF nanosheets was similar.     

Photocatalytic cascade reactions and dye degradation over CdS–metal–organic framework hybrids

Two bifunctional CdS–MOF composites have been designed and fabricated. The hybrids exhibited synergistic photocatalytic performance toward two cascade reactions under visible light integrating photooxidation activity of CdS and Lewis acids/bases of the MOF. The composite further promoted the photodegradation of dyes benefiting from effective electron transfer between the MOF and CdS.

Two bifunctional CdS–MOF composites have been successfully fabricated and exhibited synergistic photocatalytic performance toward two-step cascade reactions and dye photodegradation.

Cascade reactions are usually required for the synthesis of pharmaceuticals, pesticides and various fine chemicals,¹ especially for heterocyclic compounds.^1b Typically, benzylidene malononitrile, an essential intermediate for pharmaceutical production,^1f is normally prepared through a two-step reaction involving first oxidation of benzyl alcohol and then a Knoevenagel condensation of benzaldehyde with malononitrile.^2d Generally, the first step is mainly concentrated on the precious metal catalysts, and usually requires organic solvent, high temperatures, or high O₂ pressures, which largely limits its large-scale application.² The second Knoevenagel reaction is traditionally catalyzed by weak bases under homogeneous conditions, which is not favourable for recovery and recycling of catalysts.^2c Therefore, it is of great importance to develop a low-cost, stable and environmentally-friendly multifunctional catalyst.Solar energy, as an abundant natural resource, has attracted significant interest in photocatalytic water splitting, CO₂ or organic substrate transformations.^3,4 However, given that natural solar radiation is scattered, intermittent and constantly fluctuating, increasing the conversion rate of solar energy into chemical energy through photosensitive materials remains to be a great challenge.⁵ Significantly, a typical semiconductor material, CdS, displays excellent photocatalytic performance for many chemical reactions under light irradiation, such as photooxidation due to its a narrow band gap energy (2.4 eV) and efficient visible light absorption.⁶ However, the fact that a rapid recombination of photoelectrons and holes in CdS, and easy agglomeration of CdS nanoparticles (NPs) greatly impedes its practical application.^6d,7 Therefore, stable and effective supports should be required to stabilize pure CdS NPs.Metal organic frameworks (MOFs),⁸ featuring ordered porosities and large surface areas, have been widely used to stabilize various guest molecules, including metal nanoparticles, semiconductors and quantum dots.^7,8d,9 Recently, MOF-based composites have attracted intensive attention in photocatalysis field.^5a,9f,10 Unfortunately, most MOFs exhibit a wide bandgap and only absorb ultraviolet light region.^7,11 In addition, pure MOFs generally have a single active site, largely limiting catalytic reaction types.^9d Therefore, photoactive CdS combined with the advantages of MOFs can help construct a synergistic hybrid material.⁷Bearing above idea in mind, we have successfully fabricated a bifunctional CdS/NH₂-MIL-125 photocatalyst based on photosensitive CdS and active NH₂-MIL-125 (Scheme 1). The cooperative effect greatly improved photocatalytic performance of the composite toward the cascade reaction of selective oxidation of benzyl alcohol to benzaldehyde tandemly with a condensation of benzaldehyde with malononitrile. The superior catalytic activity mainly benefits from excellent photooxidation activity of CdS while the outer NH₂-MIL-125 plays multiple roles; it acts as a Lewis base site, accelerates the reaction by O₂ enrichment in air atmosphere, and stabilizes the CdS cores. Furthermore, effective electron transfer between MOF and CdS endows the hybrid outstanding photo-degradation performance toward organic pollutants.Open in a separate windowScheme 1Schematic illustration for the preparation of CdS/MOF hybrid.The crystallographic structure of CdS/NH₂-MIL-125 ^7c,d is analyzed and confirmed using powder X-ray diffraction (PXRD). As shown in Fig. 1a, the as-synthesized NH₂-MIL-125 has identical diffraction patterns as the simulated NH₂-MIL-125, which indicates the successful synthesis of MOF. For the diffraction patterns of CdS/NH₂-MIL-125, except for the typical diffraction peaks of MOF, two additional peaks appear at 2-theta values of 26.5° and 43.9° are assignable to CdS. And the peak intensities are enhanced along with increased CdS loadings. N₂ sorption experiments reveal that the Brunauer–Emmett–Teller (BET) surface areas of NH₂-MIL-125 and 15 wt% CdS/NH₂-MIL-125 are 956 and 613 m² g⁻¹, respectively (Fig. 1b). The decreased surface areas indicate that CdS NPs may be successfully loaded on the MOF, and are well stabilized by the pores. The morphology of 15 wt% CdS/NH₂-MIL-125 is investigated by scanning electron microscopy (SEM). Fig. 1c shows the retained octahedral morphology of MOF with an average diameter of 200–300 nm. In addition, the transmission electron microscopy (TEM) image shows uniform dispersion of CdS particles (average size, 3.7 nm) throughout MOF (Fig. 1d), further demonstrating their successful assembly. The actual contents of CdS in CdS/NH₂-MIL-125 samples have been confirmed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The percentages by weight of CdS are very close to the nominal values (Table S1, ESI^†).Open in a separate windowFig. 1(a) PXRD patterns of simulated NH₂-MIL-125, as-synthesized NH₂-MIL-125, and CdS/NH₂-MIL-125. (b) N₂ sorption isotherms of NH₂-MIL-125 and 15 wt% CdS/NH₂-MIL-125 at 77 K. (c) SEM and (d) TEM images of 15 wt% CdS/NH₂-MIL-125 and (inset in d) the corresponding size distribution of CdS NPs.The cascade reaction between benzyl alcohol and malononitrile to produce benzylidene malononitrile under visible light irradiation has been investigated by CdS/NH₂-MIL-125. The reaction involves two steps including the first photocatalytic oxidation of benzyl alcohol to form benzaldehyde, and the second Knoevenagel reaction of benzaldehyde and malononitrile. As shown in †). These results highlight the important roles of each component in CdS/NH₂-MIL-125 and their excellent synergistic effects toward cascade reaction.Cascade reactions of benzyl alcohol oxidation followed by Knoevenagel condensation^a

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.     

An efficient modulated synthesis of zirconium metal–organic framework UiO-66

The use of large amounts of deleterious solvents in the synthesis of metal–organic frameworks (MOFs) is one of the important factors limiting their application in industry. Herein, we present a detailed study of the synthesis of UiO-66, which was conducted with hydrobromic (HBr) acid as a modulator for the first time, at a high concentration of precursor solution (ZrCl₄ and H₂BDC, both 0.2 mol L⁻¹). Powder crystals with atypical cuboctahedron structure were obtained which indicated that the HBr acid modulator played roles by competitive coordination and deprotonation modulation, thereby controlling the processes of nucleation and crystal growth. The properties of the obtained materials were systematically characterized and compared with those of materials synthesized with hydrofluoric (HF) acid and hydrochloric (HCl) acid modulators. Despite the high concentration of defectivity, the UiO-66 material synthesized with the HBr acid additive has the characteristics of larger specific surface area, excellent thermal stability and higher porosity in the structure. Besides that, the present protocol has the advantages of high reaction mass efficiency (RME), and feasibility of scalable synthesis, providing a facile and sustainable route to diverse Zr-based MOFs.

The use of large amounts of deleterious solvents in the synthesis of metal–organic frameworks (MOFs) is one of the important factors limiting their application in industry.     

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.     

A new iron-based metal–organic framework with enhancing catalysis activity for benzene hydroxylation

A new Fe-based metal–organic framework (MOF), termed Fe-TBAPy Fe₂(OH)₂(TBAPy)·4.4H₂O, was solvothermally synthesized. Structural analysis revealed that Fe-TBAPy is built from [Fe(OH)(CO₂)₂]_∞ rod-shaped SBUs (SBUs = secondary building units) and 1,3,6,8-tetrakis(p-benzoate)pyrene (TBAPy⁴⁻) linker to form the frz topological structure highlighted by 7 Å channels and 3.4 Å narrow pores sandwiching between the pyrene cores of TBAPy⁴⁻. Consequently, Fe-TBAPy was used as a recyclable heterogeneous catalyst for benzene hydroxylation. Remarkably, the catalysis reaction resulted in high phenol yield and selectivity of 64.5% and 92.9%, respectively, which are higher than that of the other Fe-based MOFs and comparable with those of the best-performing heterogeneous catalysts for benzene hydroxylation. This finding demonstrated the potential for the design of MOFs with enhancing catalysis activity for benzene hydroxylation.

A new Fe-based MOFs catalyst was used for benzene hydroxylation with the high phenol yield (64.5%) and selectivity (92.9%).     

Identifying misbonded atoms in the 2019 CoRE metal–organic framework database

Databases of experimentally-derived metal–organic framework (MOF) crystal structures are useful for large-scale computational screening to identify which MOFs are best-suited for particular applications. However, these crystal structures must be cleaned to identify and/or correct various artifacts. The recently published 2019 CoRE MOF database (Chung et al., J. Chem. Eng. Data, 2019, 64, 5985–5998) reported thousands of experimentally-derived crystal structures that were partially cleaned to remove solvent molecules, to identify hundreds of disordered structures (approximately thirty of those were corrected), and to manually correct approximately 100 structures (e.g., adding missing hydrogen atoms). Herein, further cleaning of the 2019 CoRE MOF database is performed to identify structures with misbonded or isolated atoms: (i) structures containing an isolated atom, (ii) structures containing atoms too close together (i.e., overlapping atoms), (iii) structures containing a misplaced hydrogen atom, (iv) structures containing an under-bonded carbon atom (which might be caused by missing hydrogen atoms), and (v) structures containing an over-bonded carbon atom. This study should not be viewed as the final cleaning of this database, but rather as progress along the way towards the goal of someday achieving a completely cleaned set of experimentally-derived MOF crystal structures. We performed atom typing for all of the accepted structures to identify those structures that can be parameterized by previously reported forcefield precursors (Chen and Manz, RSC Adv., 2019, 9, 36492–36507). We report several forcefield precursors (e.g., net atomic charges, atom-in-material polarizabilities, atom-in-material dispersion coefficients, electron cloud parameters, etc.) for more than five thousand MOFs in the 2019 CoRE MOF database.

The 2019 CoRE MOF database was cleaned by identifying structures containing isolated atoms, overlapping atoms, misplaced hydrogens, and under/over-bonded carbons.     

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′.     

Synthesis and adsorption performance of La@ZIF-8 composite metal–organic frameworks

In this study, ZIF-8 with a rhombic dodecahedron structure was prepared by a hydrothermal method. Then La(OH)₃, was successfully loaded onto the ZIF-8 by an immersion deposition method, to form a lanthanide-based metal–organic framework (La@ZIF-8) composites. The structure and properties of La@ZIF-8 were verified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and zeta potential measurements. The optimum process conditions are discussed within the materials and methods. The effects of initial phosphorus concentration, dosage, pH and contact reaction time on the phosphorus removal performance of the nanomaterial were investigated. The results indicated that La@ZIF-8 exhibited an excellent adsorption capacity (147.63 mg g⁻¹) and its phosphorus removal efficiency could reach as high as 99.7%. Experimental data were interpreted using different adsorption kinetic and isotherm models. The kinetic behavior conformed to the pseudo-second-order kinetic model, which indicated the chemisorption of phosphorus by La@ZIF-8. The adsorption behavior of phosphorus by La@ZIF-8 fitted well to the Langmuir isotherm model, suggesting a monolayer chemical adsorption process. The majority of the adsorbed phosphate could be desorbed by NaOH (2 mol L⁻¹), and the removal efficiency of the recycled La@ZIF-8 reached 90%, even after the fifth cycle. The obtained results demonstrate the great application potential of the prepared La@ZIF-8 as a fascinating adsorbent for the removal of phosphate.

In this study, La(OH)₃ was successfully loaded on ZIF-8 by immersion deposition method, to form lanthanide-based metal–organic frameworks (La@ZIF-8) composites.     

High-performance quasi-solid-state flexible supercapacitors based on a flower-like NiCo metal–organic framework

NiCo metal–organic framework (MOF) electrodes were prepared by a simple hydrothermal method. The flower-like NiCo MOF electrode exhibited an exciting potential window of 1.2 V and an excellent specific capacitance of 927.1 F g⁻¹ at 1 A g⁻¹. The flower-like NiCo MOF//activated carbon (AC) device delivered a high energy density of 28.5 W hkg⁻¹ at a power density of 400.5 W kg⁻¹ and good cycle stability (95.4% after 5000 cycles at 10 A g⁻¹). Based on the flower-like NiCo MOF electrode, the asymmetric quasi-solid-state flexible supercapacitor (AFSC) was prepared and exhibited good capacitance retention after bending (79% after 100 bends and 64.4% after 200 bends). Furthermore, two AFSCs in series successfully lit up ten parallel red LED lights, showing great application potential in flexible and wearable energy storage devices.

The flower-like NiCo MOF prepared by a hydrothermal has a specific capacitance of 927.1 F g⁻¹ at 1 A g⁻¹ and a capacitance retention of 69.7% from 1 A g⁻¹ to 10 A g⁻¹, showing excellent electrochemical performance.     

Tailoring biocompatibility of composite scaffolds of collagen/guar gum with metal–organic frameworks

Metal–organic frameworks (MOFs) are microporous materials with high potential for biomedical applications. They are useful as drug delivery systems, antibacterials, and biosensors. Recently, composite materials comprised of polymer matrixes and MOFs have gained relevance in the biomedical field due to their high potential as materials to accelerate wound healing. In this work, we studied the potential applications of composite hydrogels containing MgMOF74, CaMOF74, and Zn(Atz)(Py). The composite hydrogels are biodegradable, being completely degraded after 15 days by the action of collagenase and papain. The composites showed high biocompatibility reaching cell viabilities up to 165.3 ± 8.6% and 112.3 ± 12.8% for porcine fibroblasts and human monocytes, respectively. The composites did not show hemolytic character and they showed antibacterial activity against Escherichia coli reaching up to 84 ± 5% of inhibition compared with amoxicillin (20 ppm). Further, the immunological assays revealed that the composites produce a favorable cell signaling stimulating the secretion of the TGF-β and MCP-1 cytokines and maintaining the secretion of TNF-α in normal levels. Finally, the composites showed potential to be used as controlled drug delivery systems reaching a release efficiency of 30.5 ± 2.5% for ketorolac. Finally, results revealed that ColGG-Zn(Atz)(Py) was the best formulation evaluated.

MOF Zn(Atz)(Py) tailored the biocompatibility of collagen/guar gum hydrogels stimulating the cell metabolism and the secretion of TGF-β and MCP-1. Further, Zn(Atz)(Py) increased the antibacterial activity and improved the drug release performance.     

CoSx/C hierarchical hollow nanocages from a metal–organic framework as a positive electrode with enhancing performance for aqueous supercapacitors

Benefiting from abundant redox chemistry and high electrochemical properties, metal sulfides have been broadly employed as electrode materials in supercapacitor systems. However, the predominant limitation in their performance, which arises from indifferent electron and ion dynamics for transportation and a rapid slash in capacitance, is of particular concern. Herein, we portray the cobalt sulfides/carbon (CoS_x/C) hierarchical hollow nanocages using ZIF-67 nanocrystals coated with carbon from resorcinol–formaldehyde (ZIF-67@RF) as a self-sacrificial template. The RF acted as a hard framework to prevent the hollow structure from breaking and was transformed to a carbon layer to enhance the charge transfer process. When used as positive electrodes in supercapacitor systems with aqueous electrolytes, the optimized CoS_x/C hierarchic hollow nanocages exhibited a considerable specific capacitance (618 F g⁻¹ at 2 A g⁻¹), superior rate performance (83.6% capacitance retention of the initial capacity when the current density was amplified from 2 A g⁻¹ to 50 A g⁻¹) and an extraordinary cycle stationarity along with an undiminished specific capacitance after 10 000 cycles. In this study, the meticulously designed hierarchical hollow structure that we conceived not only provides an outstanding electrochemical performance but also provides options for other related materials, such as various MOFs.

Benefiting from abundant redox chemistry and high electrochemical properties, metal sulfides have been broadly employed as electrode materials in supercapacitor systems.     

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.