Synthetic methods, properties and controlling roles of synthetic parameters of zeolite imidazole framework-8: A review

Development of Zeolite Imidazole Framework-Based Adsorbent for Effective Microextraction and Preconcentration of Histamine in Food Samples

This study is the first to focus on the preconcentration and determination of histamine (HIS) in food samples using zeolite imidazole frameworks (ZIFs) on a solid-phase microextraction (SPME) platform. ZIF was developed on a polypropylene hollow fiber (PPHF) substrate (ZIF@PPHF) and characterized. The extraction performance was optimized by adjusting several parameters, including pH, contact time for adsorption, and desorption conditions. Under the optimized conditions, a wide linear dynamic range (0.05-250 mg/L) with high R2 values (0.9989), low limit of detection (0.019 mg/L), and low limit of quantification (0.050 mg/L) were determined as analytical figures of merit. Additionally, a reusability study confirmed that ZIF@PPHF preconcentrated 83% of the HIS up to the fourth cycle. The developed method was used to preconcentrate HIS in fish and cheese samples. The spiked real samples confirmed the validity and accuracy of this method. The percentage mean recoveries ± relative standard deviation (% RSD, n = 3) at the concentration levels of 5, 10, and 50 mg/L of HIS and the sample amount of 5 g for intra- and inter days ranged from 97 ± 1.10 to 102.80 ± 0.90 and from 96.40 ± 1.82 to 103.40 ± 0.79, respectively. The results suggest that the analytical method validation parameters were acceptable, indicating the repeatability and sensitivity of the method.

Zeta Potential and Size Analysis of Zeolitic Imidazolate Framework-8 Nanocrystals Prepared by Surfactant-Assisted Synthesis

The crystal nucleation and growth mechanism of monodispersed metal-organic framework nanoparticles were studied using time-resolved light dynamic, electrokinetic, and powder X-ray diffraction methods. We confirmed that zeolitic imidazolate framework-8 (ZIF-8) nanocrystals follow a nonclassical crystal growth pathway, where a fast nucleation occurs with dense liquid clusters or nanocrystals forming spontaneously when two precursors are mixed. We also explored the zeta potential and solvodynamic size changes of ZIF-8 prepared by a surfactant-assisted synthesis. Three modulators, including 1-methylimidazole (1-mIm), tris(hydroxymethyl)aminomethane (THAM), and (1-hexadecyl)trimethylammonium bromide (CTAB), were studied. We found that 1-mIm dramatically increases the rate of nucleation of ZIF-8. With an increasing amount of 1-mIm, which functions as a coordination modulator, the size increases, and the zeta potential of ZIF-8 decreases. Whereas THAM, as both a coordination and a deprotonation modulator, increases the size and zeta potential of ZIF-8 simultaneously, CTAB, as a long alkyl cationic surfactant, mainly adsorbs on the surface of ZIF-8, and the zeta potential of the formed ZIF-8 is controlled by the amount of CTAB in solution compared with its critical micelle concentration. Overall, we reveal that the modulator type and concentration can be used to control the size and zeta potential of the dispersed ZIF-8 nanocrystals in a colloid system. The experiments also enable identification of the nucleation and crystal growth processes of ZIF-8. The findings will be applicable to other nanocrystals in colloid systems, which are used for heterogeneous catalysis and guest molecular loadings.

Enantioseparation by zeolitic imidazolate framework-8-silica hybrid monolithic column with sulfobutylether-β-cyclodextrin as a chiral additive in capillary electrochromatography

A zeolitic imidazolate framework (ZIF)-8-silica hybrid monolithic column was prepared by one-step sol-gel method. The stationary phase in the monolithic column was characterized by Fourier-transform infrared spectra, X-ray diffraction, thermogravimetric analysis, nitrogen adsorption/desorption, and zeta potential. The results showed that ZIF-8-silica hybrid monolithic materials had abundant functional groups, good crystallinity, large specific surface area, and good thermal stability. A capillary electrochromatography (CEC) chiral separation system was for the first time constructed with ZIF-8-silica hybrid monolithic column and sulfobutylether-β-cyclodextrin (SBE-β-CD) as a chiral additive and was applied to separate the selected single and mixed chiral compounds (13 natural amino acids and 5 chiral pesticides). Under the optimized CEC conditions, all the single analytes achieved baseline separation with resolution of 2.14-5.94 and selectivity factor of 1.06-1.49 in less than 6 min, and the mixed amino acids with similar properties were also simultaneously enantioseparated (Rs ≥ 1.82). Relative standard deviations (RSDs) of migration time and column efficiency were lower than 4.26% and did not change significantly after 200 runs, evidencing excellent reproducibility and stability. These results demonstrate that the application of SBE-β-CD as a chiral additive for ZIF-8-silica hybrid monolithic columns is a promising method for the separation of chiral compounds.

Highly-Controlled Soft-Templating Synthesis of Hollow ZIF-8 Nanospheres for Selective CO2 Separation and Storage

Global warming is an ever-rising environmental concern, and carbon dioxide (CO2) is among its major causes. Different technologies, including adsorption, cryogenic separation, and sequestration, have been developed for CO2 separation and storage/utilization. Among these, carbon capture using nano-adsorbents has the advantages of excellent CO2 separation and storage performance as well as superior heat- and mass-transfer characteristics due to their large surface area and pore volume. In this work, an environmentally friendly, facile, bottom-up synthesis of ZIF-8 hollow nanospheres (with reduced chemical consumption) was developed for selective CO2 separation and storage. During this soft-templating synthesis, a combined effect of ultra-sonication and low-temperature hydrothermal synthesis showed better control over an oil-in-water microemulsion formation and the subsequent growth of large-surface-area hollow ZIF-8 nanospheres having excellent particle size distribution. Systematic studies on the synthesis parameters were also performed to achieve fine-tuning of the ZIF-8 crystallinity, hollow structures, and sphere size. The optimized hollow ZIF-8 nanosphere sample having uniform size distribution exhibited remarkable CO2 adsorption capability (∼2.24 mmol g-1 at 0 °C and 1.75 bar), a CO2/N2 separation selectivity of 12.15, a good CO2 storage capacity (1.5-1.75 wt %), and an excellent cyclic adsorption/desorption performance (up to four CO2 adsorption/desorption cycles) at 25 °C. In addition, the samples showed exceptional structural stability with only ∼15% of overall weight loss up to 600 °C under a nitrogen environment. Therefore, the hollow ZIF-8 nanospheres as well as their highly controlled soft-templating synthesis method reported in this work are useful in the course of the development of nanomaterials with optimized properties for future CO2 capture technologies.

Real-Time Quantifying Microdroplet Synthesis of Metal-Organic Framework Colloids Using Gas-Phase Electrophoresis

A hyphenated electrospray-differential mobility analysis (ES-DMA) was developed for providing a high-resolution, real-time quantitative analysis on the metal-organic framework (MOF) colloids produced via the concept of microfluidic flow chemistry. Zeolitic imidazolate framework-8 was chosen as the representative MOF of the study. The results show that the physical size and number concentration of the MOF colloid were successfully characterized by the hyphenated ES-DMA during the microdroplet synthetic process, with 3 nm and 4% of measurement uncertainties, respectively. The effects of the synthetic temperature and the molar ratio of the organic linker to metal precursor were investigated, providing an opportunity for accurate control on the particle size (100-200 nm) of the microdroplet-synthesized MOF. The work demonstrates a powerful approach for the real-time quality assurance and material optimization in microdroplet synthesis of colloidal MOFs.

Insights into the Structure-Property-Activity Relationship of Zeolitic Imidazolate Frameworks for Acid-Base Catalysis

Zeolitic imidazolate frameworks (ZIFs) have been extensively examined for their potential in acid-base catalysis. Many studies have demonstrated that ZIFs possess unique structural and physicochemical properties that allow them to demonstrate high activity and yield products with high selectivity. Herein, we highlight the nature of ZIFs in terms of their chemical formulation and the textural, acid-base, and morphological properties that strongly affect their catalytic performance. Our primary focus is the application of spectroscopic methods as instruments for analyzing the nature of active sites because these methods can allow an understanding of unusual catalytic behavior from the perspective of the structure-property-activity relationship. We examine several reactions, such as condensation reactions (the Knoevenagel condensation and Friedländer reactions), the cycloaddition of CO₂ to epoxides, the synthesis of propylene glycol methyl ether from propylene oxide and methanol, and the cascade redox condensation of 2-nitroanilines with benzylamines. These examples illustrate the broad range of potentially promising applications of Zn-ZIFs as heterogeneous catalysts.

Nanoporous ZIF-8 Microparticles as Acetylcholinesterase and Alkaline Phosphatase Mimics for the Selective and Sensitive Detection of Ascorbic Acid Oxidase and Copper Ions

In this study, the alkaline phosphatase (ALP)-like activity of zeolitic-imidazolate framework-8 (ZIF-8) is reported for the first time. Then, colorimetric sensors for the ascorbic acid oxidase (AAO) and copper ion (Cu²⁺) detection were developed based on the acetylcholinesterase (AChE)- and ALP-like activities of ZIF-8. The ZIF-8 has good mimetic enzyme activity and exhibits high affinity to the substrates. Its AChE- and ALP-like activities also have good reusability and storage stability. Good linear dependences are obtained in the range of 1.3-250.0 μM (AChE-like activity-based) and 4.5-454.5 μM (ALP-like activity based) for Cu²⁺ detection. Furthermore, good linear dependence is also obtained based on the ALP-like activity of ZIF-8 for AAO detection in the range of 2.3-454.5 U/L. Their limits of detection (LODs) are calculated to be 0.7 µM, 2.8 µM, and 1.8 U/L, respectively. Finally, the sample spiked recoveries of Cu²⁺ in tap water, Cu²⁺, and AAO in human serum and rabbit plasma were measured, and the results are in the range of 80.0-119.3%. In short, the preparation of ZIF-8 is simple, environmentally friendly, and harmless, and can realize highly selective detection of AAO and Cu²⁺ in an efficient and fast process.

How Do Proteins Associate with Nanoscale Metal-Organic Framework Surfaces?

It is well known that colloidal nanomaterials, upon exposure to a complex biological medium, acquire biomolecules on their surface to form coronas. Porous nanomaterials present an opportunity to sequester biomolecules and/or control their orientation at the surface. In this report, a metal-organic framework (MOF) shell around gold nanorods was compared to MOF nanocrystals as potential protein sponges to adsorb several common proteins (lysozyme, beta-lactoglobulin-A, and bovine serum albumin) and potentially control their orientation at the surface. Even after correction for surface area, MOF shell/gold nanorod materials adsorbed more protein than the analogous nanoMOFs. For the set of proteins and nanomaterials in this study, all protein-surface interactions were exothermic, as judged by isothermal titration calorimetry. Protein display at the surfaces was determined from limited proteolysis experiments, and it was found that protein orientation was dependent both on the nature of the nanoparticle surface and on the nature of the protein, with lysozyme and beta-lactoglobulin-A showing distinct molecular positioning.