Metal organic framework–organic polymer monolith stationary phases for capillary electrochromatography and nano-liquid chromatography

An Imine-Based Porous 3D Covalent Organic Polymer as a New Sorbent for the Solid-Phase Extraction of Amphenicols from Water Sample

In this paper, an imine-based porous 3D covalent organic polymer (COP) was synthesized via solvothermal condensation. The structure of the 3D COP was fully characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and powder X-ray diffractometry, thermogravimetric analysis, and Brunauer-Emmer-Teller (BET) nitrogen adsorption. This porous 3D COP was used as a new sorbent for the solid-phase extraction (SPE) of amphenicol drugs, including chloramphenicol (CAP), thiamphenicol (TAP), and florfenicol (FF) in aqueous solution. Factors were investigated for their effects on the SPE efficiency, including the types and volume of eluent, washing speed, pH, and salinity of water. Under the optimized conditions, this method gave a wide linear range (0.1-200 ng/mL) with a high correlation coefficient value (R² > 0.99), low limits of detection (LODs, 0.01-0.03 ng/mL), and low limits of quantification (LOQs, 0.04-0.10 ng/mL). The recoveries ranged from 83.98% to 110.7% with RSDs ≤ 7.02%. The good enrichment performance for this porous 3D COP might contribute to the hydrophobic and π-π interactions, the size-matching effect, hydrogen bonding, and the good chemical stability of 3D COP. This 3D COP-SPE method provides a promising approach to selectively extract trace amounts of CAP, TAP, and FF in environmental water samples in ng quantities.

Ionic liquid assisted in situ growth of nano-confined ionic liquids/metal-organic frameworks nanocomposites for monolithic capillary microextraction of microcystins in environmental waters

A facile in-situ ionothermal synthesis strategy for fabrication of ionic liquids/metal-organic frameworks (MOFs) (ILs@ZIF-8) nanocomposites hybrid monolith has been proposed to facilitate highly effective capillary microextraction (CME) of ultra-trace microcystins (MCs) in environmental waters. The ZnO nanoparticles (ZnO-NPs) were initially introduced into a precursor polymer monolith, and acted as the metal sources and anchoring seeds to construct ILs@ZIF-8 nanocomposites hybrid monolith via a nanoparticle-directed in-situ growth route in confined imidazolium ionic liquids. Detailed characterization based on scanning electron microscopy (SEM), X-ray diffraction (XRD) and N₂ adsorption-desorption isotherms confirmed that both the morphology and porous structure of ZIF-8 were finely tuned by the incorporation of ILs, which acted as solvents and structure directing agent. The confinement of ILs in ZIF-8 framework endows the ILs@ZIF-8 hybrid monolith additional adsorption sites and satisfied water stability for the synergistic enhancement of adsorption efficiency of MCs via multiple interactions (including π-π stacking, hydrogen bonding, hydrophobic and electrostatic interactions). Coupling ILs@ZIF-8 hybrid monolith-based CME to LC-MS enabled an efficient and sensitive analysis of MCs in surface waters with ultra-low detection limits (LOD ≤ 1.4 ng L^-1) and satisfactory recoveries (70.2%-107.0%). This study showed great potential for feasible design and fabrication of ILs@MOFs composites with synergistic and tunable structures toward efficient sample preparation applications.

Lipase-based MIL-100(Fe) biocomposites as chiral stationary phase for high-efficiency capillary electrochromatographic enantioseparation

A novel chiral porous column was fabricated by lipase immobilized MIL-100(Fe) biocomposites as chiral stationary phase through covalent coupling and applied to capillary electrochromatographic enantioseparation. MOF-based lipase biocomposites not only enhance stereoselective activities but also improve the stability and applicability of the enzyme. The functionalized porous columns were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and powder X-ray diffraction. The performance of the porous column was evaluated by enantioseparating amino acid enantiomers, affording high resolution over 2.0. Besides, the enantio-resolutions of phenylephrine, phenylsuccinic acid, chloroquine, and zopiclone were also greater than 2.0. The relative standard deviations of run-to-run, intra-, and inter-day repeatability were within 4.0% in terms of resolution and retention time, exhibiting excellent stability of the column. Conceivably, the results show that MOF-based lipase composites as chiral stationary phase offer a highly efficient means for enantioseparation in capillary electrochromatography, attributing to the enhanced enantioselective activities of lipase by highly ordered frameworks.

Recent trends on the implementation of reticular materials in column‐centered separations

Advances in the development of column‐based analytical separations are strongly linked to the development of novel materials. Stationary phases for chromatographic separation are usually based on silica and polymer materials. Nevertheless, recent advances have been made using porous crystalline reticular materials, such as metal‐organic frameworks and covalent organic frameworks. However, the direct packing of these materials is often limited due to their small crystal size and nonspherical shape. In this review, recent strategies to incorporate porous crystalline materials as stationary phases for liquid‐phase separations are covered. Moreover, we discuss the potential future directions in their development and integration into suitable supports for analytical applications. Finally, we discuss the main challenges to be solved to take full advantage of these materials as stationary phases for analytical separations.

High-Efficiency and Versatile Approach To Fabricate Diverse Metal-Organic Framework Coatings on a Support Surface as Stationary Phases for Electrochromatographic Separation

A large number of metal-organic frameworks (MOFs) have exhibited increasingly wide utilization in the field of chromatographic separation owing to their intrinsic fascinating properties. However, the previous studies on supported MOF coating-based chromatographic separation focused only on the synthesis and chromatographic performance of a certain kind of supported MOF coatings as stationary phases using the multiple-step, complicated, and time-consuming modification methods, which severely impeded the widespread application of MOFs in separation science. Herein, a high-efficiency and versatile methodology toward diverse supported MOF coating-based stationary phases to achieve high-efficiency chromatographic separation was first reported based on the immobilized cysteine (Cys)-triggered in situ growth (ICISG) strategy. As a proof-of-concept demonstration, four types of MOF crystals consisting of different ligands and metal ions (Zn²⁺, Cu²⁺, Fe³⁺, and Zr⁴⁺) were conveniently and firmly grown on a Cys-modified capillary using the ICISG strategy and employed as the functional stationary phase for electrochromatographic separation. A broad variety of neutral, acidic, and basic compounds were all separated in a highly efficient manner on the developed four MOF-coated columns. The maximum theoretical plate number for Cys-MIL-100(Fe)@capillary was close to 1.0 × 10⁵ plates/m, and the intraday, interday, and column-to-column repeatabilities of retention times for the four MOF-modified columns were all less than 5.25%. More interestingly, the diversified separation performance of the developed MOF-coated columns indicated that the preparation strategy and the skeletal structure of the MOF coating-based stationary phases have a significant influence on the electrochromatographic separation performance and column capacity. Benefiting from the strong universality and high applicability of the developed ICISG strategy, the present study provides an effective route to facilitate the design and fabrication of novel functional MOF-based chromatographic stationary phases.

A Comprehensive Review on Metal Organic Framework Based Preconcentration Strategies for Chromatographic Analysis of Organic Pollutants

Organic pollutants (OPs) are of worldwide concern for being hazardous to human existence and natural flora and fauna in view of their contaminating nature, bio-aggregation properties and long range movement abilities in environment. Metal organic frameworks (MOFs) are a new kind of crystalline porous material, composed of metal ions and multi dentate organic ligands with well-defined co-ordination geometry exhibiting promising application respect to adsorptive evacuation of OPs for chromatographic analysis. Applications of MOFs as preconcentration material and column packing material are reviewed. Key analytical characteristics of MOF based preconcentration techniques and coupled chromatographic procedures are summarized in detail. MOF based preconcentration strategies are compared with conventional sorbent based extraction techniques for thorough evaluation of performance of MOF materials.

Metal-Organic Frameworks for Liquid Phase Applications

In the last two decades, metal-organic frameworks (MOFs) have attracted overwhelming attention. With readily tunable structures and functionalities, MOFs offer an unprecedentedly vast degree of design flexibility from enormous number of inorganic and organic building blocks or via postsynthetic modification to produce functional nanoporous materials. A large extent of experimental and computational studies of MOFs have been focused on gas phase applications, particularly the storage of low-carbon footprint energy carriers and the separation of CO2-containing gas mixtures. With progressive success in the synthesis of water- and solvent-resistant MOFs over the past several years, the increasingly active exploration of MOFs has been witnessed for widespread liquid phase applications such as liquid fuel purification, aromatics separation, water treatment, solvent recovery, chemical sensing, chiral separation, drug delivery, biomolecule encapsulation and separation. At this juncture, the recent experimental and computational studies are summarized herein for these multifaceted liquid phase applications to demonstrate the rapid advance in this burgeoning field. The challenges and opportunities moving from laboratory scale towards practical applications are discussed.

Chiral Metal-Organic Framework d-His-ZIF-8@SiO2 Core-Shell Microspheres Used for HPLC Enantioseparations

Chiral metal-organic frameworks (MOFs) have aroused great attention in the chiral separation field based on their excellent characteristics, including abundant topological structures, large surface area, adjustable pore/channel sizes, multiple active sites, and good chemical stability. However, the irregular morphology and nonuniformity of the synthesized MOF particles cause low column efficiency and high column backpressure for MOF-packed columns, which significantly affects their separation performance. Herein, we prepared a homochiral d-his-ZIF-8@SiO₂ composite by growing of d-his-ZIF-8 on the carboxylic-functionalized SiO₂ microspheres via a simple one-pot synthesis approach. The d-his-ZIF-8@SiO₂ core-shell microspheres with uniform particles and narrow size distribution were applied as the chiral stationary phase (CSP) for enantioseparations in HPLC. Various racemates were separated on the d-his-ZIF-8@SiO₂-packed columns with n-hexane/isopropanol as the mobile phase. Eighteen racemates including alcohol, phenol, amine, ketone, and organic acid were well resolved on the homochiral d-his-ZIF-8@SiO₂ CSP. The d-his-ZIF-8@SiO₂ core-shell microspheres' CSP possesses an excellent chiral resolution ability toward various racemic compounds with good reproducibility and stability. Hence, the fabrication of chiral MOF@SiO₂ core-shell microspheres is an effective strategy to improve the application of homochiral MOFs as CSPs in the field of chromatography.

Polycarbonate Microchip Containing CuBTC-Monopol Monolith for Solid-Phase Extraction of Dyes

In the present study, preparation of CuBTC-monopol monoliths for use within the microchip solid phase extraction was undertaken through a 20-min UV lamp-assisted polymerization for 2,2-dimethoxy-2-phenyl acetophenone (DMPA), butyl methacrylate (BMA), and ethylene dimethacrylate (EDMA) alongside inclusion of the porogenic solvent system (1-propanol and methanol (1 : 1)). The resultant coating underwent coating using CuBTC nanocrystals in ethanolic solution of ethanolic solution of 1,3,5-benzenetricarboxylic acid (H₃BTC, 10 mM) and 10 mM copper(II) acetate Cu(CH₃COO)₂. This paper reports enhanced extraction, characterization, and synthesis studies for porous CuBTC metal organic frameworks that are marked by different methods including SEM/EDAX analysis, atomic force microscopy (AFM), and Fourier-transform infrared spectroscopy (FT-IR). The evaluation of the microchip's performance was undertaken as sorbent through retrieval of six toxic dyes (anionic and cationic dyes). Various parameters (desorption and extraction step flow rates, volume of desorption solvent, volume of sample, and type of desorption solvent) were examined to optimize dye extraction using fabricated microchips. The result indicated that CuBTC-monopol monoliths were permeable with the ability of removing impurities and attained high toxic dye extraction recovery (83.4-99.9%). The assessment of reproducibility for chip-to-chip was undertaken by computing the relative standard deviations (RSDs) of the six dyes in extraction. The interbatch and intrabatch RSDs ranged between 3.8 and 6.9% and 2.3 and 4.8%. Such features showed that fabricated CuBTC-monopol monolithic disk polycarbonate microchips have the potential of extracting toxic dyes that could be utilized for treating wastewater.

Chiral metal-organic cages used as stationary phase for enantioseparations in capillary electrochromatography

Since some metal-organic cages (MOCs) have been synthesized in past several years, the applications of MOCs such as drug delivery, molecular recognition, separation, catalysis, and gas storage, etc. have been witnessed with a significant increase. However, to the best of our knowledge, so far no one has used MOCs as chiral stationary phase to separate chiral compounds in CEC. In this study, three MOCs were developed as the stationary phase for CEC separation of enantiomers. The MOCs coated capillary column showed good chiral recognition ability for some chiral compounds, including amine, alcohols, ketone, etc. The influence of buffer concentration, applied voltage, pH of buffer solution on the chiral separations was also investigated. The RSDs of run-to-run, day-to-day, and column-to-column for retention time were 2.1-4.67%, 1.2-4.36%, and 3.62-6.43%, respectively. This work reveals that the chiral MOCs material is feasible for the enantioseparation in CEC.

Metal-Organic Frameworks for Food Safety

Food safety is a prevalent concern around the world. As such, detection, removal, and control of risks and hazardous substances present from harvest to consumption will always be necessary. Metal-organic frameworks (MOFs), a class of functional materials, possess unique physical and chemical properties, demonstrating promise in food safety applications. In this review, the synthesis and porosity of MOFs are first introduced by some representative examples that pertain to the field of food safety. Following that, the application of MOFs and MOF-based materials in food safety monitoring, food processing, covering preservation, sanitation, and packaging is overviewed. Future perspectives, as well as potential opportunities and challenges faced by MOFs in this field will also be discussed. This review aims to promote the development and progress of MOF chemistry and application research in the field of food safety, potentially leading to novel solutions.

Metal organic framework HKUST-1 modified with carboxymethyl-β-cyclodextrin for use in improved open tubular capillary electrochromatographic enantioseparation of five basic drugs

This work shows that the metal organic framework (MOF) HKUST-1 of type Cu₃(BTC)₂ (also referred to as MOF-199; a face-centered-cubic MOF containing nanochannels) is a most viable coating for use in enantioseparation in capillary electrochromatography (CEC). A HKUST-1 modified capillary was prepared and characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra, elemental analysis and thermogravimetric analysis. CEC-based enantioseparation of the basic drugs propranolol (PRO), esmolol (ESM), metoprolol (MET), amlodipine (AML) and sotalol (SOT) was performed by using carboxymethyl-β-cyclodextrin as the chiral selector. Compared with a fused-silica capillary, the resolutions are improved (ESM: 1.79; MET: 1.80; PRO: 4.35; SOT: 1.91; AML: 2.65). The concentration of chiral selector, buffer pH value, applied voltage and buffer concentration were optimized, and the reproducibilities of the migration times and R_s values were evaluated. Graphical abstract Schematic presentation of the preparation of a HKUST-1@capillary for enantioseparation of racemic drugs. Cu(NO₃)₂ and 1,3,5-benzenetricarboxylic acid (BTC) were utilized to prepare the HKUST-1@capillary. Then the capillary was applied to construct capillary electrochromatography system with carboxymethyl-β-cyclodextrin (CM-β-CD) for separation of basic racemic drugs.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry

Porous materials confined within capillary columns/microfluidic devices are discussed, and progress in chromatographic and membrane separations and catalysis is reviewed.

Ionic liquid-copolymerized monolith incorporated with zeolitic imidazolate framework-8 as stationary phases for enhancing reversed phase selectivity in capillary electrochromatography

A novel ionic liquid (1-allyl-methylimidazolium chloride, AlMeIm⁺Cl^-) polymer monolith poly(ionic liquid-co-ethylene dimethacrylate) incorporated with zeolitic imidazolate framework-8 (ZIF-8-poly(IL-co-EDMA)) was firstly synthesized as stationary phases of monolithic column for capillary electrochromatography by one-step copolymerization. Incorporation of ZIF-8 into ionic liquid polymer monolith evidently enhanced the separation selectivity for four alkylbenzenes in reversed phase capillary electrochromatography (CEC), due to the synergistic effect derived from the same imidazole ring structure of ionic liquid and organic ligands of ZIF-8. Meanwhile, electroosmotic flow (EOF) was generated by ionic liquid in a wide range of pH values from 2.0 to 12.0. The resultant monolithic columns were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR). The results indicated that the prepared monolithic columns had good permeability and mechanism stability. The resultant monolithic columns were applied for the separation of neutral compounds, anilines and phenols. The highest column efficiency was 2.07 × 10⁵ plates m^-1 (theoretical plates, N) for toluene. Under optimal conditions, reproducibility was obtained with relative standard deviations (RSDs) of the retention time for run-to-run, day-to-day, column-to-column and batch-to-batch were in the range of 1.58 - 3.19%, 1.92 - 3.87%, 3.84 - 4.96% and 2.63 - 4.33%, respectively. Incorporation ZIF-8 into ionic liquid polymer monolith was a promising way for the application of new materials in the fabrication of novel monolithic columns.

Advancements in the preparation of high-performance liquid chromatographic organic polymer monoliths for the separation of small-molecule drugs

The various advantages of organic polymer monoliths, including relatively simple preparation processes, abundant monomer availability, and a wide application range of pH, have attracted the attention of chromatographers. Organic polymer monoliths prepared by traditional methods only have macropores and mesopores, and micropores of less than 50 nm are not commonly available. These typical monoliths are suitable for the separation of biological macromolecules such as proteins and nucleic acids, but their ability to separate small molecular compounds is poor. In recent years, researchers have successfully modified polymer monoliths to achieve uniform compact pore structures. In particular, microporous materials with pores of 50 nm or less that can provide a large enough surface area are the key to the separation of small molecules. In this review, preparation methods of polymer monoliths for high-performance liquid chromatography, including ultra-high cross-linking technology, post-surface modification, and the addition of nanomaterials, are discussed. Modified monolithic columns have been used successfully to separate small molecules with obvious improvements in column efficiency.

Organic polymer-based monolithic stationary phases with incorporated nanostructured materials for HPLC and CEC

This review article is concerned with the recent advances made in the field of organic polymer-based monoliths with incorporated nanostructured materials (NSMs) for use in liquid chromatography and capillary electrochromatography. It covers the pertinent literature published over the last 7-8 years with a total of 56 references. The present article has two distinct parts: one major part encompassing "traditional" organic polymer-based monoliths modified with NSMs and a minor part on cryogels modified with NSMs.

Recent advances in metal-organic frameworks and covalent organic frameworks for sample preparation and chromatographic analysis

In the field of analytical chemistry, sample preparation and chromatographic separation are two core procedures. The means by which to improve the sensitivity, selectivity and detection limit of a method have become a topic of great interest. Recently, porous organic frameworks, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in this research area because of their special features, and different methods have been developed. This review summarizes the applications of MOFs and COFs in sample preparation and chromatographic stationary phases. The MOF- or COF-based solid-phase extraction (SPE), solid-phase microextraction (SPME), gas chromatography (GC), high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) methods are described. The excellent properties of MOFs and COFs have resulted in intense interest in exploring their performance and mechanisms for sample preparation and chromatographic separation.

Nanoparticle-Directed Metal-Organic Framework/Porous Organic Polymer Monolithic Supports for Flow-Based Applications

A two-step nanoparticle-directed route for the preparation of macroporous polymer monoliths for which the pore surface is covered with a metal-organic framework (MOF) coating has been developed to facilitate the use of MOFs in flow-based applications. The flow-through monolithic matrix was prepared in a column format from a polymerization mixture containing ZnO-nanoparticles. These nanoparticles embedded in the precursor monolith were converted to MOF coatings via the dissolution-precipitation equilibrium after filling the pores of the monolith with a solution of the organic linker. Pore surface coverage with the microporous zeolitic imidazolate framework ZIF-8 resulted in an increase in surface area from 72 to 273 m² g^-1. Monolithic polymer containing ZIF-8 coating was implemented as a microreactor catalyzing the Knoevenagel condensation reaction and also in extraction column format enabling the preconcentration of trace levels of toxic chlorophenols in environmental waters. Our approach can be readily adapted to other polymers and MOFs thus enabling development of systems for flow-based MOF applications.

Pressure inverse solubility and polymorphism of an edible γ-cyclodextrin-based metal–organic framework

An edible metal–organic framework, γ-CD-MOF-1, has been obtained in a new trigonal form. The γ-CD-MOF-1 high-pressure dissolution as well as the cubic and trigonal polymorphs are closely related and regulated by the guest framework interactions and adsorption in the pores.