Metal-organic framework mixed-matrix disks: Versatile supports for automated solid-phase extraction prior to chromatographic separation

Extraction improvement of ionic liquid-functionalized silica by in situ anion-exchange for online solid-phase extraction of estrogens in honey

The accurate detection of analytes in honey is affected by the complex substrates, making it crucial to employ an effective sample preparation technique. In this work, an imidazolium ionic liquid was functionalized to the silica surface by a click reaction for solid-phase extraction (SPE) column, and in situ anion-exchange process was performed with different organic anions (dodecyl sulfonate, dodecyl benzene sulfonate, and naphthalene sulfonate). These SPE columns were evaluated through extracting the estrogens. The naphthalene sulfonate-based SPE column displayed the best extraction ability among these, and it was combined with high-performance liquid chromatography-diode array detection to establish an online enrichment and analysis system. Under the optimal test conditions, an online analytical method was developed, with high enrichment factors (1872-4744), wide linear ranges (0.0033-1.50, 0.0165-1.50, and 0.0330-1.50 μg g-1), and low detection limits (0.001-0.010 μg g-1). The method successfully determined several estrogens in some honey samples, and achieved satisfactory recovery results.

A membrane solid-phase extraction method based on MIL-53-mixed-matrix membrane for the determination of estrogens and parabens: polyvinylidene difluoride membrane vs. polystyrene-block-polybutadiene membrane

In this work, MIL-53(Al), as an inorganic 'filler' component, was embedded in polyvinylidene difluoride (PVDF) and polystyrene-block-polybutadiene (SBS) matrices to prepare two mixed-matrix membranes (MMMs), using a simpler method than that previously reported. The PVDF and SBS membranes retained much of the properties of PVDF, SBS, and native MIL-53(Al). The prepared MMMs were then placed in a vortex-stirred sample solution to develop a membrane solid-phase extraction method to extract estrogens and parabens which were determined by high-performance liquid chromatography with fluorescence detection. The extraction efficiencies of the two membranes were compared, with the PVDF membrane exhibiting superior performance. In addition, the PVDF membrane was more free-standing and flexible, and its preparation method was also more facile and simple. The extraction conditions were optimized, and the analytical method showed low limits of detection (0.005-0.18 ng/mL), good linearity, and high accuracy, with recoveries ranging from 90.7 to 102.5%. As a result, this membrane solid-phase extraction method indicated its potential for application in aqueous sample pretreatment. For metal-organic framework based MMM used in this method, in addition to being durable, free-standing, mechanically stable, and possessing a large area, it should also exhibit high MOF incorporation, good flexibility, and appropriate thickness and weight.

Role of Lewis Acid Metal Centers in Metal–Organic Frameworks for Ultrafast Reduction of 4-Nitrophenol

Metal–Organic Frameworks (MOFs) can be a good alternative to conventional catalysts because they are non-toxic and can be selective without compromising efficiency. Nano MOFs such as UiO-66 have proven themselves to be competitive in the catalytic family. In this study, we report the excellent catalytic behavior of UiO-66 MOF in the reduction of a model reaction: 4-Nitrophenol (4-NP) to 4-Aminophenol (4-AP) over MOF-5 (Zn-BDC) and MIL-101 (Fe-BDC). Nano UiO-66 crystals were synthesized by a hydrothermal process and characterized by Powder X-ray Diffraction, Diffused Reflectance UV-Vis spectroscopy, Scanning Electron Microscopy, and Transmission Electron Microscopy. The catalysts’ performance during the hydrogenation reduction reaction from 4-NP to 4-AP was investigated in the presence of a reducer, NaBH4. The UiO-66 nano crystals exhibited excellent catalytic behavior owing to its large surface area and Lewis acidic nature at the metal nodes. Furthermore, UiO-66 showed excellent recyclability behavior, verified during repeated consecutive use in a sequence. The catalyst yielded similar catalytic behavior during the reduction of nitrophenols at each cycle, which is a novel finding.

Zirconium-Based Metal-Organic Framework Mixed-Matrix Membranes as Analytical Devices for the Trace Analysis of Complex Cosmetic Samples in the Assessment of Their Personal Care Product Content

A device comprising a zirconium-based metal-organic framework (MOF) mixed-matrix membrane (MMM) framed in a plastic holder has been used to monitor the content of personal care products (PCPs) in cosmetic samples. Seven different devices containing the porous frameworks UiO-66, UiO-66-COOH, UiO-67, DUT-52, DUT-67, MOF-801, and MOF-808 in polyvinylidene fluoride (PVDF) membranes were studied. Optimized membranes reach high adsorption capacities of PCPs, up to 12.5 mg·g^-1 benzophenone in a 3.0 mg·L^-1 sample. The MMM adsorption kinetics, uptake measurements, and isotherm studies were carried out with aqueous standard solutions of PCPs to ensure complete characterization of the performance. The studies demonstrate the high applicability and selectivity of the composites prepared, highlighting the performance of PVDF/DUT-52 MMM that poses uptakes up to 78% for those PCPs with higher affinity while observing detection limits for the entire method down to 0.03 μg·L^-1. The PVDF/DUT-52 device allowed the detection of parabens and benzophenones in the samples, with PCPs found at concentrations of 1.9-24 mg·L^-1.

Rapid synthesis of graphite phase carbon nitride/zeolitic imidazolate framework-8 with hierarchical structure and its superior adsorption of polycyclic aromatic hydrocarbons from aqueous solution

Graphite phase carbon nitride (g-C₃N₄) incorporating zeolitic imidazolate framework-8 (ZIF-8) nanocomposite (g-C₃N₄ /ZIF-8) with hierarchical structure was synthesized successfully by simple and rapid in situ growth method at room temperature. The composites were used as an adsorbent of solid-phase extraction (SPE) and the superior adsorptive removal of polycyclic aromatic hydrocarbons (PAHs) for the first time. Under several optimum conditions, the g-C₃N₄ /ZIF-8-SPE-HPLC-FLD method show low detection limits (0.006-3.41 μg L^-1) and limit of quantification (0.02-11.3 μg L^-1), wide linear ranges from 0.02 to 1000 μg L^-1 for all compounds, correlation coefficients (r) of more than 0.9968, and satisfying reproducibility (relative standard deviations, RSDs < 4.0% for intra-day, RSDs < 8.3% for inter-day), the spiked recoveries at two levels of 10.0, 50.0 μg L^-1 were in the range of 77.4%-114% with the RSDs less than 8.66%. In addition, the g-C₃N₄/ZIF-8 nanocomposites demonstrated excellent enrichment ability and extraction efficiency for PAHs compared with commercial adsorbents, which might since there were strong π-π stacking force, hydrophobic interaction, hydrogen bonding, and more adsorption sites compared with other adsorbents. Finally, the g-C₃N₄ /ZIF-8 based SPE method was combined with high-performance liquid chromatography (HPLC) to detect fifteen PAHs in environmental water samples successfully.

A new strategy for the preparation of core-shell MOF/Polymer composite material as the mixed-mode stationary phase for hydrophilic interaction/ reversed-phase chromatography

A facile method for efficient synthesis of core-shell composite material was proposed. In this method, the silica microspheres were co-modified with metal organic framework (MOF-235) and polyethylene glycol polymer (PEG) and used as mixed-mode stationary phase (MOF-235@PEG@silica) for high-performance liquid chromatography. Elemental analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Brunauer-Emmett-Teller etc. methods were used to investigate the properties of the core-shell composite material. The MOF-235@PEG@silica stationary phase showed flexible selectivity for the separation of both hydrophilic and hydrophobic compounds especially for the separation of nine alkaloids, which showed superior hydrophilic separation performance than previous MOF-based composite stationary phases. Some factors including the pH of buffer salt, the ratio of organic phase and water phase in the mobile phase have been investigated, suggesting that the chromatographic retention mechanism of the column was a mixed mode of hydrophilic and reversed phase. The composite material also showed excellent chromatographic repeatability with the RSDs of the retention time found to be 0.2%-0.6% (n = 10) and the standard addition test in the actual sample proved that it can be used for practical sample analysis. In short, it provided a general way for preparing MOFs-based composites as mixed-mode chromatographic stationary phases, and changed the current status of MOF-based composite materials as single mode chromatographic stationary phases.

On-line enrichment and determination of aristolochic acid in medicinal plants using a MOF-based composite monolith as adsorbent

In this study, modified UiO-66-NH₂ and N-methylolacrylamide (NMA) were used as common monomers to prepare a metal organic framework (MOF)-based composite monolith through in-situ polymerization, which was used as a new adsorbent to purify and enrich aristolochic acid-I (AA-I) in medicinal plants. The MOF-based composite monolithic column was characterized by nitrogen adsorption-desorption isotherm, mercury intrusion porosimetry and scanning electron microscopy (SEM). The adsorption ability of MOF-based composite monolith for AA-I was compared with that of the polymer monolith without MOF added. The results proved that the addition of UiO-66-NH₂ can increase both the specific surface area and the permeability of the monolith. Moreover, the adsorption amount of AA-I on the monolith improved. This proposed on-line solid phase extraction (SPE) method showed good linear relationship in the range 0.044 ~ 400 μg/mL with r = 0.9994; the limit of detection (LOD) was 13.08 ng/mL and the limit of quantification (LOQ) was 44.00 ng/mL; the intra-day and inter-day accuracies were less than 0.97%; the inter-column accuracies was less than 6.11%; the recovery was in the range of 91.11%~106.48%. The method was found to be easy, accurate and convenient for on-line enrichment and purification of AA-I in medicinal plants.

3D Printing in analytical sample preparation

In the last 5 years, additive manufacturing (three-dimensional printing) has emerged as a highly valuable technology to advance the field of analytical sample preparation. Three-dimensional printing enabled the cost-effective and rapid fabrication of devices for sample preparation, especially in flow-based mode, opening new possibilities for the development of automated analytical methods. Recent advances involve membrane-based three-dimensional printed separation devices fabricated by print-pause-print and multi-material three-dimensional printing, or improved three-dimensional printed holders for solid-phase extraction containing sorbent bead packings, extraction disks, fibers, and magnetic particles. Other recent developments rely on the direct three-dimensional printing of extraction sorbents, the functionalization of commercial three-dimensional printable resins, or the coating of three-dimensional printed devices with functional micro/nanomaterials. In addition, improved devices for liquid-liquid extraction such as extraction chambers, or phase separators are opening new possibilities for analytical method development combined with high-performance liquid chromatography. The present review outlines the current state-of-the-art of three-dimensional printing in analytical sample preparation.

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.

Evaluation of metal-organic framework NH2-MIL-101(Fe) as an efficient sorbent for dispersive micro-solid phase extraction of phenolic pollutants in environmental water samples

This work proposes an application of amine-functionalized metal-organic framework (NH₂-MIL-101(Fe)) as sorbent for dispersive micro-solid phase extraction (D-μSPE) of ten priority phenolic pollutants. The sorbent was simply synthesized under facile condition. The entire D-μSPE process was optimized by studying the effect of experimental parameters affecting the extraction recovery of the target analytes. The final extract was analyzed using high performance liquid chromatography with photodiode array detector. Under the optimum condition, the proposed procedure can be applied for wide linear calibration ranges between 1.25–5000 μg L⁻¹ with the correlation coefficients of greater than 0.9900. The limits of detection (LODs) and limits of quantitation (LOQs) were in the ranges of 0.4–9.5 μg L⁻¹ and 1.25–30 μg L⁻¹, respectively. The precision evaluated in terms of the relative standard deviations (RSDs) of the intra- and inter-day determinations of the phenol compounds at their LOQ concentrations were below 13.9% and 12.2%, respectively. High enrichment factors up to 120 were reached. The developed method has been successfully applied to determine phenol residues in environmental water samples. The satisfactory recoveries obtained by spiking phenol standards at two different concentrations (near LOQs and 5 times as high as LOQs) ranged from 68.4–114.4%. The results demonstrate that the NH₂-MIL-101(Fe) material is promising sorbent in the D-μSPE of phenolic pollutants.

Woven cotton yarn-graphene oxide-layered double hydroxide composite as a sorbent for thin film microextraction of nonsteroidal anti-inflammatory drugs followed by quantitation through high performance liquid chromatography

The applicability of a highly flexible and natural cotton yarn-graphene oxide-layered double hydroxide composite (CY-GO-LDH) was introduced for the extraction of the targets in the current study. For increasing the contact area of the analytes and the prepared sorbent, the green substrate was woven and employed as the substrate for the construction of GO layers. It was proved that the prepared CY-GO-LDH film is a reliable sorbent for thin film microextraction (TFME) of the nonsteroidal anti-inflammatory drugs (NSAIDs) including acetylsalicylic acid, naproxen, diclofenac, ibuprofen and mefenamic acid in human urine and plasma. Extraction factors were optimized using multivariate optimization strategy. High adherence of GO-LDH to the natural substrate made this technique more robust for routine analysis. There are two consecutive steps to optimize the parameters influencing the extraction of analytes; First, a Plackett-Burman Design (PBD) was utilized to screen the significant factors. Second, the selected factors were optimized utilizing the Box-Behnken Design (BBD). The extracted NSAIDs were analyzed by HPLC-UV. Under the obtained optimum condition, the linearity of the method was 0.2-200  μg L^-1. Limits of detection, limits of quantification and intra-day as well as inter-day RSDs were lower than 0.25  μg L^-1, 0.72  μg L^-1 and 6.1%, respectively. The method was successfully used to determine NSAIDs in different human biological fluids.

Metal–Organic Frameworks as Key Materials for Solid-Phase Microextraction Devices—A Review

Metal–organic frameworks (MOFs) have attracted recently considerable attention in analytical sample preparation, particularly when used as novel sorbent materials in solid-phase microextraction (SPME). MOFs are highly ordered porous crystalline structures, full of cavities. They are formed by inorganic centers (metal ion atoms or metal clusters) and organic linkers connected by covalent coordination bonds. Depending on the ratio of such precursors and the synthetic conditions, the characteristics of the resulting MOF vary significantly, thus drifting into a countless number of interesting materials with unique properties. Among astonishing features of MOFs, their high chemical and thermal stability, easy tuneability, simple synthesis, and impressive surface area (which is the highest known), are the most attractive characteristics that makes them outstanding materials in SPME. This review offers an overview on the current state of the use of MOFs in different SPME configurations, in all cases covering extraction devices coated with (or incorporating) MOFs, with particular emphases in their preparation.

Carbon composite membrane derived from MIL-125-NH2 MOF for the enhanced extraction of emerging pollutants

Porous carbon derived from amine-functionalized MIL-125 metal-organic framework (C-MIL-125-NH₂) was prepared by carbonization at high temperature under inert atmosphere, and used for adsorption of bisphenol A (BPA) and 4-tert-butylphenol (4-tBP). The obtained carbon showed bimodal porosity and fast extraction of both pollutants in batch conditions following a pseudo-second-order model. The adsorption mechanism was studied by the measurement of zeta potential, and the results suggested that π-π stacking interactions between the carbon material and the phenol molecules probably are the main sorption mechanism. The prepared C-MIL-125-NH₂ was incorporated into mechanically stable membranes for flow-through solid-phase extraction of studied phenols prior to HPLC analysis. The hybrid material showed excellent permeance to flow, easy regeneration and good performance for the simultaneous enrichment of mixtures of BPA and 4-tBP, facilitating their determination when present at low concentration levels.

Magnetic metal-organic frameworks nanocomposites for negligible-depletion solid-phase extraction of freely dissolved polyaromatic hydrocarbons

The bioavailability of a pollutant is usually evaluated based on its freely dissolved concentration (C_free), which can be measured by negligible-depletion equilibrium extraction that is commonly suffered from long equilibration time. Herein, metal-organic framework (MOF) composites (Fe₃O₄@MIL-101), consists of a magnetic Fe₃O₄ core and a MIL-101 (Cr) MOF shell, is developed as sorbents for negligible-depletion magnetic solid-phase extraction (nd-MSPE) of freely dissolved polyaromatic hydrocarbons (PAHs) in environmental waters. The freely dissolved PAHs in 1000 mL water samples are extracted with 1.5 mg MOF composites, and desorbed with 0.9 mL of acetonitrile under sonication for 5 min. The MOF composites exclude the extraction of dissolved organic matter (DOM) and DOM-associated PAHs by size exclusion. Additionally, the combined interactions (hydrophobic, π-π and π-complexation) between PAHs and composites markedly reduced the extraction equilibration time to < 60 min for all the studied PAHs with logK_OW up to 5.74. Moreover, the porous coordination polymers property of the MOFs makes the proposed nd-MSPE based on the partitioning of PAHs and thus excludes the competitive adsorption of coexisting substances. The developed nd-MSPE approach provides low detection limits (0.08-0.82 ng L^-1), wide linear range (1-1000 ng L^-1) and high precision (relative standard deviations (RSDs) (3.3-4.8%) in determining C_free of PAHs. The measured C_free of PAHs in environmental waters are in good agreement with that of verified method. Given the large diversity in structure and pore size of MOFs, various magnetic MOFs can be fabricated for task-specific nd-MSPE of analytes, presenting a prospective strategy for high-efficiency measuring C_free of contaminants in environments.

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.

Direct photoimmobilization of extraction disks on "green state" 3D printed devices

Post-curing is essential to improve the mechanical properties of 3D printed parts fabricated by stereolithography (SLA), since right after 3D printing they remain in a "green state". It means that the 3D printed parts have reached their final shape, but the polymerization reaction has not been yet completed. Herein, we take advantage of the tacky partially polymerized surface of "green state" SLA 3D printed parts to immobilize extraction disks and miniature magnets, which after UV post-curing, become permanently attached to the 3D printed part resulting in a rotating-disk sorptive extraction device (RDSE). The developed "stick & cure" procedure is reagent-free and does not require any additional preparation time, specialized skills, or instrumentation. As proof of concept, 3D printed RDSE devices with immobilized chelating disks have been applied to the simultaneous extraction of 14 trace metals prior to ICP-OES determination, featuring LODs between 0.03 and 1.27 μg L^-1, and an excellent device-to-device reproducibility (n = 5, RSD = 2.7-8.3%). The developed method was validated using certified wastewater and soil reference samples, and satisfactory spiking recoveries were obtained in the analysis of highly polluted solid waste treatment plant leachates (89-110%). In addition, exploiting the versatility of 3D printing, nine RDSE devices with different shapes were fabricated. Their performance was evaluated and compared for the fast extraction of the highly toxic Cr (VI) as its 1,5-diphenylcarbazide complex in reversed-phase mode, showing different extraction performance on depending on the shape of the 3D printed RDSE device.

Magnesium-aluminum-layered double hydroxide-graphene oxide composite mixed-matrix membrane for the thin-film microextraction of diclofenac in biological fluids

Herein, the applicability of Mg-Al-layered double hydroxide-graphene oxide (LDH/GO) mixed-matrix membrane (MMM) for microextraction purposes is reported for the first time. The LDH/GO MMM was used as sorbent for the thin film microextraction (TFME) of diclofenac in human body fluids. The prepared LDH/GO composite has been incorporated into a mechanically stable polyvinylidene difluoride (PVDF) membrane. The contribution of GO in LDH/GO composites significantly improved the extraction efficiency of the TFME sorbent. After elution with methanol, diclofenac was quantified by high performance liquid chromatography-ultraviolet detection (HPLC-UV). Plackett-Burman design was used for screening the experimental factors of interest and specify the significant variables affecting the extraction efficiency. The effective factors were optimized using Box-Behnken design (BBD). Under the optimum conditions, limits of detections (LODs) were 0.14, 0.23 and 0.57 μg L^-1 in water, urine and plasma samples, respectively. Limits of quantifications (LOQs) were 0.46, 0.76 and 1.8 μg L^-1 in water, urine and plasma samples, respectively. Relative standard deviations (RSDs) at a spiked concentration of 10 μg L^-1 were 6.7, 6.9 and 7.1% (as intra-day RSD) in water, urine and plasma samples, respectively. The linear dynamic ranges (LDRs) were in the range of 0.5-200 μg L^-1. The applicability of the method was investigated by the extraction and determination of diclofenac in different biological fluids including urine and plasma samples.

Automated dispersive liquid-liquid microextraction based on the solidification of the organic phase

In this work, the dispersive liquid-liquid microextraction technique based on the solidification of the organic phase (DLLME-SFO) has been automated for the first time. DLLME-SFO is automated by hyphenating a sequential injection analysis (SIA) system with a custom-made robotic phase separator. Automated in-syringe DLLME is followed by phase separation in a 3D printed device integrating a Peltier cell set, mounted on a multi-axis robotic arm. The combined action of the flow system and the robotic arm is controlled by a single software package, enabling the solidification/melting and collection of the organic phase for further analyte quantification. As proof-of-concept, automated DLLME-SFO was applied to the extraction of parabens followed by separation using liquid chromatography, obtaining LODs between 0.3 and 1.3 µg L^-1 (4 mL of sample extracted in 1 mL of 1-dodecanol: MeOH, 15:85, v-v). The method showed a high reproducibility, obtaining intraday RSDs between 4.6% and 5.8% (n = 6), and interday RSDs between 5.6% and 8.6% (n = 6). The developed method was evaluated for the determination of parabens in water, urine, saliva, and personal care products.