Magnetic polyimide nanocomposite for analysis of parabens in cooking wine by magnetic solid-phase extraction coupled with gas chromatography - Mass spectrometry.

A magnetic polyimide (PI) nanocomposite has been synthesized by phase inversion of PI and simultaneous encapsulation of Fe3O4 nanoparticles. The Fe3O4/PI nanocomposite was characterized by a variety of characterization techniques, including infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption isotherms, and vibrating sample magnetometry. The results showed that the prepared nanocomposite had a homogeneous structure, adequate specific surface area (76.1 m2/g) and high saturation magnetization (42.9 emu/g). Using parabens as model analytes, the performance of the Fe3O4/PI nanocomposite as an adsorbent for magnetic solid-phase extraction (MSPE) was evaluated. The extracted parabens were desorbed and determined by gas chromatography-mass spectrometry (GC-MS). The parameters affecting the extraction and desorption efficiency of parabens were optimized. Under the optimal conditions, the developed MSPE/GC-MS method was successfully applied to the determination of parabens in cooking wine. The MSPE/GC-MS method exhibited broad linearity (0.2-100 µg/L), low detection limits (0.04-0.05 µg/L), and satisfactory extraction recoveries (79.2 %-113.3 %) with relative standard deviations (RSDs) ranging from 0.7 % to 10.4 %. For real cooking wine samples, the spiked recoveries ranged from 91.7 % to 118.7 % with RSDs of 1.0 %-11.2 %. The results demonstrated that the Fe3O4/PI nanocomposite was an effective adsorbent, and this work provides a novel reference for the easy preparation of magnetic adsorbent materials.

Fabrication of Highly Oriented Ultrathin Zirconium Metal-Organic Framework Membrane from Nanosheets towards Unprecedented Gas Separation.

Concurrent regulation of crystallographic orientation and thickness of zirconium metal-organic framework (Zr-MOF) membranes is challenging but promising for their performance enhancement. In this study, we pioneered the fabrication of uniform triangular-shaped, 40 nm thick UiO-66 nanosheet (NS) seeds by employing an anisotropic etching strategy. Through innovating confined counter-diffusion-assisted epitaxial growth, highly (111)-oriented 165 nm-thick UiO-66 membrane was prepared. The significant reduction in thickness and diffusion barrier in the framework endowed the membrane with unprecedented CO2 permeance (2070 GPU) as well as high CO2 /N2 selectivity (35.4), which surpassed the performance limits of state-of-the-art polycrystalline MOF membranes. In addition, highly (111)-oriented 180 nm-thick NH2 -UiO-66 membrane showing superb H2 /CO2 separation performance with H2 permeance of 1230 GPU and H2 /CO2 selectivity of 41.3, was prepared with the above synthetic procedure.

A hybrid ZIF-8/ZIF-62 glass membrane for gas separation.

Metal-organic framework (MOF) glasses have demonstrated great potential for high-performance separation. Herein a uniform hybrid MOF glass membrane was fabricated by using the liquid state of ZIF-62 to facilitate the melting of ZIF-8. The doping of ZIF-8 enhanced both the adsorption capacity as well as the ideal C3H6/C3H8 selectivity of ZIF-62 glass. As expected, the hybrid glass membrane exhibited good C3H6/C3H8 separation performance while preserving the CO2 performance of the sole ZIF-62 membrane.

Magnetic polyimide nanosheet microspheres for trace analysis of estrogens in aqueous samples by magnetic solid-phase extraction-gas chromatography-mass spectrometry.

Magnetic polyimide nanosheet microspheres (PI-NMs) were used for magnetic solid-phase extraction (MSPE) for the first time. The PI-NMs were modified with magnetic Fe3O4 nanoparticles by chemical coprecipitation to produce the PI-NM/Fe3O4 composite. The prepared composite possessed a nanosheet structure, large specific surface area (71.5 m2/g), high saturation magnetization (19.1 emu/g), large adsorption capacity (≥ 676 ng/mg for selected estrogens), and good extraction stability (> 10 times). Trace estrogens in environmental water and urine samples were extracted by the PI-NM/Fe3O4 composite, desorbed, derivatized, and analyzed by gas chromatography-mass spectrometry (GC-MS). The derivatization, extraction, and desorption conditions were optimized. Extraction equilibrium was achieved within 1 min due to the good dispersibility and large specific surface area of the PI-NM/Fe3O4 composite. Under the optimized conditions, the MSPE/GC-MS method validation results showed wide linearity (0.02-50 µg/L or 0.05-50 µg/L), high determination coefficients (R2 ≥ 0.9983), good intraday and interday precisions (expressed as relative standard deviation, RSDs ≤ 8.2%), and low limits of detection (LODs, 0.001-0.015 µg/L). For the real environmental water and urine samples, the recoveries and RSDs were 77.0-112.5% and 0.1-10.7%, respectively. The performance of the MSPE/GC-MS method proved that the PI-NM/Fe3O4 composite was a good alternative material for the extraction of organic pollutants in aqueous samples.

Bar adsorptive microextraction device coated with polyimide microsphere assembled by nanosheets combined with thermal desorption-gas chromatography for trace analysis of nitroaromatic explosives in environmental waters.

Polyimide (PI) microspheres assembled by nanosheets were used for bar adsorptive microextraction (BAµE) for the first time. The PI microsphere possessed self-organized hierarchical nanostructure, large specific surface area (170 m2/g) and good thermostability (up to 400 °C). The BAµE device was prepared by adhering the PI microspheres on a quartz bar with Kapton double sided tape. Trace nitroaromatic explosives in environmental waters were extracted by the BAµE device, desorbed by thermal desorption (TD), and analyzed by gas chromatography-mass spectrometry (GC-MS). The reproducibility of five BAµE devices prepared in parallel was less than 13.0% (expressed as relative standard deviation, RSD). The BAµE device could stand up to 30 extraction/desorption cycles without decrease of extraction efficiency. The results of method validation showed that the BAµE-TD/GC-MS method possessed wide linearity (0.05-50 µg/L or 0.05-20 µg/L), high correlation coefficients (> 0.9987), good precision (RSDs < 11.8%), low detection limits (0.005-0.013 µg/L) and high enrichment factors (528-1410). Relative recoveries were in the range of 72.2-122.6% with RSDs between 0.1% and 10.5% for real water samples. These results proved that the proposed method was a good choice for determination of organic pollutants in water samples.

A MOF Glass Membrane for Gas Separation.

Metal-organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF-62 MOF glass membrane and exploited its intrinsic gas-separation properties. The MOF glass membrane was fabricated by melt-quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF-62 MOF membrane on a porous ceramic alumina support. The molten ZIF-62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H2 /CH4 , CO2 /N2 and CO2 /CH4 mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.

A Highly Symmetric Bimetallic-Tetracarboxylate Framework: Two-Step Crystallization and Gas Separation Properties.

A novel noninterpenetrated MOF (NbU-2, NbU denotes Ningbo University) based on an enneanuclear bimetallic cluster was synthesized by postsynthetic uptake of Ni(II) ions of a sodium-based framework via two-step crystallization. This result demonstrated that the hydrolysis of metal ions in the step-by-step synthesis process is controllable and provided a new method for synthesizing high-nuclear metal cluster-based MOF materials. Note that the activated NbU-2 displays relatively high CO2/N2 selectivity, which was proved by breakthrough experiments of CO2/N2 mixtures.

Porous polyimide particle-coated adsorptive microextraction bar combined with thermal desorption-gas chromatography for rapid determination of parabens in condiments.

Bar adsorptive microextraction using polyimide (PI) particles as the extraction phase followed by thermal desorption and gas chromatography-mass spectrometry (BAµE/TD-GC-MS) was developed to detect parabens in condiments, such as soy sauce, vinegar, and cooking wine. The PI particles were prepared by pneumatic spray combined with the immersion-precipitation phase transformation method. The prepared particles have highly porous surfaces, on which the 10-60 nm open nanopores are closely packed. Particles between 250-500 µm were then sieved out and used as extraction phase for BAµE. In contrast to the smooth and dense surface of the conventional PI phase transformation bar, the macroscopic rough surface of the PI particle bar and its microscopic porous particle surfaces provided larger extraction interfaces and more surface adsorption sites, both of which enhanced the extraction mass flux. With an extraction time of 2 min, the absolute recoveries of parabens by the PI particle bar were 1.9˜2.7 times those obtained by the conventional PI phase transformation bar. The intrabatch and interbatch precisions of the PI particle bars were less than 4.6% and 7.5%, respectively, and the PI particle bar exhibited a long lifetime of more than 50 extraction/desorption cycles. To realize rapid determination of parabens, the extraction time was fixed at 2 min. The analytical performance for standard water samples showed wide linearity (0.14-50 µg/L) with good correlation coefficients (r > 0.9980), good precision (RSD < 5.6%), appropriate detection limits (0.005-0.008 µg/L), and high enrichment factors (305-626). For the analysis of parabens in diluted condiments, the relative recoveries were between 86.1% and 109.0% with RSDs ranging from 0.1%-8.7%.

Inhibition of water adsorption into polar solid-phase microextraction materials with ultrathin polydimethylsiloxane coating for thermal desorption-gas chromatography analysis.

Solid-phase microextraction (SPME) coupled with thermal desorption-gas chromatography (TD-GC) has become a powerful analysis tool for volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) in water samples. However, water adsorption into polar microextraction phase is usually unavoidable during the extraction process, and the burst of large amounts of water vapour during thermal desorption will cause serious problems to GC separation and detectors. Pawliszyn's group had demonstrated that the tens of micron-thick, defect-free polydimethylsiloxane (PDMS) coating could act as a perfect barrier for water adsorption and offer much better compatibility in complex matrices. However, the PDMS overcoat largely decreased the uptake rate of polar analytes into the inner sorbent. In order to quantify the effect of PDMS coating thickness on water adsorption amount and the extraction kinetics, ultrathin PDMS layer was used to coat the polar extraction phase with polyimide (PI) as a model in this work. It was surprising to find that the PDMS coating with the thickness less than one micron can decrease the water adsorption by 96%, while the extraction efficiency for polar analytes (phenolic compounds and nitroaromatic explosives) was decreased by less than 20% at the extraction time of 30 min. Moreover, the kinetic data showed that the thinner the PDMS coating was, the less the uptake rate of polar analytes into PI extraction phase decreased. Finally, polar poly (phthalazine ether sulfone ketone) (PPESK) extraction phase was also coated with ultrathin PDMS coating to verify the universality of the strategy. Generally, the water adsorption problem in polar SPME was overcome to a great extent, and the extraction efficiency of polar analytes was mainly preserved with this ultrathin PDMS coating, which could broaden the application of SPME in the environmental field.

Two-Dimensional Metal-Organic Framework Nanosheets for Membrane-Based Gas Separation.

Metal-organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass-transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. A modified soft-physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H2 /CO2 separation performance, with a separation factor of up to 166 and H2 permeance of up to 8×10-7  mol m-2 s-1 Pa-1 at elevated testing temperatures owing to a well-defined size-exclusion effect. This nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.

Metal-substituted zeolitic imidazolate framework ZIF-108: gas-sorption and membrane-separation properties.

A series of dual-metal zeolitic imidazolate framework (ZIF) crystals with SOD and RHO topologies was synthesised by metal substitution from ZIF-108 (Zn(2-nitroimidazolate)2 , SOD topology) as the parent material. This was based on the concept that metal substitution of ZIF-108 requires a much lower activation energy than homogenous nucleation owing to the metastability of ZIF-108. In-depth investigations of the formation processes of the daughter ZIFs indicated that the transformation of ZIF-108 is a dissolution/heterogeneous nucleation process. Typical isostructural Co(2+) substitution mainly occurs at the outer surface of ZIF-108 and results in a core-shell structure. On the contrary, the Cu(2+) -substituted ZIF has a RHO topology with a homogeneous distribution of Cu(2+) ions in the structure. Substitution with Ni(2+) resulted in a remarkable enhancement in adsorption selectivity toward CO(2) over N(2) by a factor of up to 227. With Co(2+) -substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H(2)/CH(4), CO(2)/N(2) and CO(2)/CH(4).