气体分离用MOF基混合基质膜的界面构建策略

混合基质膜结合多孔填料优异的气体分离性能和聚合物材料良好的加工性能,被认为是最具有应用前景的一种气体分离膜材料。金属-有机框架材料(MOF)由于具有高比表面积和孔隙率、可调节的孔径以及可修饰的表面性能,成为制备混合基质膜的重要多孔材料。本文针对MOF基混合基质膜制备中所面临的主要挑战,聚焦于MOF和聚合物界面缺陷问题,分析了界面缺陷的产生原因及其对性能的影响,重点阐述了改善MOF填料和聚合物基质界面相容性的策略,以期为制备具有良好的界面形态和优异的气体分离性能的混合基质膜提供借鉴思路。     

Porous polymer monoliths for small molecule separations: advancements and limitations

Porous polymer monoliths are considered to be one of the major breakthroughs in separation science. These materials are well known to be best suited for the separation of large molecules, specifically proteins, an observation most often explained by convective mass transfer and the absence of small pores in the polymer scaffold. However, this conception is not sufficient to explain the performance of small molecules. This review focuses in particular on the preparation of (macro)porous polymer monoliths by simple free-radical processes and the key events in their formation. There is special focus on the fluid transport properties in the heterogeneous macropore space (flow dispersion) and on the transport of small molecules in the swollen, and sometimes permanently porous, globule-scale polymer matrix. For small molecule applications in liquid chromatography, it is consistently found in the literature that the major limit for the application of macroporous polymer monoliths lies not in the optimization of surface area and/or modification of the material and microscopic morphological properties only, but in the improvement of mass transfer properties. In this review we discuss the effect of resistance to mass transfer arising from the nanoscale gel porosity. Gel porosity induces stagnant mass transfer zones in chromatographic processes, which hamper mass transfer efficiency and have a detrimental effect on macroscopic chromatographic dispersion under equilibrium (isocratic) elution conditions. The inherent inhomogeneity of polymer networks derived from free-radical cross-linking polymerization, and hence the absence of a rigid (meso)porous pore space, represents a major challenge for the preparation of efficient polymeric materials for the separation of small molecules.     

Direct coupling of polymer-based microchip electrophoresis to online MALDI-MS using a rotating ball inlet

We report on the coupling of a polymer-based microfluidic chip to a MALDI-TOF MS using a rotating ball interface. The microfluidic chips were fabricated by micromilling a mold insert into a brass plate, which was then used for replicating polymer microparts via hot embossing. Assembly of the chip was accomplished by thermally annealing a cover slip to the embossed substrate to enclose the channels. The linear separation channel was 50 microm wide, 100 microm deep, and possessed an 8 cm effective length separation channel with a double-T injector (V(inj) = 10 nL). The exit of the separation channel was machined to allow direct contact deposition of effluent onto a specially constructed rotating ball inlet to the mass spectrometer. Matrix addition was accomplished in-line on the surface of the ball. The coupling utilized the ball as the cathode transfer electrode to transport sample into the vacuum for desorption with a 355 nm Nd:YAG laser and analyzed on a TOF mass spectrometer. The ball was cleaned online after every rotation. The ability to couple poly(methylmethacrylate) microchip electrophoresis devices for the separation of peptides and peptide fragments produced from a protein digest with subsequent online MALDI MS detection was demonstrated.     

Detachable Porous Organic Polymers Responsive to Light and Heat

Stimuli-responsive porous materials have captured much attention due to the on-demand tunable properties. Most reported stimuli-responsive porous materials are based on molecule isomerism or host-guest interaction, and it is highly desired to develop new types based on different responsive mechanism. Herein, inspired by natural cells which have the ability to fuse and divide induced by external stimulation, we report a new type of stimuli-responsive porous material based on detachment mechanism. A detachable porous organic polymer, namely DT-POP-1, is fabricated from the polymerization of anthracene-containing monomer (AnMon) when irradiated by 365 nm UV light. DT-POP-1 can detach into the monomer AnMon when irradiated with 275 nm UV light or heat. Such polymerization/detachment is reversible. The detachment results in a big difference in porosity and adsorption capacity, making the present detachable porous polymer highly promising in adsorptive separation and drug delivery.     

Stepwise deposition of metal organic frameworks on flexible synthetic polymer surfaces

Thin films of [Cu(3)(btc)(2)](n) (btc = 1,3,5-benzenetricarboxylate) metal organic framework were deposited in a stepwise manner on surfaces of flexible organic polymers. The thickness of films can be precisely controlled. The deposition of the first cycles was monitored by UV-vis spectroscopy. The porosity was proven by the adsorption of pyrazine, which was monitored by FT-IR and thermogravimetric analysis. The deposition of MOF thin films on flexible polymer surfaces might be a new path for the fabrication of functional materials for different applications, such as protection layers for working clothes and gas separation materials in the textile industry.     

Designing Hydrogen‐Bonded Organic Frameworks (HOFs) with Permanent Porosity

Designing organic components that can be used to construct porous materials enables the preparation of tailored functionalized materials. Research into porous materials has seen a resurgence in the past decade as a result of finding of self‐standing porous molecular crystals (PMCs). Particularly, a number of crystalline systems with permanent porosity that are formed by self‐assembly through hydrogen bonding (H‐bonding) have been developed. Such systems are called hydrogen‐bonded organic frameworks (HOFs). Herein we systematically describe H‐bonding patterns (supramolecular synthons) and molecular structures (tectons) that have been used to achieve thermal and chemical durability, a large surface area, and functions, such as selective gas sorption and separation, which can provide design principles for constructing HOFs with permanent porosity.     

Composite hollow fiber membranes for gas separation: the resistance model approach

A model is developed to explain the behavior of composite gas separation membranes which consist of a porous asymmetric substrate of one polymer, and a coating of a second polymer. An analogy between gas permeation and electrical flow is made, and the various portions of the composite membrane are described in terms of their resistance to gas permeation. It is shown that substrate porosity can vary significantly without altering the separating properties of such a composite, and that substrate and coating properties can be matched to optimize the flux and separation factor. Several major problems previously associated with the development of useful hollow fibers for gas separation are discussed, and it is shown that the use of Resistance Model composites can help to resolve these problems.     

Recent progress in microporous polymers from thermally rearranged polymers and polymers of intrinsic microporosity for membrane gas separation: Pushing performance limits and revisiting trade-off lines

Polymers are promising materials for gas separation membranes. However, the trade-off relationship between gas permeability and selectivity remains an obstacle for achieving polymer membranes that exhibit high gas permeation with desirable separation efficiency. Improving polymer microporosity is of interest in gas separation membranes to enhance gas transport behavior. Polymer modifications by (a) incorporating intrinsically microporous units and/or (b) increasing chain rigidity can enhance microporosity in conventional polymer membrane materials such as polyimides. These strategies are adopted for new classes of microporous polymers, thermally rearranged (TR) polymers, and polymers with intrinsic microporosity (PIMs), to maximize gas transport properties. Their outstanding gas separation performances have redefined the traditional trade-off lines. This review aims to explore the advances in microporous polymers for gas separation applications. The approaches on TR polymers and PIMs to enhance their microporosity are listed, and their developments are evaluated in the context of revisiting performance limits for industrially relevant gas separation applications.     

Multistep liquid-phase lithography for fast prototyping of microfluidic free-flow-electrophoresis chips

We present a fast and versatile method to produce functional micro free-flow electrophoresis chips. Microfluidic structures were generated between two glass slides applying multistep liquid-phase lithography, omitting troublesome bonding steps or cost-intensive master structures. Utilizing a novel spacer-less approach with the photodefinable polymer polyethyleneglycol dimethacrylate (PEG-DA), microfluidic devices with hydrophilic channels of only 25 μm in height were generated. The microfluidic chips feature ion-permeable segregation walls between the electrode channels and the separation bed and hydrophilic surfaces. The performance of the chip is demonstrated by free-flow electrophoretic separation of fluorescent xanthene dyes and fluorescently labeled amino acids.     

Performance of HPLC/MS microchips in isocratic and gradient elution modes

We analyzed the chromatographic performance of particle‐packed, all‐polyimide high‐performance liquid chromatography/mass spectrometry (HPLC/MS) microchips in terms of their hydraulic permeabilities and separation efficiency under isocratic and gradient elution conditions. The separation channels of the chips (with ca 50 µm × 75 µm trapezoidal cross‐section and a length of 43 mm) were slurry packed with either 3.5 or 5 µm spherical porous C18‐silica particles. A custom‐built holder enveloped the chip during packing to prevent channel deformation and delamination from high pressures. It is shown that the packing conditions significantly impact the packing density of the HPLC/MS chips, which determines their performance in both, isocratic and gradient elution modes. Even with steep solvent gradients, peak shape and chromatographic resolution for the densely packed HPLC/MS chips are much improved. Our data show that the analytical power of the HPLC/MS chip is limited by the quality of the chromatographic separation. Copyright © 2010 John Wiley & Sons, Ltd.     

Highly sensitive optofluidic chips for biochemical liquid assay fabricated by 3D femtosecond laser micromachining followed by polymer coating

The demand for increased sensitivity in the concentration analysis of biochemical liquids is a crucial issue in the development of lab on a chip and optofluidic devices. We propose a new design for optofluidic devices for performing highly sensitive biochemical liquid assays. This design consists of a microfluidic channel whose internal walls are coated with a polymer and an optical waveguide embedded in photostructurable glass. The microfluidic channel is first formed by three-dimensional femtosecond laser micromachining. The internal walls of the channel are then coated by the dipping method with a polymer that has a lower refractive index than water. Subsequently, the optical waveguide is integrated with the microfluidic channel. The polymer coating on the internal walls permits the probe light, which is introduced by the optical waveguide, to propagate along the inside of the microfluidic channel. This results in a sufficiently long interaction length between the probe light and a liquid sample in the channel and thus significantly improves the sensitivity of absorption measurements. Using the fabricated optofluidic chips, we analyzed protein in bovine serum albumin to concentrations down to 7.5 mM as well as 200 nM glucose-D.     

CE chips fabricated by injection molding and polyethylene/thermoplastic elastomer film packaging methods

This study presents a new packaging method using a polyethylene/thermoplastic elastomer (PE/TPE) film to seal an injection-molded CE chip made of either poly(methyl methacrylate) (PMMA) or polycarbonate (PC) materials. The packaging is performed at atmospheric pressure and at room temperature, which is a fast, easy, and reliable bonding method to form a sealed CE chip for chemical analysis and biomedical applications. The fabrication of PMMA and PC microfluidic channels is accomplished by using an injection-molding process, which could be mass-produced for commercial applications. In addition to microfluidic CE channels, 3-D reservoirs for storing biosamples, and CE buffers are also formed during this injection-molding process. With this approach, a commercial CE chip can be of low cost and disposable. Finally, the functionality of the mass-produced CE chip is demonstrated through its successful separation of phiX174 DNA/HaeIII markers. Experimental data show that the S/N for the CE chips using the PE/TPE film has a value of 5.34, when utilizing DNA markers with a concentration of 2 ng/microL and a CE buffer of 2% hydroxypropyl-methylcellulose (HPMC) in Tris-borate-EDTA (TBE) with 1% YO-PRO-1 fluorescent dye. Thus, the detection limit of the developed chips is improved. Lastly, the developed CE chips are used for the separation and detection of PCR products. A mixture of an amplified antibiotic gene for Streptococcus pneumoniae and phiX174 DNA/HaeIII markers was successfully separated and detected by using the proposed CE chips. Experimental data show that these DNA samples were separated within 2 min. The study proposed a promising method for the development of mass-produced CE chips.     

Comparison of capillary zone electrophoresis performance of powder-blasted and hydrogen fluoride-etched microchannels in glass

The applicability of glass chips with powder-blasted microchannels for electrophoretic separations was examined, and the performance was compared to microchannels etched with hydrogen fluoride (HF), using bicarbonate buffer and rhodamine B and fluorescein as model compounds. The measured electroosmotic mobilities in all chips were comparable, with values of ca. 7 x 10(-4) cm(2) V(-1)s(-1). The effect of electrical field strength and detection length on the separation efficiency was monitored. It was found that the main source of dispersion is of the Taylor-Aris type, which was discussed in relation to channel roughness differences. Although in powder-blasted channels with a separation length of 8.20 cm, 7-9 times lower plate numbers were obtained than in a HF-etched channel with similar dimensions, successful separation of five fluorescein isothiocyanate (FITC)-labeled amino acids was obtained on a powder-blasted chip within 80 s. Efficiencies of up to 360 000 plates/m were demonstrated on this chip, when a higher buffer concentration was used at a field strength of 664 V/cm. It can be concluded that powder-blasted microchannel chips, although they have a lower separation efficiency compared to HF-etched chips, perform well enough for many applications. Powder blasting can therefore be considered a low-cost and efficient alternative to HF etching, in particular because of the possibility to fabricate access holes through the glass with the same process.     

Preconcentration and separation of double-stranded DNA fragments by electrophoresis in plastic microfluidic devices

We have evaluated double-stranded DNA separations in microfluidic devices which were designed to couple a sample preconcentration step based on isotachophoresis (ITP) with a zone electrophoretic (ZE) separation step as a method to increase the concentration limit of detection in microfluidic devices. Developed at ACLARA BioSciences, these LabCard trade mark devices are plastic 32 channel chips, designed with a long sample injection channel segment to increase the sample loading. These chips were designed to allow stacking of the sample into a narrow band using discontinuous ITP buffers, and subsequent separation in the ZE mode in sieving polymer solutions. Compared to chip ZE, the sensitivity was increased by 40-fold and we showed baseline resolution of all fragments in the PhiX174/HaeIII DNA digest. The total analysis time was 3 min/sample, or less than 100 min per LabCard device. The resolution for multiplexed PCR samples was the same as obtained in chip ZE. The limit of detection was 9 fg/microL of DNA in 0.1xpolymerase chain reaction (PCR) buffers using confocal fluorescence detection following 488 nm laser excitation with thiazole orange as the fluorescent intercalating dye.     

Low-voltage driven control in electrophoresis microchips by traveling electric field.

This paper presents the use of a physical model and numerical simulation in the investigation of traveling electric fields on capillary electrophoresis (CE) chips. The principal material transport mechanisms of electrokinetic migration, ionic concentration, fluid flow, and diffusion are all taken into consideration. Traditionally, the high electric field strength required for the separation of biological samples by microfluidic devices has involved the application of high external voltages. In contrast, this study presents a proposal for samples separation by means of a moving electric field within a low voltage-driven CE chip. Under this proposal, the separation channel is partitioned into a series of smaller separation zones by means of electrode pairs. This paper considers two different electrode configurations, namely arranged along a single side of the separation channel, and arranged on two sides of the separation channel. The quality of the separation achieved with these two configurations is then compared with the traditional straight separation channel approach. The results confirm that the proposed method is successful in maintaining an adequate field strength for separation purposes in a low-voltage driven CE chip. Furthermore, it is determined that the best separation results are obtained using electrodes arranged along both sides of the separation channel.     

Applications of covalent organic frameworks (COFs):From gas storage and separation to drug delivery

Covalent organic frameworks (COFs) are an emerging class of porous covalent organic structures whose backbones were composed of light elements (B, C, N, O, Si) and linked by robust covalent bonds to endow such material with desirable properties, i.e., inherent porosity, well-defined pore aperture, ordered channel structure, large surface area, high stability, and multi-dimension. As expected, the above-mentioned properties of COFs broaden the applications of this class of materials in various fields such as gas storage and separation, catalysis, optoelectronics, sensing, small molecules adsorption, and drug delivery. In this review, we outlined the synthesis of COFs and highlighted their applications ranging from the initial gas storage and separation to drug delivery.     

A smart surface in a microfluidic chip for controlled protein separation

The smart surface created in a microfluidic chip has shown the capability of adsorbing and releasing proteins under electrical control. The inner surface of the chip channel was first coated by a thin layer of Au through sputtering and was subsequently modified with loosely packed self-assembled monolayers (SAMs) of thiols with terminal carboxylic or amino groups. Upon application of an external electric potential to the gold substrate, reversible conformational transformation between "bent" and "straight" states for the anchored mercapto chains could be modulated, through the electrostatic effect between the ionized terminal groups and the charged gold substrate. Thus, a hydrophobic or hydrophilic channel surface was established and could be reversibly switched electrochemically. Accordingly, the microchips prepared in this way can reversibly and selectively adsorb and release differently charged proteins under electrical control. Two model proteins, avidin and streptavidin, were demonstrated to be readily adsorbed by the smart chips under negative and positive potential, respectively. Also, more than 90 % of the adsorbed proteins could be released upon an electrical command. Furthermore, these chips were applied to the controlled separation of avidin and streptavidin mixtures with 1:1 and 1:1000 molar ratios. Under specific applied potentials, the chips adsorbed a certain protein from the mixture whereas the other protein was allowed to flow out, after which the adsorbed protein could be released by switching the applied potential. Thus, two eluted protein fractions were obtained and the separation of the two proteins was achieved. For the former mixture, each eluted fraction contained up to approximately 80-90 % avidin or streptavidin. For the latter mixture, the resulting separation efficiency indicated that the molar ratio of avidin and streptavidin could be increased from 1:1000 to about 32:1 after five run separations.     

Fabrication of micro free-flow electrophoresis chip by photocurable monomer binding microfabrication technique for continuous separation of proteins and their numerical simulation

In this study, a simple, fast, and reliable method to fabricate a micro free-flow electrophoresis (μFFE) device on glass is presented. The two-dimensional depth channel in the chip was easily achieved by using a photocurable monomer (NOA?81) that served as the bonding material. In such a geometrical structure (two-dimensional depth channel), the effect of fluid behavior on the separation efficiency of micro free-flow zone electrophoresis (μFFZE) was simulated. The results of numerical simulation indicate that the pressure at the inlets may play an important role in the separation performance. Under the optimum separation conditions, four FITC-labeled amino acids were well separated, indicating the validity of the performance of the chip. Since the chip was fabricated by organic polymer bonding, it was easily recyclable through a simple re-fabrication process. The reproducibility of results from these recycling re-fabrication chips was investigated. The RSD of the resolution between FITC-l-glycine and FITC-l-phenylalanine was 5.3%. Furthermore, three FITC-labeled proteins were successfully separated with the resolution of 2.2 and 5.46, respectively, by using the coating of neutral liposome.     

Nanoporous metals with controlled multimodal pore size distribution

A simple two-step dealloying strategy is described to make free-standing metal membranes with hierarchical porous architecture. This structure has a bimodal pore size distribution composed of large porosity channels and small porosity channel walls, where each pore size can be tailored independently of the others. A new gas-phase electroless plating technique was also developed here that could be used to uniformly fill porous structures with pore size as small as 10 nm.