Showing papers on "Membrane published in 2012"

Unimpeded permeation of water through helium-leak-tight graphene-based membranes.

Abstract: Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H 2 O permeates through the membranes at least 10 10 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.

Water desalination across nanoporous graphene

Abstract: We show that nanometer-scale pores in single-layer freestanding graphene can effectively filter NaCl salt from water. Using classical molecular dynamics, we report the desalination performance of such membranes as a function of pore size, chemical functionalization, and applied pressure. Our results indicate that the membrane’s ability to prevent the salt passage depends critically on pore diameter with adequately sized pores allowing for water flow while blocking ions. Further, an investigation into the role of chemical functional groups bonded to the edges of graphene pores suggests that commonly occurring hydroxyl groups can roughly double the water flux thanks to their hydrophilic character. The increase in water flux comes at the expense of less consistent salt rejection performance, which we attribute to the ability of hydroxyl functional groups to substitute for water molecules in the hydration shell of the ions. Overall, our results indicate that the water permeability of this material is several ...

Hygro-responsive membranes for effective oil–water separation

Abstract: There is a critical need for new energy-efficient solutions to separate oil-water mixtures, especially those stabilized by surfactants. Traditional membrane-based separation technologies are energy-intensive and limited, either by fouling or by the inability of a single membrane to separate all types of oil-water mixtures. Here we report membranes with hygro-responsive surfaces, which are both superhydrophilic and superoleophobic, in air and under water. Our membranes can separate, for the first time, a range of different oil-water mixtures in a single-unit operation, with >99.9% separation efficiency, by using the difference in capillary forces acting on the two phases. Our separation methodology is solely gravity-driven and consequently is expected to be highly energy-efficient. We anticipate that our separation methodology will have numerous applications, including the clean-up of oil spills, wastewater treatment, fuel purification and the separation of commercially relevant emulsions.

Selective molecular sieving through porous graphene

Abstract: Membranes act as selective barriers and play an important role in processes such as cellular compartmentalization and industrial-scale chemical and gas purification. The ideal membrane should be as thin as possible to maximize flux, mechanically robust to prevent fracture, and have well-defined pore sizes to increase selectivity. Graphene is an excellent starting point for developing size-selective membranes because of its atomic thickness, high mechanical strength, relative inertness and impermeability to all standard gases. However, pores that can exclude larger molecules but allow smaller molecules to pass through would have to be introduced into the material. Here, we show that ultraviolet-induced oxidative etching can create pores in micrometre-sized graphene membranes, and the resulting membranes can be used as molecular sieves. A pressurized blister test and mechanical resonance are used to measure the transport of a range of gases (H(2), CO(2), Ar, N(2), CH(4) and SF(6)) through the pores. The experimentally measured leak rate, separation factors and Raman spectrum agree well with models based on effusion through a small number of angstrom-sized pores.

Synthetic Lipid Membrane Channels Formed by Designed DNA Nanostructures

Abstract: We created nanometer-scale transmembrane channels in lipid bilayers by means of self-assembled DNA-based nanostructures. Scaffolded DNA origami was used to create a stem that penetrated and spanned a lipid membrane, as well as a barrel-shaped cap that adhered to the membrane, in part via 26 cholesterol moieties. In single-channel electrophysiological measurements, we found similarities to the response of natural ion channels, such as conductances on the order of 1 nanosiemens and channel gating. More pronounced gating was seen for mutations in which a single DNA strand of the stem protruded into the channel. Single-molecule translocation experiments show that the synthetic channels can be used to discriminate single DNA molecules.

Zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes for gas separation

Abstract: As synthesised ZIF-8 nanoparticles (size ∼ 60 nm and specific surface area ∼ 1300–1600 m2 g−1) were directly incorporated into a model polymer matrix (Matrimid® 5218) by solution mixing. This produces flexible transparent membranes with excellent dispersion of nanoparticles (up to loadings of 30 wt%) with good adhesion within the polymer matrix, as confirmed by scanning electron microscopy, dynamic mechanical thermal analysis and gas sorption studies. Pure gas (H2, CO2, O2, N2 and CH4) permeation tests showed enhanced permeability of the mixed matrix membrane with negligible losses in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that an increase in the free volume of the polymer with ZIF-8 loading together with the free diffusion of gas through the cages of ZIF-8 contributed to an increase in gas permeability of the composite membrane. The gas transport properties of the composite membranes were well predicted by a Maxwell model whilst the processing strategy reported can be extended to fabricate other polymer nanocomposite membranes intended for a wide range of emerging energy applications.

Carbonized Chicken Eggshell Membranes with 3D Architectures as High‐Performance Electrode Materials for Supercapacitors

Abstract: Supercapacitor electrode materials are synthesized by carbonizing a common livestock biowaste in the form of chicken eggshell membranes. The carbonized eggshell membrane (CESM) is a three-dimensional macroporous carbon film composed of interwoven connected carbon fibers containing around 10 wt% oxygen and 8 wt% nitrogen. Despite a relatively low surface area of 221 m2 g−1, exceptional specific capacitances of 297 F g−1 and 284 F g−1 are achieved in basic and acidic electrolytes, respectively, in a 3-electrode system. Furthermore, the electrodes demonstrate excellent cycling stability: only 3% capacitance fading is observed after 10 000 cycles at a current density of 4 A g−1. These very attractive electrochemical properties are discussed in the context of the unique structure and chemistry of the material.

Super-resolution fluorescence imaging of organelles in live cells with photoswitchable membrane probes.

Abstract: Imaging membranes in live cells with nanometer-scale resolution promises to reveal ultrastructural dynamics of organelles that are essential for cellular functions. In this work, we identified photoswitchable membrane probes and obtained super-resolution fluorescence images of cellular membranes. We demonstrated the photoswitching capabilities of eight commonly used membrane probes, each specific to the plasma membrane, mitochondria, the endoplasmic recticulum (ER) or lysosomes. These small-molecule probes readily label live cells with high probe densities. Using these probes, we achieved dynamic imaging of specific membrane structures in living cells with 30–60 nm spatial resolution at temporal resolutions down to 1–2 s. Moreover, by using spectrally distinguishable probes, we obtained two-color super-resolution images of mitochondria and the ER. We observed previously obscured details of morphological dynamics of mitochondrial fusion/fission and ER remodeling, as well as heterogeneous membrane diffusivity on neuronal processes.

Elucidating membrane structure and protein behavior using giant plasma membrane vesicles.

Abstract: The observation of phase separation in intact plasma membranes isolated from live cells is a breakthrough for research into eukaryotic membrane lateral heterogeneity, specifically in the context of membrane rafts. These observations are made in giant plasma membrane vesicles (GPMVs), which can be isolated by chemical vesiculants from a variety of cell types and microscopically observed using basic reagents and equipment available in any cell biology laboratory. Microscopic phase separation is detectable by fluorescent labeling, followed by cooling of the membranes below their miscibility phase transition temperature. This protocol describes the methods to prepare and isolate the vesicles, equipment to observe them under temperature-controlled conditions and three examples of fluorescence analysis: (i) fluorescence spectroscopy with an environment-sensitive dye (laurdan); (ii) two-photon microscopy of the same dye; and (iii) quantitative confocal microscopy to determine component partitioning between raft and nonraft phases. GPMV preparation and isolation, including fluorescent labeling and observation, can be accomplished within 4 h.

Metal–organic frameworks in mixed-matrix membranes for gas separation

Abstract: Mixed-matrix membranes (MMMs) with metal–organic frameworks (MOFs) as additives (fillers) exhibit enhanced gas permeabilities and possibly also selectivities when compared to the pure polymer. Polyimides (Matrimid®) and polysulfones are popular polymer matrices for MOF fillers. Presently investigated MOFs for MMMs include [Cu(SiF6)(4,4′-BIPY)2], [Cu3(BTC)2(H2O)3] (HKUST-1, Cu-BTC), [Cu(BDC)(DMF)], [Zn4O(BDC)3] (MOF-5), [Zn(2-methylimidazolate)2] (ZIF-8), [Zn(purinate)2] (ZIF-20), [Zn(2-carboxyaldehyde imidazolate)2] (ZIF-90), Mn(HCOO)2, [Al(BDC)(μ-OH)] (MIL-53(Al)), [Al(NH2-BDC)(μ-OH)] (NH2-MIL-53(Al)) and [Cr3O(BDC)3(F,OH)(H2O)2] (MIL-101) (4,4′-BIPY = 4,4′-bipyridine, BTC = benzene-1,3,5-tricarboxylate, BDC = benzene-1,4-dicarboxylate, terephthalate). MOF particle adhesion to polyimide and polysulfone organic polymers does not represent a problem. MOF-polymer MMMs are investigated for the permeability of the single gases H2, N2, O2, CH4, CO2 and of the gas mixtures O2/N2, H2/CH4, CO2/CH4, H2/CO2, CH4/N2 and CO2/N2 (preferentially permeating gas named first). Permeability increases can be traced to the MOF porosity. Since the porosity of MOFs can be tuned very precisely, which is not possible with polymeric material, MMMs offer the opportunity of significantly increasing the selectivity compared to the pure polymeric matrix. Additionally in most of the cases the permeability is increased for MMM membranes compared to the pure polymer. Addition of MOFs to polymers in MMMs easily yields performances similar to the best polymer membranes and gives higher selectivities than those reported to date for any pure MOF membrane for the same gas separation. MOF-polymer MMMs allow for easier synthesis and handability compared to pure MOF membranes.

Cell membrane electroporation- Part 1: The phenomenon

Abstract: Each biological cell, trillions of which build our bodies, is enveloped by its plasma membrane. Composed largely of a bilayer (double layer) of lipids just two molecules thick (about 5 nm), and behaving partly as a liquid and partly as a gel, the cell plasma membrane nonetheless separates and protects the cell from its surrounding environment very reliably and stably. Embedded within the lipid bilayer, also quite stably, are a number of different proteins, some of which act as channels and pumps, providing a pathway for transporting specific molecules across the membrane. Without these proteins, the membrane would be a largely impenetrable barrier. Electrically, the cell plasma membrane can be viewed as a thin insulating sheet surrounded on both sides by aqueous electrolyte solutions. When exposed to a sufficiently strong electric field, the membrane will undergo electrical breakdown, which renders it permeable to molecules that are otherwise unable to cross it. The process of rendering the membrane permeable is called membrane electroporation. Unlike solid insulators, in which an electrical breakdown generally causes permanent structural change, the membrane, with its lipids behaving as a two-dimensional liquid, can spontaneously return to its prebreakdown state. If the exposure is sufficiently short and the membrane recovery sufficiently rapid for the cell to remain viable, electroporation is termed reversible; otherwise, it is termed irreversible. Since its discovery [1]���[3], electroporation has steadily gained ground as a useful tool in various areas of medicine and biotechnology. Today, reversible electroporation is an established method for introducing chemotherapeutic drugs into tumor cells (electrochemotherapy) [4]. It also offers great promise as a technique for gene therapy without the risks caused by viral vectors (DNA electrotransfer) [5]. In clinical medicine, irreversible electroporation is being investigated as a method for tissue ablation (nonthermal electroablation) [6], whereas in biotechnology, it is useful for extraction of biomolecules [7] and for microbial deactivation, particularly in food preservation [8]. This article, the first in a series of three focusing on electroporation, describes the phenomenon at the molecular level of the lipid bilayer, and then proceeds to the cellular level, explaining how exposure of a cell as a whole to an external electric field results in an inducement of voltage on its plasma membrane, its electroporation, and transport thorough the electroporated membrane. The second article will review the most important and promising applications of electroporation, and the third article will focus on the hardware for electroporation (pulse generators and electrodes) and on the need for standards, safety, and certification.

A Superacid-Catalyzed Synthesis of Porous Membranes Based on Triazine Frameworks for CO2 Separation

Abstract: A general strategy for the synthesis of porous, fluorescent, triazine-framework-based membranes with intrinsic porosity through an aromatic nitrile trimerization reaction is presented. The essence of this strategy lies in the use of a superacid to catalyze the cross-linking reaction efficiently at a low temperature, allowing porous polymer membrane architectures to be facilely derived. With functionalized triazine units, the membrane exhibits an increased selectivity for membrane separation of CO2 over N2. The good ideal CO2/N2 selectivity of 29 ± 2 was achieved with a CO2 permeability of 518 ± 25 barrer. Through this general synthesis protocol, a new class of porous polymer membranes with tunable functionalities and porosities can be derived, significantly expanding the currently limited library of polymers with intrinsic microporosity for synthesizing functional membranes in separation, catalysis, and energy storage/conversion.

Selective Molecular Transport through Intrinsic Defects in a Single Layer of CVD Graphene

Abstract: We report graphene composite membranes with nominal areas more than 25 mm2 fabricated by transfer of a single layer of CVD graphene onto a porous polycarbonate substrate. A combination of pressure-driven and diffusive transport measurements provides evidence of size-selective transport of molecules through the membrane, which is attributed to the low-frequency occurrence of intrinsic 1–15 nm diameter pores in the CVD graphene. Our results present the first step toward the realization of practical membranes that use graphene as the selective material.