Versatile Membrane Research Articles

Normal incidence birefringence in nanoporous alumina

There are many works and interests focused on the development of optical sensors based on porous anodic aluminum oxide membranes (AAO). Normal incidence birefringence (BR) is present in AAO when is obtained from a monocrystalline aluminum substrate but his effect has been largely ignored since it is canceled out in a poly-crystalline substrate. Nevertheless, this averaging of BR limits the optical performance of AAO membranes in optical devices. In this work, we study normal incidence BR in AAO for different crystal orientations of the aluminium substrate using a polycrystalline substrate and a simple thermo-mechanical treatment to obtain millimeter size crystalline regions. We found that normal incidence BR is minimum at [100] and [111] orientations and reaches a maximum of ∼ 1.8 ⋅ 10−3 at intermediate orientations. In this case, the BR can be associated with a slight deformation of the structure perpendicular to the direction of the pores, confirmed through scanning electron microscope images. Furthermore, we propose a simple model of elliptical section pores that quantitatively explain the origin of normal incidence BR in AAO. Considering the effect of BR, the design of sensors could be improved to obtain higher quality factors or to incorporate new optical functionalities to these already versatile membranes.

A solvent‐resistance OTS/PDA/O‐PASS composite membrane for water‐in‐oil emulsions separation

AbstractWater‐in‐oil emulsions separation plays a critical role in industrial wastewater treatment. Membrane technology has attracted much interest in water‐in‐oil emulsions separation field for its excellent separation performance and facile processing method. However, versatile membrane with better solvent‐resistance is still absent in chemical industry. In this work, oxidized polyarylene sulfide sulfone (O‐PASS) membrane was severed as substrate, on which a hydrophobicity surface was built via coating of octadecyltrichlorosilane (OTS). And polydopamine (PDA) layer was employed and acted as connector between OTS and O‐PASS membrane due to its abundant active hydroxyl group. The maximum water contact angle was 132.6° indicating good hydrophobicity of the membrane. Finally, the OTS/PDA/O‐PASS composite membranes showed good separation performance for corrosive emulsions: the rejection and flux were 93.0% and 10.0 L/m2h for water‐in‐dichloromethane emulsions, and the rejection and flux were 92.3% and 34.6 L/m2h for water‐in‐n‐hexane emulsions. The OTS/PDA/O‐PASS composite membrane is a new candidate membrane for water‐in‐oil emulsions separation. In addition, the superior performance of the composite membrane under harsh environment conditions ensures its usefulness in resistance aggressive solvent.

Hybrid membrane with controllable surface microroughness by micro-nano structure processing for diluted PM2.5 capture

Tremendous efforts have been devoted by researchers on air particulate matter pollution for the increasing harm, however, the Air Pollution Index (API) from “good” to “excellent” is something hard to achieve. Here, halloysite nanotubes/polyvinyl alcohol (HNTs/PVA) hybrid membrane with surface micro-nano structure processing using a one-step method for efficient PM2.5 capture was prepared. The filtration efficiency is 45.35% and the pressure drop is 41.57 Pa of composite membrane with a 60 wt% halloysite dosage. Specially, it resulted in a relatively safer PM index value of about 16.54, which tends to be more stringent than the restriction by Government of China (PM2.5 < 35 μg/m3). The filtration performance was mainly attributed to the controllable microroughness surface as well as the hierarchical structure constructed by one-step method, which has a functional role in obstruction and adsorption for diluted PM2.5. The methodology can employ halloysite onto various polymers, like polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyacrylonitrile (PAN) and also polycaprolactone (PCL) to yield hybrid membranes with the similar modification of surface and structure. Such versatile membrane filters can be purposely designed and scaled up, which endows the existing hybrid membrane with great potentials in both residential and public areas pollution control to achieve a healthier living environment.

Ultrathin Cellulose Voronoi-Nanonet Membranes Enable High-Flux and Energy-Saving Water Purification.

Creating a desirable porous membrane with high-flux and energy-saving properties for the purification of water containing submicron-sized contaminants, especially pathogenic microbes, is of great significance, yet a great challenge. Herein, we demonstrate a facile methodology to construct an innovative membrane with continuous cellulose Voronoi-nanonet structures via nonsolvent-induced phase separation. This approach enables cellulose Voronoi nanonets to tightly weld with electrospun nanofibrous substrates by controlling the solvent-nonsolvent mutual diffusion process. The resultant membranes exhibit integrated properties of small pore size (0.23 μm), high porosity (90.7%), good interconnectivity, and ultrathin thickness (∼600 nm, 2 orders of magnitude thinner than the conventional microfiltration membrane). As a result, the prepared membranes can effectively intercept submicron particles (∼0.3 μm) with robust rejection efficiency (>99.80%) and ultrahigh permeation flux (maximum of 8834 L m-2 h-1) under an extremely low driving pressure (≤20 kPa). More importantly, prominent bacterial rejection efficiency with a log reduction value (LRV) of 8.0 (overcoming the previous limitation of LRV <7) and outstanding antifouling function are also achieved for the membranes. The successful fabrication of such a versatile membrane may provide new insights into the development of next-generation high-performance separation materials for various applications.

Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation

Endosomal sorting complexes for transport-III (ESCRT-III) assemble in vivo onto membranes with negative Gaussian curvature. How membrane shape influences ESCRT-III polymerization and how ESCRT-III shapes membranes is yet unclear. Human core ESCRT-III proteins, CHMP4B, CHMP2A, CHMP2B and CHMP3 are used to address this issue in vitro by combining membrane nanotube pulling experiments, cryo-electron tomography and AFM. We show that CHMP4B filaments preferentially bind to flat membranes or to tubes with positive mean curvature. Both CHMP2B and CHMP2A/CHMP3 assemble on positively curved membrane tubes. Combinations of CHMP4B/CHMP2B and CHMP4B/CHMP2A/CHMP3 are recruited to the neck of pulled membrane tubes and reshape vesicles into helical “corkscrew-like” membrane tubes. Sub-tomogram averaging reveals that the ESCRT-III filaments assemble parallel and locally perpendicular to the tube axis, highlighting the mechanical stresses imposed by ESCRT-III. Our results underline the versatile membrane remodeling activity of ESCRT-III that may be a general feature required for cellular membrane remodeling processes.

GPCR Solubilization and Quality Control.

G protein-coupled receptors (GPCRs) are versatile membrane proteins involved in the regulation of many physiological processes and pathological conditions, making them interesting pharmacological targets. In order to study their structure and function, GPCRs are traditionally extracted from membranes using detergents. However, due to their hydrophobic nature, intrinsic instability in aqueous solutions, and theirdenaturing effects, theisolation of properly folded and functional GPCRs is not trivial. Therefore, it is of crucial importance to solubilize receptors under mild conditions and control the sample quality subsequently. Here we describe widely used methods for small-scale GPCR solubilization, followed by quality control based on fluorescence size-exclusion chromatography, SDS-PAGE, temperature-induced protein unfolding (CPM dye binding) and fluorescent ligand binding assay. These methods can easily be used to assess the thermostability and functionality of a GPCR sample exposed to different conditions, such as theuse of various detergents, addition of lipids and ligands, making them valuable for obtaining an optimal sample qualityfor structural and functional studies.

Polyelectrolytes self-assembly: versatile membrane fabrication strategy

Polyelectrolytes self-assembly is developing rapidly as a fascinating family of functional membrane fabrication strategies with the ability to modulate the architectures and physical–chemical characteristics of membranes at the molecular level.

Curving Cells Inside and Out: Roles of BAR Domain Proteins in Membrane Shaping and Its Cellular Implications.

Many cellular processes rely on precise and timely deformation of the cell membrane. While many proteins participate in membrane reshaping and scission, usually in highly specialized ways, Bin/amphiphysin/Rvs (BAR) domain proteins play a pervasive role, as they not only participate in many aspects of cell trafficking but also are highly versatile membrane remodelers. Subtle changes in the shape and size of the BAR domain can greatly impact the way in which BAR domain proteins interact with the membrane. Furthermore, the activity of BAR domain proteins can be tuned by external physical parameters, and so they behave differently depending on protein surface density, membrane tension, or membrane shape. These proteins can form 3D structures that mold the membrane and alter its liquid properties, even promoting scission under various circumstances.As such, BAR domain proteins have numerous roles within the cell. Endocytosis is among the most highly studied processes in which BAR domain proteins take on important roles. Over the years, a more complete picture has emerged in which BAR domain proteins are tied to almost all intracellular compartments; examples include endosomal sorting and tubular networks in the endoplasmic reticulum and T-tubules. These proteins also have a role in autophagy, and their activity has been linked with cancer. Here, we briefly review the history of BAR domain protein discovery, discuss the mechanisms by which BAR domain proteins induce curvature, and attempt to settle important controversies in the field. Finally, we review BAR domain proteins in the context of a cell, highlighting their emerging roles in cell signaling and organelle shaping.

Rational design of ion separation membranes

Synthetic membranes for desalination and ion separation processes are a prerequisite for the supply of safe and sufficient drinking water as well as smart process water tailored to its application. This requires a versatile membrane fabrication methodology. Starting from an extensive set of new ion separation membranes synthesized with a layer-by-layer methodology, we demonstrate for the first time that an artificial neural network (ANN) can predict ion retention and water flux values based on membrane fabrication conditions. The predictive ANN is used in a local single-objective optimization approach to identify manufacturing conditions that improve permeability of existing membranes. A deterministic global multi-objective optimization is performed in order to identify the upper bound (Pareto front) of the delicate trade-off between ion retention characteristics and permeability. Ultimately, a coupling of the ANN into a hybrid model enables physical insight into the influence of fabrication conditions on apparent membrane properties.

Development of Formvar-Based Membranes with Controlled Porosities for Microfluidics and Large-Area Graphene Transfer

Ultra-thin and strong formvar-based membranes with controlled morphologies were developed via a simple synthesis technique. This thin hydrophilic and oleophilic polymer possesses very interesting morphological properties including being inert to most chemicals and resistant to radiation. Tunable porous membranes (porosity ranges from 20 to 60%) were synthesized by controlling the ratios of formvar, glycerol, and chloroform; and varying the sonication time. While the porosity can be altered, the hydrophilicity and oleophilicity is fundamental to the formvar itself and is independent of synthesis conditions. The formvar-based membranes were found to have an elastic modulus of 7.8 GPa (twice as high as previously reported values) with an average thickness of 125 nm. The figure below shows the SEM images of holey formvar structure (A) and lacey structure (B) along with a 3D render AFM image of a high porosity holey formvar sample (C). To demonstrate the versatility of this membrane, we performed chemical compatibility testing revealing that only chloroform had a destructive effect fully dissolving the formvar in a few minutes. High concentrations of HCl (above 6M) were found to have minor deleterious effects after an hour of exposure. Moreover, a numerical model was used to demonstrate the feasibility of the membranes with a range of porosity in realistic conditions in nano- and micro-devices. These thorough simulations revealed that the membrane with the highest porosity of ~50% exhibited the lowest probability of failure in a pressurized system. Additionally, the unique mechanical properties and chemical resistivity of formvar, provide the opportunity for using these type of membranes in certain environmental conditions to transfer and handle various 2D materials, where other polymer types would not be suited for. More specifically, due to the ease at which formvar dissolves in chloroform, for the first time we utilize the formvar-based membranes for large-area graphene transfer. Compared to other polymer-based transfer techniques, our novel method allows for a cleaner and a faster transfer of graphene sheets onto a variety of substrates, including commercial TEM grids, silicon dioxide and glass. Various spectroscopy and microscopy analyses demonstrated the effectiveness of this formvar-based transfer technique for large-area crystalline graphene sheets. The formvar removal is one to two orders of magnitude faster than other reported PMMA-based transfer methods. Moreover, formvar can be applied to large-area graphene sheets limiting the transfer dimensions only to that of catalyst size utilized for graphene growth. Ultimately, the development of ultra-strong and versatile membrane materials can allow for major advances across a wide range of important applications in microscale to nanoscale fluidics and nano-electronic devices. Figure 1

Ultra-thin and strong formvar-based membranes with controlled porosity for micro- and nano-scale systems

We present a methodology for developing ultra-thin and strong formvar-based membranes with controlled morphologies. Formvar is a thin hydrophilic and oleophilic polymer inert to most chemicals and resistant to radiation. The formvar-based membranes are viable materials as support structures in micro- and macro-scale systems depending on thinness and porosity control. Tunable sub-micron thick porous membranes with 20%–65% porosity were synthesized by controlling the ratios of formvar, glycerol, and chloroform. This synthesis process does not require complex separation or handling methods and allows for the production of strong, thin, and porous formvar-based membranes. An expansive array of these membrane characterizations including chemical compatibility, mechanical responses, wettability, as well as the mathematical simulations as a function of porosity has been presented. The wide range of chemical compatibility allows for membrane applications in various environments, where other polymers would not be suitable. Our formvar-based membranes were found to have an elastic modulus of 7.8 GPa, a surface free energy of 50 mN m−1 and an average thickness of 125 nm. Stochastic model simulations indicate that formvar with the porosity of ∼50% is the optimal membrane formulation, allowing the most material transfer across the membrane while also withstanding the highest simulated pressure loadings before tearing. Development of novel, resilient and versatile membranes with controlled porosity offers a wide range of exciting applications in the fields of nanoscience, microfluidics, and MEMS.

Recent Advances of Membrane-Cloaked Nanoplatforms for Biomedical Applications.

In terms of the extremely small size and large specific surface area, nanomaterials often exhibit unusual physical and chemical properties, which have recently attracted considerable attention in bionanotechnology and nanomedicine. Currently, the extensive usage of nanotechnology in medicine holds great potential for precise diagnosis and effective therapeutics of various human diseases in clinical practice. However, a detailed understanding regarding how nanomedicine interacts with the intricate environment in complex living systems remains a pressing and challenging goal. Inspired by the diversified membrane structures and functions of natural prototypes, research activities on biomimetic and bioinspired membranes, especially for those cloaking nanosized platforms, have increased exponentially. By taking advantage of the flexible synthesis and multiple functionality of nanomaterials, a variety of unique nanostructures including inorganic nanocrystals and organic polymers have been widely devised to substantially integrate with intrinsic biomoieties such as lipids, glycans, and even cell and bacteria membrane components, which endow these abiotic nanomaterials with specific biological functionalities for the purpose of detailed investigation of the complicated interactions and activities of nanomedicine in living bodies, including their immune response activation, phagocytosis escape, and subsequent clearance from vascular system. In this review, we summarize the strategies established recently for the development of biomimetic membrane-cloaked nanoplatforms derived from inherent host cells (e.g., erythrocytes, leukocytes, platelets, and exosomes) and invasive pathogens (e.g., bacteria and viruses), mainly attributed to their versatile membrane properties in biological fluids. Meanwhile, the promising biomedical applications based on nanoplatforms inspired by diverse moieties, such as selective drug delivery in targeted sites and effective vaccine development for disease prevention, have also been outlined. Finally, the potential challenges and future prospects of the biomimetic membrane-cloaked nanoplatforms are also discussed.

Coiled-coil oligomerization controls localization of the plasma membrane REMORINs

REMORINs are nanodomain-organized proteins located in the plasma membrane and involved in cellular responses in plants. The dynamic assembly of the membrane nanodomains represents an essential tool of the versatile membrane barriers to control and modulate cellular functions. Nevertheless, the assembly mechanisms and protein organization strategies of nanodomains are poorly understood and many structural aspects are difficult to visualize. Using an ensemble of biophysical approaches, including solid-state nuclear magnetic resonance, cryo-electron microscopy and in vivo confocal imaging, we provide first insights on the role and the structural mechanisms of REMORIN trimerization. Our results suggest that the formation of REMORIN coiled-coil trimers is essential for membrane recruitment and promotes REMORIN assembly in vitro into long filaments by trimer-trimer interactions that might participate in nanoclustering into membrane domains in vivo.

Rapid large area fabrication of multiscale through-hole membranes.

There are many proposed mechanisms by which single cells can be trapped; among them is the through-hole membrane for the characterization of individual microorganisms. Due to the small scale of the fabricated pores, the construction of through-hole membranes on a large scale and with relatively large areas faces many difficulties. This paper describes novel fabrication methods for a large-area, freestanding micro/nano through-hole membrane constructed from versatile membrane materials using through-hole membranes on a microfluidic chip (THMMC). This process can rapidly (<20 min) fabricate membranes with high fidelity multiscale hole size without residual layers. The through-hole site was easily customizable from the micro to the nanoscale, with a low or high aspect ratio giving rise to reliable membranes. Also, the rigidity and biocompatibility of the through-hole membrane are easily tunable by simple injection of versatile membrane materials to obtain a large area (up to 3600 mm2). Membranes produced in this manner were then applied as a proof of concept for the isolation, cultivation, and quantification of individual micro-algal cells for selection with respect to the growth rate, while controlling the quorum sensing mediated metabolic and proliferative changes.

Magnetically Oriented Bicelles with Monoalkylphosphocholines: Versatile Membrane Mimetics for Nuclear Magnetic Resonance Applications.

Bicelles (bilayered micelles) are model membranes used in the study of peptide structure and membrane interactions. They are traditionally made of long- and short-chain phospholipids, usually dimyristoylphosphatidylcholine (D14PC) and dihexanoyl-PC (D6PC). They are attractive membrane mimetics because their composition and planar surface are similar to the native membrane environment. In this work, to improve the solubilization of membrane proteins and allow their study in bicellar systems, D6PC was replaced by detergents from the monoalkylphosphocholine (MAPCHO) family, of which dodecylphosphocholine (12PC) is known for its ability to solubilize membrane proteins. More specifically 12PC, tetradecyl- (14PC), and hexadecyl-PC (16PC) have been employed. To verify the possibility of making bicelles with different hydrophobic thicknesses to better accommodate membrane proteins, D14PC was also replaced by phospholipids with different alkyl chain lengths: dilauroyl-PC (D12PC), dipalmitoyl-PC (D16PC), distearoyl-PC (D18PC), and diarachidoyl-PC (D20PC). Results obtained by 31P solid-state nuclear magnetic resonance (NMR) and isothermal titration calorimetry (ITC) at several lipid-to-detergent molar ratios (q) and temperatures indicate that these new MAPCHO bicelles can be formed under a variety of conditions. The quality of their alignment is similar to that of classical bicelles, and the low critical micelle concentration (CMC) of the surfactants and their miscibility with phospholipids are likely to be advantageous for the reconstitution of membrane proteins.

PACSIN1 regulates the dynamics of AMPA receptor trafficking.

Dynamic trafficking of AMPA receptors (AMPARs) into and out of synapses plays an important role in synaptic plasticity. We previously reported that the protein kinase C and casein kinase II substrate in neurons (PACSIN) forms a complex with AMPARs through its interaction with the protein interacting with C-kinase 1 (PICK1) to regulate NMDA receptor (NMDAR)-induced AMPAR endocytosis and cerebellar long-term depression. However, the molecular mechanism by which PACSIN regulates the dynamics of AMPAR trafficking remains unclear. Using a pH-sensitive green fluorescent protein, pHluorin, tagged to the extracellular domain of the GluA2 subunit of AMPARs, we demonstrate dual roles for PACSIN1 in controlling the internalization and recycling of GluA2 after NMDAR activation. Structure and function analysis reveals a requirement for the PACSIN1 F-BAR and SH3 domains in controlling these NMDAR-dependent processes. Interestingly, the variable region, which binds to PICK1, is not essential for NMDAR-dependent GluA2 internalization and is required only for the correct recycling of AMPARs. These results indicate that PACSIN is a versatile membrane deformation protein that links the endocytic and recycling machineries essential for dynamic AMPAR trafficking in neurons.

Nanoporous Gold Membranes as Robust Constructs for Selectively Tunable Chemical Transport

Nanopores are promising structures for small-volume separations, but they often require complex top-down fabrication and are not easily (re)configurable to allow for dynamically tuned separations. Herein we report the first use of nanoporous gold (NPG) membranes as tunable, charge-based separation elements. NPG is fabricated into a robust network of interconnecting pores via simple solution de-alloying. We demonstrate that control over the selective permeation of small-molecule analytes can be controlled via three different methods: (1) anion adsorption at the Au surface; (2) pH-tuning of self-assembled monolayer (SAM) charge; and (3) electrochemical oxidation of a redox-active SAM. This simple and versatile membrane system is promising as a dynamically tunable element for small-volume separations.

Robust Chemical Synthesis of Membrane Proteins through a General Method of Removable Backbone Modification.

Chemical protein synthesis can provide access to proteins with post-translational modifications or site-specific labelings. Although this technology is finding increasing applications in the studies of water-soluble globular proteins, chemical synthesis of membrane proteins remains elusive. In this report, a general and robust removable backbone modification (RBM) method is developed for the chemical synthesis of membrane proteins. This method uses an activated O-to-N acyl transfer auxiliary to install in the Fmoc solid-phase peptide synthesis process a RBM group with switchable reactivity toward trifluoroacetic acid. The method can be applied to versatile membrane proteins because the RBM group can be placed at any primary amino acid. With RBM, the membrane proteins and their segments behave almost as if they were water-soluble peptides and can be easily handled in the process of ligation, purification, and mass characterizations. After the full-length protein is assembled, the RBM group can be readily removed by trifluoroacetic acid. The efficiency and usefulness of the new method has been demonstrated by the successful synthesis of a two-transmembrane-domain protein (HCV p7 ion channel) with site-specific isotopic labeling and a four-transmembrane-domain protein (multidrug resistance transporter EmrE). This method enables practical synthesis of small- to medium-sized membrane proteins or membrane protein domains for biochemical and biophysical studies.

Compartmentalization and Transport in Synthetic Vesicles.

Nanoscale vesicles have become a popular tool in life sciences. Besides liposomes that are generated from phospholipids of natural origin, polymersomes fabricated of synthetic block copolymers enjoy increasing popularity, as they represent more versatile membrane building blocks that can be selected based on their specific physicochemical properties, such as permeability, stability, or chemical reactivity. In this review, we focus on the application of simple and nested artificial vesicles in synthetic biology. First, we provide an introduction into the utilization of multicompartmented vesosomes as compartmentalized nanoscale bioreactors. In the bottom-up development of protocells from vesicular nanoreactors, the specific exchange of pathway intermediates across compartment boundaries represents a bottleneck for future studies. To date, most compartmented bioreactors rely on unspecific exchange of substrates and products. This is either based on changes in permeability of the coblock polymer shell by physicochemical triggers or by the incorporation of unspecific porin proteins into the vesicle membrane. Since the incorporation of membrane transport proteins into simple and nested artificial vesicles offers the potential for specific exchange of substances between subcompartments, it opens new vistas in the design of protocells. Therefore, we devote the main part of the review to summarize the technical advances in the use of phospholipids and block copolymers for the reconstitution of membrane proteins.

Characterization of fast-tumbling isotropic bicelles by PFG diffusion NMR.

Small isotropic bicelles are versatile membrane mimetics, which, in contrast to micelles, provide a lipid bilayer and are at the same time suitable for solution-state NMR studies. The lipid composition of the bilayer is flexible allowing for incorporation of various head groups and acyl chain types. In bicelles, lipids are solubilized by detergents, which are localized in the rim of the disk-shaped lipid bilayer. Bicelles have been characterized by a broad array of biophysical methods, pulsed-field gradient NMR (PFG NMR) being one of them. PFG NMR can readily be used to measure diffusion coefficients of macromolecules. It is thus employed to characterize bicelle size and morphology. Even more importantly, PFG NMR can be used to study the degree of protein association to membranes. Here, we present the advances that have been made in producing small, fast-tumbling isotropic bicelles from a variety of lipids and detergents, together with insights on the morphology of such mixtures gained from PFG NMR. Furthermore, we review approaches to study protein-membrane interaction by PFG NMR. Copyright © 2015 John Wiley & Sons, Ltd.