Self-assembled Membranes Research Articles

Layer-by-layer self-assembled chitosan/PAA nanofiltration membranes

Abstract Polyelectrolyte composite nanofiltration membranes were prepared using layer-by-layer self-assembly of chitosan and poly(acrylic acid) on a microporous polyethersulfone substrate. Membranes with a layer-by-layer structure were formed by sequential depositions of cationic chitosan and anionic poly(acrylic acid) alternately. The impacts of parameters involved in the membrane preparation process on the separation performance of the resulting membrane were investigated. The membranes showed good separation performance with an over 90% rejections to sulfate salts (i.e., MgSO4 and Na2SO4) and nearly 60% rejections to chloride salts (i.e., MgCl2 and NaCl). In spite of commonly observed trade-off relationship between salt rejection and water permeability, the membrane fabrication conditions can be tailored to enhance the overall performance of the nanofiltration membranes. The LbL self-assembled membranes may not afford a long-term use for nanofiltration of aqueous solutions due to extensive membrane swelling over a prolonged period of operation, and thermal and chemical post-treatments (i.e., heat treatment and chemical crosslinking) can be used to improve the membrane stability.

Silica Metal Oxide Vesicles Catalyze Comprehensive Prebiotic Chemistry

It has recently been demonstrated that mineral self-assembled structures catalyzing prebiotic chemical reactions may form in natural waters derived from serpentinization, a geological process widespread in the early stages of Earth-like planets. We have synthesized self-assembled membranes by mixing microdrops of metal solutions with alkaline silicate solutions in the presence of formamide (NH2 CHO), a single-carbon molecule, at 80 °C. We found that these bilayer membranes, made of amorphous silica and metal oxide/hydroxide nanocrystals, catalyze the condensation of formamide, yielding the four nucleobases of RNA, three amino acids and, several carboxylic acids in a single-pot experiment. Besides manganese, iron and magnesium, two abundant elements in the earliest Earth crust that are key in serpentinization reactions, are enough to produce all these biochemical compounds. These results suggest that the transition from inorganic geochemistry to prebiotic organic chemistry is common on a universal scale and, most probably, occurred earlier than ever thought for our planet.

Preparation of triazole compounds via click chemistry reaction and formation of the protective self-assembled membrane against copper corrosion

The triazole inhibitors of 2-(1-tosyl-1H-1,2,3-triazol-4-yl)-ethanol (TTE) and 2-(1-tosyl-1H-1,2,3-triazol-4-yl)-propan-2-ol (TTP) were synthesized via the click chemistry reaction. Self-assembling technique was used to form self-assembled membrane (SAM) of TTE and TTP on copper surface. The electrochemical measurement results indicate that the TTE and TTP film can strongly suppress copper corrosion in 3 wt.% NaCl solution and the inhibition efficiency of TTE and TTP are 89.4% and 93.1%, respectively. TTP shows better inhibition performance than TTE. Polarization curves show that TTE and TTP predominantly retard the anodic process of the corrosion reaction. The results of quantum chemical calculations and molecule dynamics (MD) simulations reveal that the formation of TTE and TTP SAM on copper surface is mainly achieved by the adsorption of triazole ring and O atoms at horizontal orientation.

YxSi1-xO2-SO3H self-assembled membrane formed on phosphorylated YxSi1-xO2/Al2O3 for oily seawater partial desalination and deep cleaning

In order to save energy consumption of reverse osmosis (RO) process via partial desalination and deep cleaning of oily seawater, YxSi1-xO2-SO3H (YSS) particles were synthesized through co-hydrolysis, silanization and sulfonation, then employed as a functional layer on phosphorylated YxSi1-xO2/Al2O3 (PYSA) to form YSS self-assembled membrane. YSS particles were characterized by SEM, FT-IR and XRD, while the membranes were analyzed through SEM. The YSS self-assembled membranes formed under the optimum conditions were used to partially desalinate and deeply clean oily seawater. The results indicate that YSS particles were successfully synthesized and particle size is homogeneously distributed between 3 and 5 µm. Furthermore, YSS self-assembled membranes not only perform attractive ultrafiltration properties, but also display a desirable partial desalination ratio of 28.6% through Donnan effect under low operating pressure of 0.14 MPa, and contribute to an energy saving of 254.7 W (about 22.0%) for RO process. Besides, the self-assembled membranes can be recycled by calcination, and still partially desalinate and clean oily seawater. This work suggests that YSS self-assembled membranes with partial desalination property are promising alternatives for oily seawater treatment.

Self-Assembly of Thiourea-Crosslinked Graphene Oxide Framework Membranes toward Separation of Small Molecules.

The poor mechanical strength of graphene oxide (GO) membranes, caused by the weak interlamellar interactions, poses a critical challenge for any practical application. In addition, intrinsic but large-sized 2D channels of stacked GO membranes lead to low selectivity for small molecules. To address the mechanical strength and 2D channel size control, thiourea covalent-linked graphene oxide framework (TU-GOF) membranes on porous ceramics are developed through a facile hydrothermal self-assembly synthesis. With this strategy, thiourea-bridged GO laminates periodically through the dehydration condensation reactions via NH2 and/or SH with OCOH as well as the nucleophilic addition reactions of NH2 to COC, leading to narrowed and structurally well-defined 2D channels due to the small dimension of the covalent TU-link and the deoxygenated processes. The resultant TU-GOF/ceramic composite membranes feature excellent sieving capabilities for small species, leading to high hydrogen permselectivities and nearly complete rejections for methanol and small ions in gas, solvent, and saline water separations. Moreover, the covalent bonding formed at the GO/support and GO/GO interfaces endows the composite membrane with significantly enhanced stability.

A dimyristoyl phosphatidylcholine/polydiacetylene biomimetic assembly for the selective screening of progesterone

We have reported a hybrid polymer composite, consisting of a supramolecular assembly of polydiacetylene (PDA) and phospholipid, that undergoes chromatic and fluorogenic changes after specific interactions with progesterone, the pregnancy hormone. Progesterone was selectively detected among 10 steroids by the mixed polymer of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and PDA at an optimized ratio of 1:9. The triggering effect of progesterone on the membrane mimetic system were investigated using UV–vis, fluorescence, circular dichroism spectroscopy, Raman scattering, dynamic light scattering, transmission electron microscopy, and computational method. These results offer insight into self-assembly disruption and membrane targeting antibiotics as well as PDA-based sensing technology.

Vapor-Deposited Porous Polymers for the Fabrication of Giant Lipid Vesicles

Giant unilamellar vesicles (GUVs) are self-assembled biomimetic model membrane systems useful for examining biomembrane properties and building artificial cell membranes for membrane protein incorporation and encapsulation of other biomolecules. Hydrogel-assisted rehydration is an emerging technique to form GUVs under physiological conditions at high yields circumventing the shortcomings of traditional techniques such as electroformation and gentle hydration. Here, we present a new hydrogel material, porous negatively charged poly (methacrylic acid-co-ethylene glycol diacrylate) (xPMAA) for the hydrogel-assisted rehydration method. Porous xPMAA membranes are fabricated using an unconventional solvent-free initiated chemical vapor deposition technique and utilized as hydrogel substrates for vesicle formation. Zwitterionic and charged lipid mixtures are deposited on hydrogel membranes as thin lipid films and subsequently swollen in an aqueous hydration buffer. Vesicle yield and size are controlled by surface roughness, density and charge of the polymer. Our findings show that high hydrogel porosity and reduced electrostatic interactions between the polymer and the lipid are preferred for vesicle formation. In addition, the presence of ions in the hydration buffer affects the yield of GUV formation due to Debye screening of the polymer charge by buffer ions.

Stability of membrane-induced self-assemblies of spherical nanoparticles.

The self-assembly of spherical nanoparticles, resulting from their adhesion on tensionless lipid membranes, is investigated through molecular dynamics simulations of a coarse-grained implicit-solvent model. Our simulations indicate that, with increasing adhesion strength, while reshaping the membrane, the nanoparticles aggregate into a sequence of self-assemblies corresponding to in-plane chains, two-row tubular (bitube) chains, annular (ring) chains, and single-row tubular (tube) chains. Annealing scans, with respect to adhesion strength, show that the transitions between the various phases are highly first-order with significant hystereses. Free energy calculations indicate that the gas and single-row tubular chains are stable over wide ranges of adhesion strength. In contrast, the in-plane chains are only stable for small aggregates of NPs, and the bitube and ring chains are long-lived metastable states over a wide range of adhesion strength.

Self-assembled soft nanoparticle membranes with programmed free volume hierarchy

A self-assembled polyelectrolyte complex nanoparticle membrane was prepared, featuring programed free volume hierarchy, 8 times enhanced fractional free volume, and highly improved molecular separation performance in ethanol dehydration.

Modeling biomembranes and red blood cells by coarse-grained particle methods

In this work, the previously developed coarse-grained (CG) particle models for biomembranes and red blood cells (RBCs) are reviewed, and the advantages of the CG particle methods over the continuum and atomistic simulations for modeling biological phenomena are discussed. CG particle models can largely increase the length scale and time scale of atomistic simulations by eliminating the fast degrees of freedom while preserving the mesoscopic structures and properties of the simulated system. Moreover, CG particle models can be used to capture the microstructural alternations in diseased RBCs and simulate the topological changes of biomembranes and RBCs, which are the major challenges to the typical continuum representations of membranes and RBCs. The power and versatility of CG particle methods are demonstrated through simulating the dynamical processes involving significant topological changes, e.g., lipid self-assembly vesicle fusion and membrane budding.

Mechanically stable solvent-free lipid bilayers in nano- and micro-tapered apertures for reconstitution of cell-free synthesized hERG channels

The self-assembled bilayer lipid membrane (BLM) is the basic component of the cell membrane. The reconstitution of ion channel proteins in artificially formed BLMs represents a well-defined system for the functional analysis of ion channels and screening the effects of drugs that act on them. However, because BLMs are unstable, this limits the experimental throughput of BLM reconstitution systems. Here we report on the formation of mechanically stable solvent-free BLMs in microfabricated apertures with defined nano- and micro-tapered edge structures. The role of such nano- and micro-tapered structures on the stability of the BLMs was also investigated. Finally, this BLM system was combined with a cell-free synthesized human ether-a-go-go-related gene channel, a cardiac potassium channel whose relation to arrhythmic side effects following drug treatment is well recognized. Such stable BLMs as these, when combined with a cell-free system, represent a potential platform for screening the effects of drugs that act on various ion-channel genotypes.

A self-assembled clavanin A-coated amniotic membrane scaffold for the prevention of biofilm formation by ocular surface fungal pathogens

Amniotic membrane (AM) is frequently used in ophthalmologic surgery for rapid ocular surface reconstruction. Sometimes it may create a major problem with associated infections after biofilm formation over the membrane. To overcome this problem, AM was coated with the antimicrobial peptide clavanin A. The antifungal activity of clavanin A in the native and self-assembled form was determined against the common ocular surface pathogens Candida albicans, Aspergillus fumigatus, Alternaria sp. and Fusarium sp. Biofilm formation over the coated surface was significantly reduced in comparison with the uncoated membrane. The coated membrane revealed effectiveness in terms of biocompatibility, cell attachment colonization when tested in non-cancerous 3T3 and human embryonic kidney (HEK)-293 cell lines. Clavanin A-coated AM also exhibited excellent physical, morphological and antifungal characteristics, indicating potential applicability for ocular surface infection control.

Probing Elastic and Viscous Properties of Phospholipid Bilayers Using Neutron Spin Echo Spectroscopy.

The elastic and viscous properties of self-assembled amphiphilic membranes dictate the intricate hierarchy of their structure and dynamics ranging from the diffusion of individual molecules to the large-scale deformation of the membrane. We previously demonstrated that neutron spin echo spectroscopy measurements of model amphiphilic membranes can access the naturally occurring submicrosecond membrane motions, such as bending and thickness fluctuations. Here we show how the experimentally measured fluctuation parameters can be used to determine the inherent membrane properties and demonstrate how membrane viscosity and compressibility modulus are influenced by lipid composition in a series of simple phosphatidylcholine bilayers with different tail lengths as a function of temperature. This approach highlights the interdependence of the bilayer elastic and viscous properties and the collective membrane dynamics and opens new avenues to investigating the mechanical properties of more complex and biologically inspired systems.

Sequential coating of nanopores with charged polymers: A general approach for controlling pore properties of self-assembled block copolymer membranes

A general approach is developed for controlling the pore size and pore chemistry of integral isoporous membranes derived from the assembly of polystyrene-b-poly-4-vinyl pyridine (PS-b-P4VP) diblock copolymer. In this approach, initially, the sub-50 nm pore surface, decorated with poly-4-vinyl pyridine (P4VP) polymer brush, is coated with polyacrylic acid (PAA). PAA offers a majority of anionic and a minority of neutral acid functionalities. The neutral acid groups adhere to the P4VP segments of the pore wall through hydrogen bonding interactions and the anionic sites remain free. The availability of the anionic sites sets the stage for a layer-by-layer deposition of cationic and anionic polymers, in a sequential manner, on the pore-wall surface through a continuous flow of a dilute polyelectrolyte solution through the nanoporous membrane. In this way, multiple layers, stabilized through electrostatic interactions, can be deposited leading to a continuous decrease in the pore size and a known surface charge. Due to the known facile nature of the large area isoporous asymmetric membrane formation and modular nature of the polyelectrolyte assembly, the present approach is anticipated to yield new block copolymer membranes with tailored separation, sensing, and catalytic properties.

Thermomechanical Response of Self-Assembled Nanoparticle Membranes.

Monolayers composed of colloidal nanoparticles, with a thickness of less than 10 nm, have remarkable mechanical moduli and can suspend over micrometer-sized holes to form free-standing membranes. In this paper, we discuss experiments and coarse-grained molecular dynamics simulations characterizing the thermomechanical properties of these self-assembled nanoparticle membranes. These membranes remain strong and resilient up to temperatures much higher than previous simulation predictions and exhibit an unexpected hysteretic behavior during the first heating-cooling cycle. We show this hysteretic behavior can be explained by an asymmetric ligand configuration from the self-assembly process and can be controlled by changing the ligand coverage or cross-linking the ligand molecules. Finally, we show the screening effect of water molecules on the ligand interactions can strongly affect the moduli and thermomechanical behavior.

Flexible self-assembled membrane electrodes based on eco-friendly bamboo fibers for supercapacitors

Here we report a flexible supercapacitor electrode fabricated via a simple method by applying a continuous polyaniline (PANi) coating on the surface of a cellulose nanofibers (CNFs) film. On one hand, we can recycle a large amount of bamboo fiber waste. On the other hand, we greatly improve the processing performance of PANi by making full use of CNFs’ superior mechanical properties. The flexible eco-friendly CNFs/PANi film can be directly used as an electrode without binder. SEM and FTIR measurements indicate there is a very secure combination between CNFs and PANi. The CNFs/PANi electrode boasts fine mechanical properties, excellent reversible performance and good specific capacitance (254.7 F/g at current density of 0.2 A/g). We expect the hybrid electrode will be used for flexible supercapacitor application.

Self-Assembled TEMPO Cellulose Nanofibers: Graphene Oxide-Based Biohybrids for Water Purification.

Nanocellulose, graphene oxide (GO), and their combinations there off have attracted great attention for the application of water purification recently because of their unique adsorption capacity, mechanical characteristics, coordination with transition metal ions, surface charge density, and so on. In the current study, (2,2,6,6-tetramethylpiperidine-1-oxylradical) (TEMPO)-mediated oxidized cellulose nanofibers (TOCNF) and GO sheets or graphene oxide nanocolloid (nanoGO) biohybrids were prepared by vacuum filtration method to obtain self-assembled adsorbents and membranes for water purification. The porous biohybrid structure, studied using advanced microscopy techniques, revealed a unique networking and self-assembling of TOCNF, GO, and nanoGO, driven by the morphology of the GO phase and stabilized by the intermolecular H-bonding between carboxyl groups and hydroxyl groups. The biohybrids exhibited a promising adsorption capacity toward Cu(II) due to TOCNF and formed a unique "arrested state" in water because of ionic cross-linking between adsorbed Cu(II) and the negatively charged TOCNF and GO phase. The mechanical performance of the freestanding biohybrid membranes investigated using PeakForce Quantative NanoMechanics characterization confirmed the enhanced modulus of the hybrid membrane compared to that of the TOCNF membrane. Besides, the TOCNF+nanoGO membrane shows unique hydrolytic stability and recyclability even under several cycles of adsorption and desorption and strong sonication. This study shows that TOCNF and nanoGO hybrids can generate new water-cleaning membranes with synergistic properties because of their high adsorption capacity, flexibility, hydrolytic stability, and mechanical robustness.

Formation of phosphorylated ZrxSi1−xO2/Al2O3 self-assembled membrane for cleaning oily seawater

In order to deeply clean seawater containing oil, hydrolyzed polyacrylamide (HPAM) and suspended solids, phosphorylated ZrxSi1−xO2/Al2O3 (PZSA) particles were firstly prepared through co-hydrolysis, silanization and phosphorylation, followed by coating of Al2O3 and then employed as a functional layer to form PZSA self-assembled membranes on porous supports. PZSA particles were characterized by scanning electron microscope (SEM), energy dispersive X-ray detector (EDX), Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) while the self-assembled membranes were analyzed using SEM. Moreover, the optimum formation conditions of PZSA self-assembled membrane thickness were determined. The membranes formed under the optimal conditions were used to treat oily seawater and the quality of permeate water was investigated and analyzed. The results indicate that the particle size of PZSA is mainly distributed between 400 and 600nm, Zr elements have been successfully doped into the lattice of SiO2 and γ-Al2O3 has been coated on the surface of ZrxSi1−xO2. Meanwhile, PZSA self-assembled membranes with desirable separation and selective adsorption properties perform attractive oil and COD retention ratio and water yield. Therefore, PZSA self-assembled membranes have the potential application in treating and cleaning oily seawater.

Effectively suppressing vanadium permeation in vanadium redox flow battery application with modified Nafion membrane with nacre-like nanoarchitectures

Abstract A novel self-assembled composite membrane, Nafion-[PDDA/ZrP]n with nacre-like nanostructures was successfully fabricated by a layer-by-layer (LbL) method and used as proton exchange membrane for vanadium redox flow battery applications. Poly(diallyldimethylammonium chloride) (PDDA) with positive charges and zirconium phosphate (ZrP) nanosheets with negative charges can form ultra-thin nacre-like nanostructure on the surface of Nafion membrane via the ionic crosslinking of tightly folded macromolecules. The lamellar structure of ZrP nanosheets and Donnan exclusion effect of PDDA can greatly decrease the vanadium ion permeability and improve the selectivity of proton conductivity. The fabricated Nafion-[PDDA/ZrP]4 membrane shows two orders of magnitude lower vanadium ion permeability (1.05 × 10−6 cm2 min−1) and 12 times higher ion selectivity than those of pristine Nafion membrane at room temperature. Consequently, the performance of vanadium redox flow batteries (VRFBs) assembled with Nafion-[PDDA/ZrP]3 membrane achieved a highly coulombic efficiency (CE) and energy efficiency (EE) together with a very slow self-discharge rate. When comparing with pristine Nafion VRFB, the CE and EE values of Nafion-[PDDA/ZrP]3 VRFB are 10% and 7% higher at 30 mA cm−2, respectively.

Liquid-crystal self-assembly of lipid membranes on solutions: A dissipative particle dynamic simulation study

The self-assembly of the amphiphilic lipid molecules in aqueous solution is investigated via dissipative particle dynamic simulation. Lipid bilayer and perforated bilayer membranes, together with the micelles and vesicles, which couple the spatial inhomogeneity and orientation ordering, are observed in equilibrium states and dynamics processes. For the equilibrium states, the phase diagrams are arranged by the lengths of head and tail chains in various lipid-water mixtures, where the influence of chain length and water composition on formation of bilayer and perforated bilayer membranes is carefully analyzed. In the dynamic processes, we investigate the bilayer membranes by analyzing the energies of systems, which indicates that the formations of bilayers and perforated bilayers strongly depended on the initial chain distributions and lipid types. Moreover, we also investigate the dependence of elastic modulus on the chain lengths for the bilayer membranes and the tension for the perforated pores in the perforated bilayer membranes. The observations on these liquid-crystalline self-assembly provided a promising approach that could be used to design and understand the biomembrane based on the lipid molecules.