Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation

Polyelectrolyte composite membranes were prepared by the electrostatic layer-by-layer self-assembly of oppositely charged polyelectrolytes, and the membranes were used for dehydration of isopropanol by pervaporation. The effects of membrane preparation conditions on the separation performance of the resulting membranes were investigated. It was found that a high charge density of the polyelectrolyte was favorable to the formation of permselective membranes and that the polyelectrolyte molecules should be sufficiently larger than the pore size of the microporous substrate in order to reduce the number of polyelectrolyte depositions required to form a defect-free membrane. It has been demonstrated that using the appropriate substrate and under suitable conditions for polyelectrolyte deposition, a permselective membrane can be formed with as few as two polyelectrolyte bilayers, which is much less than those commonly used in the literature (e.g., 60–90 bilayers). At 70 °C, the polyelectrolyte composite membrane with two bilayers exhibited a flux of 1.8 kg/(m² h) and a permeate water concentration of over 98 wt.% for the dehydration of isopropanol containing 9 wt.% water, which corresponds to a separation factor of greater than 495.     

Self-assembled virus-membrane complexes

Anionic polyelectrolytes and cationic lipid membranes can self-assemble into lamellar structures ranging from alternating layers of membranes and polyelectrolytes to 'missing layer' superlattice structures. We show that these structural differences can be understood in terms of the surface-charge-density mismatch between the polyelectrolyte and membrane components by examining complexes between cationic membranes and highly charged M13 viruses, a system that allowed us to vary the polyelectrolyte diameter independently of the charge density. Such virus-membrane complexes have pore sizes that are about ten times larger in area than DNA-membrane complexes, and can be used to package and organize large functional molecules; correlated arrays of Ru(bpy)(3)(2+) macroionic dyes have been directly observed within the virus-membrane complexes using an electron-density reconstruction. These observations elucidate fundamental design rules for rational control of self-assembled polyelectrolyte-membrane structures, which have applications ranging from non-viral gene therapy to biomolecular templates for nanofabrication.     

Self-assembled nano-structured polyelectrolyte composite membranes for pervaporation

This work is concerned with nano-structured polyelectrolyte composite membranes for solvent dehydration by pervaporation. The membranes were prepared by the electrostatic layer-by-layer deposition of polyethylenimine and poly(acrylic acid) onto a microporous polyacrylonitrile substrate membrane. Each cycle of alternating deposition of the cationic and anionic polymers formed a bilayer of polyelectrolyte, and multiple deposition cycles were used to achieve the permselectivity of the membrane. The polyacrylonitrile substrate membrane was partially hydrolyzed to improve the initial deposition of polyelectrolytes. It was proposed to use a relatively dilute concentration of the polyelectrolytes in the first few cycles of deposition, followed by depositions with more concentrated polyelectrolyte solutions (but still far below the critical overlapping concentration to ensure well extended conformation of the polyelectrolyte molecules). It was demonstrated that using this technique a good permselectivity could be achieved with less than 10 cycles of deposition, which was much less than the number of cycles used in the literature (e.g. 60–90). The membrane showed good separation performance for separation of water from isopropanol; at a feed water concentration of 8–10 wt.%, a permeate concentration of over 99 wt.% water was achieved with a permeation flux of about 0.6 kg/m² h.     

Mechanically responding nanovalves based on polyelectrolyte multilayers

The alternate deposition of exponentially and linearly growing polyelectrolyte multilayers leads to the formation of multicompartment films. In this study, a new system consisting in nanometer-sized multilayer barriers deposited on or between multilayer compartments was designed to respond to mechanical stimuli and to act as nanovalves. The diffusion of polyelectrolytes through the barrier from one compartment to another can be switched on/off by tuning the mechanical stretching and thereby opening or closing nanopores in the barrier. This work represents a first step toward the design of chemically or biologically active films responding to mechanical stresses.     

Shapes of polyelectrolyte titration curves. 1. Well-behaved strong polyelectrolytes

The charge content of aqueous polymers is measured routinely by polyelectrolyte titrations in which an unknown polymer is titrated with an oppositely charged standard polymer solution, usually poly(diallyldimethyl ammonium chloride) or potassium poly(vinyl sulfate). Polyelectrolyte titration end points are frequently determined with a streaming current detector (SCD). The shapes of polyelectrolyte titration curves from polyelectrolytes with fixed electrical charges were simulated by a diffuse electrical double layer model. Well-behaved titration curves obtained with fixed charged polyelectrolytes were fit by the modeling, giving support for the basic hypothesis that the net charge of material adsorbed on the SCD wall is a linear function of the volume of added titrant. The shapes of titration curves from deviant systems such as poly(vinyl alcohol)-borate cannot be predicted by the model.     

Engineering Nanoassemblies of Polysaccharides

Polysaccharides offer a wealth of biochemical and biomechanical functionality that can be used to develop new biomaterials. In mammalian tissues, polysaccharides often exhibit a hierarchy of structure, which includes assembly at the nanometer length scale. Furthermore, their biochemical function is determined by their nanoscale organization. These biological nanostructures provide the inspiration for developing techniques to tune the assembly of polysaccharides at the nanoscale. These new polysaccharide nanostructures are being used for the stabilization and delivery of drugs, proteins, and genes, the engineering of cells and tissues, and as new platforms on which to study biochemistry. In biological systems polysaccharide nanostructures are assembled via bottom‐up processes. Many biologically derived polysaccharides behave as polyelectrolytes, and their polyelectrolyte nature can be used to tune their bottom‐up assembly. New techniques designed to tune the structure and composition of polysaccharides at the nanoscale are enabling researchers to study in detail the emergent biological properties that arise from the nanoassembly of these important biological macromolecules.     

Controlling Ion‐Transport Selectivity in Gold Nanotubule Membranes

We have developed a new class of synthetic membranes that consist of a porous polymeric support. This support contains an ensemble of gold nanotubules that span the complete thickness of the support membrane. The support is a commercially available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubules are prepared via electroless deposition of Au onto the pore walls, and tubules that have inside diameters of molecular dimensions (<1 nm) can be prepared. Hence, these membranes are a new class of molecular sieves. We review in this paper the ion‐transport properties of these Au nanotubule membranes. We will show that these membranes can be cation‐permselective or anion‐permselective, and that the permselectivity can be reversibly switched between these two states. Ion permselectivity can be introduced by two different routes. The first entails chemisorption of an ionizable thiol, e.g., a carboxylated or ammonium‐containing thiol to the Au tubule walls. If the thiol contains both of these functionalities (e.g., the amino acid cysteine), the permselectivity can be reversibly switched by varying the pH of the contacting solution phase. Ion permselectivity can also be introduced by potentiostatically charging the membrane in an electrolyte solution. By applying excess negative charge, cation permselective membranes are obtained, and excess positive charge yields anion permselective membranes. In this case the permselectivity can be reversibly switched by changing the potential applied to the membrane.     

Hinged nanorods made using a chemical approach to flexible nanostructures

The fabrication of multifunctional nanomaterials and their subsequent use for novel applications in various branches of nanotechnology has been under intense scrutiny. Particularly in the area of nanomechanics, the design of multicomponent nanostructures with an integrated multifunctionality would enable the construction of building blocks for nanoscale analogues of macroscopic objects. Here, we introduce a new class of flexible nanostructures: metallic nanorods with polyelectrolyte hinges, synthesized using layer-by-layer electrostatic self-assembly of oppositely charged polyelectrolytes on barcode metal nanorods followed by segment-selective chemical etching. Nanorods with hinges that consist of one polyelectrolyte bilayer display considerable flexibility, but with a greater number of bilayers the flexibility of the hinge is significantly reduced. Magnetically induced bending about the polymer hinge is illustrated through the incorporation of nickel segments into the barcodes and the application of an external fluctuating magnetic field.     

Characterizing the Lateral Friction of Nanoparticles on On‐Chip Integrated Black Lipid Membranes

The use of nanoparticles (NPs) in biomedical applications creates a need for appropriate model systems to systematically investigate NP–membrane interactions under well‐defined conditions. Black lipid membranes (BLMs) are free‐floating membranes with defined composition that are ideally suited for characterizing NP–membrane interactions free of any potential perturbation through a supporting substrate. Herein, arrays of microfabricated BLMs are integrated into a chip‐based platform that is compatible with high‐speed optical NP tracking. This system is used to investigate the lateral diffusion of 40 nm gold spheres tethered to biotinylated lipids through antibody‐functionalized ligands (single‐stranded DNA or polyethylene glycol). Although the NPs show an almost free and ergodic diffusion, their lateral motion is subject to substantial drag at the membrane surface, which leads to systematically smaller diffusion coefficients than those obtained for lipids in the membrane through fluorescence recovery after photobleaching. The lateral mobility of the NPs is influenced by the chemical composition and salt concentration at the NP‐membrane interface, but is independent of the ligand density in the membrane. Together with the observation that nanoprisms, which have a larger relative contact area with the membrane than spherical NPs, show an even slower diffusion, these findings indicate that the lateral mobility of NPs tethered in close vicinity to a membrane is significantly reduced by the friction at the NP‐membrane interface.     

Controlled Synthesis of One‐Dimensional Inorganic Nanostructures Using Pre‐Existing One‐Dimensional Nanostructures as Templates

Template‐directed strategy has become one of the most popular methods for the fabrication of one‐dimensional (1D) nanostructures with uniform size and controllable physical dimensions in recent years. This Review article describes the recent progress in the synthesis of 1D inorganic nanostructures by using suitable templates. A brief survey on the templating method based on the organic templates and porous membrane is firstly given. Then, the article is focused on recent emerging synthetic strategies by templating against the pre‐existing 1D nanostructures using different physical and chemical transformation techniques, including epitaxial growth, nonepitaxial growth, direct chemical transformation, solid‐state interfacial diffusion reaction, and so on. The important reactivity role of the 1D nanostructures will be emphasized in such transformation process. Finally, we conclude this paper with some perspectives and outlook on this research topic.     

Phosphorus‐Mediated MoS2 Nanowires as a High‐Performance Electrode Material for Quasi‐Solid‐State Sodium‐Ion Intercalation Supercapacitors

Molybdenum disulfide (MoS₂) is a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacity and fascinating 2D layered structure. However, its sluggish kinetics for ionic diffusion and charge transfer limits its practical applications. Here, a promising strategy is reported for enhancing the Na⁺‐ion charge storage kinetics of MoS₂ for supercapacitors. In this strategy, electrical conductivity is enhanced and the diffusion barrier of Na⁺ ion is lowered by a facile phosphorus‐doping treatment. Density functional theory results reveal that the lowest energy barrier of dilute Na‐vacancy diffusion on P‐doped MoS₂ (0.11 eV) is considerably lower than that on pure MoS₂ (0.19 eV), thereby signifying a prominent rate performance at high Na intercalation stages upon P‐doping. Moreover, the Na‐vacancy diffusion coefficient of the P‐doped MoS₂ at room temperatures can be enhanced substantially by approximately two orders of magnitude (10^?6–10^?4 cm² s^?1) compared with pure MoS₂. Finally, the quasi‐solid‐state asymmetrical supercapacitor assembled with P‐doped MoS₂ and MnO₂, as the positive and negative electrode materials, respectively, exhibits an ultrahigh energy density of 67.4 W h kg^?1 at 850 W kg^?1 and excellent cycling stability with 93.4% capacitance retention after 5000 cycles at 8 A g^?1.     

The Role of Temperature and Lipid Charge on Intake/Uptake of Cationic Gold Nanoparticles into Lipid Bilayers

Understanding the molecular mechanisms governing nanoparticle–membrane interactions is of prime importance for drug delivery and biomedical applications. Neutron reflectometry (NR) experiments are combined with atomistic and coarse‐grained molecular dynamics (MD) simulations to study the interaction between cationic gold nanoparticles (AuNPs) and model lipid membranes composed of a mixture of zwitterionic di‐stearoyl‐phosphatidylcholine (DSPC) and anionic di‐stearoyl‐phosphatidylglycerol (DSPG). MD simulations show that the interaction between AuNPs and a pure DSPC lipid bilayer is modulated by a free energy barrier. This can be overcome by increasing temperature, which promotes an irreversible AuNP incorporation into the lipid bilayer. NR experiments confirm the encapsulation of the AuNPs within the lipid bilayer at temperatures around 55 °C. In contrast, the AuNP adsorption is weak and impaired by heating for a DSPC–DSPG (3:1) lipid bilayer. These results demonstrate that both the lipid charge and the temperature play pivotal roles in AuNP–membrane interactions. Furthermore, NR experiments indicate that the (negative) DSPG lipids are associated with lipid extraction upon AuNP adsorption, which is confirmed by coarse‐grained MD simulations as a lipid‐crawling effect driving further AuNP aggregation. Overall, the obtained detailed molecular view of the interaction mechanisms sheds light on AuNP incorporation and membrane destabilization.     

Chemically tunable electrochemical dissolution of noncontinuous polyelectrolyte assemblies: an in situ study using ecAFM

The electrochemically triggered dissolution of noncontinuous polyelectrolyte assemblies presenting distinct nanomorphologies and its tuning by chemical cross-linking were monitored locally, in situ, by electrochemical atomic force microscopy. Poly-l-lysine and hyaluronic acid deposited layer-by-layer on indium tin oxide electrodes at specific experimental conditions formed well-defined nanostructures whose morphologies could be easily and precisely followed along the dissolution process. In addition to shrinkage of polyelectrolyte nanodroplets, ecAFM images revealed the faster dissolution of coalesced structures compared to droplet-like complexes, and the readsorption of dissolved polyelectrolytes onto slower dissolving neighboring structures. Covalently cross-linked PLL/HA assemblies dissolved only partially, and exhibited slower dissolution rates compared to native multilayers, with a clear dependence on the cross-link density. Tuning the electrochemical dissolution of polyelectrolyte multilayers through chemical cross-linking opens new prospects for future biomedical applications, such as the development of advanced drug or gene delivery platforms allowing for tightly controlled releases of different compounds at specific rates.     

Probing Membrane Viscosity and Interleaflet Friction of Supported Lipid Bilayers by Tracking Electrostatically Adsorbed,Nano‐Sized Vesicles

Particle tracking is used to measure the diffusional motion of nanosized (≈100 nm), lipid vesicles that are electrostatically adsorbed onto a solid supported lipid bilayer. It is found that the motion of membrane‐adhering vesicles is Brownian and depends inversely on the vesicle size, but is insensitive to the vesicle surface charge. The measured diffusivity agrees well with the Evans–Sackmann model for the diffusion of inclusions in supported, fluidic membranes. The agreement implies that the vesicle motion is coupled to that of a nanoscopic lipid cluster in the upper leaflet, which slides over the lower leaflet. The diffusivity of membrane‐adhering vesicles is therefore predominantly governed by the interleaflet friction coefficient, while the diffusivity of single lipids is mainly governed by the membrane viscosity. Combined with fluorescence recovery after photobleaching analysis, the interleaflet friction coefficient and the membrane viscosity are determined by applying the Evans–Sackmann model to the measured diffusivity of membrane adhering vesicles and that of supported membrane lipids. This approach provides an alternative to existing methods for measuring the interleaflet friction coefficient and the membrane viscosity.     

Modulated Triple‐Material Nano‐Heterostructures: Where Gold Influenced the Chemical Activity of Silver in Nanocrystals

For efficient charge separations, multimaterial hetero‐nanostructures are being extensively studied as photocatalysts. While materials with one heterojunction are widely established, the chemistry of formation of multijunction heterostructures is not explored. This needs a more sophisticated approach and modulations. To achieve these, a generic multistep seed mediated growth following controlled ion diffusion and ion exchange is reported which successfully leads to triple‐material hetero‐nanostructures with bimetallic‐binary alloy‐binary/ternary semiconductors arrangements. Ag₂S nanocrystals are used as primary seeds for obtaining AuAg‐AuAgS bimetallic‐binary alloyed metal–semiconductor heterostructures via partial reduction of Ag(I) using Au(III) ions. These are again explored as secondary seeds for obtaining a series of triple‐materials heterostructures, AuAg‐AuAgS‐CdS (or ZnS or AgInS₂), with introduction of different divalent and trivalent ions. Chemistry of each step of the gold ion–induced changes in the rate of diffusion and/or ion exchanges are investigated and the formation mechanism for these nearly monodisperse triple material heterostructures are proposed. Reactions without gold are also performed, and the change in the reaction chemistry and growth mechanism in presence of Au is also discussed.     

Nanoporous anodic alumina obtained without protective oxide layer by hard anodization

In this study, a fast and cost-effective approach is applied for fabricating nanoporous anodic alumina membranes under hard conditions without a protective oxide layer. This structural characteristic is a result of using a two-step anodization strategy under specific hard conditions during the second anodization step (i.e. high stirring rate, low acid electrolyte temperature and concentration). Notice that, after the anodization process, the membrane is detached from the aluminium substrate at the same time that pores are opened. So, after the fabrication process, no additional stages are required for removing both the protective oxide layer and the oxide barrier layer from the top and the bottom of the membrane, respectively. The resulting nanostructures obtained by this approach are defect-free nanoporous anodic alumina membranes with well-defined pores from the top to the bottom. This makes it possible to directly use those membranes in later applications (e.g. templates for replicating nanostructures, filters, optoelectronic devices and so forth) without additional processes and costs.     

Kinetic modulation of pulsed chronopotentiometric polymeric membrane ion sensors by polyelectrolyte multilayers

Polymeric membrane ion-selective electrodes are normally interrogated by zero current potentiometry, and their selectivity is understood to be primarily dependent on an extraction/ion-exchange equilibrium between the aqueous sample and polymeric membrane. If concentration gradients in the contacting diffusion layers are insubstantial, the membrane response is thought to be rather independent of kinetic processes such as surface blocking effects. In this work, the surface of calcium-selective polymeric ion-selective electrodes is coated with polyelectrolyte multilayers as evidenced by zeta potential measurements, atomic force microscopy, and electrochemical impedance spectroscopy. Indeed, such multilayers have no effect on their potentiometric response if the membranes are formulated in a traditional manner, containing a lipophilic ion exchanger and a calcium-selective ionophore. However, drastic changes in the potential response are observed if the membranes are operated in a recently introduced kinetic mode using pulsed chronopotentiometry. The results suggest that the assembled nanostructured multilayers drastically alter the kinetics of ion transport to the sensing membrane, making use of the effect that polyelectrolyte multilayers have different permeabilities toward ions with different valences. The results have implications to the design of chemically selective ion sensors since surface-localized kinetic limitations can now be used as an additional dimension to tune the operational ion selectivity.     

Catalytic membranes prepared using layer-by-layer adsorption of polyelectrolyte/metal nanoparticle films in porous supports

Layer-by-layer adsorption of polyelectrolytes and gold nanoparticles within porous supports provides a convenient method for forming catalytic membranes. The polyelectrolyte film effectively immobilizes the gold nanoparticles without inhibiting access to catalytic sites, as shown by the similar rate constants for nanoparticle-catalyzed 4-nitrophenol reduction in solution and in membranes. Modified alumina membranes reduce >99% of 0.4 mM 4-nitrophenol at linear flow rates of 0.98 cm/s, and the modification process is also applicable to track-etched polycarbonate supports.     

Templates as Shadow Masks to Tune the Magnetic Anisotropy in Nanostructured CoCrPt/Ti Bilayer Films

Off‐axis deposition of Ti and CoCrPt films onto lithographically patterned templates has been used to make nanostructures with a lateral thickness variation that allows the tuning of the magnetic anisotropy. CoCrPt rectangles of 1 μm × 725 nm without a thickness variation show an out‐of‐plane easy axis and a single‐domain configuration after demagnetization. On the other hand, rectangles with a thickness variation along their longer dimension show an out‐of‐plane multidomain state, but an in‐plane vortex configuration occurs when the thickness variation is along the shorter dimension. The evolution of the magnetic behavior is understood from the change in both Ti and CoCrPt thicknesses and their effects on the magnetic anisotropy, and provides a simple method for controlling the magnetic state and reversal process of patterned nanostructures.     

Functional Barrier Performance of a Polyamide‐6 Membrane Towards n‐Alkanes and 1‐Alcohols

Polymeric barrier membranes are widely used in the food packaging area. So‐called functional barriers prevent the packed food from contamination of permeants from the environment or from other packaging components. Aim of the study was to determine barrier properties of non‐polar n‐alkanes (n‐pentane up to n‐tetradecane) and polar 1‐alcohols (1‐propanol to 1‐octanol) of a 12 µm biaxially oriented PA6 film at temperatures between 70°C and 100°C. From the experimentally determined lag times, the diffusion coefficients D_P as well as the partition coefficients K of the investigated permeants were calculated. From the correlation between the molecular volume V and the diffusion coefficients D_P, the lag times for lower temperatures can be predicted for temperatures between 20°C and 70°C. In conclusion, the investigated 12 µm PA6 film is a sufficient barrier towards organic compounds. The permeation of substances with molecular volumes above 200 ų is negligible for a shelf life of three years and temperatures below of 60°C. These storage conditions are sufficient for nearly all kinds of food packaging applications. Copyright © 2016 John Wiley & Sons, Ltd.