In-Situ Gas Permeation-Driven Ionic Current Rectification of Heterogeneously Charged Nanopore Arrays.

Ionic diodes provide ionic current rectification (ICR), which is useful for micro-/nanofluidic devices for ionic current-mediated applications. However, the modulation of ICR is not fully developed, and current challenges include limited active control and localized modulation for further multiplexing of micro-/nanofluidic ionic diodes. Herein, a microfluidic device integrated with particle-assembly-based ionic diodes (PAIDs) and a gas-flow channel above them is presented. Exploiting in-situ gas permeation through a polymeric film, precise control over the physiochemical conditions of the nanopores within the PAIDs, leading to the modulation of ICR is demonstrated. The investigation not only characterizes the rectification properties of the PAIDs but also unveils their capacitor-like behavior and the ability to actively modulate ICR using various gas flows. Furthermore, the reversible modulation of ICR through dynamic switching of gas-dissolved solutions, enabling ion-signal amplification is showcased. This pioneering approach of in situ gas-permeation offers programmable manipulation of ion transport along PAIDs, thereby positioning ionic diodes as versatile nanofluidic components. Looking ahead, the development of multiplexed PAIDs in an addressable manner on a chip holds promise for practical applications across diverse fields, including ion signaling, ion-based logic, chemical reactors, and (bio)chemical sensing.

Molecular Permeation in Freestanding Bilayer Silica.

Graphene and other single-layer structures are pursued as high-flux separation membranes, although imparting porosity endangers their crystalline integrity. In contrast, bilayer silica composed of corner-sharing (SiO4) units is foreseen to be permeable for small molecules due to its intrinsic lattice openings. This study sheds light on the mass transport properties of freestanding 2D SiO2 upon using atomic layer deposition (ALD) to grow large-area films on Au/mica substrates followed by transfer onto Si3N4 windows. Permeation experiments with gaseous and vaporous substances reveal the suspended material to be porous, but the membrane selectivity appears to diverge from the size exclusion principle. Whereas the passage of inert gas molecules is hindered with a permeance below 10-7 mol·s-1·m-2·Pa-1, condensable species like water are found to cross vitreous bilayer silica a thousand times faster in accordance with their superficial affinity. This work paves the way for bilayer oxides to be addressed as inherent 2D membranes.

The effect of layer number on the gas permeation through nanopores within few-layer graphene.

Few-layer graphene has been widely regarded as an efficient filter for gas separation, but the effect of the layer number on the gas permeation process is still unclear. To explore the layer number effect, we perform molecular dynamics simulations to investigate the gas permeation through a nanopore within the few-layer graphene. Our numerical simulations show that the permeation constant decreases with increasing layer number, which is analyzed based on the macroscopic Kennard empirical model. The macroscopic model is in good agreement with the numerical result in the limit of large layer number, but there are obvious deviations for the medium layer number. We generalize the macroscopic model by considering the nanoscale effect from the surface morphology of the nanoscale pore, which can well describe the layer number dependence for the gas permeation constant in the full range. These results provide valuable information for the application of few-layer graphene in the gas permeation field.

A novel poly (4-methyl-1-pentene)/polypropylene (PMP/PP) thin film composite (TFC) artificial lung membrane for enhanced gas transport and excellent hemo-compatibility.

Extracorporeal membrane oxygenation is a technique that provides short-term supports to the heart and lungs. It removes CO2 from the blood and provides enough oxygen, which is a huge help in the fight against COVID-19. As the key component, the artificial lung membranes have evolved in three generations including silicon, polypropylene and poly (4-methyl-1-pentene). Herein, we for the first time design and fabricate a novel poly (4-methyl-1-pentene)/polypropylene (PMP/PP) thin film composite (TFC) membrane with the anticoagulant coating composed of poly (sodium 4-styrenesulfonate) and cross-linked poly (vinyl alcohol). Poly (sodium 4-styrenesulfonate) provides sulfonic acid groups to inhibit the coagulant factors (FVIII and FXII), and cross-linked poly (vinyl alcohol) increase the stability of the anticoagulant coating and further improve the hydrophilicity via abundant hydroxyl groups to depress the protein adsorption. Long-term anticoagulant property was demonstrated by whole human blood for 28 days. Blood compatibility was evaluated by hemolysis rate, anticoagulation activity (APTT, TT and PT), complement activation, platelet activation and contact activation. Pure CO2, O2 and N2 permeation rates were determined to evaluate the mass transfer properties of PMP/PP TFC membranes. Gas permeation results revealed that gas permeation flux increased in the TFC membranes because of the decrease of crystallinity. Overall, the so prepared PMP/PP membrane shows good CO2/O2 selectivity and blood compatibility as novel TFC artificial lung membrane.

Determination of Gas Permeation Properties in Polymer Using Capacitive Electrode Sensors.

The objective of this work was to develop an effective technique for characterizing the permeation properties of various gases, including H2, He, N2, and Ar, that are absorbed in polymers. Simultaneous three-channel real-time techniques for measuring the sorption content and diffusivity of gases emitted from polymers are developed after exposure to high pressure and the subsequent decompression of the corresponding gas. These techniques are based on the volumetric measurement of released gas combined with the capacitance measurement of the water content by both semi-cylindrical and coaxial-cylindrical electrodes. This minimizes the uncertainty due to the varying temperature and pressure of laboratory environments. The gas uptake and diffusivity are determined as a function of the exposed pressure and gas spices in nitrile butadiene rubber (NBR) and ethylene propylene diene monomer (EPDM) polymers. The pressure-dependent gas transport behaviors of four different gases are presented and compared with those obtained by different techniques. A linear correlation between the logarithmic diffusivity and kinetic diameter of molecules in the gas is found between the two polymers.

Selective Gas Permeation in Defect-Engineered Bilayer Graphene.

Defective graphene holds great potential to enable the permeation of gas molecules at high rates with high selectivity due to its one-atom thickness and resultant atomically small pores at the defect sites. However, precise control and tuning of the size and density of the defects remain challenging. In this work, we introduce atomic-scale defects into bilayer graphene via a decoupled strategy of defect nucleation using helium ion irradiation followed by defect expansion using hydrogen plasma treatment. The cotreated membranes exhibit high permeability and simultaneously high selectivity compared to those singly treated by ion irradiation or hydrogen plasma only. High permeation selectivity values for H2/N2 and H2/CH4 of 495 and 877, respectively, are achieved for optimally cotreated membranes. The method presented can also be scaled up to prepare large-area membranes for gas separation, e.g., for hydrogen purification and recovery from H2/CH4 and H2/N2 mixtures.

Mass Transfer in Boronate Ester 2D COF Single Crystals.

Unlike graphene and similar structures, 2D covalent organic frameworks (2D COFs) exhibit intrinsic porosity with a high areal density of well-defined and uniform openings. Given the pore size adjustability, 2D COFs are likely to outperform artificially perforated inorganic layers with respect to their prospects in membrane separation. Yet, exploring the mass transport in 2D COFs is hidden by the lack of laterally extended free-standing membranes. This work reports on direct molecular permeation measurements with single crystals of an interfacially synthesized boronate ester 2D COF. In accordance with the material topography, the atmospheric and noble gases readily pass the suspended nanosheets while their areal porosity is quantified to be almost 40% exceeding that in any 2D membranes known. However, bulkier aromatic hydrocarbons are found to deviate substantially from Graham's law of diffusion. Counterintuitively, the permeation rate is demonstrated to rise from benzene to toluene and further to xylene despite the increase in the molecular mass and dimensions. The results are interpreted in terms of adsorption-mediated flow that appears to be an important transport mechanism for microporous planar nanomaterials.

Unprecedentedly Low CO2 Transport through Vertically Aligned, Conical Silicon Nanotube Membranes.

Nanotube membranes could show significantly enhanced permeance and selectivity for gas separations. Up until now, studies have primarily focused on applying carbon nanotubes to membranes to achieve ultrafast mass transport. Here, we report the first preparation of silicon nanotube (SiNT) membranes via a template-assisted method and investigate the gas transport behavior through these SiNT membranes using single- and mixed-gas permeation experiments. The SiNT membranes consist of conical cylinder-shaped nanotubes vertically aligned on a porous silicon wafer substrate. The diameter of the SiNT pore mouths are 10 and 30 nm, and the average inner diameter of the tube body is 80 nm. Interestingly, among the gases tested, we found an unprecedentedly low CO2 permeance through the SiNT membranes in single-gas permeation experiments, exceeding the theoretical Knudsen selectivity toward small gases/CO2 separation. This behavior was caused by the reduction of CO2 permeability through the blocking effect of CO2 adsorbed in the narrow pore channels of the SiNT cone regions, indicating that CO2 molecules have a high affinity to the native silicon oxide layer (∼2 nm) that is formed on the inner walls of SiNTs. SiNT membranes also exhibited enhanced gas permeance and water flux as compared to classic theoretical models and, as such, may prove useful as a new type of nanotube material for use in membrane applications.

Stable, Temperature-Dependent Gas Mixture Permeation and Separation through Suspended Nanoporous Single-Layer Graphene Membranes.

Graphene membranes with nanometer-scale pores could exhibit an extremely high permeance and selectivity for the separation of gas mixtures. However, to date, no experimental measurements of gas mixture separation through nanoporous single-layer graphene (SLG) membranes have been reported. Herein, we report the first measurements of the temperature-dependent permeance of gas mixtures in an equimolar mixture feed containing H2, He, CH4, CO2, and SF6 from 22 to 208 °C through SLG membranes containing nanopores formed spontaneously during graphene synthesis. Five membranes were fabricated by transfer of CVD graphene from catalytic Cu film onto channels framed in impermeable Ni. Two membranes exhibited gas permeances on the order of 10-6 to 10-5 mol m-2 s-1 Pa-1 as well as gas mixture selectivities higher than the Knudsen effusion selectivities predicted by the gas effusion mechanism. We show that a new steric selectivity mechanism explains the permeance data and selectivities. This mechanism predicts a mean pore diameter of 2.5 nm and an areal pore density of 7.3 × 1013 m-2, which is validated by experimental observations. A third membrane exhibited selectivities lower than the Knudsen effusion selectivities, suggesting a combination of effusion and viscous flow. A fourth membrane exhibited increasing permeance values as functions of temperature from 27 to 200 °C, and a CO2/SF6 selectivity > 20 at 200 °C, suggestive of activated translocation through molecular-sized nanopores. A fifth membrane exhibited no measurable permeance of any gas above the detection limit of our technique, 2 × 10-7 mol m-2 s-1 Pa-1, indicating essentially a molecularly impermeable barrier. Overall, these data demonstrate that SLG membranes can potentially provide a high mixture separation selectivity for gases, with CVD synthesis alone resulting in nanometer-scale pores useful for gas separation. This work also shows that temperature-dependent permeance measurements on SLG can be used to reveal underlying permeation mechanisms.

On Dioxygen and Substrate Access to Soluble Methane Monooxygenases: An all-Atom Molecular Dynamics Investigation in Water Solution.

In a preliminary exploration of the dummy model for diiron proteins, random-acceleration molecular dynamics (RAMD) revealed that a pure four-helix bundle structure, like hemerythrin, constitutes an efficient cage for dioxygen (O2 ), which can only leave from defined, albeit very broad, gates. However, this well ordered structure does not constitute an archetype on which to compare O2 permeation of other diiron proteins, like the complex of soluble methane monooxygenase hydroxylase with the regulatory protein (sMMOH-MMOB). The reason is that with this complex, unlike hemerythrin, the four helices of the four-helix bundle are heavily bent, and RAMD showed that most traps for O2 lie outside them. It was also observed that, in spite of a nearly identical van der Waals radius for O2 and the natural substrate CH4 , the latter behaves under RAMD as a bulkier molecule than O2 , requiring a higher external force to be brought out of sMMOH-MMOB along trajectories of viable length. All that determined with sMMOH-MMOB multiple gates and multiple pathways to each of them through several binding pockets, for both O2 and CH4 . Of the two equally preferred pathways for O2 , at right angle with one another, one proved to be in accordance with the Xe-atom mapping for sMMOH. In contrast, none of the pathways identified for CH4 proved to be in accordance with such mapping, CH4 looking for more open avenues instead.

Effect of amine structure on CO2 capture by polymeric membranes.

Poly(amidoamine)s (PAMAMs) incorporated into a cross-linked poly(ethylene glycol) exhibited excellent CO2 separation properties over H2. However, the CO2 permeability should be increased for practical applications. Monoethanolamine (MEA) used as a CO2 determining agent in the current CO2 capture technology at demonstration scale was readily immobilized in poly(vinyl alcohol) (PVA) matrix by solvent casting of aqueous mixture of PVA and the amine. The resulting polymeric membranes can be self-standing with the thickness above 3 µm and the amine fraction less than 80 wt%. The gas permeation properties were examined at 40 °C and under 80% relative humidity. The CO2 separation performance increased with increase of the amine content in the polymeric membranes. When the amine fraction was 80 wt%, the CO2 permeability coefficient of MEA containing membrane was 604 barrer with CO2 selectivity of 58.5 over H2, which was much higher than the PAMAM membrane (83.7 barrer and 51.8, respectively) under the same operation conditions. On the other hand, ethylamine (EA) was also incorporated into PVA matrix to form a thin membrane. However, the resulting polymeric membranes exhibited slight CO2-selective gas permeation properties. The hydroxyl group of MEA was crucial for high CO2 separation performance.

The influence of few-layer graphene on the gas permeability of the high-free-volume polymer PIM-1.

Gas permeability data are presented for mixed matrix membranes (MMMs) of few-layer graphene in the polymer of intrinsic microporosity PIM-1, and the results compared with previously reported data for two other nanofillers in PIM-1: multiwalled carbon nanotubes functionalized with poly(ethylene glycol) (f-MWCNTs) and fused silica. For few-layer graphene, a significant enhancement in permeability is observed at very low graphene content (0.05 vol.%), which may be attributed to the effect of the nanofiller on the packing of the polymer chains. At higher graphene content permeability decreases, as expected for the addition of an impermeable filler. Other nanofillers, reported in the literature, also give rise to enhancements in permeability, but at substantially higher loadings, the highest measured permeabilities being at 1 vol.% for f-MWCNTs and 24 vol.% for fused silica. These results are consistent with the hypothesis that packing of the polymer chains is influenced by the curvature of the nanofiller surface at the nanoscale, with an increasingly pronounced effect on moving from a more-or-less spherical nanoparticle morphology (fused silica) to a cylindrical morphology (f-MWCNT) to a planar morphology (graphene). While the permeability of a high-free-volume polymer such as PIM-1 decreases over time through physical ageing, for the PIM-1/graphene MMMs a significant permeability enhancement was retained after eight months storage.

Computing membrane-AQP5-phosphatidylserine binding affinities with hybrid steered molecular dynamics approach.

In order to elucidate how phosphatidylserine (PS6) interacts with AQP5 in a cell membrane, we developed a hybrid steered molecular dynamics (hSMD) method that involved: (1) Simultaneously steering two centers of mass of two selected segments of the ligand, and (2) equilibrating the ligand-protein complex with and without biasing the system. Validating hSMD, we first studied vascular endothelial growth factor receptor 1 (VEGFR1) in complex with N-(4-Chlorophenyl)-2-((pyridin-4-ylmethyl)amino)benzamide (8ST), for which the binding energy is known from in vitro experiments. In this study, our computed binding energy well agreed with the experimental value. Knowing the accuracy of this hSMD method, we applied it to the AQP5-lipid-bilayer system to answer an outstanding question relevant to AQP5's physiological function: Will the PS6, a lipid having a single long hydrocarbon tail that was found in the central pore of the AQP5 tetramer crystal, actually bind to and inhibit AQP5's central pore under near-physiological conditions, namely, when AQP5 tetramer is embedded in a lipid bilayer? We found, in silico, using the CHARMM 36 force field, that binding PS6 to AQP5 was a factor of 3 million weaker than "binding" it in the lipid bilayer. This suggests that AQP5's central pore will not be inhibited by PS6 or a similar lipid in a physiological environment.

On the Permeation by Dioxygen of Urate Oxidase from Aspergillus flavus in Complex with Xanthine Anion: Dioxygen Pathways and a Portrait of the Enzyme Cavities from Molecular Dynamics Simulations in Water Solution.

This work describes molecular dynamics (MD) simulations in aqueous media for the complex of the homotetrameric urate oxidase (UOX) from Aspergillus flavus with xanthine anion (5) in the presence of dioxygen (O2 ). After 196.6 ns of trajectory from unrestrained MD, a O2 molecule was observed leaving the bulk solvent to penetrate the enzyme between two subunits, A/C. From here, the same O2 molecule was observed migrating, across subunit C, to the hydrophobic cavity that shares residue V227 with the active site. The latter was finally attained, after 378.3 ns of trajectory, with O2 at a bonding distance from 5. The reverse same O2 pathway, from 5 to the bulk solvent, was observed as preferred pathway under random acceleration MD (RAMD), where an external, randomly oriented force was acting on O2 . Both MD and RAMD simulations revealed several cavities populated by O2 during its migration from the bulk solvent to the active site or backwards. Paying attention to the last hydrophobic cavity that apparently serves as O2 reservoir for the active site, it was noticed that its volume undergoes ample fluctuations during the MD simulation, as expected from the thermal motion of a flexible protein, independently from the particular subunit and no matter whether the cavity is filled or not by O2 .

On the Permeation by Dioxygen of the Cofactor-Independent Unusual Oxygenase RhCC, in Complex with Substrate 4-Hydroxyphenylenolpyruvate. A Molecular Dynamics Investigation.

This work deals with a trimeric bacterial protein, RhCC, which, although belonging to the tautomerase superfamily, shows oxygenase activity. A model of the complex from RhCC and substrate 4-hydroxyphenylenolpyruvate (4HPP), fitting the observation of extra electron densities from X-ray diffraction of the crystal, could be built by autodocking. When subjected to molecular dynamics (MD) aided by an external random force applied to a O2 molecule placed above 4HPP, this model evolved with O2 egressing toward the bulk solvent from two nearly opposite gates. These were located between the nearly parallel helices 75 - 91 and 15 - 33 of either chain C (gate SE) or chain B (gate FL). Alternatively, with four O2 molecules in the bulk solvent, unbiased MD led to O2 entering the protein from gate SE and getting to 4HPP, while forming a stabilizing salt bridge between the 4HPP carboxylate and P1.C (+) NH2 , thus providing scientific ground for a refined model of the complex.

Unveiling the Pathways of Dioxygen Through the C2 Component of the Environmentally Relevant Monooxygenase p-Hydroxyphenylacetate Hydroxylase from Acinetobacter baumannii: A Molecular Dynamics Investigation.

In this work, models of the homotetrameric C2 component of the monooxygenase p-hydroxyphenylacetate hydroxylase from Acinetobacter baumannii, in complex with dioxygen (O2 ) and, or not, the substrate p-hydroxyphenylacetate (HPA) were built. Both models proved to be amenable to random-acceleration molecular dynamics (RAMD) simulations, whereby a tiny randomly oriented external force, acting on O2 at the active site in front of flavin mononucleotide (FMNH(-) ), accelerated displacement of O2 toward the bulk solvent. This allowed us to carry out a sufficiently large number of RAMD simulations to be of statistical significance. The two systems behaved very similarly under RAMD, except for O2 leaving the active site more easily in the absence of HPA, but then finding similar obstacles in getting to the gate as when the active site was sheltered by HPA. This challenges previous conclusions that HPA can only reach the active center after that the C4aOOH derivative of FMNH(-) is formed, requiring uptake of O2 at the active site before HPA. According to these RAMD simulations, O2 could well get to FMNH(-) also in the presence of the substrate at the active site.

On the Quest of Dioxygen by Monomeric Sarcosine Oxidase. A Molecular Dynamics Investigation.

It is reported here on random acceleration molecular dynamics (RAMD) simulations with the 2GF3 bacterial monomeric sarcosine oxidase (MSOX), O2 , and furoic acid in place of sarcosine, solvated by TIP3 H2 O in a periodic box. An external tiny force, acting randomly on O2 , accelerated its relocation, from the center of activation between residue K265 and the si face of the flavin ring of the flavin adenine dinucleotide cofactor, to the surrounding solvent. Only three of the four O2 gates previously described for this system along a composite method technique were identified, while two more major O2 gates were found. The RAMD simulations also revealed that the same gate can be reached by O2 along different pathways, often involving traps for O2 . Both the residence time of O2 in the traps, and the total trajectory time for O2 getting to the solvent, could be evaluated. The new quick pathways discovered here suggest that O2 exploits all nearby interstices created by the thermal fluctuations of the protein, not having necessarily to look for the permanent large channel used for uptake of the FADH cofactor. To this regard, MSOX resembles closely KijD3 N-oxygenase. These observations solicit experimental substantiation, in a long term aim at discovering whether gates and pathways for the small gaseous ligands inside the proteins are under Darwinian functional evolution or merely stochastic control operates.

Stretchable gas barrier achieved with partially hydrogen-bonded multilayer nanocoating.

Super gas barrier nanocoatings are recently demonstrated by combining polyelectrolytes and clay nanoplatelets with layer-by-layer deposition. These nanobrick wall thin films match or exceed the gas barrier of SiOx and metallized films, but they are relatively stiff and lose barrier with significant stretching (≥ 10% strain). In an effort to impart stretchability, hydrogen-bonding polyglycidol (PGD) layers are added to an electrostatically bonded thin film assembly of polyethylenimine (PEI) and montmorillonite (MMT) clay. The oxygen transmission rate of a 125-nm thick PEI-MMT film increases more than 40x after being stretched 10%, while PGD-PEI-MMT trilayers of the same thickness maintain its gas barrier. This stretchable trilayer system has an OTR three times lower than the PEI-MMT bilayer system after stretching. This report marks the first stretchable high gas barrier thin film, which is potentially useful for applications that require pressurized elastomers.

Binding pockets and pathways for dioxygen through the KijD3 N-oxygenase in complex with flavin mononucleotide cofactor and a 3-aminoglucose substrate: predictions from molecular dynamics simulations.

In this work, two protein systems, Kij3D-FMN-AKM-O2 and Kij3D-FMN-O2 , made of KijD3 N-oxygenase, flavin mononucleotide (FMN) cofactor, dTDP-3-amino-2,3,6-trideoxy-4-keto-3-methyl-D-glucose (AKM) substrate, and dioxygen (O2), have been assembled by adding a molecule of O2, and removing (or not) AKM, to crystal data for the Kij3D-FMN-AKM complex. Egress of AKM and O2 from these systems was then investigated by applying a tiny external random force, in turn, to their center of mass in the course of molecular dynamics in explicit H2 O. It turned out that the wide AKM channel, even when emptied, does not constitute the main route for O2 egress. Other routes appear to be also viable, while various binding pockets (BPs) outside the active center are prone to trap O2. By reversing the reasoning, these can also be considered as routes for uptake of O2 by the protein, before or after AKM uptake, while BPs may serve as reservoirs of O2. This shows that the small molecule O2 is capable of permeating the protein by exploiting all nearby interstices that are created on thermal fluctuations of the protein, rather than having necessarily to look for farther, permanent channels.

Laser Patterning of Porous Support Membranes to Enhance the Effective Surface Area of Thin-Film Composite-Facilitated Transport Membranes for CO2 Separation.

Although thin-film composite membranes have achieved great success in CO2 separation, further improvements in the CO2 permeance are required to reduce the size and cost of the CO2 separation process. Herein, we report the fabrication of composite membranes with high CO2 permeability using a laser-patterned porous membrane as the support membrane. High-aspect-ratio micropatterns with well-defined micropores on their surface were carved on microporous polymer supports by a direct laser writing process using a short-pulsed laser. By using a Galvano scanner and optimizing the laser conditions and target materials, in-plane micropatterns, such as microhole arrays, microline grating, microlattices, and out-of-plane hierarchical micropatterns, were created on porous membranes. An aqueous suspension of hydrogel microparticles doped with an amine-based mobile carrier was sprayed onto the patterned surface to form a defect-free thin separation layer. The surface area of the separation layer on the patterned support is up to 80% larger than that of flat pristine membranes, resulting in a 52% higher CO2 permeance (1106 GPU) with a CO2/N2 selectivity of 172. The laser-patterned porous membranes allow the development of inexpensive and high-performance functional membranes not only for CO2 separation but also for other applications, such as water treatment, cell culture, micro-TAS, and membrane reactors.