Recent Developments in Graphene-Based Membranes: Structure, Mass-Transport Mechanism and Potential Applications

Tunable Release of Ions from Graphene Oxide Laminates for Sustained Antibacterial Activity in a Biomimetic Environment

Silver has long been recognized for its potent antimicrobial properties, but achieving a slow and longer-term delivery of silver ions presents significant challenges. Previous efforts to control silver ion dosages have struggled to sustain release for extended periods in biomimetic environments, especially in the presence of complex proteins. This challenge is underscored by the absence of technology for sustaining antimicrobial activity, especially in the context of orthopedic implants where long-term efficacy, extending beyond 7 days, is essential. In this study, the tunable, slow, and longer-term release of silver ions from the two-dimensional (2D) nanocapillaries of graphene oxide (GO) laminates incorporated with silver ions (Ag-GO) for antimicrobial applications are successfully demonstrated. To closely mimic a physiologically relevant serum-based environment, a novel in vitro study model using 100% fetal bovine serum (FBS) is introduced as the test medium for microbiology, biocompatibility, and bioactivity studies. To emulate fluid circulation in a physiological environment, the in vitro studies are challenged with serum exchange protocols on different days. The findings show that the Ag-GO coating can sustainably release silver ions at a minimum dosage of 10 µg cm-2 day-1, providing an effective and sustained antimicrobial barrier for over ten days.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Ionic Covalent Organic Framework-Based Membranes for Selective and Highly Permeable Molecular Sieving

Two-dimensional covalent organic frameworks (COFs) with uniform pores and large surface areas are ideal candidates for constructing advanced molecular sieving membranes. However, a fabrication strategy to synthesize a free-standing COF membrane with a high permselectivity has not been fully explored yet. Herein, we prepared a free-standing TpPa-SO3H COF membrane with vertically aligned one-dimensional nanochannels. The introduction of the sulfonic acid groups on the COF membrane provides abundant negative charge sites in its pore wall, which achieve a high water flux and an excellent sieving performance toward water-soluble drugs and dyes with different charges and sizes. Furthermore, the COF membrane exhibited long-term stability, fouling resistance, and recyclability in rejection performance. We envisage that this work provides new insights into the effect of ionic ligands on the design of a broad range of COF membranes for advanced separation applications.

Macroscopic assembly of 2D materials for energy storage and seawater desalination

Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted widespread attention due to their excellent physical and chemical properties in the fields of energy, environment, catalysis, and optoelectronics. However, there are still many key problems in the process of practical application. To further promote the potential of 2D materials for practical applications, macroscopic assembly of 2D materials is crucial for the continued development of 2D materials, especially in the fields of energy storage and seawater desalination. Therefore, this review focuses on the latest progress and current status related to the macroscopic assembly of 2D materials, including 1D fibers, 2D films, and 3D architectures. In addition, the application of macroscopic bodies assembled based on 2D materials in the fields of energy storage and seawater desalination is also introduced. Finally, future directions for the macroscopic assembly of 2D materials and their applications are prospected.

Nanoparticles and nanofiltration for wastewater treatment: From polluted to fresh water

Water pollution poses significant threats to both ecosystems and human health. Mitigating this issue requires effective treatment of domestic wastewater to convert waste into bio-fertilizers and gas. Neglecting liquid waste treatment carries severe consequences for health and the environment. This review focuses on intelligent technologies for water and wastewater treatment, targeting waterborne diseases. It covers pollution prevention and purification methods, including hydrotherapy, membrane filtration, mechanical filters, reverse osmosis, ion exchange, and copper-zinc cleaning. The article also highlights domestic purification, field techniques, heavy metal removal, and emerging technologies like nanochips, graphene, nanofiltration, atmospheric water generation, and wastewater treatment plants (WWTPs)-based cleaning. Emphasizing water cleaning's significance for ecosystem protection and human health, the review discusses pollution challenges and explores the integration of wastewater treatment, coagulant processes, and nanoparticle utilization in management. It advocates collaborative efforts and innovative research for freshwater preservation and pollution mitigation. Innovative biological systems, combined with filtration, disinfection, and membranes, can elevate recovery rates by up to 90%, surpassing individual primary (<10%) or biological methods (≤50%). Advanced treatment methods can achieve up to 95% water recovery, exceeding UN goals for clean water and sanitation (Goal 6). This progress aligns with climate action objectives and safeguards vital water-rich habitats (Goal 13). The future holds promise with advanced purification techniques enhancing water quality and availability, underscoring the need for responsible water conservation and management for a sustainable future.

Interface engineering induced charge rearrangement boosting reversible oxygen electrocatalysis activity of heterogeneous FeCo-MnO@N-doped carbon nanobox

The advancement of bifunctional oxygen catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative yet challenging for the optimization of Zn-air batteries. In this study, we reported the successful incorporation of a novel Mott-Schottky catalytic site within a MnO-FeCo heterojunction into an N-doping carbon nanobox, taking into consideration the effects of the intrinsic electric field and hollow/porous support carriers for electrocatalyst design. As expected, the resulting heterogeneous catalyst exhibited an encouraging half-wave potential of 0.88 V and an impressive limiting-current density of 5.62 mA/cm2 for the ORR, as well as a minimal overpotential of 271 mV at 10 mA/cm2 for the OER, both in alkaline conditions. Furthermore, the Zn-air battery constructed with the heterojunction nanobox product displayed a decent potential gap of 0.621 V, an outstanding power density of 253 mW/cm2, a considerable specific capacity of 761 mAh/gZn, and exceptional stability, with up to 336 h of cycling charging and discharging operation. Consequently, this method of modulating the catalyst's surface charge distribution through an internal electric field at the interface and facilitating mass transport offers a novel avenue for the development of robust bifunctional oxygen catalysts.

Extreme Ion-Transport Inorganic 2D Membranes for Nanofluidic Applications

Inorganic 2D materials offer a new approach to controlling mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. The recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology is herein discussed. The fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications are highlighted. Finally, the fundamental issues of thermodynamics and kinetics are outlined along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels.

Glass-like behavior of intercalated organic solvents in graphite oxide detected by spin-probe EPR

Membranes based on graphite oxide (GO) are promising materials for the separation of polar liquids and gases. Understanding the properties of solvents immersed in GO is important for the development of various technological applications. Here, the molecular motions of the TEMPO nitroxide spin probe in acetonitrile intercalated into the GO inter-plane space were studied using electron paramagnetic resonance (EPR), including its pulsed version, and electron spin echo (ESE). For a sample containing 75% acetonitrile relative to equilibrium sorption at room temperature, ESE-detected stochastic librations were observed for TEMPO molecules above 135 K. Since these librations are an inherent property of molecular glasses, this fact indicates that intercalated acetonitrile forms a two-dimensional glass state. Above 225 K, an acceleration of stochastic librations was observed, indicating the manifestation of a glass-like dynamical cross-over. Continuous wave (CW) EPR spectra of TEMPO showed the absence of overall tumbling motions in the entire investigated temperature range of up to 340 K, indicating that the intercalated acetonitrile does not behave as a bulk liquid (the melting point of acetonitrile is 229 K). Dynamical librations of TEMPO molecules detected by CW EPR were found to accelerate above 240 K.

Adsorptive separation of CH4, H2, CO2, and N2 using fullerene pillared graphene nanocomposites: Insights from molecular simulations

CONTEXT

The adsorptive separation performances of fullerene pillared graphene nanocomposites (FPGNs) with tunable micro and meso porous morphology are investigated for the binary mixtures of CH4, H2, CO2 and N2 by using grand canonical Monte Carlo (GCMC) simulations. Different fullerene types are considered in designs as pillar to investigate the effects of porosity on the gas separation performances of FPGNs, and the GCMC simulations are performed for an equimolar binary mixture of CO2/H2, CO2/CH4, CO2/N2 and CH4/H2 inspired by industrial gas mixtures. It is found that CO2/N2, CO2/H2 and CH4/H2 selectivity of FPGNs are about 72, 410 and 145 at 298 K and 1 bar, which are higher than those for several adsorbent materials reported.

METHODS

Five different FPGN models which contain covalently bonded periodical fullerene and graphene units were constructed using C60, C180, C320, C540 and C720 fullerenes, followed by geometry optimization using Open Babel. All GCMC simulations of adsorption were performed in the RASPA. The adsorption isotherms of FPGNs for pure gases are comparatively examined, and their performances are discussed based on the pore structure and isosteric heat of adsorption. Then, the separation factors of FPGNs for equimolar binary mixtures of these gases are elucidated from the difference in the heat of adsorption and the adsorption selectivity.

Multifunctional Bacterial Cellulose/Covalent Organic Framework Composite Membranes with Antifouling and Antibacterial Properties for Dye Separation

Covalent organic frameworks (COFs) have a wide application prospect in wastewater treatment because of their unique structure and properties; however, the preparation of pure COF membranes remains a great challenge by reason of the insolubility and unprocessability of COF powders formed at high temperature and high pressure. In this study, a continuous and defect-free bacterial cellulose/covalent organic framework composite membrane was prepared by using bacterial cellulose (BC) and a porphyrin-based COF with their unique structures and hydrogen bonding forces. The dye rejection rate of this composite membrane toward methyl green and congo red was up to 99%, and the permeance was about 195 L m^-2 h^-1 bar^-1. It showed excellent stability under different pH conditions, long-time filtration, and cyclic experimental conditions. In addition, the hydrophilicity and surface negativity of the BC/COF composite membrane made it have certain antifouling performance, and the flux recovery rate can reach 93.72%. More importantly, the composite membrane exhibited excellent antibacterial properties due to the doping of the porphyrin-based COF, and the survival rates of both Escherichia coli and Staphylococcus aureus were less than 1% after exposure to visible light. The self-supporting BC/COF composite membrane synthesized by this strategy also has outstanding antifouling and antibacterial properties, in addition to excellent dye separation effects, which greatly broaden the application of COF materials in water treatment.

Side-Chain-Dependent Functional Intercalations in Graphene Oxide Membranes for Selective Water and Ion Transport

Subnanometer interlayer space in graphene oxide (GO) laminates is desirable for use as permselective membrane nanochannels. Although the facile modification of the local structure of GO enables various nanochannel functionalizations, precisely controlling nanochannel space is still a challenge, and the roles of confined nanochannel chemistry in selective water/ion separation have not been clearly defined. In this study, macrocyclic molecules with consistent basal plane but varying side groups were used to conjunct with GO for modified nanochannels in laminates. We demonstrated the side-group dependence of both the angstrom-precision tunability for channel free space and the energy barrier setting for ion transport, which challenges the permeability-selectivity trade-off with a slightly decreased permeance from 1.1 to 0.9 L m^-2 h^-1 bar^-1 but an increased salt rejection from 85% to 95%. This study provides insights into the functional-group-dependent intercalation modifications of GO laminates for understanding laminate structural control and nanochannel design.

Mechanical Mechanism of Ion and Water Molecular Transport through Angstrom-Scale Graphene Derivatives Channels: From Atomic Model to Solid-Liquid Interaction

Ion and water transport at the Angstrom/Nano scale has always been one of the focuses of experimental and theoretical research. In particular, the surface properties of the angstrom channel and the solid-liquid interface interaction will play a decisive role in ion and water transport when the channel size is small to molecular or angstrom level. In this paper, the chemical structure and theoretical model of graphene oxide (GO) are reviewed. Moreover, the mechanical mechanism of water molecules and ions transport through the angstrom channel of GO are discussed, including the mechanism of intermolecular force at a solid/liquid/ion interface, the charge asymmetry effect and the dehydration effect. Angstrom channels, which are precisely constructed by two-dimensional (2D) materials such as GO, provide a new platform and idea for angstrom-scale transport. It provides an important reference for the understanding and cognition of fluid transport mechanism at angstrom-scale and its application in filtration, screening, seawater desalination, gas separation and so on.

Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation

A new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters is presented. The method is based on fast mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench far below the glass transition temperature T_g . The process is first designed generally by simulation and then verified for the example of polystyrene films. The methodology does not need any chemical processing, supporting substrate, or self assembly process and is solely based on polymer inherent entanglement effects. Pore diameters are of the order of ten polymer reptation tube diameters. The resulting membranes are stable over months at ambient conditions and display remarkable elastic properties.

Recent advance in biomass membranes: Fabrication, functional regulation, and antimicrobial applications

Both inorganic and polymeric membranes have been widely applied for antimicrobial applications. However, these membranes exhibit low biocompatibility, weak biodegradability, and potential toxicity to human being and environment. Biomass materials serve as excellent candidates for fabricating functional membranes to address these problems due to their unique physical, chemical, and biological properties. Here we present recent progress in the fabrication, functional regulation, and antimicrobial applications of various biomass-based membranes. We first introduce the types of biomass membranes and their fabrication methods, including the phase inversion, vacuum filtration, electrospinning, layer-by-layer self-assembly, and coating. Then, the strategies on functional regulation of biomass membranes by adding 0D, 1D, and 2D nanomaterials are presented and analyzed. In addition, antibacterial, antifungal, and antiviral applications of biomass-based functional membranes are summarized. Finally, potential development aspects of biomass membranes are discussed and prospected. This comprehensive review is valuable for guiding the design, synthesis, structural/functional tailoring, and sustainable utilization of biomass membranes.

The recent advance of precisely designed membranes for sieving

Developing new membranes with both high selectivity and permeability is critical in membrane science since conventional membranes are often limited by the trade-off between selectivity and permeability. In recent years, the emergence of advanced materials with accurate structures at atomic or molecular scale, such as metal organic framework, covalent organic framework, graphene, has accelerated the development of membranes, which benefits the precision of membrane structures. In this review, current state-of-the-art membranes are first reviewed and classified into three different types according to the structures of their building blocks, including laminar structured membranes, framework structured membranes and channel structured membranes, followed by the performance and applications for representative separations (liquid separation and gas separation) of these precisely designed membranes. Last, the challenges and opportunities of these advanced membranes are also discussed.

Liquid-like and solid-like acetonitrile intercalated into graphite oxide as studied by the spin probe technique

The molecular mobility of acetonitrile intercalated into the inter-plane space of graphite oxide was studied using the spin probe technique. It was revealed that two types of intercalated substance - liquid-like and solid-like - are simultaneously present in between the oxidized graphene planes, and their ratio depends on temperature. The micro-viscosity of liquid-like intercalated acetonitrile was found to be higher than that of bulk acetonitrile and depends on the amount of intercalated liquid.

Membranes Coated with Graphene-Based Materials: A Review

Graphene is a popular material with outstanding properties due to its single layer. Graphene and its oxide have been put to the test as nano-sized building components for separation membranes with distinctive structures and adjustable physicochemical attributes. Graphene-based membranes have exhibited excellent water and gas purification abilities, which have garnered the spotlight over the past decade. This work aims to examine the most recent science and engineering cutting-edge advances of graphene-based membranes in regard to design, production and use. Additional effort will be directed towards the breakthroughs in synthesizing graphene and its composites to create various forms of membranes, such as nanoporous layers, laminates and graphene-based compounds. Their efficiency in separating and decontaminating water via different techniques such as cross-linking, layer by layer and coating will also be explored. This review intends to offer comprehensive, up-to-date information that will be useful to scientists of multiple disciplines interested in graphene-based membranes.

Sorption of Polar Sorbents into GO Powders and Membranes

The comparative study of sorption of polar substances acetonitrile and water into powders and membranes (>10 μm thick) of modified Hummers (HGO) and Brodie (BGO) graphite oxides was performed using isopiestic method (IM) and differential scanning calorimetry (DSC). Additional sorption data were obtained for pyridine and 1-octanol. Sorption measurements were accompanied by conventional XRD and XPS control. Electron paramagnetic resonance (EPR) was additionally used to characterize ordering of the membranes. The impact on sorption of synthetic procedure (Brodie or Hummers), method of making membranes, chemical nature of the sorbent, and method of sorption was systematically examined. It was demonstrated that variations in synthetic procedures within both Hummers and Brodie methods did not lead to changes in the sorption properties of the corresponding powders. Sorption of acetonitrile and pyridine was reduced by approximately half when switching from powders to membranes at ambient temperature. DSC measurements at a lower temperature gave equal sorption of acetonitrile into HGO powder and membranes. Water has demonstrated unique sorption properties. Equal sorption of water was measured for HGO membranes and powders at T = 298 K and at T = 273 K. It was demonstrated that lowering the orientational alignment of the membranes led to the increase of sorption. In practice this could allow one to tune sorption/swelling and transport properties of the GO membranes directly by adjusting their internal ordering without the use of any composite materials.

Aqueous Proton Transportation in Graphene-Based Nanochannels

Graphene oxide (GO) has been unveiled to exhibit high proton conductivity in a humidified or aqueous environment, making it a promising candidate to construct proton conduction nanochannels. In this work, we systematically investigate how the confinement effect and surface chemistry influence the proton transportation behavior in graphene-based nanochannels via extensive ReaxFF MD simulations. Graphene (GE), graphane (GA), and hydroxygraphane (HG) sheets were employed to mimic the graphitic and functionalized region of GO and construct nanochannels with different interlayer distances. We find that confined water molecules are stratified and their orientation is influenced by the surface chemistry, thus impacting the distribution of protons. Surface chemistry makes the compression of the hydrogen-bond network induced by the confinement effect more variable. The hydrogen-bond network between GE slabs is crushed by extreme confinement and ultrafast proton transportation behavior mainly achieved via vehicle mechanism. Meanwhile, the hydrogen-bond network and solvation structure can be kept more complete with the existence of functional groups. The hydrogen bonds formed with surface functional groups impede the transportation of water molecules but allow more Grotthuss hopping of protons to different extents. Our work clarified the proton transportation mechanism in graphene-based nanochannels with different interlayer distances and surface chemistry and can guide the future design of proton conduction devices such as proton exchange membranes.

Bioinspired Dual-Driven Binary Heterogeneous Nanofluidic Ionic Diodes

Recently, bioinspired 2D material-based nanofluidic systems with unique properties and advantages have been receiving considerable research interest and getting rapid development. However, it remains a huge challenge to integrate adaptive responsiveness to external stimuli and asymmetric ion transport characteristics into the 2D nanofluidic systems. Herein, we report a dual-driven switchable asymmetric ionic transport phenomenon through a graphene oxide-based heterogeneous 2D nanofluidic membrane. Taking advantage of the formation of a charge heterojunction induced by the variation of pH or UV irradiation, a maximum ionic current rectification (ICR) ratio of ca. 56 for pH or 140 for light was achieved. Such smart nanofluidic devices with pH and light dual-responsiveness and asymmetric ion transport behaviors provide a universal strategy for potential applications in chemical sensing, water treatment, and energy conversion and establish a promising platform for exploring advanced quantum ionics biodevices with ultrafast signal transmission, nanochannel-structured bioreactors with high efficiency, etc.

Graphene-Based Membranes for Water Desalination: A Literature Review and Content Analysis

Graphene-based membranes have unique nanochannels and can offer advantageous properties for the water desalination process. Although tremendous efforts have been devoted to heightening membrane performance and broadening their application, there is still lack of a systematic literature review on the development and future directions of graphene-based membranes for desalination. In this mini-review, literature published between 2011 and 2022 were analyzed by using the bibliometric method. We found that the major contributors to these publications and the highest citations were from China and the USA. Nearly 80% of author keywords in this analysis were used less than twice, showing the broad interest and great dispersion in this field. The recent advances, remaining gaps, and strategies for future research, were discussed. The development of new multifunctional nanocomposite materials, heat-driven/solar-driven seawater desalination, and large-scale industrial applications, will be important research directions in the future. This literature analysis summarized the recent development of the graphene-based membranes for desalination application, and will be useful for researchers in gaining new insights into this field.

Metal-free graphene-based catalytic membranes for persulfate activation toward organic pollutant removal: a review

Owing to their ultrathin two-dimensional structure and efficient catalytic ability for persulfate activation, graphene-based nanocarbons exhibit considerable application potential in fabricating carbonaceous composite membranes for in situ catalytic oxidation to remove organic pollutants. This approach offers significant advantages over conventional batch systems. However, the relationships between the physicochemical properties of carbon mats and performance of graphene-based catalytic membranes in water purification remain ambiguous. Herein, we summarize the main mechanisms of in situ catalytic oxidation and the facile fabrication strategies of carbonaceous composite membranes. Different factors influencing the performance of graphene-based catalytic membranes are comprehensively discussed. The defective level, heteroatom doping, and stacking morphology of carbon mats and operational conditions during filtration play critical roles in the oxidative degradation of target pollutants. Long-term operation leads to the deterioration of catalytic activity and transmembrane pressure, especially in the complex water matrix. Finally, the present challenges and future perspectives are presented to improve the anti-fouling performance and catalytic stability of membranes and develop scalable fabrication methods to promote the engineering applications of in situ catalytic oxidation in real water purification.

Ionic Liquid-Reduced Graphene Oxide Membrane with Enhanced Stability for Water Purification

There has been a growing interest in water purification by graphene oxide (GO) laminate membranes due to their exceptional hydrophilicity, high throughput, and extraordinary separation performance originating from their two-dimensional and well-defined nanostructure. However, the swelling and stability in an aqueous environment are areas of concern for the GO laminate membranes. Here, a novel methylimidazolium ionic liquid-reduced GO (mimG)-assembled GO laminate membrane (mimG-GO) with remarkable stability was fabricated by a vacuum-assisted strategy for water purification. Methylimidazolium-based ionic liquid-reduced graphene oxide (mimG) was prepared by a facile nucleophilic ring-opening mechanism. Fabricated membranes were thoroughly characterized for stability, structural, permeance, and rejection properties in an aqueous environment. A combination of cationic mimG and GO nanosheets improves membrane stability in the aqueous environment via cation-π interactions and creates nanofluidic channels for facile water transport while yielding significant enhancement in the salt and dye separation performance. The pore size and the number of nanofluidic channels were precisely controlled via material deposition and laminate thickness to remove salts from water. The mimG-GO laminate membrane containing 72.2 mg m^-2 deposition showed a permeance of 14.9 LMH bar^-1, 50% higher than 9.7 LMH bar^-1 of the neat GO laminate membrane, in addition to an increase in Na₂SO₄ salt rejection from 46.6 to 77.4%, overcoming the flux-rejection trade-off. The mimG-GO laminate membrane also rejected various anionic dyes (i.e., 99.9% for direct red 80 (DR 80), 96.8% for reactive black 5 (RB 5), and 91.4% for methyl orange (MO)). The mimG-GO laminate membrane containing 361.0 mg m^-2 deposition showed the highest rejection for Na₂SO₄ (92.1%) and 99.9% rejection for DR 80, 99.0% rejection for RB 5, and 98.1% rejection for MO dyes keeping a flux of 2.6 LMH bar^-1. Partial reduction and covalent grafting of ionic liquid moieties on GO helped to enhance the cation-π interaction between GO laminates, which showed enhanced stability, frictionless water transport, with high salt and dye rejection. Moreover, a simultaneous improvement in water permeance and solute rejection reveals the great potential of ionic liquid-functionalized GO laminate membranes for water-based applications.

Functionalized Graphene Oxide-Based Lamellar Membranes with Tunable Nanochannels for Ionic and Molecular Separation

Graphene oxide (GO)-based membranes with tunable microstructure and controlled nanochannels have attracted an increasing interest for various applications in wastewater treatment, desalination, gas separation, organic nanofiltration, etc. However, they showed limited use in water desalination due to their lower stability and separation efficiency. In this work, a class of two-dimensional (2D) GO lamellar membranes have been prepared with controlled pores for efficient and fast separation of ions and dye molecules. The GO membranes are fucntionalized with a star-like 6-armed poly(ethylene oxide) using the simple amidation route under mild conditions. The as-prepared covalently cross-linked networks are chemically steady in aqueous medium and show remarkable selectivity (∼100%) for several probe molecules and 10-100 higher permeance than those of the reported GO-based membranes. Further, such membranes are also used for salt separation and show more than 80% rejection for Pb²⁺ and Ni²⁺ salts. Moreover, a 1360 nm-thick membrane shows >99% rejection for NaCl with a good water permeance of up to 120 L m^-2 h^-1 bar^-1. Additionally, these membranes are stable for more than 20 days under different conditions.

Graphene: A Path-Breaking Discovery for Energy Storage and Sustainability

The global energy situation requires the efficient use of resources and the development of new materials and processes for meeting current energy demand. Traditional materials have been explored to large extent for use in energy saving and storage devices. Graphene, being a path-breaking discovery of the present era, has become one of the most-researched materials due to its fascinating properties, such as high tensile strength, half-integer quantum Hall effect and excellent electrical/thermal conductivity. This paper presents an in-depth review on the exploration of deploying diverse derivatives and morphologies of graphene in various energy-saving and environmentally friendly applications. Use of graphene in lubricants has resulted in improvements to anti-wear characteristics and reduced frictional losses. This comprehensive survey facilitates the researchers in selecting the appropriate graphene derivative(s) and their compatibility with various materials to fabricate high-performance composites for usage in solar cells, fuel cells, supercapacitor applications, rechargeable batteries and automotive sectors.

Nanoparticle Effects on Stress Response Pathways and Nanoparticle-Protein Interactions

Nanoparticles (NPs) are increasingly used in a wide variety of applications and products; however, NPs may affect stress response pathways and interact with proteins in biological systems. This review article will provide an overview of the beneficial and detrimental effects of NPs on stress response pathways with a focus on NP-protein interactions. Depending upon the particular NP, experimental model system, and dose and exposure conditions, the introduction of NPs may have either positive or negative effects. Cellular processes such as the development of oxidative stress, the initiation of the inflammatory response, mitochondrial function, detoxification, and alterations to signaling pathways are all affected by the introduction of NPs. In terms of tissue-specific effects, the local microenvironment can have a profound effect on whether an NP is beneficial or harmful to cells. Interactions of NPs with metal-binding proteins (zinc, copper, iron and calcium) affect both their structure and function. This review will provide insights into the current knowledge of protein-based nanotoxicology and closely examines the targets of specific NPs.

Strain-boosted hyperoxic graphene oxide efficiently loading and improving performances of microcystinase

Harmful Microcystis blooms (HMBs) and microcystins (MCs) that are produced by Microcystis seriously threaten water ecosystems and human health. This study demonstrates an eco-friendly strategy for simultaneous removal of MCs and HMBs by adopting unique hyperoxic graphene oxides (HGOs) as carrier and pure microcystinase A (PMlrA) as connecting bridge to form stable HGOs@MlrA composite. After oxidation, HGOs yield inherent structural strain effects for boosting the immobilization of MlrA by material characterization and density functional theory calculations. HGO₅ exhibits higher loading capacities for crude MlrA (1,559 mg·g⁻¹) and pure MlrA (1,659 mg·g⁻¹). Moreover, the performances of HGO₅@MlrA composite, including the capability of removing MCs and HMBs, the ecological and human safety compared to MlrA or HGO₅ treatment alone, have been studied. These results indicate that HGO₅ can be used as a promising candidate material to effectively improve the application potential of MlrA in the simultaneous removal of MCs and HMBs.

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Hyperoxic graphene oxide (HGO₅) provides inherent strain effects

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HGO₅ exhibits an impressive loading capacity for MlrA

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A new assembly mechanism for the HGO₅@MlrA composite is proposed

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HGO₅@MlrA composite shows excellent capability and ecological safety

The coming of age of water channels for separation membranes: from biological to biomimetic to synthetic

Water channels are one of the key pillars driving the development of next-generation desalination and water treatment membranes. Over the past two decades, the rise of nanotechnology has brought together an abundance of multifunctional nanochannels that are poised to reinvent separation membranes with performances exceeding those of state-of-the-art polymeric membranes within the water-energy nexus. Today, these water nanochannels can be broadly categorized into biological, biomimetic and synthetic, owing to their different natures, physicochemical properties and methods for membrane nanoarchitectonics. Furthermore, against the backdrop of different separation mechanisms, different types of nanochannel exhibit unique merits and limitations, which determine their usability and suitability for different membrane designs. Herein, this review outlines the progress of a comprehensive amount of nanochannels, which include aquaporins, pillar[5]arenes, I-quartets, different types of nanotubes and their porins, graphene-based materials, metal- and covalent-organic frameworks, porous organic cages, MoS₂, and MXenes, offering a comparative glimpse into where their potential lies. First, we map out the background by looking into the evolution of nanochannels over the years, before discussing their latest developments by focusing on the key physicochemical and intrinsic transport properties of these channels from the chemistry standpoint. Next, we put into perspective the fabrication methods that can nanoarchitecture water channels into high-performance nanochannel-enabled membranes, focusing especially on the distinct differences of each type of nanochannel and how they can be leveraged to unlock the as-promised high water transport potential in current mainstream membrane designs. Lastly, we critically evaluate recent findings to provide a holistic qualitative assessment of the nanochannels with respect to the attributes that are most strongly valued in membrane engineering, before discussing upcoming challenges to share our perspectives with researchers for pathing future directions in this coming of age of water channels.

Structural and Chemical Peculiarities of Nitrogen-Doped Graphene Grown Using Direct Microwave Plasma-Enhanced Chemical Vapor Deposition

Chemical vapor deposition (CVD) is an attractive technique which allows graphene with simultaneous heteroatom doping to be synthesized. In most cases, graphene is grown on a catalyst, followed by the subsequent transfer process. The latter is responsible for the degradation of the carrier mobility and conductivity of graphene due to the presence of the absorbants and transfer-related defects. Here, we report the catalyst-less and transfer-less synthesis of graphene with simultaneous nitrogen doping in a single step at a reduced temperature (700 °C) via the use of direct microwave plasma-enhanced CVD. By varying nitrogen flow rate, we explored the resultant structural and chemical properties of nitrogen-doped graphene. Atomic force microscopy revealed a more distorted growth process of graphene structure with the introduction of nitrogen gas—the root mean square roughness increased from 0.49 ± 0.2 nm to 2.32 ± 0.2 nm. Raman spectroscopy indicated that nitrogen-doped, multilayer graphene structures were produced using this method. X-ray photoelectron spectroscopy showed the incorporation of pure pyridinic N dopants into the graphene structure with a nitrogen concentration up to 2.08 at.%.

Ultrafast Macroscopic Assembly of High-Strength Graphene Oxide Membranes by Implanting an Interlaminar Superhydrophilic Aisle

A macroscopic-assembled graphene oxide (GO) membrane with sustainable high strength presents a bright future for its applications in ionic and molecular filtration for water purification or fast force response for sensors. Traditionally, the bottom-up macroscopic assembly of GO sheets is optimized by widening the interlaminar space for expediting water passage, frequently leading to a compromise in strength, assembly time, and ensemble thickness. Herein, we rationalize this strategy by implanting a superhydrophilic bridge of cobalt-based metal-organic framework nanosheets (NMOF-Co) as an additional water "aisle" into the interlaminar space of GO sheets (GO/NMOF-Co), resulting in a high-strength macroscopic membrane ensemble with tunable thickness from the nanometer scale to the centimeter scale. The GO/NMOF-Co membrane assembly time is only 18 s, 30800 times faster than that of pure GO (154 h). More importantly, the obtained membrane attains a strength of 124.4 MPa, which is more than 3 times higher than that of the GO membrane prepared through filtration. The effect of hydrophilicity on membrane assembly is also investigated by introducing different intercalants, suggesting that, except for the interlamellar spacing, the interlayered hydrophilicity plays a more decisive role in the macroscopic assembly of GO membranes. Our results give a fundamental implication for fast macroscopic assembly of high-strength 2D materials.

Tuning Transport in Graphene Oxide Membrane with Single-site Copper (II) Cations

Controlling the ion transport through graphene oxide (GO) membrane is challenging, particularly in the aqueous environment due to its strong swelling tendency. Fine-tuning the interlayer spacing and chemistry is critical to create highly selective membranes. We investigate the effect of single-site divalent cations in tuning GO membrane properties. Competitive ionic permeation test indicates that Cu²⁺ cations dominate the transport through the 2D channels of GO membrane over other cations (Mg²⁺/Ca²⁺/Co²⁺). Without/With the single-site M²⁺ modifications, pristine GO, Mg-GO, Ca-GO, and Cu-GO membranes show interlayer spacings of ∼13.6, 15.6, 14.5, and 12.3 Å in wet state, respectively. The Cu-GO membrane shows a two-fold decrease of NaCl (1 M) permeation rate comparing to pristine GO, Mg-GO, and Ca-GO membranes. In reverse osmosis tests using 1000 ppm NaCl and Na₂SO₄ as feeds, Cu-GO membrane shows rejection of ∼78% and ∼94%, respectively, which are 5%–10% higher than its counterpart membranes.

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Single-site Cu²⁺ decreases the interlayer spacing of wet graphene oxide membrane

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Single-site Cu²⁺ modifications can enhance salt rejection