Strong Differential Monovalent Anion Selectivity in Narrow Diameter Carbon Nanotube Porins

Ion Coupling, Bonding, and Transfer in Narrow Carbon Nanotubes

Narrow carbon nanotubes (nCNT) are unique mimics of biological channels with water-ion selectivity attractive for applications such as water purification and osmotic energy harvesting, yet their understanding is still incomplete. Here, an ab initio computation is employed to develop the full picture of ion transfer in nCNT including specificity and coupling between ions. The thermodynamic costs of ion transfer are computed for single ions and ion pairs and used to evaluate different local coupling scenarios including strong (pairing) and weak (free-ion) coupling as well as "electroneutrality breakdown" (EB), possible for cations only due to their chemisorption-like interaction with nCNT. The results also indicate that nCNT behaves as a highly polarizable metal-like shell, which eliminates the dielectric energy when CNT accommodates coupled cation and anion. This allows facile computation and comparison of the full transfer costs, including translation entropy, for different ions in different coupling modes to identify the dominant regime. EB transfer appears most favorable for K+, while anions strongly favor transfer as pairs, except for chloride which favors weak coupling and, at neutral pH, transfers as a trace ion coupled to both cation and OH-. The results demonstrate that, in general, observed ion permeation and conduction in nCNT, especially for anions, reflect a complex ion-specific and composition-dependent interplay between different ions.

Bio-Inspired Functional Materials for Environmental Applications

With the global population expected to reach 9.7 billion by 2050, there is an urgent need for advanced materials that can address existing and developing environmental issues. Many current synthesis processes are environmentally unfriendly and often lack control over size, shape, and phase of resulting materials. Based on knowledge from biological synthesis and assembly processes, as well as their resulting functions (e.g., photosynthesis, self-healing, anti-fouling, etc.), researchers are now beginning to leverage these biological blueprints to advance bio-inspired pathways for functional materials for water treatment, air purification and sensing. The result has been the development of novel materials that demonstrate enhanced performance and address sustainability. Here, an overview of the progress and potential of bio-inspired methods toward functional materials for environmental applications is provided. The challenges and opportunities for this rapidly expanding field and aim to provide a valuable resource for researchers and engineers interested in developing sustainable and efficient processes and technologies is discussed.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

Unifying the Conversation: Membrane Separation Performance in Energy, Water, and Industrial Applications

Dense polymer membranes enable a diverse range of separations and clean energy technologies, including gas separation, water treatment, and renewable fuel production or conversion. The transport of small molecular and ionic solutes in the majority of these membranes is described by the same solution-diffusion mechanism, yet a comparison of membrane separation performance across applications is rare. A better understanding of how structure-property relationships and driving forces compare among applications would drive innovation in membrane development by identifying opportunities for cross-disciplinary knowledge transfer. Here, we aim to inspire such cross-pollination by evaluating the selectivity and electrochemical driving forces for 29 separations across nine different applications using a common framework grounded in the physicochemical characteristics of the permeating and rejected solutes. Our analysis shows that highly selective membranes usually exhibit high solute rejection, rather than fast solute permeation, and often exploit contrasts in the size and charge of solutes rather than a nonelectrostatic chemical property, polarizability. We also highlight the power of selective driving forces (e.g., the fact that applied electric potential acts on charged solutes but not on neutral ones) to enable effective separation processes, even when the membrane itself has poor selectivity. We conclude by proposing several research opportunities that are likely to impact multiple areas of membrane science. The high-level perspective of membrane separation across fields presented herein aims to promote cross-pollination and innovation by enabling comparisons of solute transport and driving forces among membrane separation applications.

Dielectric exclusion, an éminence grise

Dielectric exclusion has long been well-established as the key mechanism in membrane desalination, critical for delivering the required levels of salt rejection, also playing important role in electro-membrane processes, nanofluidics, and biomimetics. Unfortunately, its elusive nature and many features, such as dependence on the pore size, membrane hydration, and ion size and charge, make it deceivingly similar to the other ion exclusions mechanisms, steric and Donnan, which has led to much controversy and misconceptions. Starting from the Born model and the concept of self-energy, the present paper reviews and highlights the physical basis of dielectric exclusion, its main features and the ways it may be looked at. It discusses what makes the dielectric exclusion both similar and distinctly different from the other mechanism and its synergy and intimate connection with other phenomena, such as Donnan exclusion, permeability-selectivity upper-bound, and selectivity of charged membranes towards uncharged solutes. The paper also addresses subjects that still cause much controversy at present, such as appropriate measures of ionic radii and the subtle distinction between the dielectric exclusion and primary ion hydration. It also points to gaps that need to be bridged towards more complete theory. The points addressed here are important for understanding, modeling and development of various next-generation separation technologies including water purification, resource recovery and reuse, and green energy generation and storage.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H2 and O2 Evolution

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO₂ films. It is shown that even for SiO₂ pores with a width as small as 0.8 nm, water does not experience any energy barrier for permeating into the pores due to a favorable interaction with hydrophilic silanol groups on the oxide surface. In contrast, permeation of Fe(III) and Fe(II) into microporous SiO₂ pores is limited due to high energy barriers, which stem from a combination of distortion and dehydration of the second and third ion solvation shells. Our simulations and experimental results show that SiO₂ coatings can effectively mitigate undesired Fe(III)/Fe(II) redox reactions at underlying electrodes by attenuating permeation of iron cations, while allowing water to permeate and thus participate in water splitting reactions. In a broader context, our study demonstrates that selectivity of solvated cations can be manipulated by controlling the pore size and surface chemistry of oxide films.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Nanopores: synergy from DNA sequencing to industrial filtration - small holes with big impact

Nanopores in thin membranes play important roles in science and industry. Single nanopores have provided a step-change in portable DNA sequencing and understanding nanoscale transport while multipore membranes facilitate food processing and purification of water and medicine. Despite the unifying use of nanopores, the fields of single nanopores and multipore membranes differ - to varying degrees - in terms of materials, fabrication, analysis, and applications. Such a partial disconnect hinders scientific progress as important challenges are best resolved together. This Viewpoint suggests how synergistic crosstalk between the two fields can provide considerable mutual benefits in fundamental understanding and the development of advanced membranes. We first describe the main differences including the atomistic definition of single pores compared to the less defined conduits in multipore membranes. We then outline steps to improve communication between the two fields such as harmonizing measurements and modelling of transport and selectivity. The resulting insight is expected to improve the rational design of porous membranes. The Viewpoint concludes with an outlook of other developments that can be best achieved by collaboration across the two fields to advance the understanding of transport in nanopores and create next-generation porous membranes tailored for sensing, filtration, and other applications.

Breakdown of the Nernst-Einstein relation in carbon nanotube porins

For over 100 years, the Nernst-Einstein relation has linked a charged particle's electrophoretic mobility and diffusion coefficient. Here we report experimental measurements of diffusion and electromigration of K⁺ ions in narrow 0.8-nm-diameter single-walled carbon nanotube porins (CNTPs) and demonstrate that the Nernst-Einstein relation in these channels breaks down by more than three orders of magnitude. Molecular dynamics simulations using polarizable force fields show that K⁺ ion diffusion in CNTPs in the presence of a single-file water chain is three orders of magnitude slower than bulk diffusion. Intriguingly, the simulations also reveal a disintegration of the water chain upon application of electric fields, resulting in the formation of distinct K⁺-water clusters, which then traverse the CNTP at high velocity. Finally, we show that although individual ion-water clusters still obey the Nernst-Einstein relation, the overall relation breaks down because of two distinct mechanisms for ion diffusion and electromigration.

Carbon Nanotubes-Based Nanofluidic Devices: Fabrication, Property and Application

With the rapid development of nanofluidics, more and more unexpected behaviors and bizarre properties have been discovered, which brings more possibility to solve the water and energy problem. Carbon nanotubes (CNTs) with nanoscale diameter and ultrasmooth hydrophobic surface provide strong confinement and unusual water-carbon couple which lead to many exotic properties, such as flow enhancement, strong ion exclusion, ultrafast proton transport and phase transition. This article reviews the recent progresses of CNT-based nanofluidic devices in fabrication, property, and applications. Moreover, challenges and opportunities of the CNT-based nanofluidic devices are discussed.

Protein-Mimicking Nanoparticles in Biosystems

Proteins are essential elements for almost all life activities. The emergence of nanotechnology offers innovative strategies to create a diversity of nanoparticles (NPs) with intrinsic capacities of mimicking the functions of proteins. These artificial mimics are produced in a cost-efficient and controllable manner, with their protein-mimicking performances comparable or superior to those of natural proteins. Moreover, they can be endowed with additional functionalities that are absent in natural proteins, such as cargo loading, active targeting, membrane penetrating, and multistimuli responding. Therefore, protein-mimicking NPs have been utilized more and more often in biosystems for a wide range of applications including detection, imaging, diagnosis, and therapy. To highlight recent progress in this broad field, herein, representative protein-mimicking NPs that fall into one of the four distinct categories are summarized: mimics of enzymes (nanozymes), mimics of fluorescent proteins, NPs with high affinity binding to specific proteins or DNA sequences, and mimics of protein scaffolds. This review covers their subclassifications, characteristic features, functioning mechanisms, as well as the extensive exploitation of their great potential for biological and biomedical purposes. Finally, the challenges and prospects in future development of protein-mimicking NPs are discussed.

Nanotube Functionalization: Investigation, Methods and Demonstrated Applications

This review presents an update on nanotube functionalization, including an investigation of their methods and applications. The review starts with the discussion of microscopy and spectroscopy investigations of functionalized carbon nanotubes (CNTs). The results of transmission electron microscopy and scanning tunnelling microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy and resistivity measurements are summarized. The update on the methods of the functionalization of CNTs, such as covalent and non-covalent modification or the substitution of carbon atoms, is presented. The demonstrated applications of functionalized CNTs in nanoelectronics, composites, electrochemical energy storage, electrode materials, sensors and biomedicine are discussed.

Putting together the puzzle of ion transfer in single-digit carbon nanotubes: mean-field meets ab initio

Nature employs channel proteins to selectively pass water across cell membranes, which inspires the search for bio-mimetic analogues. Carbon nanotube porins (CNTPs) are intriguing mimics of water channels, yet ion transport in CNTPs still poses questions. As an alternative to continuum models, here we present a molecular mean-field model that transparently describes ion coupling, yet unlike continuum models, computes ab initio all required thermodynamic quantities for the KCl salt and H⁺ and OH^- ions present in water. Starting from water transfer, the model considers the transfer of free ions, along with ion-pair formation as a proxy of non-mean-field ion-ion interactions. High affinity to hydroxide, suggested by experiments, making it a dominant charge carrier in CNTPs, is revealed as an exceptionally favorable transfer of KOH pairs. Nevertheless, free ions, coexisting with less mobile ion-pairs, apparently control ion transport. The model well explains the observed effects of salt concentration and pH on conductivity, transport numbers, anion permeation and its activation energies, and current rectification. The proposed approach is extendable to other sub-nanochannels and helps design novel osmotic materials and devices.

Comparison Study on Single Nucleotide Transport Phenomena in Carbon Nanotubes

To be considered as a promising candidate for mimicking biological nanochannels, carbon nanotubes (CNTs) have been used to explore the mass transport phenomena in recent years. In this study, the single nucleotide transport phenomena are comparatively studied using individual CNTs with a length of ∼15 μm and diameters ranging from 1.5 to 2.5 nm. In the case of CNTs with a diameter of 1.57-1.98 nm, the current traces of nucleotide transport are independent with the metallicity of CNTs and consist of single peak current pulses, whereas extraordinary stepwise current signals are observed in CNT with a diameter of 2.33 nm. It suggests that there is only one molecule in the nanochannel at a time until the diameter of CNT increases to 2.33 nm. Furthermore, it also demonstrates that the single nucleotides can be identified statistically according to their current pulses, indicating the potential application of CNT-based sensors for nucleotides identification.

Electroneutrality breakdown in nanopore arrays

The electrostatic screening of charge in one-dimensional confinement leads to long-range breakdown in electroneutrality within a nanopore. Through a series of continuum simulations, we demonstrate the principles of electroneutrality breakdown for electrolytes in one-dimensional confinement. We show how interacting pores in a membrane can counteract the phenomenon of electroneutrality breakdown, eventually returning to electroneutrality. Emphasis is placed on applying simplifying formulas to reduce the multidimensional partial differential equations into a single ordinary differential equation for the electrostatic potential. Dielectric mismatch between the electrolyte and membrane, pore aspect ratio, and confinement dimensionality are studied independently, outlining the relevance of electroneutrality breakdown in nanoporous membranes for selective ion transport and separations.

Membrane Materials for Selective Ion Separations at the Water-Energy Nexus

Synthetic polymer membranes are enabling components in key technologies at the water-energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water-energy nexus require ion selectivity, or separation of specific ionic species from other similar species. Here, the ion selectivity of conventional polymeric membrane materials is assessed and recent progress in enhancing selective transport via tailored free volume elements and ion-membrane interactions is described. In view of the limitations of polymeric membranes, three material classes-porous crystalline materials, 2D materials, and discrete biomimetic channels-are highlighted as possible candidates for ion-selective membranes owing to their molecular-level control over physical and chemical properties. Lastly, research directions and critical challenges for developing bioinspired membranes with molecular recognition are provided.

Electrostatic gating of ion transport in carbon nanotube porins: A modeling study

Carbon nanotube porins (CNTPs) are biomimetic membrane channels that demonstrate excellent biocompatibility and unique water and ion transport properties. Gating transport in CNTPs with external voltage could increase control over ion flow and selectivity. Herein, we used continuum modeling to probe the parameters that enable and further affect CNTP gating efficiency, including the size and composition of the supporting lipid membrane, slip flow in the carbon nanotube, and the intrinsic electronic properties of the nanotube. Our results show that the optimal gated CNTP device consists of a semiconducting CNTP inserted into a small membrane patch containing an internally conductive layer. Moreover, we demonstrate that the ionic transport modulated by gate voltages is controlled by the charge distribution along the CNTP under the external gate electric potential. The theoretical understanding developed in this study offers valuable guidance for the design of gated CNTP devices for nanofluidic studies, novel biomimetic membranes, and cellular interfaces in the future.

Water and Ion Transfer to Narrow Carbon Nanotubes: Roles of Exterior and Interior

Narrow carbon nanotubes (CNTs) desalinate water, mimicking water channels of biological membranes, yet the physics behind selectivity, especially the effect of the membrane embedding CNTs on water and ion transfer, is still unclear. Here, we report ab initio analysis of the energies involved in transfer of water and K⁺ and Cl^- ions from solution to empty and water-filled 0.68 nm CNTs for different dielectric constants (ϵ) of the surrounding matrix. The transfer energies computed for 1 ≤ ϵ < ∞ permit a transparent breakdown of the transfer energy to three main contributions: binding to CNT, intra-CNT hydration, and dielectric polarization of the matrix. The latter scales inversely with ϵ and is of the order 10²/ϵ kJ/mol for both ions, which may change ion transfer from favorable to unfavorable, depending on ion, ϵ, and CNT diameter. This may have broad implications for designing and tuning selectivity of nanochannel-based devices.

Snap-through in Graphene Nanochannels: With Application to Fluidic Control

Recent studies on the structure and transport behaviors of water confined within lamellar graphene have attracted intense interest in filtration technology, but the mechanism of water transport in complex membrane nanostructures remains an open question. For example, similar systems but at much larger scales have indicated that the instabilities of an elastic structure, such as snap-through, play an essential role in controlling the fluid flow. Graphene sheets, which have an atomic thickness, often appear highly wrinkled in nanofluidic devices and so are vulnerable to elastic instabilities. However, it remains unclear how does the flexible wrinkled structure affect the transport of water and filtration efficiency or whether such an effect can be exploited in devices. In this work, we explore the flow-induced snap-through in graphene nanochannels by combining molecular simulations with the theoretical analysis. We further demonstrate its applications to passive control of fluid flow and to ion/molecule selection. By introducing a flexible arch embedded within a graphene nanochannel, we observe the "snap" of the arched graphene wall from one stable state to another by varying the fluid flux (i.e., velocity); the critical velocity of this snap transition is found to depend nonmonotonically on the geometric size of the channel and the arch. We also demonstrate reversible snap-through by fixing the end parts of the flexible arch. These results suggest the potential of flow-induced snap-through in graphene-based nanochannels for ion/molecule selection applications in, for example, the design of a foul-resistant, easy-to-clean, reusable filter membrane.

Selectivity of nitrate and chloride ions in microporous carbons: the role of anisotropic hydration and applied potentials

Understanding ion transport in porous carbons is critical for a wide range of technologies, including supercapacitors and capacitive deionization for water desalination, yet many details remain poorly understood. For instance, an atomistic understanding of how ion selectivity is influenced by the molecular shape of ions, morphology of the micropores and applied voltages is largely lacking. In this work, we combined molecular dynamics simulations with enhanced sampling methods to elucidate the mechanism of nitrate and chloride selectivity in subnanometer graphene slit-pores. We show that nitrate is preferentially adsorbed over chloride in the slit-like micropores. This preferential adsorption was found to stem from the weaker hydration energy and unique anisotropy of the ion solvation of nitrate. Beside the effects of ion dehydration, we found that applied potential plays an important role in determining the ion selectivity, leading to a lower selectivity of nitrate over chloride at a high applied potential. We conclude that the measured ion selectivity results from a complex interplay between voltage, confinement, and specific ion effects-including ion shape and local hydration structure.

Pathways and Challenges for Biomimetic Desalination Membranes with Sub-Nanometer Channels

Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrices on a robust support. While biological components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers. However, the experimental performance of biomimetic membranes remains far below that of biological membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection. We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examination of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m^-2 h^-1 bar^-1, which is comparable to that of current state-of-the-art polymeric desalination membranes. Through a quantitative assessment of biomimetic membrane formats, we analytically show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains observed subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.