Nanofluidic Ion Transport Research Articles

(Invited) Nanofluidic Ion Transport in Carbon Nanotube Porin Channels

Extreme spatial confinement in narrow fluidic channels strongly influences their transport properties and enables unconventional selectivity mechanisms that can often mimic some of the selectivity and transport characteristics of biological membrane channels. More than any modern nanomaterial, carbon nanotubes have enabled experimental platforms that can recreate such extreme confinement in a channel with defined geometry and controllable electronic properties. I will discuss ion transport in canon nanotube porin channels and show how confinement phenomena, coupled with the different electronic effects in these channels can shape their transport properties and ion selectivity characteristics. Overall, these observations contribute to our understanding of nanoscale transport and pave the way for developing a new generation of precision separation platforms for biomedical and industrial use.

Silk Fibroin Fibers with In Situ Growth of ZIF-67 Nanoparticles for Membranes with Highly Efficient Nanofluidic Ion Transport

Silk Fibroin Fibers with In Situ Growth of ZIF-67 Nanoparticles for Membranes with Highly Efficient Nanofluidic Ion Transport

Local Electric Potential-Driven Nanofluidic Ion Transport for Ultrasensitive Biochemical Sensing.

Nanofluidic biosensors have been widely used for detection of analytes based on the change of system resistance before and after target-probe interactions. However, their sensitivity is limited when system resistance barely changes toward low-concentration targets. Here, we proposed a strategy to address this issue by means of target-induced change of local membrane potential under relatively unchanged system resistance. The local membrane potential originated from the directional diffusion of photogenerated carriers across nanofluidic biosensors and gated photoinduced ionic current signal before and after target-probe interactions. The sensitivity of such biosensors for the detection of biomolecules such as circulating tumor DNA (ctDNA) and lysozyme exceeds that of applying a traditional strategy by more than 3 orders of magnitude under unchanged system resistance. Such biosensors can specifically detect the small molecule biomarker in the blood sample between prostate cancer patients and healthy humans. The key advantages of such nanofluidic biosensors are therefore complementary to traditional nanofluidic biosensors, with potential applications in a point-of-care analytical tool.

Ion transport in nanofluidics under external fields.

Nanofluidic channels with tailored ion transport dynamics are usually used as channels for ion transport, to enable high-performance ion regulation behaviors. The rational construction of nanofluidics and the introduction of external fields are of vital significance to the advancement and development of these ion transport properties. Focusing on the recent advances of nanofluidics, in this review, various dimensional nanomaterials and their derived homogeneous/heterogeneous nanofluidics are first briefly introduced. Then we discuss the basic principles and properties of ion transport in nanofluidics. As the major part of this review, we focus on recent progress in ion transport in nanofluidics regulated by external physical fields (electric field, light, heat, pressure, etc.) and chemical fields (pH, concentration gradient, chemical reaction, etc.), and reveal the advantages and ion regulation mechanisms of each type. Moreover, the representative applications of these nanofluidic channels in sensing, ionic devices, energy conversion, and other areas are summarized. Finally, the major challenges that need to be addressed in this research field and the future perspective of nanofluidics development and practical applications are briefly illustrated.

Gated ion transport in disjoint carbon nanotubes by a water bridge

Nanofluidic ion transport in carbon nanotubes (CNTs) has numerous applications, including membrane separation, energy conversion, and ionic circuits. To imitate the natural gated ion channel systems, CNTs require a more complex structure beyond a single intact tube. In this study, we present molecular dynamics simulations demonstrating the gated ion transport in two disjoint CNTs with a nanogap. The observation of nonlinear current-voltage characteristics, featuring onset and plateau voltages for ion transport, indicates that the nanogap serves as a gate. Ions cannot transport through the disjoint CNTs below the onset voltage. Interestingly, a water bridge forms occasionally within the nanogap above the onset voltage, allowing ions to transport through coupling with water molecules. Above the plateau voltage, a stable water bridge forms and ions can transport in disjoint CNTs with the same efficiency as in the intact CNT. The formation of water bridges is due to that the electric stress on polarized water molecules near the nanogap overcomes the surface tension. Furthermore, the gated ion transport behavior is dependent on the nanogap length, CNT diameter, and CNT wettability, but not on the ion concentration. This work offers a viable approach for developing CNT-based nanofluidic devices with finely tuned ion transport efficiency.

Optimizing Membranes for Osmotic Power Generation

AbstractThe design of ion‐selective membranes is the key towards efficient reverse electrodialysis‐based osmotic power conversion. The tradeoff between ion selectivity (output voltage) and ion permeability (output current) in existing porous membranes, however, limits the upgradation of power generation efficiency for practical applications. Thus, we provide the simple guidelines based on fundamentals of ion transport in nanofluidics for promoting osmotic power conversion. In addition, we discuss strategies for optimizing membrane performance through analysis of various material parameters in membrane design, such as pore size, surface charge, pore density, membrane thickness, ion pathway, pore order, and ionic diode effect. Lastly, a perspective on the future directions of membrane design to further maximize the efficiency of osmotic power conversion is outlined.

Optimizing Membranes for Osmotic Power Generation.

The design of ion-selective membranes is the key towards efficient reverse electrodialysis-based osmotic power conversion. The tradeoff between ion selectivity (output voltage) and ion permeability (output current) in existing porous membranes, however, limits the upgradation of power generation efficiency for practical applications. Thus, we provide the simple guidelines based on fundamentals of ion transport in nanofluidics for promoting osmotic power conversion. In addition, we discuss strategies for optimizing membrane performance through analysis of various material parameters in membrane design, such as pore size, surface charge, pore density, membrane thickness, ion pathway, pore order, and ionic diode effect. Lastly, a perspective on the future directions of membrane design to further maximize the efficiency of osmotic power conversion is outlined.

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.

Photothermal regulated ion transport in nanofluidics: From fundamental principles to practical applications

Photothermal regulated ion transport in nanofluidics: From fundamental principles to practical applications

Ultrathin and Ultrastrong Kevlar Aramid Nanofiber Membranes for Highly Stable Osmotic Energy Conversion.

An ion‐selective membrane can directly convert the osmotic energy to electricity through reverse electrodialysis. However, developing an advanced membrane that simultaneously possesses high power density, excellent mechanical strength, and convenient large‐scale production for practical osmotic energy conversion, remains challenging. Here, the fabrication of ultrathin and ultrastrong Kevlar aramid nanofiber (KANF) membranes with interconnected three‐dimensional (3D) nanofluidic channels via a simple blade coating method is reported. The negatively charged 3D nanochannels show typical surface‐charge‐governed nanofluidic ion transport and exhibit excellent cation selectivity. When applied to osmotic energy conversion, the power density of the KANF membrane‐based generator reaches 4.8 W m–2 (seawater/river water) and can be further increased to 13.8 W m–2 at 328 K, which are higher than most of the state‐of‐the‐art membranes. Importantly, a 4‐µm‐thickness KANF membrane shows ultrahigh tensile strength (565 MPa) and Young's modulus (25 GPa). This generator also exhibits ultralong stability over 120 days, showing great potential in practical energy conversions.

Oriented Two-Dimensional Covalent Organic Framework Membranes with High Ion Flux and Smart Gating Nanofluidic Transport.

Nanofluidic ion transport holds high promise in bio-sensing and energy conversion applications. However, smart nanofluidic devices with high ion flux and modulable ion transport capabilities remain to be realised. Herein, we demonstrate smart nanofluidic devices based on oriented two-dimensional covalent organic framework (2D COF) membranes with vertically aligned nanochannel arrays that achieved a 2-3 orders of magnitude higher ion flux compared with that of conventional single-channel nanofluidic devices. The surface-charge-governed ion conductance is dominant for electrolyte concentration up to 0.01 M. Moreover, owing to the customisable pH-responsivity of imine and phenol hydroxyl groups, the COF-DT membranes attained an actively modulable ion transport with a high pH-gating on/off ratio of ≈100. The customisable structure and rich chemistry of COF materials will offer a promising platform for manufacturing nanofluidic devices with modifiable ion/molecular transport features.

MXene‐Based Membranes for Separation Applications

MXenes are a new type of 2D material, featuring numerous favorable properties, such as excellent flexibility, hydrophilicity, abundant functional groups, and high conductivity. Currently, MXene is a hot topic in the field of membrane separation processes. This timely review presents the recent progress in MXene‐based membrane in terms of structural engineering and versatile applications. First, the preparation methods for MXene nanosheets and the fabrication technologies for MXene‐based membranes are categorized. Then, the separation‐based applications of MXene membranes, including gas separation, water treatment, organic solvent purification, nanofluidic ion transport, and osmotic energy conversion, are summarized. Finally, the bottlenecks that limit the application of MXene‐based membranes are outlined, and future research prospects are discussed.

Assembly of Nanofluidic MXene Fibers with Enhanced Ionic Transport and Capacitive Charge Storage by Flake Orientation.

MXenes are an emerging class of highly conductive two-dimensional (2D) materials with electrochemical storage features. Oriented macroscopic Ti3C2Tx fibers can be fabricated from a colloidal 2D nematic phase dispersion. The layered conductive Ti3C2Tx fibers are ideal candidates for constructing high-speed ionic transport channels to enhance the electrochemical capacitive charge storage performance. In this work, we assemble Ti3C2Tx fibers with a high degree of flake orientation by a wet spinning process with controlled spinning speeds and morphology of the spinneret. In addition to the effects of cross-linking of magnesium ions between Ti3C2Tx flakes, the electronic conductivity and mechanical strength of the as-prepared fibers have been improved to 7200 S cm-1 and 118 MPa, respectively. The oriented Ti3C2Tx fibers present a volumetric capacitive charge storage capability of up to 1360 F cm-3 even in a Mg-ion based neutral electrolyte, with contributions from both nanofluidic ion transport and Mg-ion intercalation pseudocapacitance. The oriented 2D Ti3C2Tx driven nanofluidic channels with great electronic conductivity and mechanical strength endows the MXene fibers with attributes for serving as conductive ionic cables and active materials for fiber-type capacitive electrochemical energy storage, biosensors, and potentially biocompatible fibrillar tissues.

Harnessing Ionic Power from Equilibrium Electrolyte Solution via Photoinduced Active Ion Transport through van-der-Waals-Like Heterostructures.

Nanofluidic ion transport through van der Waals heterostructures, composed of two or more types of reconstructed 2D nanomaterials, gives rise to fascinating opportunities for light-energy harvesting, due to coupling between the optoelectronic properties of the layered constituents and ion transport in between the atomic layers. Here, a photoinduced active ion transport phenomenon through transition metal dichalcogenides (TMDs)-based van-der-Waals-like multilayer heterostructures is reported for harnessing ionic power from equilibrium electrolyte solution. The binary heterostructure comprises sequentially stacked 2D-WS2 and 2D-MoS2 multilayers with sub-1 nm interlayer spacing. Upon visible-light illumination, a net ionic flow is initiated through the Janus membrane, suggesting a directional cationic transport from WS2 to MoS2 part. The transport mechanism is explained in terms of a photovoltaic effect due to type II band alignment of WS2 /MoS2 heterostructures. The driving mechanism can be generally applied to a variety of heterogeneous TMD membranes with type II semiconductor heterojunctions. In equilibrium ionic solutions, the maximum ionic photoresponse approaches ≈21 µAcm-2 and ≈45mV under one sun equivalent excitation. Under optimized conditions, the harvested power density reaches 2mWm-2 . The proof-of-concept demonstration of photonic-to-ionic power generation within angstrom-scale confinement anticipates potential for light-controlled ionic circuits, artificial photosynthesis, and biomimetic energy conversion.

Neuromorphic van der Waals crystals for substantial energy generation

Controlling ion transport in nanofluidics is fundamental to water purification, bio-sensing, energy storage, energy conversion, and numerous other applications. For any of these, it is essential to design nanofluidic channels that are stable in the liquid phase and enable specific ions to pass. A human neuron is one such system, where electrical signals are transmitted by cation transport for high-speed communication related to neuromorphic computing. Here, we present a concept of neuro-inspired energy harvesting that uses confined van der Waals crystal and demonstrate a method to maximise the ion diffusion flux to generate an electromotive force. The confined nanochannel is robust in liquids as in neuron cells, enabling steady-state ion diffusion for hundred of hours and exhibiting ion selectivity of 95.8%, energy conversion efficiency of 41.4%, and power density of 5.26 W/m2. This fundamental understanding and rational design strategy can enable previously unrealisable applications of passive-type large-scale power generation.

Flexible fiber-shaped lithium and sodium-ion batteries with exclusive ion transport channels and superior pseudocapacitive charge storage

Tungstate/rGO fiber was engineered and fabricated for flexible lithium and sodium-ion batteries, with exclusive 2D nanofluidic ion transport channels, fast 3D interconnected ion transport tunnels, and efficient pseudocapacitive charge storage.

Light-Powered Directional Nanofluidic Ion Transport in Kirigami-Made Asymmetric Photonic-Ionic Devices.

Nacre-mimetic 2D nanofluidic materials with densely packed sub-nanometer-height lamellar channels find widespread applications in water-, energy-, and environment-related aspects by virtue of their scalable fabrication methods and exceptional transport properties. Recently, light-powered nanofluidic ion transport in synthetic materials gained considerable attention for its remote, noninvasive, and active control of the membrane transport property using the energy of light. Toward practical application, a critical challenge is to overcome the dependence on inhomogeneous or site-specific light illumination. Here, asymmetric photonic-ionic devices based on kirigami-tailored graphene oxide paper are fabricated, and directional nanofluidic ion transport properties therein powered by full-area light illumination are demonstrated. The in-plane asymmetry of the graphene oxide paper is essential to the generation of photoelectric driving force under homogeneous illumination. This light-powered ion transport phenomenon is explained based on a modified carrier diffusion model. In asymmetric nanofluidic structures, enhanced recombination of photoexcited charge carriers at the membrane boundary breaks the electric potential balance in the horizontal direction, and thus drives the ion transport in that direction under symmetric illumination. The kirigami-based strategy provides a facile and scalable way to fabricate paper-like photonic-ionic devices with arbitrary shapes, working as fundamental elements for large-scale light-harvesting nanofluidic circuits.

Decoupling Ionic and Electronic Pathways in Low-Dimensional Hybrid Conductors.

The construction of two-dimensional (2D) layered compounds for nanofluidic ion transport has recently attracted increasing interest due to the facile fabrication, tunable channel size, and high flux of these materials. Here we design a nacre-mimetic graphite-based nanofluidic structure in which the nanometer-thick graphite flakes are wrapped by negatively charged nanofibrillated cellulose (NFC) fibers to form multiple 2D confined spacings as nanochannels for rapid cation transport. At the same time, the graphite-NFC structure exhibits an ultralow electrical conductivity (σe ≤ 10-9 S/cm), even when the graphite concentration is up to 50 wt %, well above the percolation threshold (∼1 wt %). By tuning the hydration degree of graphite-NFC composites, the surface-charge-governed ion transport in the confined ∼1 nm spacings exhibits nearly 12 times higher ionic conductivity (1 × 10-3 S/cm) than that of a fully swollen structure (∼1.5 nm, 8.5 × 10-5 S/cm) at salt concentrations up to 0.1 M. The resulting charge selective conductor shows intriguing features of both high ionic conductivity and low electrical conductivity. Moreover, the inherent stability of the graphite and NFC components contributes to the strong functionality of the nanofluidic ion conductors in both acidic and basic environments. Our work demonstrates this 1D-2D material hybrid system as a suitable platform to study nanofluidic ion transport and provides a promising strategy to decouple ionic and electronic pathways, which is attractive for applications in new nanofluidic device designs.

Large-scale orientated self-assembled halloysite nanotubes membrane with nanofluidic ion transport properties

One of the significant challenges in nanoscience and nanotechnology is how to make macroscopic devices by assembling nano-building blocks while maintaining their extraordinary properties. Here we show a large-scale orientated membrane based on halloysite nanotubes which can be self-assembled on glass substrate by using isothermal heating evaporation induced method. The halloysite nanotubes exhibit the maximum alignment degree when the halloysite dispersion was dried at 80 °C with the concentration of 10 mg mL−1 and the pH of 6. The birefringence observed over the membrane indicates the presence of the nematic liquid crystalline phase with aligned nanotubes. The uniform space between the nanotubes in the orientated membranes forms nanochannels which could be used for ion transport. Surface-charge-governed ion transport behavior is observed through these nanochannels at KCl concentrations below 10−4 M. The ease of constructing large-scale and stable nanochannel arrays offer many new exciting opportunities for studies of ion transport as well as new device designs.

Fabrication of Stable and Flexible Nanocomposite Membranes Comprised of Cellulose Nanofibers and Graphene Oxide for Nanofluidic Ion Transport

Two-dimensional membranes with nanofluidic channels and a high chemical stability are strongly needed in many practical applications. We present a facile vacuum filtration method to fabricate a lam...