Nanofluidic Channels Research Articles

Directed charge boosting by polyelectrolyte nanorods on a graphene oxide membrane for High-Performance blue energy harvesting

Blue energy harvesting is conversion of the Gibbs free energy of mixing due to the salinity gradient between fresh and salt water into electrical energy. Two-dimensional (2D) material-based nanofluidic channels offer unique features and distinct advantages for blue energy harvesting, and they can be prepared via simple fabrication and modification methods. However, because of their lower ion transport selectivity and conductivity than theoretical estimation due to intrinsic and structural defects, 2D nanochannels exhibit low power density and extractable power. In this study, we developed a scalable blade-coating method for the shear-induced alignment of polyelectrolyte nanorods in 2D nanochannels. Based on the increase in the directional charge density via structural modifications, devices had the developed nanochannel structure and achieved a high osmotic power density and energy conversion efficiency of 13.12 W m−2 and 35.64%, respectively. Finally, a hybrid nanofluidic tandem cell was used to operate a handheld electrical device. This study demonstrates the potential of coupling of improved charge density and modified ion pathway in the nanochannel by introducing a charge booster for osmotic energy conversion.

Mapping fluid structuration to flow enhancement in nanofluidic channels.

Fluid flow in miniature devices is often characterized by a boundary "slip" at the wall, as opposed to the classical paradigm of a "no-slip" boundary condition. While the traditional mathematical description of fluid flow as expressed by the differential forms of mass and momentum conservation equations may still suffice in explaining the resulting flow physics, one inevitable challenge against a correct quantitative depiction of the flow velocities from such considerations remains in ascertaining the correct slip velocity at the wall in accordance with the complex and convoluted interplay of exclusive interfacial phenomena over molecular scales. Here, we report an analytic engine that applies combined physics-based and data-driven modeling to arrive at a quantitative depiction of the interfacial slip via a molecular-dynamics-trained machine learning algorithm premised on fluid structuration at the wall. The resulting mapping of the system parameters to a single signature data that bridges the molecular and continuum descriptions is envisaged to be a preferred computationally inexpensive route as opposed to expensive multi-scale or molecular simulations that may otherwise be inadequate to resolve the flow features over experimentally tractable physical scales.

Intrinsic Interface Adsorption Drives Selectivity in Atomically Smooth Nanofluidic Channels.

Specific molecular interactions underlie unexpected and useful phenomena in nanofluidic systems, but these require descriptions that go beyond traditional macroscopic hydrodynamics. In this letter, we demonstrate how equilibrium molecular dynamics simulations and linear response theory can be synthesized with hydrodynamics to provide a comprehensive characterization of nanofluidic transport. Specifically, we study the pressure driven flows of ionic solutions in nanochannels comprised of two-dimensional crystalline substrates made from graphite and hexagonal boron nitride. While simple hydrodynamic descriptions do not predict a streaming electrical current or salt selectivity in such simple systems, we observe that both arise due to the intrinsic molecular interactions that act to selectively adsorb ions to the interface in the absence of a net surface charge. Notably, this emergent selectivity indicates that these nanochannels can serve as desalination membranes.

Ionic Thermoelectric Properties of Reconstructed Lamellar Vanadium Pentoxide Membranes

AbstractIn recent years, the application of ionic thermoelectric (TE) materials to convert low‐grade waste heat into electricity has become a subject of intense scientific research. However, most of the efforts are focused on organic polyelectrolytes or ionic‐liquids embedded in polymeric gels. Here, for the first time, it is demonstrated that nanofluidic membranes of reconstructed layered materials like vanadium pentoxide (V2O5) exhibit excellent ionic‐TE characteristics. The high Seebeck coefficient (S = 14.5 ± 0.5 mV K‐1) of the V2O5 membrane (VO‐M) is attributed to temperature gradient‐induced unidirectional transport of protons through the percolated network of 2D nanofluidic channels. The TE characteristics of VO‐M show nearly 80% improvement (S = 26.3 ± 0.7 mV K‐1) upon functionalizing its percolated network with ionic polymers like poly(4‐styrenesulfonic acid) (PSS). Further, unlike organic polymer‐based TE systems, VO‐M not only sustains exposure to high temperatures (≈200 °C, 5 min) but also protects the PSS molecules intercalated into its interlayer space. Moreover, V2O5‐based TE materials can self‐repair any damage to their physical structure with the help of a tiny water droplet. Thus, nanofluidic membranes of reconstructed layered materials like VO‐Ms demonstrate vast robustness and great ionic‐TE performance, which can provide a novel platform for scientific studies and futuristic applications.

Ion filling of a one-dimensional nanofluidic channel in the interaction confinement regime.

Ion transport measurements are widely used as an indirect probe for various properties of confined electrolytes. It is generally assumed that the ion concentration in a nanoscale channel is equal to the ion concentration in the macroscopic reservoirs it connects to, with deviations arising only in the presence of surface charges on the channel walls. Here, we show that this assumption may break down even in a neutral channel due to electrostatic correlations between the ions arising in the regime of interaction confinement, where Coulomb interactions are reinforced due to the presence of the channel walls. We focus on a one-dimensional channel geometry, where an exact evaluation of the electrolyte's partition function is possible with a transfer operator approach. Our exact solution reveals that in nanometer-scale channels, the ion concentration is generally lower than in reservoirs and depends continuously on the bulk salt concentration, in contrast to the conventional mean-field theory that predicts an abrupt filling transition. We develop a modified mean-field theory taking into account the presence of ion pairs that agrees quantitatively with the exact solution and provides predictions for experimentally relevant observables, such as the ionic conductivity. Our results will guide the interpretation of nanoscale ion transport measurements.

Quantitative characterization of liquids flowing in geometrically controlled sub-100nm nanofluidic channels.

With development of nanotechnologies, applications exploiting nanospaces such as single-molecule analysis and high-efficiency separation have been reported, and understanding properties of fluid flows in 101nm to 102nm scale spaces becomes important. Nanofluidics has provided a platform of nanochannels with defined size and geometry, and revealed various unique liquid properties including higher water viscosity with dominant surface effects in 102nm spaces. However, experimental investigation of fluid flows in 101nm spaces is still difficult owing to lack of fabrication procedure for 101nm nanochannels with smooth walls and precisely controlled geometry. In the present study, we established a top-down fabrication process to realize fused-silica nanochannels with 101nm scale size, 100nm roughness and rectangular cross-sectional shape with an aspect ratio of 1. Utilizing a method of mass flowmetry developed by our group, accurate measurements of ultra-low flow rates in sub-100nm nanochannels with sizes of 70nm and 100nm were demonstrated. The results suggested that the viscosity of water in these sub-100nm nanochannels was approximately 5 times higher than that in the bulk, while that of dimethyl sulfoxide was similar to the bulk value. The obtained liquid permeability in the nanochannels can be explained by a hypothesis of loosely structured liquid phase near the wall generated by interactions between the surface silanol groups and protic solvent molecules. The present results suggest the importance of considering the species of solvent, the surface chemical groups, and the size and geometry of nanospaces when designing nanofluidic devices and membranes.

Nanofluidics: Transport in chirality-controlled carbon nanotube porins (CNTPs) and fluorescence ultrashort nanotubes (FUNs).

Carbon nanotubes (CNTs) with different chirality are great models to study the influence of channel electronic properties on ion transport through nanofluidic channels. We prepared short chirality-separated pure (6,5), (7,4), and (7,5)/(8,4) CNT porins (CNTPs), which have different electronic properties but are almost identical in diameter. Single channel conductance from potassium transport through these CNTPs indicates that electronic properties have a weak effect on ion transport. Sodium and lithium ion transport through different chirality CNTPs have also been studied. Chemical functionalities and defects on the CNTP walls can also influence transport properties. To address that possibility, we measured ion transport through fluorescence ultrashort nanotubes (FUNs) - functionalized channels with defective walls dispersed by surfactants such as CHAPS and SDOC. These channels have significantly lower conductance compared to the pristine (6,5) CNTPs. Finally, we discuss other factors that influence conductance in these nanofluidic channels.

Nanobubble-governed membrane with nanofluidic channels for efficient molecule/ion sieving

Nanobubbles play key roles in determining various interfacial properties. However, investigation of water/ions transport behavior in nanobubble-governed nanofluidic channel was limited by the resolution of current in-situ observation techniques. Here, we investigated selective nanofluidic transport in nanobubble-plugged nanochannel during cross-flow forward osmosis (FO) process with a combined approach of simulations and experiments. Nanobubbles were generated by mechanical vibration during FO process. A nanobubble-plugged nanochannel model established via finite element modeling was used to explore nanobubble-governed selective water/ions transport. FO experimental results also verified nanobubble-plugged nanochannels with different asymmetric wettability and nanopore sizes could achieve unidirectional water transport while near-zero salt diffusion. Ion sieving tests further demonstrated nanobubble-plugged nanochannel could impede salt ions by nanobubble gating with size sieving, hydrophobic interaction and electrostatic interaction between nanobubbles and salt ions. These findings have implications for understanding selective nanofluidic transport in nanobubble-governed nanofluidic channel, which also provide new routes for designing potential osmosis membranes for high performance desalination.

Stability of enzyme immobilized on the nanofluidic channel surface.

The lifetime of an enzyme is critical to prevent system failure and optimize maintenance schedules in biological and analytical chemistry. The lifetime metrics of an enzyme can be evaluated from enzyme activity in terms of catalytic cycles per enzyme at various storage times. Trypsin, which is a gold-standard enzyme in proteomics, has been known to decrease activity due to self-digestion. To improve the activity of trypsin, enzyme reactors have developed by immobilizing in micro and nanospace. However, an evaluation method for the catalytic cycle has not been established due to major issues such as nonuniform space, unstable liquid transport, and self-digestion during immobilization in conventional work. To solve these issues, we have previously developed an ultra-fast enzyme reactor with a well-defined nanofabrication method, stable liquid transport, and partial enzyme modification. Here, we aimed to investigate catalytic cycles in a nanochannel. To extend enzyme lifetime efficiently, we have evaluated the optimal immobilization process and catalytic cycles of trypsin. As a result, immobilized enzyme densities by the trypsinogen immobilization process were increased at all concentrations compared to the trypsin immobilization process. To evaluate the lifetime of trypsin, the immobilized enzyme densities and activities were almost the same before and after 72h of enzyme storage, and the calculated catalytic cycles were 1740. These results indicated that self-digestion of the immobilized enzyme was highly suppressed. Consequently, the reaction efficiency has been evaluated depending on the catalytic cycles from the substrate for the first time, while preventing self-digestion by trypsin.

Long-term memory and synapse-like dynamics in two-dimensional nanofluidic channels

Fine-tuned ion transport across nanoscale pores is key to many biological processes, including neurotransmission. Recent advances have enabled the confinement of water and ions to two dimensions, unveiling transport properties inaccessible at larger scales and triggering hopes of reproducing the ionic machinery of biological systems. Here we report experiments demonstrating the emergence of memory in the transport of aqueous electrolytes across (sub)nanoscale channels. We unveil two types of nanofluidic memristors depending on channel material and confinement, with memory ranging from minutes to hours. We explain how large time scales could emerge from interfacial processes such as ionic self-assembly or surface adsorption. Such behavior allowed us to implement Hebbian learning with nanofluidic systems. This result lays the foundation for biomimetic computations on aqueous electrolytic chips.

Tunable Ion Transport in Two-Dimensional Nanofluidic Channels

Layered two-dimensional (2D) materials with interlayer channels at the nanometer scale offer an ideal platform to control ion transport behaviors, including high-precision separation, ultrafast diffusion, and tunable permeation flux, which show great potential for energy conversion and storage, water treatment, catalysis, biosynthesis, and sensing. Recent advances in controlling the structure and functionality of 2D nanofluidic channels sustainably open doors for more revolutionary applications. In this Perspective, we first present a brief introduction to the fundamental mechanisms for ion transport in 2D nanofluidic channels and an overview of state-of-the-art assembly technologies of nanochannel membranes. We then point out new avenues for developing advanced nanofluidics, combining molecular-level cross-linking, and surface modification in nanoconfinement. Finally, we outline the potential applications of these 2D nanofluidic channel membranes and their technical challenges that need to be addressed to afford for practical applications.

Probing physical properties of single amyloid fibrils using nanofluidic channels.

Amyloid fibril formation is central to the pathology of many diseases, including neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Amyloid fibrils can also have functional and scaffolding roles, for example in bacterial biofilms, and have also been exploited as useful biomaterials. Despite being linear protein homopolymers, amyloid fibrils can exhibit significant structural and morphological polymorphism, making it relevant to study them on the single fibril level. We here introduce the concept of nanofluidic channel analysis to the study of single, fluorescently-labeled amyloid fibrils in solution, monitoring the extension and emission intensity of individual fibrils confined in nanochannels with a depth of 300 nm and a width that gradually increases from 300 to 3000 nm. The change in fibril extension with channel width permitted accurate determination of the persistence length of individual fibrils using Odijk's theory for strongly confined polymers. The technique was applied to amyloid fibrils prepared from the Alzheimer's related peptide amyloid-β(1-42) and the Parkinson's related protein α-synuclein, obtaining mean persistence lengths of 5.9 ± 4.5 μm and 3.0 ± 1.6 μm, respectively. The broad distributions of fibril persistence lengths indicate that amyloid fibril polymorphism can manifest in their physical properties. Interestingly, the α-synuclein fibrils had lower persistence lengths than the amyloid-β(1-42) fibrils, despite being thicker. Furthermore, there was no obvious within-sample correlation between the fluorescence emission intensity per unit length of the labelled fibrils and their persistence lengths, suggesting that stiffness may not be proportional to thickness. We foresee that the nanofluidics methodology established here will be a useful tool to study amyloid fibrils on the single fibril level to gain information on heterogeneity in their physical properties and interactions.

Ionic current driven by a viscosity gradient.

Gradients of voltage, pressure, temperature, and salinity can transport objects in micro- and nanofluidic systems by well-known mechanisms. This paper explores the dynamics of particles in a viscosity gradient with numerical simulations. The different stochastic rules used to integrate the random motion of Brownian particles affect the steady-state distribution of particles in a diffusivity gradient. Importantly, the simulations illuminate the important role that the boundary conditions play, disallowing a steady-state flux when the boundary conditions mimic those of a closed container, but allowing flux when they mimic electrodes. These results provide an interpretation for measurements of a steady ionic current flowing between electrodes separated by a nanofluidic channel with a liquid viscosity gradient.

Electrochemically addressed FET-like nanofluidic channels with dynamic ion-transport regimes.

Nanofluidic channels in which the ionic transport can be modulated by the application of an external voltage to the nanochannel walls have been described as nanofluidic field effect transistors (nFETs) because of their analogy with electrolyte-gated field effect transistors. The creation of nFETs is attracting increasing attention due to the possibility of controlling ion transport by using an external voltage as a non-invasive stimulus. In this work, we show that it is possible to extend the actuation range of nFETs by using the supporting electrolyte as a "chemical effector". For this aim, a gold-coated poly(ethylene terephthalate) (PET) membrane was modified with electroactive poly-o-aminophenol. By exploiting the interaction between the electroactive poly-o-aminophenol and the ions in the electrolyte solution, the magnitude and surface charge of the nanochannels were fine-tuned. In this way, by setting the electrolyte nature it has been possible to set different ion transport regimes, i.e.: cation-selective or anion-selective ion transport, whereas the rectification efficiency of the ionic transport was controlled by the gate voltage applied to the electroactive polymer layer. Remarkably, under both regimes, the platform displays a reversible and rapid response. We believe that this strategy to preset the actuation range of nFETs by using the supporting electrolyte as a chemical effector can be extended to other devices, thus offering new opportunities for the development of stimulus-responsive solid-state nanochannels.

Heat and osmosis cooperatively driven power generation in robust two-dimensional hybrid nanofluidic channels

Heat and osmosis cooperatively driven power generation in robust two-dimensional hybrid nanofluidic channels is shown.

A nanofluidic sensing platform based on robust and flexible graphene oxide/chitosan nanochannel membranes for glucose and urea detection.

We present the development of a health-monitoring nanofluidic membrane utilizing biocompatible and biodegradable graphene oxide, chitosan, and graphene quantum dots. The nanoconfinement provided by graphene oxide nanolayers encapsulates chitosan molecules, allowing for their conformational changes and switchable hydrophobic-hydrophilic behavior in response to pH variations. This low-dimensional membrane operates as an array of nanofluidic channels that can release quantum dots upon pH change. The photoluminescence emission from quantum dots enables rapid and reliable optical visualization of pH changes, facilitating efficient human health monitoring. To ensure fouling prevention and enable multiple usages, we adopt a design approach that avoids direct contact between biomarkers and the nanochannels. This design strategy, coupled with good mechanical properties (Young's modulus of 5.5 ± 0.7 GPa), preserves the integrity and functionality of the sensors for repeated sensing cycles. Furthermore, leveraging the memory effect, our sensors can be reloaded with graphene quantum dots multiple times without significant loss of selectivity, achieving reusability. The wide-ranging capabilities of 2D materials and stimuli-responsive polymers empower our sustainable approach to designing low-dimensional, robust, and flexible sensing materials. This approach allows for the integration of various biorecognition elements and signal transduction modes, expanding the versatility and applications of the designed materials.

Two-dimensional nanofluidic channels towards ion transport

Two-dimensional nanofluidic channels towards ion transport

Bioinspired Three-Dimensional Nanoporous Membranes for Salinity-Gradient Energy Harvesting

ConspectusSalinity-gradient energy represents a widespread, clean, environmentally friendly, and sustainable source of renewable energy, which has attracted great attention in the past years. To harness this energy, interdisciplinary efforts from chemistry, materials science, environmental science, and nanotechnology have been made to develop efficient and low-cost approaches and materials for energy conversion. Conventional reverse electrodialysis (RED) systems are generally based on ion-exchange membranes, which usually suffer from ineffective mass transport, high membrane resistance, limited pore size, and concentration polarization, resulting in low output power density and poor energy-conversion efficiency. As one promising material, nanofluidic channels with their unique transport properties, which can be attributed to nanoconfinement effect, enable high-performance reverse electrodialysis to efficiently harvest salinity-gradient energy. Due to the unique porous architectures, three-dimensional (3D) nanoporous membranes demonstrate great potential for harvesting salinity-gradient power. It is generally known that the porous membranes can be prepared by many methods; however, there are some shortcomings such as high costs, poor ion conductance, and fragility limiting the practical application. Several simple and versatile approaches to low-cost fabrication of 3D nanoporous membranes have been developed in recent years. For example, self-assembly provides an effective route of constructing functional materials and organizing them into 3D architectures. In this Account, we mainly review our recent progress in the design and fabrication of bioinspired 3D nanoporous membranes for salinity-gradient energy harvesting. First, we give a brief introduction to bioinspired nanochannel membranes (BNMs) with diverse structural dimensions, and nanofluidic channel membranes may lead to technological breakthroughs and thus act as an emerging platform for harvesting salinity-gradient energy. Subsequently, we discuss the typical preparation approaches for bioinspired 3D nanoporous membranes. To tackle the bottlenecks of the conventional membrane-based power generator and extrapolate single-channel devices to macroscopic materials, our group have developed a series of 3D nanoporous membranes for power generation via various simple and versatile methods. We highlight the design and fabrication of several types of 3D nanoporous membranes, i.e., heterogeneous and homogeneous membranes, with tunable surface charge and porosity. The proof-of-concept demonstration of bioinspired 3D porous membranes shows that these nanofluidic platforms have the potential to overcome the selectivity-permeability trade-off and have impressive osmotic-energy-harvesting performance. Specifically, the scale-up Janus 3D porous membranes maintained high selectivity and rectified current in a hypersaline environment, which benefitted effective energy conversion and high output power density when seawater and river water were mixed. Finally, we give an outlook for future challenges and perspectives on the development of 3D nanofluidics for salinity-gradient energy conversion. We expect that this Account will spark further efforts on the development of bioinspired 3D nanoporous membranes for large-scale (typical side length of more than 10 cm) energy conversion and new opportunities for the applications in water desalination, dialysis, and ionic circuitries.

Surfactant-Assisted Sulfonated Covalent Organic Nanosheets: Extrinsic Charge for Improved Ion Transport and Salinity-Gradient Energy Harvesting.

Charge-governed ion transport is the vital property of nanofluidic channels for salinity-gradient energy harvesting and other electrochemical energy conversion technologies. 2D nanofluidic channels constructed by nanosheets exhibit great superiority in ion selectivity, but a high ion transport rate remains challenging due to the insufficiency of intrinsic surface charge density in nanoconfinement. Herein, extrinsic surface charge into nanofluidic channels composed of surfactant-assisted sulfonated covalent organic nanosheets (SCONs), which enable tunable ion transport behaviors, is demonstrated. The polar moiety of surfactant is embedded in SCONs to adjust in-plane surface charges, and the aggregation of nonpolar moiety results in the sol-to-gel transformation of SCON solution for membrane fabrication. The combination endows SCON/surfactant membranes with considerable water-resistance, and the designable extrinsic charges promise fast ion transport and high ion selectivity. Additionally, the SCON/surfactant membrane, serving as a power generator, exhibits huge potential in harvesting salinity-gradient energy where corresponding output power density can reach up to 9.08W m-2 under a 50-fold salinity gradient (0.5m NaCl|0.01m NaCl). The approach to extrinsic surface charge provides new and promising insight into regulating ion transport behaviors.

Cross-Linked Nanocellulose Membranes for Nanofluidic Osmotic Energy Harvesting

Osmotic energy generated from the salinity gradient is a kind of clean and renewable energy source, where the ion-exchange membranes play a critical role in its operation. The nanofluidic technique is emerging to overcome the limitations of high resistance and low mass transport of traditional ion-exchange membranes and thus improve osmotic power conversion. However, the currently reported nanofluidic materials suffer from high cost and complicated fabrication processes, which limits their practical application. Here, we report low-cost nanocellulose membranes that can be facilely prepared by a chemical cross-linking approach. The obtained membranes exhibit excellent ion transport characteristics as high-performance nanofluidic osmotic power generators. The control of cross-linker dosage enables the simultaneous tunability of the surface charge density and size of nanofluidic channels created between the interwoven cellulose nanofibrils. The maximum osmotic power generated by the membrane is reached when the cross-linker weight content is 20 wt %. Furthermore, the cross-linked nanocellulose membranes exhibit long-term working stability in osmotic energy harvesting under a wide range of pH values (3.2–9.7). This nanocellulose membrane derived from green and sustainable natural materials demonstrates a promising potential for renewable osmotic energy harvesting.