Massively Enhanced Charge Selectivity, Ion Transport, and Osmotic Energy Conversion by Antiswelling Nanoconfined Hydrogels.

Developing a nanofluidic membrane with simultaneously enhanced ion selectivity and permeability for high-performance osmotic energy conversion has largely been unexplored. Here, we tackle this issue by the confinement of highly space-charged hydrogels within an orderedly aligned nanochannel array membrane. The nanoconfinement effect endows the hydrogel-based membrane with excellent antiswelling property. Furthermore, experimental and simulation results demonstrate that such a nanoconfined hydrogel membrane exhibits massively enhanced cation selectivity and ion transport properties. Consequently, an amazingly high power density up to ∼52.1 W/m2 with an unprecedented energy conversion efficiency of 37.5% can be reached by mixing simulated salt-lake water (5 M NaCl) and river water (0.01 M NaCl). Both efficiency indexes surpass those of most of the state-of-the-art nanofluidic membranes. This work offers insights into the design of highly ion-selective membranes to achieve ultrafast ion transport and high-performance osmotic energy harvesting.

Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators.

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3 nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1 m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5 nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

Enhancing Ionic Selectivity and Osmotic Energy by Using an Ultrathin Zr-MOF-Based Heterogeneous Membrane with Trilayered Continuous Porous Structure.

Designing a nanofluidic membrane with high selectivity and fast ion transport property is the key towards high-performance osmotic energy conversion. However, most of reported membranes can produce power density less than commercial benchmark (5 W/m2), due to the imbalance between ion selectivity and permeability. Here, we report a novel nanoarchitectured design of a heterogeneous membrane with an ultrathin and dense zirconium-based UiO-66-NH2 metal-organic framework (MOF) layer and a highly aligned and interconnected branched alumina nanochannel membrane. The design leads to a continuous trilayered pore structure of large geometry gradient in the sequence from angstrom-scale to nano-scale to sub-microscale, which enables the enhanced directional ion transport, and the angstrom-sized (~6.6-7 Å) UiO-66-NH2 windows render the membrane with high ion selectivity. Consequently, the novel heterogeneous membrane can achieve a high-performance power of ~8 W/m2 by mixing synthetic seawater and river water. The power density can be largely upgraded to an ultrahigh ~17.1 W/m2 along with ~48.5 % conversion efficiency at a 50-fold KCl gradient. This work not only presents a new membrane design approach but also showcases the great potential of employing the zirconium-based MOF channels as ion-channel-mimetic membranes for highly efficient blue energy harvesting.

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.

Single Mesopores with High Surface Charges as Ultrahigh Performance Osmotic Power Generators.

Numerous studies on osmotic power generators with nanoscale pores are conducted. However, their performance output is limited because of the finite osmotic current and conductance from such tiny pores. Here, a proof-of-concept study demonstrating that the rectified mesopore (sub-micrometer-scale pore) with high surface charges can be applied in osmotic energy conversion is reported. A single conical mesopore of ≈405 nm in tip diameter, which can reach an osmotic conductance as high as 0.284 µS (corresponding to a current of 27.5 nA and voltage of 97 mV), enables a record-high power of 667 pW under a 1000-fold salinity gradient, more than doubling all of the state-of-the-art single-pore osmotic power generators reported. This work extends the knowledge of osmotic energy with solid-state pores from nanoscale to mesoscale and opens up a promising avenue toward ultrahigh performance osmotic power.

Rectification of Concentration Polarization in Mesopores Leads To High Conductance Ionic Diodes and High Performance Osmotic Power.

Nanopores exhibit a set of interesting transport properties that stem from interactions of the passing ions and molecules with the pore walls. Nanopores are used, for example, as ionic diodes and transistors, biosensors, and osmotic power generators. Using nanopores is however disadvantaged by their high resistance, small switching currents in nA range, low power generated, and signals that can be difficult to distinguish from the background. Here, we present a mesopore with ionic conductance reaching µS that rectifies ion current in salt concentrations as high as 1 M. The mesopore is conically shaped, and its region close to the narrow opening is filled with high molecular weight poly-l-lysine. To elucidate the underlying mechanism of ion current rectification (ICR), a continuum model based on a set of Poisson-Nernst-Planck and Stokes-Brinkman equations was adopted. The results revealed that embedding the polyelectrolyte in a conical pore leads to rectification of the effect of concentration polarization (CP) that is induced by the polyelectrolyte, and observed as voltage polarity-dependent modulations of ionic concentrations in the pore, and consequently ICR. Our work reveals the link between ICR and CP, significantly extending the knowledge of how charged polyelectrolytes modulate ion transport on nano- and mesoscales. The osmotic power application is also demonstrated with the developed polyelectrolyte-filled mesopores, which enable a power of up to ∼120 pW from one pore, which is much higher than the reported values using single nanoscale pores.

Anomalous pH-Dependent Nanofluidic Salinity Gradient Power.

Previous studies on nanofluidic salinity gradient power (NSGP), where energy associated with the salinity gradient can be harvested with ion-selective nanopores, all suggest that nanofluidic devices having higher surface charge density should have higher performance, including osmotic power and conversion efficiency. In this manuscript, this viewpoint is challenged and anomalous counterintuitive pH-dependent NSGP behaviors are reported. For example, with equal pH deviation from its isoelectric point (IEP), the nanopore at pH < IEP is shown to have smaller surface charge density but remarkably higher NSGP performance than that at pH > IEP. Moreover, for sufficiently low pH, the NSGP performance decreases with lowering pH (increasing nanopore charge density). As a result, a maximum osmotic power density as high as 5.85 kW m-2 can be generated along with a conversion efficiency of 26.3% achieved for a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Using the rigorous model with considering the surface equilibrium reactions on the pore wall, it is proved that these counterintuitive surface-charge-dependent NSGP behaviors result from the pH-dependent ion concentration polarization effect, which yields the degradation in effective concentration ratio across the nanopore. These findings provide significant insight for the design of next-generation, high-performance NSGP devices.

Gate modulation of proton transport in a nanopore.

Proton transport in confined spaces plays a crucial role in many biological processes as well as in modern technological applications, such as fuel cells. To achieve active control of proton conductance, we investigate for the first time the gate modulation of proton transport in a pH-regulated nanopore by a multi-ion model. The model takes into account surface protonation/deprotonation reactions, surface curvature, electroosmotic flow, Stern layer, and electric double layer overlap. The proposed model is validated by good agreement with the existing experimental data on nanopore conductance with and without a gate voltage. The results show that the modulation of proton transport in a nanopore depends on the concentration of the background salt and solution pH. Without background salt, the gated nanopore exhibits an interesting ambipolar conductance behavior when pH is close to the isoelectric point of the dielectric pore material, and the net ionic and proton conductance can be actively regulated with a gate voltage as low as 1 V. The higher the background salt concentration, the lower is the performance of the gate control on the proton transport.

Salt gradient driven ion transport in solid-state nanopores: the crucial role of reservoir geometry and size.

Modern applications of nanotechnology such as salinity gradient power and ionic diodes usually involve the transport of ionic species in a system comprising a nanopore connecting two large reservoirs. The charge properties on the nanopore surface plays a key role, and they need to be estimated by fitting a mathematical model for the system to measurable quantities such as ionic current or conductance. This model can also be used to simulate the system behavior under various conditions. However, the large difference between the linear size of a nanopore and that of a reservoir makes relevant analyses difficult. Considering numerical efforts, the impact of the computational domain for the reservoir geometry and size on the system behavior is almost always overlooked in previous studies, where the computational domain for a reservoir is often assumed to have a relatively small size. Taking salinity gradient ionic current as an example, we show for the first time that the performance of a reservoir-nanopore-reservoir system is influenced appreciably by the computational domain for the reservoir geometry and size, especially when a voltage bias is not applied. Using the reported experimental data for the osmotic current in a single boron nitride nanopore, we show that its surface charge density can be estimated realistically by choosing an appropriate computational domain for reservoir geometry and size. Numerical simulation also reveals that choosing appropriate reservoir geometry and size is necessary; otherwise, the results obtained might be unreliable, or even misleading. To avoid this, we suggest that for the nanopore with the pore length smaller than 1000 nm, the size of the computational domain of a reservoir, (length × radius), with equal length and radius, should exceed 800 × 800 nm.

Electrophoresis of pH-regulated nanoparticles: impact of the Stern layer.

A multi-ion model taking into account the Stern layer effect and the surface chemistry reactions is developed for the first time to investigate the surface charge properties and electrophoresis of pH-regulated silica nanoparticles (NPs). The applicability of the model is validated by comparing its prediction to the experimental data of the electrophoretic mobility of silica NPs available from the literature. Results show that if the particle size is fixed, the Stern layer effect on the surface charge properties of the NP is notable at high pH and background salt concentration; however, that effect on the particle mobility is significant when pH is around neutrality and the salt concentration is medium high (ca. 0.07 M) because of the double-layer polarization effect. Moreover, if pH and the background salt concentration are fixed, the Stern layer effect on the zeta potential and electrophoretic mobility of the NP becomes more significant for smaller particle size. Neglecting the Stern layer effect could result in the overestimation of the zeta potential, surface charge density, and electrophoretic mobility of a NP on the order of several times.

pH-regulated ionic conductance in a nanochannel with overlapped electric double layers.

Accurately and rapidly analyzing the ionic current/conductance in a nanochannel, especially under the condition of overlapped electric double layers (EDLs), is of fundamental significance for the design and development of novel nanofluidic devices. To achieve this, an analytical model for the surface charge properties and ionic current/conductance in a pH-regulated nanochannel is developed for the first time. The developed model takes into account the effects of the EDL overlap, electroosmotic flow, Stern layer, multiple ionic species, and the site dissociation/association reactions on the channel walls. In addition to good agreement with the existing experimental data of nanochannel conductance available from the literature, our analytical model is also validated by the full model with the Poisson-Nernst-Planck and Navier-Stokes equations. The EDL overlap effect is significant at small nanochannel height, low salt concentration, and medium low pH. Neglecting the EDL overlap effect could result in a wrong estimation in the zeta potential and conductance of the nanochannel in a single measurement.

Regulating Current Rectification and Nanoparticle Transport Through a Salt Gradient in Bipolar Nanopores.

Tuning of ion and nanoparticle transport is validated through applying a salt gradient in two types of nanopores: the inner wall of a nanopore has bipolar charges and its outer wall neutral (type I), and both the inner and outer walls of a nanopore have bipolar charges (type II). The ion current rectification (ICR) behavior of these nanopores can be regulated by an applied salt gradient: if it is small, the degree of ICR in type II nanopore is more significant than that in type I nanopore; a reversed trend is observed at a sufficiently large salt gradient. If the applied salt gradient and electric field have the same direction, type I nanopore exhibits two significant features that are not observed in type II nanopore: (i) a cation-rich concentration polarization field and an enhanced funneling electric field are present near the cathode side of the nanopore, and (ii) the magnitude of the axial electric field inside the nanopore is reduced. These features imply that applying a salt gradient to type I nanopore is capable of simultaneously enhancing the nanoparticle capture into the nanopore and reducing its translocation velocity inside, so that high sensing performance and resolution can be achieved.

Tuning ion transport and selectivity by a salt gradient in a charged nanopore.

Inspired by ion channels in biological cells where the intracellular and extracellular ionic concentrations are typically different, a salt concentration gradient through a charged nanopore is proposed to actively regulate its ion transport and selectivity. Results obtained show that, in addition to the ion current rectification phenomenon, a reversed ion selectivity of the nanopore occurs when the concentration gradient is sufficiently large. In addition, if the directions of the applied concentration gradient and electric field are identical, a reversed magnified electric field occurs near the cathode side of the nanopore. This induced field can be used to enhance the capture rate of biomolecules and is therefore capable of improving the performance of single biomolecule sensing using nanopores.

Proton enhancement in an extended nanochannel.

Proton enhancement in an extended nanochannel is investigated by a continuum model consisting of three-dimensional Poisson-Nernst-Planck equations for the ionic mass transport of multiple ionic species with the consideration of surface chemistry on the nanochannel wall. The model is validated by the existing experimental data of the proton distribution inside an extended silica nanochannel. The proton enhancement behavior depends substantially on the background salt concentration, pH, and dimensions of the nanochannel. The proton enrichment at the center of the nanochannel is significant when the bulk pH is medium high (ca. 8) and the salt concentration is relatively low. The results gathered are informative for the development of biomimetic nanofluidic apparatuses and the interpretation of relevant experimental data.

Effects of lipid composition on physicochemical characteristics and cytotoxicity of vesicles composed of cationic and anionic dialkyl lipids.

The behaviors of cat-anionic vesicles composed of dioctadecyldimethylammonium bromide (DODAB) and dihexadecyl phosphate (DHP) with varying lipid composition were investigated through the measurements of size, zeta potential and fluorescence polarization, morphological observations, determination of thermotropic phase behavior, cell viability assay, and examination of entrapment efficiency and colloid stability. DODAB is miscible with DHP in the bilayer domain, which expresses a non-ideal mixing characteristic. The DODAB-rich vesicles show a smaller particle size, higher positive zeta potential, lower main transition temperature, less angular structure, better storage stability, and higher encapsulation efficiency than the DHP-rich ones. Introduction of DODAB into DHP vesicles enhances the membrane fluidity in the ripple and liquid crystalline phases. The membrane fluidity of mixed DODAB-DHP vesicles with the near charge might have a significant effect on the survival of nontransformed human skin fibroblast Hs68 cells. The degree of the cytotoxicity of Hs68 cells is dominated mainly by the charge nature of DODAB-DHP vesicles with varying lipid composition. The results gathered provide necessary information for future drug/gene delivery applications.

Programmable ionic conductance in a pH-regulated gated nanochannel.

An analytical model for the ionic conductance in a pH-regulated nanochannel gated by a field effect transistor is derived for the first time. In contrast to the existing studies, the developed model takes into account the practical effects of multiple ionic species, surface chemistry reactions, the Stern layer, and electroosmotic flow. The model is validated by the experimental data of ionic conductance available in the literature. Results show that the performance of the field effect control of the ionic conductance in the gated silica nanochannel is remarkable when the solution pH and salt concentration are low. In addition, the Stern layer effect on the ionic conductance is significant when the salt concentration is low.

Ion transport in a pH-regulated nanopore.

Fundamental understanding of ion transport phenomena in nanopores is crucial for designing the next-generation nanofluidic devices. Due to surface reactions of dissociable functional groups on the nanopore wall, the surface charge density highly depends upon the proton concentration on the nanopore wall, which in turn affects the electrokinetic transport of ions, fluid, and particles within the nanopore. Electrokinetic ion transport in a pH-regulated nanopore, taking into account both multiple ionic species and charge regulation on the nanopore wall, is theoretically investigated for the first time. The model is verified by the experimental data of nanopore conductance available in the literature. The results demonstrate that the spatial distribution of the surface charge density at the nanopore wall and the resulting ion transport phenomena, such as ion concentration polarization (ICP), ion selectivity, and conductance, are significantly affected by the background solution properties, such as the pH and salt concentration.

Electrophoresis of deformable polyelectrolytes in a nanofluidic channel.

The influence of the shape of a polyelectrolyte (PE) on its electrophoretic behavior in a nanofluidic channel is investigated by considering the translocation of a deformable ellipsoidal PE along the axis of a cylindrical nanochannel. A continuum model comprising a Poisson equation for the electric potential, Nernst-Planck equations for the ionic concentrations, and modified Stokes equations for the flow field is adopted. The effects of the PE shape, boundary, bulk ionic concentration, counterion condensation, electroosmotic retardation flow, and electroosmotic flow (EOF) on the PE mobility are discussed. Several interesting behaviors are observed. For example, if the nanochannel is uncharged and the double layer is thick, then the PE mobility increases (decreases) with increasing double-layer thickness for a smaller (larger) boundary, which has not been reported previously. If the nanochannel is negatively charged and the double layer is thick, then a negatively charged PE moves in the direction of the applied electric field. The results gathered provide necessary information for both the interpretation of experimental data and the design of nanochannel-based sensing devices.

Electrophoresis of a soft sphere in a necked cylindrical nanopore.

The influence of boundary shape on electrophoresis is modeled by considering a soft spherical particle comprising a positively charged rigid core and an uncharged membrane layer on the axis of a necked cylindrical pore with its throat positively charged. The presence of the throat makes the associated flow and electric fields nonuniform, yielding several interesting behaviors. In general, the reduction in the cross-section area of the pore intensifies the local electric field and, therefore, accelerates the particle. It also makes the ionic distribution nonuniform, and the electric field induced accelerates the particle. The maximum mobility occurs at the center of a throat, and the higher the charge density of the throat the larger the ratio of maximum mobility/mobility far away from the throat. This result is informative for the design of separation devices having variable cross sectional area.

Controlling pH-regulated bionanoparticles translocation through nanopores with polyelectrolyte brushes.

A novel polyelectrolyte (PE)-modified nanopore, comprising a solid-state nanopore functionalized by a nonregulated PE brush layer connecting two large reservoirs, is proposed to regulate the electrokinetic translocation of a soft nanoparticle (NP), comprising a rigid core covered by a pH-regulated, zwitterionic, soft layer, through it. The type of NP considered mimics bionanoparticles such as proteins and biomolecules. We find that a significant enrichment of H(+) occurs near the inlet of a charged solid-state nanopore, appreciably reducing the charge density of the NP as it approaches there, thereby lowering the NP translocation velocity and making it harder to thread the nanopore. This difficulty can be resolved by the proposed PE-modified nanopore, which raises effectively both the capture rate and the capture velocity of the soft NP and simultaneously reduces its translocation velocity through the nanopore so that both the sensing efficiency and the resolution are enhanced. The results gathered provide a conceptual framework for the interpretation of relevant experimental data and for the design of nanopore-based devices used in single biomolecules sensing and DNA sequencing.