Low Concentration Side Research Articles

Segmental-porosity regulation of nanochannel membranes with anomalously improved ion selectivity for enhanced osmotic power generation

Nanochannel-based osmotic energy conversion can directly convert salinity gradient energy between solutions with different ion concentrations into electric energy. In the energy conversion process, the high-concentration side of the nanochannel membrane has high ion permeability but low ion selectivity, whereas the low-concentration side exhibits opposite properties, which is the inherent limitation of osmotic power. However, the relationship between ion selectivity and permeability is unclear, which hinders the improvement of the output power density. This paper reports a segmental-porosity regulation method to break through the inherent constraints of the ion selectivity–permeability trade-off. The nanochannel membrane was divided into three regions along the ion transport direction as upstream, midstream, and downstream. The porosities of these three regions were separately regulated to optimize the structures for ion selectivity–permeability matching. When a high porosity was adopted for the downstream region and a small and mediate porosity combination was used for the upstream and midstream regions, the highest output power density was achieved, which was 78.44% higher than that of a straight nanochannel with homogeneous porosity under a 100-fold concentration ratio. The outstanding osmotic power generation performance of the optimized segmental structure was attributed to the alleviated ion concentration polarization and unexpectedly improved ion selectivity. Finally, the process and underlying mechanisms of segmental-porosity regulation are also proposed. This paper reports a novel methodology for designing the structure and porosity of nanochannel membranes to enhance osmotic power generation.

Uni-directional release of ibuprofen from an asymmetric fibrous membrane enables effective peritendinous anti-adhesion

Drug-loaded porous membranes have been deemed to be effective physicochemical barriers to separate postoperative adhesion-prone tissues in tendon healing. However, cell viability and subsequent tissue regeneration might be severely interfered with the unrestricted release and the locally excessive concentration of anti-inflammatory drugs. Herein, we report a double-layered membrane with sustained and uni-directional drug delivery features to prevent peritendinous adhesion without hampering the healing outcome. A vortex-assisted electrospinning system in combination with ibuprofen (IBU)-in-water emulsion was utilized to fabricate IBU-loaded poly-ʟ-lactic-acid (PLLA) fiber bundle membrane (PFB-IBU) as the anti-adhesion layer. The resultant highly porous structure, oleophilic and hydrophobic nature of PLLA fibers enabled in situ loading of IBU with a concentration gradient across the membrane thickness. Aligned collagen nanofibers were further deposited at the low IBU concentration side of the membrane for regulating cell growth and achieving uni-directional release of IBU. Drug release kinetics showed that the release amount of IBU from the high concentration side reached 79.32% at 14 d, while it was only 0.35% at the collagen side. Therefore, fibroblast proliferation at the high concentration side was successfully inhibited without affecting the oriented growth of tendon-derived stem cells at the other side. In vivo evaluation of the rat Achilles adhesion model confirmed the successful peritendinous anti-adhesion of our double-layered membrane, in that the macrophage recruitment, the inflammatory factor secretion and the deposition of pathological adhesion markers such as α-SMA and COL-III were all inhibited, which greatly improved the peritendinous fibrosis and restored the motor function of tendon.

Estimation of cancer cell migration in biomimetic random/oriented collagen fiber microenvironments

Increasing data indicate that cancer cell migration is regulated by extracellular matrixes and their surrounding biochemical microenvironment, playing a crucial role in pathological processes such as tumor invasion and metastasis. However, conventional two-dimensional cell culture and animal models have limitations in studying the influence of tumor microenvironment on cancer cell migration. Fortunately, the further development of microfluidic technology has provided solutions for the study of such questions. We utilize microfluidic chip to build a random collagen fiber microenvironment (RFM) model and an oriented collagen fiber microenvironment (OFM) model that resemble early stage and late stage breast cancer microenvironments, respectively. By combining cell culture, biochemical concentration gradient construction, and microscopic imaging techniques, we investigate the impact of different collagen fiber biochemical microenvironments on the migration of breast cancer MDA-MB-231-RFP cells. The results show that MDA-MB-231-RFP cells migrate further in the OFM model compared to the RFM model, with significant differences observed. Furthermore, we establish concentration gradients of the anticancer drug paclitaxel in both the RFM and OFM models and find that paclitaxel significantly inhibits the migration of MDA-MB-231-RFP cells in the RFM model, with stronger inhibition on the high concentration side compared to the low concentration side. However, the inhibitory effect of paclitaxel on the migration of MDA-MB-231-RFP cells in the OFM model is weak. These findings suggest that the oriented collagen fiber microenvironment resembling the late-stage tumor microenvironment is more favorable for cancer cell migration and that the effectiveness of anticancer drugs is diminished. The RFM and OFM models constructed in this study not only provide a platform for studying the mechanism of cancer development, but also serve as a tool for the initial measurement of drug screening.

Biomimetic asymmetric GO/polymer nanocomposite membrane for energy harvesting

Energy harvesting based on salinity gradients between oceans and rivers can provide a clean, stable, and continuous electric output. However, it is highly dependent on the excellent performance of biomimetic ion-selective membranes. The graphene oxide (GO) membrane, as a 2D material-based layered-structure membrane, offers the possibility of efficient salinity gradient energy conversion. However, unstable stratification in aqueous solutions and the limited number of charged functional groups make practical applications difficult. A conventional symmetric structure will also reduce the effective salinity gradient owing to ion enrichment on the low-concentration side. In this study, we pre-modified GO to create an asymmetric ion-selective membrane with opposing charges. Hyperbranched polyethyleneimine (PEI) and polyacrylic acid (PAA) as surface charge donors and crosslinkers stabilise the interlayer spacing of GO and increase charge density, achieving up to 3.4 W/m2 power density under sea water and river water. Furthermore, theoretical calculations show that membranes with asymmetric structure and opposite charges can eliminate the effect of ion enrichment on the low-concentration side, thereby avoiding power consumption. This system contributes to a further step toward the application of salinity energy conversion.

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Anomalous temperature dependence of ion transport under osmotic pressure in graphene oxide membranes

Graphene oxide (GO) membranes have attracted broad interest because of their unique mass transport properties. Towards the controllable ionic transport in GO membranes, physical fields or external driving forces are induced to control the behavior of ionic migration in situ. However, the adjustable ionic transport regulated by temperature and osmotic pressure in GO materials is still absent. Herein, we report the anomalous temperature dependence of ion transport under osmotic pressure in GO membranes. The ions can diffuse spontaneously along the concentration gradient or the temperature gradient. Intriguingly, it is found that the reverse temperature difference can promote ion transport driven by osmotic pressure. Theoretical analysis reveals that the anomalous temperature dependence of ion transport stems from the thermal-diffusion-assisted ion concentration polarization (ICP). The high temperature in the low-concentration side largely enhances the ionic thermal diffusion and suppresses the ICP, which eventually strengthens the ion current along the concentration gradient. The finding can be developed into the temperature sensor for aqueous solutions and bring inspiration to the application involving ion transport under thermodynamic and osmotic driven forces.

Impacts of transmembrane pH gradient on nanofluidic salinity gradient energy conversion

Solution pH can impact the nanochannel surface charge density, thus, to regulate the transmembrane ion transportation. Here, the performance of nanofluidic salinity gradient energy conversion is investigated, where the solution pH at the high/low concentration side varies separately, rendering asymmetric pH configurations. Results reveal that the solution pH at the low concentration side (pHL) exhibits a significant impact on the salinity gradient energy conversion. When pHL < Isoelectric Point (IEP), the energy conversion indicators are most stable under various solution pH at the high concentration side (pHH). The surface charge density of the nanochannel at the low concentration end determines the transmembrane ion transportation characteristics, dominating the energy conversion performance. The energy conversion performance via nanochannels of different lengths under asymmetric pH configurations is also studied. There exists optimal nanochannel length corresponding to the maximum electric power, which differs under various pH configurations. For longer nanochannels, the ion concentration polarization effect is weakened, allowing for a more effective energy extraction performance via pH-adjusted salinity gradients. At the length of 2000 nm, the electric power and energy conversion efficiency under the co-direction configuration is 78% and 97.9%, respectively, larger than those under the reverse direction configuration.

Surface Charge Regulated Asymmetric Ion Transport in Nanoconfined Space.

The asymmetric ion transport in the nanoconfined space, similar to that of natural ion channels, has attracted broad interest in sensor, energy conversion, and other related fields. Among these systems, the surface charge plays an important role in regulating ion transport behaviors. Herein, this surface charge-regulated asymmetric ion transport behavior is systematically explored in the nanoconfined space and the influence on the performance of nanofluidic energy conversion system. The ion transport behaviors in the nanoconfined space are classified into pure diffusion, electrical double layer, and the polarization controlled state. The asymmetric solution environment or surface charge distribution induces asymmetric ion transport behavior which is largely controlled by the low concentration side. The ion-selectivity and the energy conversion performance can be effectively enhanced by improving the local apparent surface charge (more active sites and higher charge strength) or introducing a selective layer with dense surface charge on the low concentration side. These material design concepts for asymmetric ion transport are further supported by both simulation and experiment. The results provide a significant comprehension for ion behaviors in nanoconfined space and the development of high-performance energy storage and conversion systems.

High-performance nanofluidic osmotic power generation enabled by exterior surface charges under the natural salt gradient

High-performance osmotic energy conversion (OEC) requires both high ionic selectivity and permeability in nanopores. Here, through systematical explorations of influences from individual charged nanopore surfaces on the performance of OEC, we find that the charged exterior surface on the low-concentration side (surfaceL) is essential to achieve high-performance osmotic power generation, which can significantly improve the ionic selectivity and permeability simultaneously. Detailed investigation of ionic transport indicates that electric double layers near charged surfaces provide high-speed passages for counterions. The charged surfaceL enhances cation diffusion through enlarging the effective diffusive area, and inhibits anion transport by electrostatic repulsion. Different areas of charged exterior surfaces have been considered to mimic membranes with different porosities in practical applications. Through adjusting the width of the charged ring region on the surfaceL, electric power in single nanopores increases from 0.3 to 3.4 pW with a plateau at the width of ~200 nm. The power density increases from 4200 to 4900 W/m2 and then decreases monotonously that reaches the commercial benchmark at the charged width of ~480 nm. While, energy conversion efficiency can be promoted from 4% to 26%. Our results provide useful guide in the design of nanoporous membranes for high-performance osmotic energy harvesting.

Ionic thermal up-diffusion in nanofluidic salinity-gradient energy harvesting.

Advances in nanofabrication and materials science give a boost to the research in nanofluidic energy harvesting. Contrary to previous efforts on isothermal conditions, here a study on asymmetric temperature dependence in nanofluidic power generation is conducted. Results are somewhat counterintuitive. A negative temperature difference can significantly improve the membrane potential due to the impact of ionic thermal up-diffusion that promotes the selectivity and suppresses the ion-concentration polarization, especially at the low-concentration side, which results in dramatically enhanced electric power. A positive temperature difference lowers the membrane potential due to the impact of ionic thermal down-diffusion, although it promotes the diffusion current induced by decreased electrical resistance. Originating from the compromise of the temperature-impacted membrane potential and diffusion current, a positive temperature difference enhances the power at low transmembrane-concentration intensities and hinders the power for high transmembrane-concentration intensities. Based on the system's temperature response, we have proposed a simple and efficient way to fabricate tunable ionic voltage sources and enhance salinity-gradient energy conversion based on small nanoscale biochannels and mimetic nanochannels. These findings reveal the importance of a long-overlooked element—temperature—in nanofluidic energy harvesting and provide insights for the optimization and fabrication of high-performance nanofluidic power devices.

High-efficiency power generation in hyper-saline environment using conventional nanoporous membrane

Here we introduce the new approach to high-efficiency power generation from a salinity difference using conventional nanoporous Nafion membrane. When access areas on each side of nanoporous Nafion membrane are set to be asymmetric, the ratio of ionic current upon a voltage bias of the different polarity also becomes asymmetric, resulting in ionic diode phenomena. When this geometrical ionic diode effect is combined with a salinity gradient, it can help significantly improve the energy conversion efficiency from a salinity difference even under a hyper-saline environment with a large salinity difference, e.g. ∼41% conversion efficiency and ∼120 nW power generation with 1 M KCl and 1000-fold salinity difference, both of which are comparable with the best performances reported in the previous studies. We propose that the decrease in ion concentration polarization at a low salt concentration side is responsible for the enhanced power generation with the membrane having asymmetric access areas. Our approach is simple to implement and can be applicable to any nanoporous membrane to enhance the power generation from a salinity difference.

&lt;i&gt;In Situ&lt;/i&gt; XRD Experiments on the Growth of Expanded Austenite Using Different Process Gases

In this work we show our result of in-situ nitrocarburizing and nitriding treatments AISI316L specimens. Part of the samples have been depassivated ex-situ and coated with a Ni layer, while other specimens received in-situ depassivation. Processing was carried out in a custom built reaction chamber attached to a Bruker D8 Advance diffractometer. We monitored the 111 peak of both the base material and expanded austenite. From the shrinkage of the base material peak the total thickness of the expanded austenite can be determined. Applying both N and C resulted in a more than 10 times faster growth of the expanded austenite than with N only. The growth is thermally activated. The activation energy for nitrocarburizing is 164 kJ/mol. This is in agreement with the activation energy of the diffusion of interstitials. Detailed analysis of the expanded austenite peak allowed the derivation of a “master curve” for the composition depth profile. This suggest that two interacting process controls the evolution. The width of the reaction zone is limited by the diffusion at low concentration side. The total concentration is determined by the reaction at the interface.

On the origin of membrane potential in membranes with polarizable nanopores

We report a new mechanism for the generation of membrane potential in polarizable nanoporous membranes separating electrolytes with different concentrations. The electric field generated by diffusion of ions with different mobilities induces a non–uniform surface charge, which results in charge separation inside the nanopore. The corresponding Donnan potentials appear at the pore entrance and exit leading to a dramatic enhancement of membrane potential in comparison with an uncharged non–polarizable membrane. At high concentration contrast, the interaction between electric field and uncompensated charge at a low concentration side results in the development of electrokinetic vortices. The theoretical predictions are based on the Space–Charge model, which is extended to nanopores with polarizable conductive surface for the first time. This model is validated against full Navier–Stokes, Nernst–Planck, and Poisson equations, which are solved in a high aspect ratio nanopore connecting two reservoirs. The experimental measurements of membrane potential of dielectric and conductive membranes in KCl and NaCl aqueous solutions confirm the theoretical results. The membranes are prepared from Nafen nanofibers with ∼10nm in diameter and modified by depositing a conductive carbon layer. It is shown theoretically that the membrane potential enhancement becomes greater with decreasing the electrolyte concentration and pore radius. A high sensitivity of membrane potential to the ratio of ion diffusion coefficients is demonstrated. The described phenomenon may find applications in precise determination of ion mobilities, electrochemical and bio–sensing, as well as design of nanofluidic and bioelectronic devices.

Growth of Si0.5Ge0.5 Single Crystals by the Traveling Liquidus-zone Method and their Structural Characterization

We have succeeded in growing compositionally homogeneous Si0.5Ge0.5 crystals with a newly developed growth method named the travelling liquidus-zone (TLZ) method. In this method, a narrow liquidus-zone saturated by a solute is formed at relatively low temperature gradients, 5 – 15°C/cm. Since the solubility depends on temperature, solute concentration gradient is established at the freezing interface. The concentration gradient causes diffusion of the solute towards a low concentration side. At the freezing interface, crystal growth proceeds due to the decrease in solute. As a result, solute concentration increases at the opposite side of the zone by transported solute and part of the remaining feed is dissolved. Thus, the zone travels under the temperature gradient spontaneously. Here, we report on 30mm diameter homogeneous Si0.5Ge0.5 crystal growth by the TLZ method. Compositional homogeneity of grown crystals was excellent but the length of single crystal was limited to about 2mm and origins of polycrystallization were discussed.

Diffusiophoresis of a nonuniformly charged sphere in an electrolyte solution

The diffusiophoresis of a rigid, nonuniformly charged spherical particle in an electrolyte solution is analyzed theoretically focusing on the influences of the thickness of double layer, the surface charge distribution, the effect of electrophoresis, and the effect of double-layer polarization. We show that the nonuniform charge distribution on the particle surface yields complicated effect of double-layer polarization, leading to interesting diffusiophoretic behaviors. For example, if the sign of the middle part of the particle is different from that of its left- and right-hand parts, then depending upon the charge density and the fraction of the middle part, the particle can move either to the high-concentration side or to the low-concentration side. Both the diffusiophoretic velocity and its direction can be manipulated by the distribution of the surface charge density. In particular, if the electrophoresis effect is significant, then those properties are governed by the averaged surface charge density of the particle. A dipolelike particle, where its left- (right-) hand half is negatively (positively) charged, always migrates toward the low-concentration (left-hand) side, that is, it has a negative diffusiophoretic velocity. In addition, that diffusiophoretic velocity has a negative local minimum as the thickness of double layer varies.

Entransy dissipation minimization for a class of one-way isothermal mass transfer processes

A class of one-way isothermal mass transfer processes with Fick’s diffusive mass transfer law [g ∝ Δ(c)] is investigated in this paper. Based on the definition of the mass entransy, the entransy dissipation function which reflects the irreversibility of mass transfer ability loss is derived. The optimal concentration allocations of the key components corresponding to the highand low-concentration sides for the minimum entransy dissipation of the mass transfer process are obtained by applying optimal control theory and compared with the strategies of the minimum entropy generation, constant mass transfer flux (constant concentration difference), and constant concentration ratio (constant chemical potential difference). The results are as follows. For the optimal mass transfer strategy of the minimum entransy dissipation, the product of the square of the key component concentration difference between the high- and the low-concentration sides and the inert component concentration at the low-concentration side is a constant, while for that of the minimum entropy generation, the ratio of the square of the key component concentration difference between the high- and the low-concentration sides to the key component concentration at the low-concentration side is a constant; when the mass transfer process is not involved in energy conversion process, the optimization principle should be the minimum entransy dissipation; the mass transfer strategy of constant concentration difference is superior to that of constant concentration ratio. The results obtained in this paper can provide some theoretical guidelines for optimal designs and operations of practical mass transfer processes.

Diffusiophoresis of an Ellipsoid along the Axis of a Cylindrical Pore

The influences of a boundary and the shape of a particle on its diffusiophoretic behavior are examined by considering the diffusiophoresis of a charged ellipsoid along the axis of an uncharged cylindrical pore filled with electrolyte solutions. The diffusiophoretic mobility of the particle under various conditions is evaluated through varying the particle aspect ratio, the size of the particle, the thickness of the double layer, the diffusivities of co-ions and counterions, and the level of the surface potential. Because the double-layer polarization, the electrophoresis effect, the boundary effect, and the electrical interaction between the particle and the co-ions outside the double layer all play a role, the diffusiophoretic behavior of the particle is complicated. For a fixed particle volume, the relative magnitude of the diffusiophoretic mobilities of particles of various shapes depends highly on the degree of the boundary effect. It is possible for a particle to migrate toward the low-concentration side, regardless of the level of the surface potential, which is not observed in the case of a sphere in a planar slit or in a spherical cavity.

Protrusion and rising of a gel-based precipitation layer

Two types of unstable growth of a precipitation layer in gel are discussed. A cation and an anion that are reactive diffuse from opposite ends of the gel to its center. A white turbid zone forms due to their reactions. When the concentration ratios for both the ions are far from stoichiometry, the turbid zone expands toward the lower-concentration side. However, when the ratio is nearly stoichiometric, unstable growth occurs. In a glass tube, a protrusion of the precipitation region from the turbid zone grows, which forms a long needle-like shape. When a free surface is present on the gel, the precipitation region protrudes from the gel surface to form a rising structure. Mapping the growing structure on a concentration diagram and using scanning electron microscopy to examine contained particles suggest that the reaction is restricted to a narrow region and the reaction product migrates through a path formed in the protrusive structure to form a bulk solid at the edge.

Diffusiophoresis of a sphere along the axis of a cylindrical pore

The diffusiophoresis of a charged spherical particle along the axis of an uncharged cylindrical pore filled with an electrolyte solution is analyzed theoretically. The influence of chemiphoresis, which includes two types of double-layer polarization, and that of electrophoresis arising from the difference in the diffusivities of the ionic species on the diffusiophoretic behavior of the particle are discussed. We have underlined the important difference between two cases: the first is a possibility for a particle to migrate to the low concentration side at low surface potentials (∼25 mV) along the axis of a cylindrical pore, while the second is that this migration occurs at high surface potentials (∼150 mV) in the case of a sphere in a spherical cavity.

Hard Sphere Diffusion Behaviour of Polymer Translocating through Interacting Pores

The translocation of polymer chain through a small pore from a high concentration side (cis side) to a low concentration side (trans side) is simulated by using Monte Carlo technique. The effect of the polymer-pore interaction on the translocation is studied. We find a special interaction at which the decay of the number of polymer chain, N, at the cis side obeys Fick's law, i.e. N decreases exponentially with time. The behaviour is analogous to the diffusion of hard sphere.