Effect of charge density distribution of polyelectrolyte layer on electroosmotic flow and ion selectivity in a conical soft nanochannel

Ion Transport in Intelligent Nanochannels: A Comparative Analysis of the Role of Electric Field

This research delves into investigating ion transport behavior within nanochannels, enhanced through modification with a negatively charged polyelectrolyte layer (PEL), aimed at achieving superior control. The study examines two types of electric fields─direct current and alternating current with square, sinusoidal, triangular, and sawtooth waveforms─to understand their impact on ion transport. Furthermore, the study compares symmetric (cylindrical) and asymmetric (conical) nanochannel geometries to assess the influence of overlapping electrical double layers (EDLs) in generating specific electrokinetic behaviors such as ionic current rectification (ICR) and ion selectivity. The research employs the finite element method to solve the coupled Poisson-Nernst-Planck and Navier-Stokes equations under unsteady-state conditions. By considering factors such as electrolyte concentration, soft layer charge density, and electric field type, the study evaluates ion transport performance in charged nanochannels, investigating effects on concentration polarization, electroosmotic flow (EOF), ion current, rectification, and ion selectivity. Notably, the study accounts for ion partitioning between the PEL and electrolyte to simulate real conditions. Findings reveal that conical nanochannels, due to improved EDL overlap, significantly enhance ion transport and related characteristics compared to cylindrical ones. For instance, under ηε = ηD = 0.8, ημ = 2, C0 = 20 mM, and NPEL/NA = 80 mol m-3 conditions, the average EOF for conical and cylindrical geometries is 0.1 and 0.008 m/s, respectively. Additionally, the study explores ion selectivity and rectification based on the electric field type, unveiling the potential of nanochannels as ion gates or diodes. In cylindrical nanochannels, the ICR remains at unity, with lower ion selectivity across waveforms compared to conical channels. Furthermore, rectification and ion selectivity trends are identified as Rf,square > Rf,DC > Rf,triangular > Rf,sinusoidal > Rf,sawtooth and Ssawtooth > Ssinusoidal > Striangular > SDC > Ssquare for conical nanochannels. Our study of ion transport control in nanochannels, guided by tailored electric fields and unique geometries, offers versatile applications in the field of Analytical Chemistry. This includes enhanced sample separation, controlled drug delivery, optimized pharmaceutical analysis, and the development of advanced biosensing technologies for precise chemical analysis and detection. These applications highlight the diverse analytical contributions of our methodology, providing innovative solutions to challenges in chemical analysis and biosensing.

Smart nanochannels: tailoring ion transport properties through variation in nanochannel geometry

This research explores ion transport behavior and functionality in a hybrid nanochannel that consists of two conical and cylindrical parts. The numerical investigation focuses on analyzing the length of each part in the nanochannel. The nanochannels are hybrid cavities embedded in a membrane, where the size of the conical part varies as equal to, larger than, or smaller than the cylindrical part. The nanochannel is coated with a polyelectrolyte layer that exhibits a dense charge density distribution. The charge density of the soft layer is described using the soft step distribution function. We study the electroosmotic flow, ionic current, rectification, and selectivity of the nanochannel versus bulk electrolyte concentration, the charge density of the polyelectrolyte layer, and decay length, while considering the effect of ionic partitioning. The steady-state Poisson-Nernst-Planck and Navier-Stokes equations are solved using the finite element method. The findings reveal that the nanochannel with a more extensive conical section demonstrates increased rectification, with the rectification factor rising from 1.4 to 2 at a bulk concentration of 100 mM. Additionally, the nanochannel with a longer cylindrical part exhibits improved selectivity under negative voltage conditions, while positive voltage introduces a different situation. The nanochannel with equal cylindrical and conical parts significantly affects conductivity by modifying the charge density in the soft layer, resulting in a 3.125-fold increase in conductivity under positive voltage when the charge density in the polyelectrolyte layer is raised from 25 to 100 mol m-3. This research focuses on creating intelligent nanochannels by controlling mass concentration, charge density, and collapse length, improving system performance, and optimizing properties. It also offers valuable insights into ion transport mechanisms in nanochannel systems, advancing our understanding in this field.

Layer-by-Layer Nanofluidic Membranes for Promoting Blue Energy Conversion

Access to and use of energy resources are now crucial components of modern human existence thanks to the exponential growth of technology. Traditional energy sources provide significant challenges, such as pollution, scarcity, and excessive prices. As a result, there is more need than ever before to replace depleting resources with brand-new, reliable, and environmentally friendly ones. With the aid of reverse electrodialysis, the salinity gradient between rivers and seawater as a clean supply with easy and infinite availability is a viable choice for energy generation. The development of nanofluidic-based reverse electrodialysis (NRED) as a novel high-efficiency technology is attributable to the progress of nanoscience. However, understanding the predominant mechanisms of this process at the nanoscale is necessary to develop and disseminate this technology. One viable option to gain insight into these systems while saving expenses is to employ simulation tools. In this study, we looked at how a layer-by-layer (LBL) soft layer influences ion transport and energy production in charged nanochannels. We solved the steady-state Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations for three different types of nanochannels with a trumpet geometry, where the narrow part is covered with a built-up LbL soft layer and the rest is a hard wall with a surface charge density of σ = -10, 0, or +10 mC/m2. The findings show that in type (I) nanochannels, at NPEL/NA = 100 mol/m3 and pH = 7, the maximum power output rises 675-fold as the concentration ratio rises from 10 to 1000. The results of this study can aid in a better understanding of energy harvesting processes using nanofluidic-based reverse electrodialysis in order to identify optimal conditions for the design of an intelligent route with great controllability and minimal pollution.

Salinity Gradient-Induced Power Generation in Nanochannels: The Role of pH-Sensitive Polyelectrolyte Layers

By varying the pH values (pHR) and types of salt solution, we investigate the salinity gradient-induced electrical and mechanical flow energies inside a reservoir-connected charged nanochannel with a grafted pH-sensitive polyelectrolyte layer (PEL) on the inner surfaces. The aqueous solutions of KCl, LiCl, BaCl2, BeCl2, AlCl3, and Co(en)3Cl3 salts are used as the working fluid in the current investigation. We examine the associated ionic transport and flow field, aiming to understand the underlying physics behind the generation of electrical and hydraulic energy through alterations in pHR and types of salt solution. Our results reveal that the PEL space charge density decreases with increasing pHR at lower values, while it remains almost insensitive to higher pHR values. The electrical conductance and maximum pore power of the Co(en)3Cl3 solution are significantly higher compared to salts with monovalent and divalent cations. Furthermore, the magnitude of these two parameters decreases with lower pHR and becomes insensitive to higher pHR values. The results illustrate that the maximum electrical energy conversion efficiency enhances with pHR, reaching its highest level for the Co(en)3Cl3 solution. We expect that the findings of the current work will have a significant bearing on the design and development of a state-of-the-art salinity gradient-based energy convertor as a potential candidate for renewable energy sources.

Enhanced Ionic Current Rectification through Innovative Integration of Polyelectrolyte Bilayers and Charged-Wall Smart Nanochannels

The tools utilized by humans continue to shrink and speed up. Lab-on-a-chip (LOC) is one of the most recent techniques for decreasing the size of chemical systems. Today, LOCs have made substantial strides in developing nanomaterial fabrication techniques. Controlling and regulating the fluid and ion mobility in these systems is crucial. Layer-by-layer (LBL) soft layers are one of the most effective strategies for controlling fluid flow in channels. In light of the present constraints for developing these systems and the high expense of experimental investigations, it is vital to employ modeling to minimize costs and comprehend their underlying ideas and operations. In this study, we examined the influence of the LBL soft layer's presence in the charged nanochannels on the ion transport parameters. To examine the effect of the coating length of the LBL soft layer, we first examined three lengths of coating: one with a length greater than half (type (I)), one with a length equal to half (type (II)), and one with a length less than half (type (III)) of the nanochannel length. Then, by solving Poisson-Nernst-Planck and Navier-Stokes equations, we determined the influences of pH, soft layer charge density (N_PEL/N_A), bulk concentration (C₀), and hard surface charge density (σ) on the ionic current rectification (R_f) and selectivity (S) of the nanochannel. The maximum rectification of 30.65 was achieved using a nanochannel of type (III) and σ = +10 mC/m². The current results demonstrate a promising hybrid architecture consisting of an LBL soft layer and a smart charged nanochannel for enhanced rectification.