Tunable Anion-Selective Transport through Monolayer Graphene and Hexagonal Boron Nitride

Angstrom-Scale 2D Channels Designed For Osmotic Energy Harvesting

Confronting the impending exhaustion of traditional energy, it is urgent to devise and deploy sustainable clean energy alternatives. Osmotic energy contained in the salinity gradient of the sea-river interface is an innovative, abundant, clean, and renewable osmotic energy that has garnered considerable attention in recent years. Inspired by the impressively intelligent ion channels in nature, the developed angstrom-scale 2D channels with simple fabrication process, outstanding design flexibility, and substantial charge density exhibit excellent energy conversion performance, opening up a new era for osmotic energy harvesting. However, this attractive research field remains fraught with numerous challenges, particularly due to the complexities associated with the regulation at angstrom scale. In this review, the latest advancements in the design of angstrom-scale 2D channels are primarily outlined for harvesting osmotic energy. Drawing upon the analytical framework of osmotic power generation mechanisms and the insights gleaned from the biomimetic intelligent devices, the design strategies are highlighted for high-performance angstrom channels in terms of structure, functionalization, and application, with a particular emphasis on ion selectivity and ion transport resistance. Finally, current challenges and future prospects are discussed to anticipate the emergence of more anomalous properties and disruptive technologies that can promote large-scale power generation.

Interpreting the power spectral density of a fluctuating colloidal current

The transport of molecules through biological and synthetic nanopores is governed by multiple stochastic processes that lead to noisy, fluctuating currents. Disentangling the characteristics of different noise-generating mechanisms is central to better understanding molecular transport at a fundamental level but is extremely challenging in molecular systems due to their complexity and relative experimental inaccessibility. Here, we construct a colloidal model microfluidic system for the experimental measurement of particle currents, where the governing physical properties are directly controllable and particle dynamics directly observable, unlike in the molecular case. Currents of hard spheres fluctuate due to the random arrival times of particles into the channel and the distribution of particle speeds within the channel, which results in characteristic scalings in the power spectral density. We rationalize these scalings by quantitatively comparing to a model for shot noise with a finite transit time, extended to include the distribution of particle speeds. Particle velocity distributions sensitively reflect the confining geometry, and we interpret and model these in terms of the underlying fluid flow profiles. Finally, we explore the extent to which details of these distributions govern the form of the resulting power spectral density, thereby establishing concrete links between the power spectral density and underlying mechanisms for this experimental system. This paves the way for establishing a more systematic understanding of the links between characteristics of transport fluctuations and underlying molecular mechanisms in driven systems such as nanopores.

Giant Blue Energy Harvesting in Two-Dimensional Polymer Membranes with Spatially Aligned Charges

Blue energy between seawater and river water is attracting increasing interest, as one of the sustainable and renewable energy resources that can be harvested from water. Within the reverse electrodialysis applied in blue energy conversion, novel membranes with nanoscale confinement that function as selective ion transport mediums are currently in high demand for realizing higher power density. The primary challenge lies in constructing well-defined nanochannels that allow for low-energy barrier transport. This work proposes a concept for nanofluidic channels with a simultaneous dual electrostatic effect that can enhance both ion selectivity and flux. To actualize this, this work has synthesized propidium iodide-based two-dimensional polymer (PI-2DP) membranes possessing both skeleton charge and intrinsic space charge, which are spatially aligned along the ion transport pathway. The dual charge design of PI-2DP significantly enhances the electrostatic interaction between the translocating anions and the cationic polymer framework, and a high anion selectivity coefficient (≈0.8) is reached. When mixing standard artificial seawater and river water, this work achieves a considerable power density of 48.4 W m-2, outperforming most state-of-the-art nanofluidic membranes. Moreover, when applied between the Mediterranean Sea and the Elbe River, an output power density of 42.2 W m-2 is achieved by the PI-2DP. This nanofluidic membrane design with dual-layer charges will inspire more innovative development of ion-selective channels for blue energy conversion that will contribute to global energy consumption.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Multifunctional graphene heterogeneous nanochannel with voltage-tunable ion selectivity

Ion-selective nanoporous two-dimensional (2D) materials have shown extraordinary potential in energy conversion, ion separation, and nanofluidic devices; however, different applications require diverse nanochannel devices with different ion selectivity, which is limited by sample preparation and experimental techniques. Herein, we develop a heterogeneous graphene-based polyethylene terephthalate nanochannel (GPETNC) with controllable ion sieving to overcome those difficulties. Simply by adjusting the applied voltage, ion selectivity among K⁺, Na⁺, Li⁺, Ca²⁺, and Mg²⁺ of the GPETNC can be immediately tuned. At negative voltages, the GPETNC serves as a mono/divalent ion selective device by impeding most divalent cations to transport through; at positive voltages, it mimics a biological K⁺ nanochannel, which conducts K⁺ much more rapidly than the other ions with K⁺/ions selectivity up to about 4.6. Besides, the GPETNC also exhibits the promise as a cation-responsive nanofluidic diode with the ability to rectify ion currents. Theoretical calculations indicate that the voltage-dependent ion enrichment/depletion inside the GPETNC affects the effective surface charge density of the utilized graphene subnanopores and thus leads to the electrically controllable ion sieving. This work provides ways to develop heterogeneous nanochannels with tunable ion selectivity toward broad applications.

Nanoporous 2D materials have shown promising potential for ion sieving applications due to their physical and chemical properties. Here authors develop a heterogeneous graphene-based polyethylene terephthalate nanochannel with ion sieving ability that is controlled by adjusting the applied voltage.

Cation-selective two-dimensional polyimine membranes for high-performance osmotic energy conversion

Two-dimensional (2D) membranes are emerging candidates for osmotic energy conversion. However, the trade-off between ion selectivity and conductivity remains the key bottleneck. Here we demonstrate a fully crystalline imine-based 2D polymer (2DPI) membrane capable of combining excellent ionic conductivity and high selectivity for osmotic energy conversion. The 2DPI can preferentially transport cations with Na⁺ selectivity coefficient of 0.98 (Na⁺/Cl^- selectivity ratio ~84) and K⁺ selectivity coefficient of 0.93 (K⁺/Cl^- ratio ~29). Moreover, the nanometer-scale thickness (~70 nm) generates a substantially high ionic flux, contributing to a record power density of up to ~53 W m^-2, which is superior to most of nanoporous 2D membranes (0.8~35 W m^-2). Density functional theory unveils that the oxygen and imine nitrogen can both function as the active sites depending on the ionization state of hydroxyl groups, and the enhanced interaction of Na⁺ versus K⁺ with 2DPI plays a significant role in directing the ion selectivity.

High-Performance Osmotic Power Generators Based on the 1D/2D Hybrid Nanochannel System

Extracting clean energy by converting the salinity gradient between river and sea into energy is an effective way to reduce the global pollution and carbon emissions. Reverse electrodialysis (RED) is of great importance to realize the energy conversion assisting the ion-selective membrane. However, its higher ion resistance and lower conversion efficiency results in the undesirable power conversion performance. Here, we demonstrate a 1D/2D hybrid nanochannel system to achieve high osmotic energy conversion and output power. This heterogeneous structure is composed of two structures, in which the subnanometer nanochannels in graphene oxide membrane (GOM) can serve as a selective layer and reduce the ion diffusion energy barrier, while the nanochannel in the polymer can introduce asymmetry to enhance ionic rectification and conversion efficiency. This heterogeneous membrane exhibits excellent cation selectivity and enhanced ionic current rectification (ICR) performance. The application of the GOM/PET hybrid nanochannel system in osmotic energy harvesting is evaluated, and the output power can reach up to 118.2 pW with the energy conversion efficiency of 40.3%. Theoretical calculation indicates that the 1D/2D hybrid system can effectively take the advantage of excellent cation selectivity of 2D lamellar nanochannels to improve its RED performance significantly.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an "electrochemical ion (proton) pump cell" mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20-60 localized sites across a ≈0.003 mm² area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites "opening" and a small number of sites "closing" under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure-property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Cation-Gated Ion Transport at Nanometer Scale for Tunable Power Generation

Gated ion channels in biological cell membranes allow efficient tuning of cross-membrane ion transport with enhanced permeation and selectivity, converting ionic signals into various forms of electrical signals and energies on demands, which functionalities though are still difficult to achieve in artificial membranes. Here, we report cation-gated ion transport through synthesized porous aromatic films containing nanometer-scale ionic channels together with -NH₂ groups at interiors. Ion selectivity and permeability is greatly tuned by gating cations, up to 2 orders of magnitude, and as a consequence, the membrane efficiently produces switchable electricity output from salinity gradients. The results are attributed to positively charged cations binding at -NH₂ groups, which screens the intrinsic negative surface charge at channels' interiors and inverts charge polarity there. Our work adds understanding to ion gating effects at nanoscale and offers strategies of developing smart membranes and their heterostructures for separation, energy conversion, cell membrane mimics, and related technologies.

Kinetics of high density functional polymer nanocomposite formation by tuning enthalpic and entropic barriers

High density functional polymer nanocomposites (PNCs) with high degree of dispersion have recently emerged as novel materials for various thermo-mechanical, optical and electrical applications. The key challenge is to attain a high loading while maintaining reasonable dispersion to attain maximum possible benefits from the functional nanoparticle additives. Here, we report a facile method to prepare polymer grafted nanoparticle (PGNP)-based high density functional polymer nanocomposites using thermal activation of a high density PGNP monolayer to overcome entropic or enthalpic barriers to insertion of PGNPs into the underlying polymer films. We monitor the temperature-dependent kinetics of penetration of a high density PGNP layer and correlate the penetration time to the effective enthalpic/entropic barriers. The experimental results are corroborated by coarse-grained molecular dynamics simulations. Repeated application of the methodology to insert nanoparticles by appropriate control over temperature, time and graft-chain properties can lead to enhanced densities of loading in the PNC. Our method can be engineered to produce a wide range of high density polymer nanocomposite membranes for various possible applications including gas separation and water desalination.

Anion-Selective Layered Double Hydroxide Composites-Based Osmotic Energy Conversion for Real-Time Nutrient Solution Detection

Nanofluidic channels based on 2D nanomaterials are promising to harvest osmotic energy for their high ion selectivity and osmotic conductivity. However, anion-selective nanofluidic channels are rare and chemical modification is necessary through fabrication. Here, a naturally anion-selective composite membrane is reported, that is, NiAl-Layered double hydroxide (LDH) coated anodic aluminum oxide (LDH@AAO), using a simple precipitant-free in situ growth technique. Positively charged LDH plates growing in channels of AAO function as screening layers for anions. Both experiments and theoretical simulations are enforced to certify the vital role of LDH growth in ion distribution and salinity gradient energy conversion. The composite membrane achieves high output performance and long-term stability. Furthermore, novel applications of nanofluidic channels are explored in hydroponic production and design a real-time detecting system based on LDH@AAO composite membranes for nutrient solution. This work provides insights into naturally anion-selective nanofluidic channels for osmotic energy harvesting and broadens the application in agricultural information sensing.

Self-Limited Formation of Bowl-Shaped Nanopores for Directional DNA Translocation

Solid-state nanopores of on-demand dimensions and shape can facilitate desired sensor functions. However, reproducible fabrication of arrayed nanopores of predefined dimensions remains challenging despite numerous techniques explored. Here, bowl-shaped nanopores combining properties of ultrathin membrane and tapering geometry are manufactured using a self-limiting process developed on the basis of standard silicon technology. The upper opening of the bowl-nanopores is 60-120 nm in diameter, and the bottom orifice reaches sub-5 nm. Current-voltage characteristics of the fabricated bowl-nanopores display insignificant rectification indicating weak ionic selectivity, in accordance to numerical simulations showing minor differences in electric field and ionic velocity upon the reversal of bias voltages. Simulations reveal, concomitantly, high-momentum electroosmotic flow downward along the concave nanopore sidewall. Collisions between the left and right tributaries over the bottom orifice drive the electroosmotic flow both up into the nanopore and down out of the nanopore through the orifice. The resultant asymmetry in electrophoretic-electroosmotic force is considered the cause responsible for the experimentally observed strong directionality in λ-DNA translocation with larger amplitude, longer duration, and higher frequencies for the downward movements from the upper opening than the upward ones from the orifice. Thus, the resourceful silicon nanofabrication technology is shown to enable nanopore designs toward enriching sensor applications.

Rational ion transport management mediated through membrane structures

Unique membrane structures endow membranes with controlled ion transport properties in both biological and artificial systems, and they have shown broad application prospects from industrial production to biological interfaces. Herein, current advances in nanochannel-structured membranes for manipulating ion transport are reviewed from the perspective of membrane structures. First, the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage is introduced. Second, nanochannel-structured membranes are highlighted according to the nanochannel dimensions, including single-dimensional nanochannels (i.e., 1D, 2D, and 3D) functioning by the controllable geometrical parameters of 1D nanochannels, the adjustable interlayer spacing of 2D nanochannels, and the interconnected ion diffusion pathways of 3D nanochannels, and mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) tuned through asymmetric factors (e.g., components, geometric parameters, and interface properties). Then, ultrathin membranes with short ion transport distances and sandwich-like membranes with more delicate nanochannels and combination structures are reviewed, and stimulus-responsive nanochannels are discussed. Construction methods for nanochannel-structured membranes are briefly introduced, and a variety of applications of these membranes are summarized. Finally, future perspectives to developing nanochannel-structured membranes with unique structures (e.g., combinations of external macro/micro/nanostructures and the internal nanochannel arrangement) for mediating ion transport are presented.

Membrane Materials for Selective Ion Separations at the Water-Energy Nexus

Synthetic polymer membranes are enabling components in key technologies at the water-energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water-energy nexus require ion selectivity, or separation of specific ionic species from other similar species. Here, the ion selectivity of conventional polymeric membrane materials is assessed and recent progress in enhancing selective transport via tailored free volume elements and ion-membrane interactions is described. In view of the limitations of polymeric membranes, three material classes-porous crystalline materials, 2D materials, and discrete biomimetic channels-are highlighted as possible candidates for ion-selective membranes owing to their molecular-level control over physical and chemical properties. Lastly, research directions and critical challenges for developing bioinspired membranes with molecular recognition are provided.

Membrane-Suspended Nanopores in Microchip Arrays for Stochastic Transport Recording and Sensing

The transport of nutrients, xenobiotics, and signaling molecules across biological membranes is essential for life. As gatekeepers of cells, membrane proteins and nanopores are key targets in pharmaceutical research and industry. Multiple techniques help in elucidating, utilizing, or mimicking the function of biological membrane-embedded nanodevices. In particular, the use of DNA origami to construct simple nanopores based on the predictable folding of nucleotides provides a promising direction for innovative sensing and sequencing approaches. Knowledge of translocation characteristics is crucial to link structural design with function. Here, we summarize recent developments and compare features of membrane-embedded nanopores with solid-state analogues. We also describe how their translocation properties are characterized by microchip systems. The recently developed silicon chips, comprising solid-state nanopores of 80 nm connecting femtoliter cavities in combination with vesicle spreading and formation of nanopore-suspended membranes, will pave the way to characterize translocation properties of nanopores and membrane proteins in high-throughput and at single-transporter resolution.

Recent Advances in Immunosafety and Nanoinformatics of Two-Dimensional Materials Applied to Nano-imaging

Two-dimensional (2D) materials have emerged as an important class of nanomaterials for technological innovation due to their remarkable physicochemical properties, including sheet-like morphology and minimal thickness, high surface area, tuneable chemical composition, and surface functionalization. These materials are being proposed for new applications in energy, health, and the environment; these are all strategic society sectors toward sustainable development. Specifically, 2D materials for nano-imaging have shown exciting opportunities in in vitro and in vivo models, providing novel molecular imaging techniques such as computed tomography, magnetic resonance imaging, fluorescence and luminescence optical imaging and others. Therefore, given the growing interest in 2D materials, it is mandatory to evaluate their impact on the immune system in a broader sense, because it is responsible for detecting and eliminating foreign agents in living organisms. This mini-review presents an overview on the frontier of research involving 2D materials applications, nano-imaging and their immunosafety aspects. Finally, we highlight the importance of nanoinformatics approaches and computational modeling for a deeper understanding of the links between nanomaterial physicochemical properties and biological responses (immunotoxicity/biocompatibility) towards enabling immunosafety-by-design 2D materials.

Heterointerface Effects on Lithium-Induced Phase Transitions in Intercalated MoS2

The intercalation-induced phase transition of MoS₂ from the semiconducting 2H to the semimetallic 1T' phase has been studied in detail for nearly a decade; however, the effects of a heterointerface between MoS₂ and other two-dimensional (2D) crystals on the phase transition have largely been overlooked. Here, ab initio calculations show that intercalating Li at a MoS₂-hexagonal boron nitride (hBN) interface stabilizes the 1T phase over the 2H phase of MoS₂ by ∼100 mJ m ^-2, suggesting that encapsulating MoS₂ with hBN may lower the electrochemical energy needed for the intercalation-induced phase transition. However, in situ Raman spectroscopy of hBN-MoS₂-hBN heterostructures during the electrochemical intercalation of Li⁺ shows that the phase transition occurs at the same applied voltage for the heterostructure as for bare MoS₂. We hypothesize that the predicted thermodynamic stabilization of the 1T'-MoS₂-hBN interface is counteracted by an energy barrier to the phase transition imposed by the steric hindrance of the heterointerface. The phase transition occurs at lower applied voltages upon heating the heterostructure, which supports our hypothesis. Our study highlights that interfacial effects of 2D heterostructures can go beyond modulating electrical properties and can modify electrochemical and phase transition behaviors.

Scalable synthesis of nanoporous atomically thin graphene membranes for dialysis and molecular separations via facile isopropanol-assisted hot lamination

Scalable graphene synthesis and facile large-area membrane fabrication are imperative to advance nanoporous atomically thin membranes (NATMs) for molecular separations. Although chemical vapor deposition (CVD) allows for roll-to-roll high-quality monolayer graphene synthesis, facile transfer with atomically clean interfaces to porous supports for large-area NATM fabrication remains extremely challenging. Sacrificial polymer scaffolds commonly used for graphene transfer typically leave polymer residues detrimental to membrane performance and transfers without polymer scaffolds suffer from low yield resulting in high non-selective leakage through NATMs. Here, we systematically study the factors influencing graphene NATM fabrication and report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination (IHL) process that enables scalable, facile and clean transfer of CVD graphene on to polycarbonate track etched (PCTE) supports with coverage ≥99.2%, while preserving support integrity/porosity. We demonstrate fully functional centimeter-scale graphene NATMs that show record high permeances (∼2-3 orders of magnitude higher) and better selectivity than commercially available state-of-the-art polymeric dialysis membranes, specifically in the 0-1000 Da range. Our work highlights a scalable approach to fabricate graphene NATMs for practical applications and is fully compatible with roll-to-roll manufacturing processes.

Understanding Mono- and Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials

Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl₂, CaCl₂, and CuCl₂ as well as monovalent KCl and NH₄Cl salts. For all salts, except CuCl₂, clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg²⁺ and Ca²⁺, the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH₄Cl membrane potential curves. For CuCl₂, the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts.

Ions and Water Dancing through Atom-Scale Holes: A Perspective toward "Size Zero"

We provide an overview of atom-scale apertures in solid-state membranes, from "pores" and "tubes" to "channels", with characteristic sizes comparable to the sizes of ions and water molecules. In this regime of ∼1 nm diameter pores, water molecules and ions are strongly geometrically confined: the size of water molecules (∼0.3 nm) and the size of "hydrated" ions in water (∼0.7-1 nm) are similar to the pore diameters, physically limiting the ion flow through the hole. The pore sizes are comparable to the classical Debye screening length governing the spatial range of electrostatic interaction, ∼0.3 to 1 nm for 1 to 0.1 M KCl. In such small structures, charges can be unscreened, leading to new effects. We discuss experiments on ∼1 nm diameter nanopores, with a focus on carbon nanotube pores and ion transport studies. Finally, we present an outlook for artificial "size zero" pores in the regime of small diameters and small thicknesses. Beyond mimicking protein channels in nature, solid-state pores may offer additional possibilities where sensing and control are performed at the pore, such as in electrically and optically addressable solid-state materials.

Impact of Surface Charge Directionality on Membrane Potential in Multi-ionic Systems

The membrane potential (V_mem), defined as the electric potential difference across a membrane flanked by two different salt solutions, is central to electrochemical energy harvesting and conversion. Also, V_mem and the ionic concentrations that establish it are important to biophysical chemistry because they regulate crucial cell processes. We study experimentally and theoretically the salt dependence of V_mem in single conical nanopores for the case of multi-ionic systems of different ionic charge numbers. The major advances of this work are (i) to measure V_mem using a series of ions (Na⁺, K⁺, Ca²⁺, Cl^-, and SO₄^2-) that are of interest to both energy conversion and cell biochemistry, (ii) to describe the physicochemical effects resulting from the nanostructure asymmetry, (iii) to develop a theoretical model for multi-ionic systems, and (iv) to quantify the contributions of the liquid junction potentials established in the salt bridges to the total cell membrane potential.