Ionic Selectivity of Single Nanochannels

Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy' 1-3 . Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient 4 or by an osmotic potential resulting from a salt concentration gradient 5 . For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness 5-7 . Here we demonstrate the use of single-layer molybdenum disulfide (MoS 2 ) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10 6 watts per square metre-a current that can be attributed mainly to the atomically thin membrane of MoS 2 . Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment 8-11 -for example, a piezoelectric zinc oxide nanowire array 8 or single-layer MoS 2 (ref. 12). We use our MoS 2 nanopore generator to power a MoS 2 transistor, thus demonstrating a self-powered nanosystem.MoS 2 nanopores have already demonstrated better water-transport behaviour than graphene 13,14 owing to the enriched hydrophilic surface sites (provided by the molybdenum) that are produced following either irradiation with transmission electron microscopy (TEM) 15 or electrochemical oxidation 16 . The osmotic power is generated by separating two reservoirs containing potassium chloride (KCl) solutions of different concentrations with a freestanding MoS 2 membrane, into which a single nanopore has been introduced 13 . A chemical potential gradient arises at the interface of these two liquids at a nanopore in a 0.65-nm-thick, single-layer MoS 2 membrane, and drives ions spontaneously across the nanopore, forming an osmotic ion flux towards the equilibrium state (Fig. 1a). The presence of surface charges on the pore screens the passing ions according to their charge polarity, and thus results in a net measurable osmotic current, known as reverse electrodialysis 1 . This cation selectivity can be better understood by analysing the concentration of each ion type (potassium and chloride) as a function of the radial distance from the centre of the pore, as we show here through molecular-dynamics simulations (Fig. 1b).We fabricated MoS 2 nanopores either by TEM 13 (Fig. 1c) Distance from the centre of the pore (Å)C max /C min = 1,000C max /C min = 500C max /C min = 100 LETTER RESEARCHosmotic current can be expected, owing to the long time required for the system to reach equilibrium. We measured the osmotic current and voltage across the pore by using a pair of Ag/AgCl electrodes to characterize the current-voltage (I-V) response of the nanopore.To gain a better insight into the performance of the MoS 2 nanopore power generator, ...

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Section: Letter Researchmentioning

confidence: 84%

“…As shown in Extended Data Fig. 3d, with a concentration gradient of 10 mM/1 mM in a 5-nm pore, the ion selectivity approaches nearly 1, presenting the conditions for ideal cation selectivity 23 .To test the cation-selective behaviour of the pore further, we investigated the relationship between power generation and pore size. As shown in Fig.…”

mentioning

confidence: 99%

Single-layer MoS2 nanopores as nanopower generators

Feng

Gräf

Liu

et al. 2016

Nature

978

855

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“…This is presumably related to the short ionic channels as suggested by simulations. 4,32 The overall effect on rectifying factor taken between jI D j at 0.4 V and À0.4 V can be modulated from 2 to 11.…”

mentioning

confidence: 99%

Field effect modulated nanofluidic diode membrane based on Al2O3/W heterogeneous nanopore arrays

Wildhaber

Bertsch

et al. 2013

Applied Physics Letters

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We developed Al2O3/W heterogeneous nanopore arrays for field effect modulated nanofluidic diodes. They are fabricated by transferring self-organized nanopores of anodic aluminium oxide into a W thin film. The nanopores are ∼20 nm in diameter and 400 nm in length. After mild oxidation, approximately 10 nm WO3 grows on the surface of W, forming a conformal and dense dielectric layer. It allows the application of an electrical field through the surrounding W electrode to modulate the ionic transport across the entire membrane. Our experimental findings have potential applications in high throughput controlled delivery and electrostatic sorting of biomolecules.

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“…Using a continuum treatment of the problem, in the absence of chemical reactions, the flux of each of these ionic species is described by Nernst-Planck equation, which represents the balance between convection, diffusion, and ionic drift due to presence electric field. 33,36 @C i @t ¼ r:…”

Section: A Theoretical Formulationmentioning

confidence: 99%

“…Together they are known as Poisson-Nernst-Plank (PNP) equations, which should be solved numerically, since analytical solutions are not available for their nonlinear form. 33 Using standard boundary conditions for continuity of w and C i and of their first derivatives, we used COMSOL Multiphysics to solve these equations over 2D space to find the concentration distribution of ionic species at any point in the device. The only exceptions are at the constriction tip and within the walls of the perm-selective region, wherein the electrical boundary condition is set to account for the surface charge density (q c )…”

Section: A Theoretical Formulationmentioning

confidence: 99%

Nanoslit design for ion conductivity gradient enhanced dielectrophoresis for ultrafast biomarker enrichment in physiological media

et al. 2016

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Selective and rapid enrichment of biomolecules is of great interest for biomarker discovery, protein crystallization, and in biosensing for speeding assay kinetics and reducing signal interferences. The current state of the art is based on DC electrokinetics, wherein localized ion depletion at the microchannel to nanochannel interface is used to enhance electric fields, and the resulting biomarker electromigration is balanced against electro-osmosis in the microchannel to cause high degrees of biomarker enrichment. However, biomarker enrichment is not selective, and the levels fall off within physiological media of high conductivity, due to a reduction in ion concentration polarization and electro-osmosis effects. Herein, we present a methodology for coupling AC electrokinetics with ion concentration polarization effects in nanoslits under DC fields, for enabling ultrafast biomarker enrichment in physiological media. Using AC fields at the critical frequency necessary for negative dielectrophoresis of the biomarker of interest, along with a critical offset DC field to create proximal ion accumulation and depletion regions along the perm-selective region inside a nanoslit, we enhance the localized field and field gradient to enable biomarker enrichment over a wide spatial extent along the nanoslit length. While enrichment under DC electrokinetics relies solely on ion depletion to enhance fields, this AC electrokinetic mechanism utilizes ion depletion as well as ion accumulation regions to enhance the field and its gradient. Hence, biomarker enrichment continues to be substantial in spite of the steady drop in nanostructure perm-selectivity within physiological media. Published by AIP Publishing. [http://dx

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Ionic Selectivity of Single Nanochannels

Cited by 422 publications

References 27 publications

Single-layer MoS2 nanopores as nanopower generators

Single-layer MoS2 nanopores as nanopower generators

Field effect modulated nanofluidic diode membrane based on Al2O3/W heterogeneous nanopore arrays

Nanoslit design for ion conductivity gradient enhanced dielectrophoresis for ultrafast biomarker enrichment in physiological media

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