Ultrathin Nanopores Research Articles

Water compression induced ionic negative differential resistance in nanopores.

The mass transport behavior through nanoscale channels, greatly influenced by the structures and dynamics of nanoconfined water, plays an essential role in many biophysical processes. However, the dynamics of nanoconfined water under an external field and its effects are still not fully understood. Here, on the basis of molecular dynamics simulations, we theoretically show that the ionic current of [Bmim][PF6] through narrow pores in graphene membrane exhibits an ionic negative differential resistance effect-the ionic current decreases as the voltage increases over a certain threshold. This effect arises from the violation of traditional fluid dynamics as the assumption of continuity and homogeneity of fluids is no longer effective in ultrathin nanopores. The gradient of electric field around the atomic-thin layer produces a strong gradient force on the polarized water inside the nanopore. This dielectrophoretically compressed water leads to a hydrostatic force that repels ions from entering the nanopore. Our findings may advance the understanding of hydrostatic mechanism, which governs ion transport through nanopores.

Assembly Nanoparticle Arrays Decorated with Ultrathin AAO Nanopores for Highly Sensitive SERS Substrate

Assembly Nanoparticle Arrays Decorated with Ultrathin AAO Nanopores for Highly Sensitive SERS Substrate

Design and Fabrication of Ultrathin Nanoporous Donor-Acceptor Copolymer-Based Organic Field-Effect Transistors for Enhanced VOC Sensing Performance.

The development of organic field-effect transistor (OFET) chemical sensors with high sensing performance and good air stability has remained a persistent challenge, thereby hindering their practical application. Herein, an OFET sensor based on a donor-acceptor copolymer is shown to provide high responsivity, sensitivity, and selectivity toward polar volatile organic compounds, as well as good air stability. In detail, a polymer blend of N-alkyl-diketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) and polystyrene is coated onto an FET substrate via shearing-assisted phase separation (SAPS) combined with selective solvent etching to fabricate the DPP-DTT-based OFET device having an ultrathin nanoporous structure suitable for gas sensing applications. This is achieved via optimization of the film morphology by varying the shear rate to adjust the dynamic balance between the shear and capillary forces to obtain an ultrathin thickness (∼8 nm) and nanopore size (80 nm) that are favorable for the efficient diffusion and interaction of analytes with the active layer. In particular, the sensor presents high responsivities toward methanol (∼70%), acetone (∼51.3%), ethanol (∼39%), and isopropyl alcohol (IPA) (∼29.8%), along with fast response and recovery times of ∼80 and 234 s, respectively. Moreover, the average sensitivity was determined to be 5.75%/ppm from the linear plot of the responsivity against the methanol concentration in the range of 1-100 ppm. Importantly, the device also exhibits excellent long-term (30-day) air and thermal storage stability, thereby demonstrating its high potential for practical applications.

Dry Synthesis of Pure and Ultrathin Nanoporous Metallic Films.

Nanoporous metals possess unique properties attributed to their high surface area and interconnected nanoscale ligaments. They are mostly fabricated by wet synthetic methods that are not universal to various metals and not free from impurities due to solution-based etching processes. Here, we demonstrate that the plasma treatment of metal nanoparticles formed by physical vapor deposition is a general route to form such films with many metals including the non-noble ones. The resultant nanoporous metallic films are free of impurities and possess highly curved ligaments and nanopores. The metal films are ultrathin, yet remarkably robust and very well connected, and thus are highly promising for various applications such as transparent conducting electrodes.

Ultrafast Polymer Dynamics through a Nanopore.

Ultrathin nanopore sensors allow single-molecule and polymer measurements at sub-microsecond time resolution enabled by high current signals (∼10-30 nA). We demonstrate for the first time the experimental probing of the ultrafast translocation and folded dynamics of double-stranded DNA (dsDNA) through a nanopore at 10 MHz bandwidth with acquisition of data points per 25 ns (150 MB/s). By introducing a rigorous algorithm, we are able to accurately identify each current level present within translocation events and elucidate the dynamic folded and unfolded behaviors. The remarkable sensitivity of this system reveals distortions of short-lived folded states at a lower bandwidth. This work revisits probing of dsDNA as a model polymer and develops broadly applicable methods. The combined improvements in sensor signals, instrumentation, and large data analysis methods uncover biomolecular dynamics at unprecedentedly small time scales.

Constructing efficient ternary PtTeCu nano-catalysts with 2D ultrathin-sheet structures for oxidation reaction of alcohols

Ternary Pt-based nanomaterials have arisen much interest due to their outstanding electro-catalytic performance to reactions in fuel cells as well as the decrease of the utilization of Pt atoms. The construction of two-dimensional (2D) ternary Pt-based nanocatalysts by combining the advantages of alloy and geometric structure were appealing. In this work, we prepared free-standing ultrathin and highly flexible PtTeCu nanosheets (NSs) by a one-pot polyol method. The ultrathin feature and nanopore structure at the edges allowed the exposure of a large number of Pt atoms, supplying sufficient active sites for the catalytic reaction. The obtained PtTeCu NSs exhibited significantly higher ethylene glycol oxidation reaction (EGOR) and methanol oxidation reaction (MOR) performance reaching mass activities of 7.1 A/mg and 4.9 A/mg, compared to 6.5 and 6.1 times that of commercial Pt/C, respectively. The freestanding PtTeCu NS were subjected to long-term stability tests (200 cycles) where activity decay and structural changes were negligible. DFT calculations showed that the introduction of Cu and Te into Pt resulted in a significant downward shift of the d-band center of Pt. This weakened the adsorption energy of the products on PtTeCu NSs and accelerated the desorption of the products to the catalyst surface, enhancing the catalyst activity and stability. The unique ultrathin nano-sheet structures could provide sufficient active sites for the catalytic reaction, which also contributed to the excellent catalytic performance.

Sparse multi-nanopore osmotic power generators

Ultrathin nanopore membrane is an emerging energy harvesting system capable of generating electricity from salinity gradients. Here, we report on the evaluation of its practical feasibility by exploring the energy conversion efficiency of single to densely packed multi-pores in a thin silicon nitride. The ionic current characteristics of single pores reveal a quasi-perfect cation selectivity when shrinking the diameter to 20 nm. The perm-selective nanopore is shown to yield osmotic power of 160 pW under a 1,000-fold transmembrane salt concentration difference. Meanwhile, whereas larger energy is gained by parallelly integrating multiple pores, excessive porosity also led to degraded energy conversion efficiency, thereby demonstrating an optimal power density of 100 W per square meter for 100 nm-sized multi-nanopores with a grid spacing of 1 μm. The present findings offer a guide to design highly efficient nanopore membrane osmotic power generators.

PH-Responsive Ultrathin Nanoporous SiO2 Films for Selective Ion Permeation.

Ultrathin nanoporous (NP) films are an emerging field for selective and effective ion/molecular separation and electrochemical sensing applications. We describe selective ion permeation in surface-functionalized ultrathin NP SiO2 films (NP SiO2-NH2). The ultrathin NP SiO2 films with ca. 8 nm thickness were prepared from silsesquioxane-containing blend polymer Langmuir-Blodgett films (nanosheets) using the photo-oxidation method. The porous SiO2 surface was modified with a pH-responsive amine-containing silane coupling agent. Selective ion permeation was demonstrated under acidic pH conditions (pH ≤ 6) using two equally sized redox probes: negative (Fe(CN)63-/4-) and positive (Ru(NH3)62+/3+) ions. The current density for Fe(CN)63-/4- decreased as the pH value increased to pH = 6, whereas it increased for Ru(NH3)62+/3+. Control measurements revealed that the probes can penetrate the pores of nonfunctionalized SiO2 films irrespective of pH values, indicating that both the size and the surface charge response contributed to selective ion permeation. Results obtained from this study pave the way for new applications in molecular separation and sensing applications based on ultrathin nanoporous films (<10 nm) and tailored surfaces.

Dynamic and weak electric double layers in ultrathin nanopores.

The unique properties of aqueous electrolytes in ultrathin nanopores have drawn a great deal of attention in a variety of applications, such as power generation, water desalination, and disease diagnosis. Inside the nanopore, at the interface, properties of ions differ from those predicted by the classical ionic layering models (e.g., Gouy-Chapman electric double layer) when the thickness of the nanopore approaches the size of a single atom (e.g., nanopores in a single-layer graphene membrane). Here, using extensive molecular dynamics simulations, the structure and dynamics of aqueous ions inside nanopores are studied for different thicknesses, diameters, and surface charge densities of carbon-based nanopores [ultrathin graphene and finite-thickness carbon nanotubes (CNTs)]. The ion concentration and diffusion coefficient in ultrathin nanopores show no indication of the formation of a Stern layer (an immobile counter-ionic layer) as the counter-ions and nanopore atoms are weakly correlated in time compared to the strong correlation observed in thick nanopores. The weak correlation observed in ultrathin nanopores is indicative of a weak adsorption of counter-ions onto the surface compared to that of thick pores. The vanishing counter-ion adsorption (ion-wall correlation) in ultrathin nanopores leads to several orders of magnitude shorter ionic residence times (picoseconds) compared to the residence times in thick CNTs (seconds). The results of this study will help better understand the structure and dynamics of aqueous ions in ultrathin nanopores.

Wetting of nanopores probed with pressure.

Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of solid state nanopores. We show how pressure can be used to wet pores, and how it allows control over bubbles or other contaminants in the nanometer scale range normally unachievable using only an electrical driving force. Molybdenum disulfide is then used as a typical example of a 2D material on which we demonstrate wetting and bubble induced nonlinear and linear conductance in the regimes typically used with these experiments. We show that by using pressure one can identify and evade wetting artifacts.

Joule Heating Effects on Transport-Induced-Charge Phenomena in an Ultrathin Nanopore.

Transport-induced-charge (TIC) phenomena, in which the concentration imbalance between cations and anions occurs when more than two chemical potential gradients coexist within an ultrathin dimension, entail numerous nanofluidic systems. Evidence has indicated that the presence of TIC produces a nonlinear response of electroosmotic flow to the applied voltage, resulting in complex fluid behavior. In this study, we theoretically investigate thermal effects due to Joule heating on TIC phenomena in an ultrathin nanopore by computational fluid dynamics simulation. Our modeling results show that the rise of local temperature inside the nanopore significantly enhances TIC effects and thus has a significant influence on electroosmotic behavior. A local maximum of the solution conductivity occurs near the entrance of the nanopore at the high salt concentration end, resulting in a reversal of TIC across the nanopore. The Joule heating effects increase the reversal of TIC with the synergy of the negatively charged nanopore, and they also enhance the electroosmotic flow regardless of whether the nanopore is charged. These theoretical observations will improve our knowledge of nonclassical electrokinetic phenomena for flow control in nanopore systems.

Revisiting Sampson's theory for hydrodynamic transport in ultrathin nanopores

Sampson's theory for hydrodynamic resistance across a zero-length orifice was developed over a century ago. Although a powerful theory for entrance/exit resistance in nanopores, it lacks accuracy for relatively small-radius pores since it does not account for the molecular interface chemistry. Here, Sampson's theory is revisited for the finite slippage and interfacial viscosity variation near the pore wall. The corrected Sampson's theory can accurately predict the hydrodynamic resistance from molecular dynamics simulations of ultrathin nanopores.6 MoreReceived 19 February 2020Accepted 1 October 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043153Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFlows in porous mediaGeophysical fluid dynamicsGranular flowsInteratomic & molecular potentialsInterfacial flowsMicrofluidicsVan der Waals interactionFluid DynamicsAtomic, Molecular & Optical

Nanoconfined Water Dynamics in Multilayer Graphene Nanopores

Water dynamics in frictionless carbon nanotubes and across ultrathin graphene nanopores have been extensively studied. In contrast, the fundamental properties of nanoconfined water in multilayer gr...

Water Transport through Ultrathin Nanopores with Highly Polar Rims

Molecular dynamics simulations are used to study water transport through nanopores with highly polar rims formed in atomically thin membranes, where the partial charges are determined by the types of atoms and chemical groups attached at the nanopore rims. Our simulations reveal that the dynamics of water passing through nanopores can be dramatically affected by its Coulomb coupling with the nanopore rims. Highly polar rims, especially with negatively charged edge atoms, can transiently bind neighboring waters by hydrogen bonds and help them pass through the nanopores, which can increase the effective cross-sections of nanopores for passing water. On the other hand, strong binding of water to the rims can block its transport through the nanopores. These observations can be implemented in designs of synthetic nanopores with a controllable molecular transport.

Light-Enhanced Blue Energy Generation Using MoS2 Nanopores

Summary Blue energy relies on the chemical potential difference between solutions of high and low ionic concentrations, potentially providing an independent energy source at estuaries around the world. The energy conversion relies on reverse electrodialysis via ion-selective membranes. A novel generation of these membranes is based on nanopores in atomically thin material. Single nanopores in molybdenum disulfide (MoS2)-based membranes have shown record-high power outputs in alkaline conditions. By increasing the surface charge of MoS2 membranes by light, we can double the osmotic power generated by a single nanopore at a neutral pH. The increased surface charge at the pore rim enhances the ion selectivity and leads to a larger osmotic voltage (dominating in small pores), while the increased surface charge of the membrane enhances the surface conductance, leading to a larger osmotic current (dominating in larger pores). The combination of these effects could efficiently boost the energy generation using membranes containing arrays of nanopores of varying sizes.

Detecting topological variations of DNA at single-molecule level

In addition to their use in DNA sequencing, ultrathin nanopore membranes have potential applications in detecting topological variations in deoxyribonucleic acid (DNA). This is due to the fact that when topologically edited DNA molecules, driven by electrophoretic forces, translocate through a narrow orifice, transient residings of edited segments inside the orifice modulate the ionic flow. Here we utilize two programmable barcoding methods based on base-pairing, namely forming a gap in dsDNA and creating protrusion sites in ssDNA for generating a hybrid DNA complex. We integrate a discriminative noise analysis for ds and ss DNA topologies into the threshold detection, resulting in improved multi-level signal detection and consequent extraction of reliable information about topological variations. Moreover, the positional information of the barcode along the template sequence can be determined unambiguously. All methods may be further modified to detect nicks in DNA, and thereby detect DNA damage and repair sites.

Breakdown of continuum model for water transport and desalination through ultrathin graphene nanopores: insights from molecular dynamics simulations.

In the quest for identifying a graphene membrane for efficient water desalination, molecular dynamics simulations were performed for the pressure-driven flow of salty water across a multilayer graphene membrane. Water transport through the graphene membranes was tuned as a function of pore size, external pressure, and salt concentration. The results predicted that water permeability through the graphene channel (width of h = 7 Å) is two orders of magnitude higher than that through the conventional thin film membranes. The breaking of continuum assumption in graphene nanopores was captured by the appearance of a layered water structure and plug-like velocity profiles. Furthermore, the fluidity under nano confinement of graphene was examined in terms of shear viscosity, friction coefficient, and slip length, which were found to depend on the separation of the confining graphene walls and the external pressure. Furthermore, the MD results revealed that the macroscopic water flux through the graphene nanopores can be linked to the microscopic diffusion of water. The calculated viscosity and diffusion coefficient under the graphene pores did not follow the Stokes-Einstein relation, indicating the failure of the hydrodynamic theory. The confined state of water in the graphene pores was also explored via the translational density of states (TDOS) and entropy, which displayed a significant change in the translational entropy with change in the pore size and applied pressure and thus revealed the interconnectivity of the structure, dynamics, thermodynamics, and hydrodynamics of water in the graphene nanopores. Such a linking of the microscopic parameters with the macroscopic profiles provides direct evidence to the experiments of pressure-driven flow through the graphene membranes and might be helpful in examining the performance of graphene membranes for the factors that have large implications on their application in the reverse osmosis (RO) process and other biological channels.

Photothermally Assisted Thinning of Silicon Nitride Membranes for Ultrathin Asymmetric Nanopores.

Sculpting solid-state materials at the nanoscale is an important step in the manufacturing of numerous types of sensor devices, in particular solid-state nanopore sensors. Here we present mechanistic insight into laser-induced thinning of low-stress silicon nitride (SiN x) membranes and films. In a recent study, we observed that focusing a visible wavelength laser beam on a SiN x membrane results in efficient localized heating, and we used this effect to control temperature at a solid-state nanopore sensor. A side-effect of the observed heating was that the pores expand/degrade under prolonged high-power illumination, prompting us to study the mechanism of this etching process. We find that SiN x can be etched under exposure to light of ∼107 W/cm2 average intensity, with etch rates that are influenced by the supporting electrolyte. Combining this controlled etching with dielectric breakdown, an electrokinetic process for making pores, nanopores of arbitrary dimensions as small as 1-2 nm in diameter and thickness can easily be fabricated. Evidence gathered from biomolecule-pore interactions suggests that the pore geometries obtained using this method are more funnel-like, rather than hourglass-shaped. Refined control over pore dimensions can expand the range of applications of solid-state nanopores, for example, biopolymer sequencing and detection of specific biomarkers.

Theory of Transport-Induced-Charge Electroosmotic Pumping toward Alternating Current Resistive Pulse Sensing.

In this work, we study transport-induced-charge electroosmosis toward alternating current resistive pulse sensing for the next generation of biomedical applications. Transport-induced-charge electroosmosis, being a new class of electrokinetic phenomenon, occurs as a salt concentration gradient works in synergy with an electric field in ultrathin nanopores. Apart from the conventional electric double layer-governed electroosmotic flow in which the flow behavior is subject to the surface charge, it is found that the transport-induced-charge electroosmotic flow behaves independently of surface charge magnitude but can be linearly regulated by the bulk salt concentration bias. The reversal of the electric field simultaneously inverses the induced charge allowing the establishment of a unidirectional flow under the application of a periodic alternating current field. This unique phenomenon permits continuous water and nanoparticles pumping through a two-dimensional material nanopore in spite of the reversal of the electric field. Built upon this mechanism, we propose a theoretical prototype of alternating current resistive pulse sensing in a two-dimensional nanopore system.

Temporal Response of Ionic Current Blockade in Solid-State Nanopores

Signal delay is a crucial factor in resistive pulse analyses using low-thickness-to-diameter aspect-ratio pores that aim to detect fine features in the ionic current blockade during the fast translocation of individual analytes to attain single-molecule tomography. Here we report on evaluations of the ionic current response to dynamic motions of nanoparticles in ultrathin solid-state nanopores. We systematically investigated the effects of pore resistance and membrane capacitance on resistive pulse waveforms under different salt concentration conditions and device configurations. The results revealed substantial modifications in the resistive pulse waveforms due to a slow charging/discharging processes at the water-touching thin dielectrics in the solid-state nanopore chips. We also provide a device design to improve the temporal resolution without compromising the spatial sensitivity. The present findings offer a breakthrough toward nanoporescopy to measure the nanoscopic shape of single-bioparticles and -molecules in electrolyte solution.