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

Photonic Interpenetrating Polymer Network Fibers Comprising Intertwined Solid-State Cholesteric Liquid Crystal and Polyelectrolyte Networks for Sensor Applications

Uniform-sized photonic interpenetrating polymer network (IPN) fibers comprising intertwined solid-state cholesteric liquid crystal (CLCsolid) and anionic poly(acrylic acid) (PAA) or cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) networks (photonic IPNPAA or IPNPDMAEMA fibers) were developed for sensor applications. IPNPAA or IPNPDMAEMA fibers with a perfect photonic structure were fabricated inside Teflon tube templates without any treatments for realizing a planar orientation in those fibers. The dominant wavelength of the photonic color from a photograph taken with a cellular phone was used to measure the photonic color change. Photonic IPNPAA fibers treated with KOH (IPNKOH fibers) were used for sensing humidity and divalent metal ions. The linear ranges for relative humidity and Ca2+ detection were 21-92% and 0.5-3.5 mM, and their limits of detection (LODs) were 7.86% and 0.07 mM, respectively. The photonic IPNPAA (or IPNPDMAEMA) fiber immobilized with urease (IPNPAA-urease) (or glucose oxidase (IPNPDMAEMA-GOx)) was used for urea (or glucose) biosensor application. The photonic IPNPAA-urease (or IPNPDMAEMA-GOx) fiber was red-shifted in response to urea (or glucose) in the linear range of 10-60 mM (or 2-16 mM) with an LOD of 2.54 mM (or 0.76 mM). These photonic IPN fibers are promising because of their easy fabrication and miniaturization, battery-free device, cost-effectiveness, and visual detection without using sophisticated analytical instruments. The developed photonic IPN fibers provide new possibilities for the widespread use of photonic sensors in cutting-edge wearable technology and beyond.

Anti-Arrhenius passage of gaseous molecules through nanoporous two-dimensional membranes

The passage of molecules through membranes is known to follow an Arrhenius-like kinetics, i.e. the flux is accelerated upon heating and vice versa. There exist though stepwise processes whose rates can decrease with temperature if, for example, adsorbed intermediates are involved. In this study, we perform temperature-variable permeation experiments in the range from -50 to +50 °C and observe anti-Arrhenius behaviour of water and ammonia permeating in two-dimensional freestanding carbon nanomembranes (CNMs). The permeation rate of water vapour is found to decrease many-fold with warming, while the passage of ammonia molecules strongly increases when the membrane is cooled down to the dew point. Liquefaction of isobutylene shows no enhancement for its transmembrane flux which is consistent with the material's pore architecture. The effects are described by the Clausius-Clapeyron relationship and highlight the key role of gas-surface interactions in two-dimensional membranes.

Hydrogen Isotope Separation Using Graphene-Based Membranes in Liquid Water

Hydrogen isotope separation has been effectively achieved electrochemically by passage of gaseous H₂/D₂ through graphene/Nafion composite membranes. Nevertheless, deuteron nearly does not exist in the form of gaseous D₂ in nature but as liquid water. Thus, it is a more feasible way to separate and enrich deuterium from water. Herein, we have successfully transferred monolayer graphene to a rigid and porous polymer substrate, PITEM (polyimide track-etched membrane), which could avoid the swelling problem of the Nafion substrate as well as keep the integrity of graphene. Meanwhile, defects in the large area of CVD graphene could be successfully repaired by interfacial polymerization resulting in a high separation factor. Moreover, a new model was proposed for the proton transport mechanism through monolayer graphene based on the kinetic isotope effect (KIE). In this model, graphene plays a significant role in the H/D separation process by completely breaking the O-H/O-D bond, which can maximize the KIE, leading to increased H/D separation performance. This work suggests a promising application for using monolayer graphene in the industry and proposes a pronounced understanding of proton transport in graphene.

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.

Scale Effect on Simple Liquid Transport through a Nanoporous Graphene Membrane

The transport mechanism of a simple liquid through nanoporous graphene membranes (NPGMs) with pores of various diameters has been explored by utilizing nonequilibrium molecular dynamics (NEMD) simulation. The flow is initiated using a pressure-driven flow mechanism that moves the specular reflection wall at a constant velocity. Both the local density peak near the membrane and the pressure drop are dependent on the pore diameter. For accurate calculation of the velocity profile inside the nanopore, we implemented three boundary approaches and local nanoscale variants to see the effect of these factors on the nature of the nanoscale flow. We found an optimized definition of the pore boundary, which minimizes the deviation between MD results and slip-viscosity-modified Sampson's prediction for nanopores of various diameters. Additionally, we observed that with decreasing pore size, the pore center velocity increases, as does the slip velocity, which we attributed to van der Waals interaction between the liquid and wall atoms inside the nanopore. However, the effects of slip velocity, interfacial viscosity, and pore boundary decay exponentially with increasing pore diameter because of the dominance of van der Waals repulsive forces at the molecular level.

Charge Regulation at a Nanoporous Two-Dimensional Interface

In this work, we have studied the pH-dependent surface charge nature of nanoporous graphene. This has been investigated by membrane potential and by streaming current measurements, both with varying pH. We observed a lowering of the membrane potential with decreasing pH for a fixed concentration gradient of potassium chloride (KCl) in the Donnan dominated regime. Interestingly, the potential reverses its sign close to pH 4. The fitted value of effective fixed ion concentration (C̅ R) in the membrane also follows the same trend. The streaming current measurements show a similar trend with sign reversal around pH 4.2. The zeta potential data from the streaming current measurement is further analyzed using a 1-pK model. The model is used to determine a representative pK (acid-base equilibrium constant) of 4.2 for the surface of these perforated graphene membranes. In addition, we have also theoretically investigated the effect of the PET support in our membrane potential measurement using numerical simulations. Our results indicate that the concentration drop inside the PET support can be a major contributor (up to 85%) for a significant deviation of the membrane potential from the ideal Nernst potential.