Facilitated Water Transport through Graphene Oxide Membranes Functionalized with Aquaporin-Mimicking Peptides

Water purification by membranes is widely investigated to address concerns related to the scarcity of clean water. Achieving high flux and rejection simultaneously is a difficult challenge using such membranes because these properties are mutually exclusive in common artificial membranes. Nature has developed a method for this task involving water-channel membrane proteins known as aquaporins. Here, the design and fabrication of graphene oxide (GO)-based membranes with a surface-tethered peptide motif designed to mimic the water-selective filter of natural aquaporins is reported. The short RF8 (RFRFRFRF, where R and F represent arginine and phenylalanine, respectively) octapeptide is a concentrated form of the core component of the Ar/R (aromatic/arginine) water-selective filter in aquaporin. The resulting GO-RF8 shows superior flux and high rejection similar to natural aquaporins. Molecular dynamics simulation reveal the unique configuration of RF8 peptides and the transport of water in GO-RF8 membranes, supporting that RF8 effectively emulates the core function of aquaporins.

Transport of water molecules through noncylindrical pores in multilayer nanoporous graphene

The permeability inside a multilayer hourglass-shaped pore depends on the length of the flow path of the water molecules.

Colored spectrum characteristics of thermal noise on the molecular scale

Thermal noise is of fundamental importance to many processes. Traditionally, thermal noise has been treated as white noise on the macroscopic scale. Using molecular dynamics simulations and power spectrum analysis, we show that the thermal noise of solute molecules in water is non-white on the molecular scale, which is in contrast to the conventional theory. In the frequency domain from 2 × 10¹¹ Hz to 10¹³ Hz, the power spectrum of thermal noise for polar solute molecules resembles the spectrum of 1/f noise. The power spectrum of thermal noise for non-polar solute molecules deviates only slightly from the spectrum of white noise. The key to this phenomenon is the existence of hydrogen bonds between polar solute molecules and solvent water molecules. Furthermore, for polar solute molecules, the degree of power spectrum deviation from that of white noise is associated with the average lifetime of the hydrogen bonds between the solute and the solvent molecules.

Temperature dependence of the transport of single-file water molecules through a hydrophobic channel

Although great effort has been made on the transport properties of water molecules through nanometer channels, our understanding on the effect of some basic parameters are still rather poor. In this article, we use molecular dynamics simulations to study the temperature effect on the transport of single-file water molecules through a hydrophobic channel. Of particular interest is that the water flow and average translocation time both exhibit exponential relations with the temperature. Based on the continuous-time random-walk model and Arrhenius equation, we explore some new physical insights on these exponential behaviors. With the increase of temperature, the water dipoles flip more frequently, since the estimated flipping barrier is less than 2 kB T. Specifically, the flipping frequency also shows an exponential relation with the temperature. Furthermore, the water-water interaction and water occupancy demonstrate linear relations with the temperature, and the water density profiles along the channel axis can be slightly affected by the temperature. These results not only enhance our knowledge about the temperature effect on the single-file water transport, but also have potential implications for the design of controllable nanofluidic machines.

Autonomous pump against concentration gradient

Using non-equilibrium molecular dynamics and Monte Carlo methods, we have studied the molecular transport in asymmetric nanochannels. The efficiency of the molecular pump depends on the angle and apertures of the asymmetric channel, the environmental temperature and average concentration of the particles. The pumping effect can be explained as the competition between the molecular force field and the thermal disturbance. Our results provide a green approach for pumping fluid particles against the concentration gradient through asymmetric nanoscale thin films without any external forces. It indicates that pumping vacuum can be a spontaneous process.

Water and ion permeability of a claudin model: A computational study

At present, the three-dimensional structure of the multimeric paracellular claudin pore is unknown. Using extant biophysical data concerning the size of the pore and permeation of water and cations through it, two three-dimensional models of the pore are constructed in silico. Molecular Dynamics (MD) calculations are then performed to compute water and sodium ion permeation fluxes under the influence of applied hydrostatic pressure. Comparison to experiment is made, under the assumption that the hydrostatic pressure applied in the simulations is equivalent to osmotic pressure induced in experimental measurements of water/ion permeability. One model, in which pore-lining charged is distributed evenly over a selectivity filter section 10-16 Å in length, is found to be generally consistent with experimental data concerning the dependence of water and ion permeation on channel pore diameter, pore length, and the sign and magnitude of pore lining charge. The molecular coupling mechanism between water and ion flow under conditions where hydrostatic pressure is applied is computationally elucidated.

Water desalination by electrical resonance inside carbon nanotubes

By using molecular dynamics simulations, we not only design one new type of carbon nanotube-based device for efficient water desalination, but also reveal the underlying mechanism of the ion blockage.

Nanofluidic Diode for Simple Fluids without Moving Parts

The fabrication of small scale, fixed structure fluidic diodes for simple fluids is quite challenging and has not yet been achieved. Here, we fabricate a moving part-free nanofluidic diode for simple fluids using heterogeneous nanochannels, half of which is hydrophilic and the other half is hydrophobic. It accepts water flows in the forward (from hydrophilic to hydrophobic) direction, while the flows in the backward direction are blocked for pressure drop range 0<ΔP<0.63 MPa. The diode is ensured by a potential energy barrier at the channel entrance on the hydrophobic side due to the molecular interactions between the water and channel surface. As the upstream pressure becomes higher than 0.63 MPa, the fluidic diode turns to be a rectifier, which allows flows in both the forward and backward directions but with different flow rates. At sufficiently high driving pressures, the fluidic system fails in flow rectification, analogous to the breakdown of electronic diodes. The three different flow modes (diode, rectifier, and breakdown) of the fluidic chip and the underlying rectification mechanisms are confirmed by molecular dynamics simulations.