Haiping Fang

Electrostatic gating of a nanometer water channel

Water permeation across a single-walled carbon nanotube (SWNT) under the influence of a mobile external charge has been studied with molecular dynamics simulations. This designed nanopore shows an excellent on–off gating behavior by a single external charge (of value +1.0e): it is both sensitive to the available charge signal when it is close (less than a critical distance of 0.85 Å or about half the size of a water molecule) and effectively resistant to charge noise, i.e., the effect on the flow and net flux across the channel is found to be negligible when the charge is >0.85 Å away from the wall of the nanopore. This critical distance can be estimated from the interaction balance for the water molecule in the SWNT closest to the imposed charge with its neighboring water molecules and with the charge. The flow and net flux decay exponentially with respect to the difference between these two interaction energies when the charge gets closer to the wall of the SWNT and reaches a very small value once the charge crosses the wall, suggesting a dominating effect on the permeation properties from local water molecules near the external charge. These findings might have biological implications because membrane water channels share a similar single-file water chain inside these nanoscale channels.

A charge-driven molecular water pump

Understanding and controlling the transport of water across nanochannels is of great importance for designing novel molecular devices, machines and sensors and has wide applications, including the desalination of seawater. Nanopumps driven by electric or magnetic fields can transport ions and magnetic quanta, but water is charge-neutral and has no magnetic moment. On the basis of molecular dynamics simulations, we propose a design for a molecular water pump. The design uses a combination of charges positioned adjacent to a nanopore and is inspired by the structure of channels in the cellular membrane that conduct water in and out of the cell (aquaporins). The remarkable pumping ability is attributed to the charge dipole-induced ordering of water confined in the nanochannels, where water can be easily driven by external fields in a concerted fashion. These findings may provide possibilities for developing water transport devices that function without osmotic pressure or a hydrostatic pressure gradient.

Plugging into proteins: poisoning protein function by a hydrophobic nanoparticle.

Nanoscale particles have become promising materials in many fields, such as cancer therapeutics, diagnosis, imaging, drug delivery, catalysis, as well as biosensors. In order to stimulate and facilitate these applications, there is an urgent need for the understanding of the nanoparticle toxicity and other risks involved with these nanoparticles to human health. In this study, we use large-scale molecular dynamics simulations to study the interaction between several proteins (WW domains) and carbon nanotubes (one form of hydrophobic nanoparticles). We have found that the carbon nanotube can plug into the hydrophobic core of proteins to form stable complexes. This plugging of nanotubes disrupts and blocks the active sites of WW domains from binding to the corresponding ligands, thus leading to the loss of the original function of the proteins. The key to this observation is the hydrophobic interaction between the nanoparticle and the hydrophobic residues, particularly tryptophans, in the core of the domain. We believe that these findings might provide a novel route to the nanoparticle toxicity on the molecular level for the hydrophobic nanoparticles.

Graphene on Au(111): A Highly Conductive Material with Excellent Adsorption Properties for High-Resolution Bio/Nanodetection and Identification

Based on numerical simulations and experimental studies, we show that a composite material which consists of a sheet of graphene on a Au(111) surface exhibits both an excellent conductivity and the ability to stably adsorb biomolecules. If we use this material as a substrate, the signal-to-noise ratios can be greatly enhanced. The key to this unique property is that graphene can stably adsorb carbon-based rings, which are widely present in biomolecules, due to pi-stacking interactions while the substrate retains the excellent conductivity of gold. Remarkably, the signal-to-noise ratio is found to be so high that the signal is clearly distinguishable for different nucleobases when an ssDNA is placed on this graphene-on-Au(111) material. Our finding opens opportunities for a range of bio/nano-applications including single-DNA-molecule-based biodevices and biosensors, particularly, high-accuracy sequencing of DNA strands with repeating segments.

Water-mediated signal multiplication with Y-shaped carbon nanotubes

Molecular scale signal conversion and multiplication is of particular importance in many physical and biological applications, such as molecular switches, nano-gates, biosensors, and various neural systems. Unfortunately, little is currently known regarding the signal processing at the molecular level, partly due to the significant noises arising from the thermal fluctuations and interferences between branch signals. Here, we use molecular dynamics simulations to show that a signal at the single-electron level can be converted and multiplied into 2 or more signals by water chains confined in a narrow Y-shaped nanochannel. This remarkable transduction capability of molecular signal by Y-shaped nanochannel is found to be attributable to the surprisingly strong dipole-induced ordering of such water chains, such that the concerted water orientations in the 2 branches of the Y-shaped nanotubes can be modulated by the water orientation in the main channel. The response to the switching of the charge signal is very rapid, from a few nanoseconds to a few hundred nanoseconds. Furthermore, simulations with various water models, including TIP3P, TIP4P, and SPC/E, show that the transduction capability of the Y-shaped carbon nanotubes is very robust at room temperature, with the interference between branch signals negligible.

Ion sieving in graphene oxide membranes via cationic control of interlayer spacing

Graphene oxide membranes—partially oxidized, stacked sheets of graphene—can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution. These materials have shown potential in a variety of applications, including water desalination and purification, gas and ion separation, biosensors, proton conductors, lithium-based batteries and super-capacitors. Unlike the pores of carbon nanotube membranes, which have fixed sizes, the pores of graphene oxide membranes—that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)—are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution. These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with ångström precision using K+, Na+, Ca2+, Li+ or Mg2+ ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. First-principles calculations and ultraviolet absorption spectroscopy reveal that the location of the most stable cation adsorption is where oxide groups and aromatic rings coexist. Previous density functional theory computations show that other cations (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) should have a much stronger cation–π interaction with the graphene sheet than Na+ has, suggesting that other ions could be used to produce a wider range of interlayer spacings.

Dynamics of single-file water chains inside nanoscale channels: physics, biological significance and applications

Transportation of water across nanochannels is of great importance for biological activities as well as for designing novel molecular devices/machines/sensors, which has wide applications in nanotechnology. With the development of experimental and computational facilities and technologies, it becomes possible to study the water dynamics inside and across the nanoscale channels by both experiments and numerical simulations. When the radius of a nanochannel is appropriate, the water molecules inside the channel form a single-file structure. Water confined in these nanoscale channels usually exhibits different dynamics not seen in the bulk system, including the wet - dry transition due to confinement, concerted hydrogen-bond orientations and flipping, concerted motion of water molecules and wavelike density distribution pattern. The permeation of water across the channels also shows unique behaviours, such as extra-high permeability, excellent on - off gating behaviour with response to the external mechanical and electrical signals and noises, reduction and enhancement by charge distributions on the channel walls, as well as directional transportation by a combination of charges close to a channel. In this review, we examine some of the recent advances in the dynamics of these single-file water molecules inside very narrow nanochannels.

Direct Three‐Dimensional Imaging of the Buried Interfaces between Water and Superhydrophobic Surfaces

The investigation of how water interacts with hydrophobic solids is crucial for understanding many natural and technological phenomena, such as hydrophobic collapse in protein folding, the formation of hydrophobic clays, and superhydrophobicity. While it is generally believed that air is trapped at the interfaces between water and superhydrophobic (SH) surfaces, direct experimental evidence for the presence of air at the microscopic level is rare. Herein we present an in situ, nondestructive approach to the direct 3D imaging of the buried interfaces between water droplets and superhydrophobic surfaces by confocal laser scanning microscopy (CLSM). A 10 mm thick air cushion trapped at the interface is quantitatively visualized and shown to be responsible for the ultralow-resistance fluid flow. The two intangible hydrophobic states, that is, Wenzel and Cassie states, can be distinctly identified by this advanced technique. Our new approach also opens a pathway to explore complex phenomena and regulate subtle processes that occur at the liquid–vapor–solid interface for various basic research and industrial applications. Inspired by natural examples (e.g., lotus leaves and water strider legs), the design of artificial SH surfaces by fabricating rough surface microstructures and decorating the microstructured surface with chemicals of low surface free energy has become a popular research focus, since SH surfaces have found important applications in fields from aquatic devices, microfluidic transport, and protective coatings, to proteins and DNA analysis. The superhydrophobicity of the surfaces has been explained by two major mechanisms, assuming that water exists in either Wenzel or Cassie states on hydrophobic surfaces. In the Wenzel state, water is in intimate contact with the rough surface; the contact area is dominated by the liquid–solid interface, and water droplets adhere to the solid with both large contact angles (CAs) and rolling angles (RAs). In the Cassie state, the existence of the additional liquid–vapor interface means that water is partial contact with the solid, so that droplets have large CAs but small RAs. Although the two mechanisms have long been proposed for the analysis of SH surfaces, it is still a matter of debate as to which state dominates in different circumstances, owing to the lack of a direct, quantitative observation method and in-depth insights into the buried interface. The interface below the liquid cannot be imaged by using techniques such as SEM, TEM, and AFM, while conventional 2D optical observation provides only limited and nonquantitative information that is insufficient for constructing the whole picture of the topologically complex buried interface.

Manipulating biomolecules with aqueous liquids confined within single-walled nanotubes.

Confinement of molecules inside nanoscale pores has become an important method for exploiting new dynamics not happening in bulk systems and for fabricating novel structures. Molecules that are encapsulated in nanopores are difficult to control with respect to their position and activity. On the basis of molecular dynamics simulations, we have achieved controllable manipulation, both in space and time, of biomolecules with aqueous liquids inside a single-walled nanotube by using an external charge or a group of external charges. The remarkable manipulation abilities are attributed to the single-walled structure of the nanotube that the electrostatic interactions of charges inside and outside the single-walled nanotube are strong enough, and the charge-induced dipole-orientation ordering of water confined in the nanochannel so that water has a strong interaction with the external charge. These designs are expected to serve as lab-in-nanotube for the interactions and chemical reactions of molecules especially biomolecules, and have wide applications in nanotechnology and biotechnology.

Galilean invariant fluid-solid interfacial dynamics in lattice Boltzmann simulations

Galilean invariance is a fundamental property; however, although the dynamics of lattice Boltzmann equation in the hydrodynamic limit is Galilean invariant, this property is usually not taken into account in the treatment of the fluid-solid interface. Here, we show that consideration of Galilean invariance in fluid-solid interfacial dynamics can greatly enhance the computational accuracy and robustness in a numerical simulation. Surprisingly, simulations are so vastly improved that the force fluctuation is very small and a time average becomes unnecessary.