Unveiling Interactions of Tumor-Targeting Nanoparticles with Lipid Bilayers Using a Titratable Martini Model.

pH-responsive nanoparticles are ideal vehicles for drug delivery and are widely used in cell imaging in targeted therapy of cancer, which usually has a weakly acidic microenvironment. In this work, we constructed a titratable molecular model for nanoparticles grafted with ligands of pH-sensitive carboxylic acids and investigated the interactions between the nanoparticles and the lipid bilayer in varying pH environments. We mainly examined the effect of the grafting density of the pH-sensitive ligands of the nanoparticles on the interactions of the nanoparticles with the lipid bilayer. The results show that the nanoparticles can penetrate the lipid bilayer only when the pH value is lower than a critical value, which can be readily modulated to the specific pH value of the tumor microenvironment by changing the ligand grafting density. This work provides some insights into modulating the interactions between the pH-sensitive nanoparticles and cellular membranes to realize targeted drug delivery to tumors based on their specific pH environment.

Molecular Mechanisms of the Generation and Accumulation of Gas at the Interface.

Gas-evolving reactions are widespread in chemical and energy fields. However, the generated gas will accumulate at the interface, which reduces the rate of gas generation. Understanding the microscopic processes of the generation and accumulation of gas at the interface is crucial for improving the efficiency of gas generation. Here, we develop an algorithm to reproduce the process of catalytic gas generation at the molecular scale based on the all-atom molecular dynamics simulations and obtain the quantitative evolution of the gas generation, which agrees well with the experimental results. In addition, we demonstrate that under an external electric field, the generated gas molecules do not accumulate at the electrode surface, which implies that the electric field can significantly increase the rate of the gas generation. The results suggest that the external electric field changes the structure of the water molecules near the electrode surface, making it difficult for gas molecules to accumulate on the electrode surface. Furthermore, it is found that gas desorption from the electrode surface is an entropy-driven process, and its accumulation at the electrode surface depends mainly on the competition between the entropy and the enthalpy of the water molecules under the influence of the electric field. These results provide deep insight into gas generation and inhibition of gas accumulation.

Self-propelled continuous transport of nanoparticles on a wedge-shaped groove track.

Controlling the directional motion of nanoparticles on the surface is particularly important for human life, but achieving continuous transport is a time-consuming and demanding task. Here, a spontaneous movement of nanoflakes on a wedge-shaped groove track is demonstrated by using all-atom molecular dynamics (MD) simulations. Moreover, an optimized track, where one end of the substrate is cut into an angle, is introduced to induce a sustained directional movement. It is shown that the wedge-shaped interface results in a driving force for the nanoflakes to move from the diverging to the converging end, and the angular substrate provides an auxiliary driving force at the junction to maintain continuous transport. A force analysis is carried out in detail to reveal the driving mechanism. Moreover, the sustained transport is sensitive to the surface energy and structural characteristics of the track: the nanoflakes are more likely to move continuously on the track with lower surface energy and a smaller substrate and groove opening angle. The present findings are useful for designing nanodevices to control the movement of nanoparticles.

Wetting-State-Induced Turning of Water Droplet Moving Direction on the Surface.

The spontaneous directional movement of water droplets on a wedge-shaped groove has gained extensive attention due to the advantage of not requiring energy input and its potential wide applications. However, manipulating the direction of movement of water droplets on a wedge-shaped groove has been not fully achieved, and the fundamental understanding of its underlying mechanism remains unclear. Here, molecular dynamics simulations and theoretical analyses are combined to reveal the mechanism of movement in opposite directions of a water droplet at the same position on the wedge-shaped groove interface. It is shown that the moving direction of the water droplet is related to its wetting state on the surface, i.e., the Wenzel and the Cassie states. A water droplet initially in the Wenzel and Cassie states will move toward the diverging and the converging ends, respectively. This phenomenon is attributed to the opposite roles played by the groove substrate and the upper layers in the two wetting states. Moreover, it is found that the water droplet is likely to move faster on a surface with a higher groove, larger opening angle and stronger hydrophobicity. These findings are expected to be of benefit for fully understanding droplet movement and shedding light on the regulation of the direction of movement of the droplets on the groove surface.

Perspective clipping and fast rendering of light field images for holographic stereograms using RGBD data.

The production of holographic stereogram (HS) requires a huge amount of light field data. How to efficiently clip and render these image data remains a challenge in the field. This work focuses on the perspective clipping and fast rendering algorithm for light field images using RGBD data without explicit 3D reconstruction. The RGBD data is expanded to RGBDθ data by introducing a light cone for each point, which gives a new degree of freedom for light field image rendering. Using the light cone and perspective coherence, the visibility of 3D image points can be clipped programmatically. Optical imaging effects including mirror imaging and half mirror imaging effects of 3D images can also be rendered with the help of light cones during the light field rendering process. The perspective coherence is also used to accelerate the rendering, which has been shown to be on average 168% faster than traditional DIBR algorithms. A homemade holographic printing system was developed to make the HSs using the rendered light field images. The vivid 3D effects of the HS have validated the effectiveness of the proposed method. It can also be used in holographic dynamic 3D display, augmented reality, virtual reality, and other fields.

The dynamics of chemically propelled dimer motors on a pinning substrate.

The dynamics of self-propelled micro-motors, in a thin fluid film containing an attractive substrate, is investigated by means of a particle-based simulation. A chemically powered sphere dimer, consisting of a catalytic and a noncatalytic sphere, may be captured by a trap on the substrate and consequently rotates around the trap center. A pair of trapped dimers spontaneously forms various configurations, including anti-parallel aligned doublets and head-to-tail rotating doublets. Small traps randomly distributed on the substrate are capable of pinning the dimers. The diffusion coefficient decreases with increasing pinning force or the pinning density, and it falls quickly at a certain critical pinning force beyond which the dimer motor is pinned completely. It is found that the pin array on the substrate gives rise to the formation of clusters of dimers and the underlying mechanism is discussed.

Initial-position-driven opposite directional transport of a water droplet on a wedge-shaped groove.

The transport direction of water droplets on a functionalized surface is of great significance due to its wide applications in microfluidics technology. The prevailing view is that a water droplet on a wedge-shaped groove always moves towards the wider end. In this paper, however, molecular dynamics simulations show that a water droplet can move towards the narrower end if placed at specific positions. It is found that the direction of water droplet transport on a grooved surface is related to its initial position. The water droplet moves towards the wider end only when it is placed near the wider end initially. If the water droplet is placed near the narrower end, it will move in the opposite direction. The novel phenomenon is attributed to the opposite interactions of the groove substrate and the groove upper layers with water droplets. Two effective models are proposed to exploit the physical origin of different transport directions of water droplets on a wedge-shaped groove surface. The study provides an insight into the design of nanostructured surfaces to effectively control the droplet motion.

Electric field-induced gas dissolving in aqueous solutions.

Gas dissolution or accumulation regulating in an aqueous environment is important but difficult in various fields. Here, we performed all-atom molecular dynamics simulations to study the dissolution/accumulation of gas molecules in aqueous solutions. It was found that the distribution of gas molecules at the solid-water interface is regulated by the direction of the external electric field. Gas molecules attach and accumulate to the interface with an electric field parallel to the interface, while the gas molecules depart and dissolve into the aqueous solutions with a vertical electric field. The above phenomena can be attributed to the redistribution of water molecules as a result of the change of hydrogen bonds of water molecules at the interface as affected by the electric field. This finding reveals a new mechanism of regulating gas accumulation and dissolution in aqueous solutions and can have tremendous applications in the synthesis of drugs, the design of microfluidic device, and the extraction of natural gas.

Correction: Nanoporous two-dimensional MoS2 membranes for fast saline solution purification.

Correction for 'Nanoporous two-dimensional MoS2 membranes for fast saline solution purification' by Jianlong Kou et al., Phys. Chem. Chem. Phys., 2016, 18, 22210-22216.

Mixture Composition Effect on Hydrocarbon-Water Transport in Shale Organic Nanochannels.

Understanding molecule transport through nanochannels is fundamental to geophysics, bioengineering, and physical chemistry. Here, molecular dynamics simulations combined with theoretical analysis are conducted to investigate hydrocarbon-water mixture flow in organic nanochannels. The flow is sensitive to the mixture compositions. The total flux decreases sharply with hydrocarbon content before the critical value, while it almost holds constant after the critical value, which is attributed to the spontaneous adsorption of hydrocarbon on the organic surface. An effective theory based on updating the Navier-Stokes equation with a slip boundary is proposed and validated to describe the mixture flow in nanochannels. The established quantitative relations between the total flux/slip length and the mixture composition are consistent with the molecular dynamics results.

A controllable water signal transistor.

We performed molecular dynamics simulations to study the regulating ability of water chains confined in a Y-shaped nanochannel. It was shown that a signal at the molecular level could be controlled by two other charge-induced signals when the water chains were confined in a Y-shaped nanochannel, demonstrating promising applications as water signal transistors in nanosignal systems. The mechanism of a water signal transistor is similar to a signal logic device. This remarkable ability to control the water signal is attributed to the strong dipole-ordering of the water chains in the nanochannel. The controllable water signal process of the Y-shaped nanochannel provides opportunities for future application in the design of molecular-scale signal devices.

Nanoporous two-dimensional MoS2 membranes for fast saline solution purification.

Finding a membrane with both high permeability and high salt rejection is very important for saline solution purification. Here, we report the performance of molybdenum disulfide (MoS2) membranes with nanoscale pores for saline solution purification via all-atom molecular dynamics simulations. It was found that the nanoporous two-dimensional MoS2 membrane can impede salt ions, while allowing highly efficient permeation of water molecules. By engineering the appropriate sizes of the nanopores within two-dimensional MoS2 membranes, their water permeability can be tens of times as high as that of conventional reverse osmosis membranes, while still maintaining a high salt rejection rate. These remarkable water permeability and salt rejection properties of the nanoporous monolayer MoS2 membranes are attributed to the formation of single chain hydrogen bonds, which link the water molecules within the nanopores and those at the immediate exteriors of the nanopores, causing significant reduction in the resistance of water molecules passing through the nanopores, which are small enough for any salt ions to pass through. Therefore such nanoporous monolayer MoS2 membranes have great potential for saline solution purification.

Current inversions induced by resonant coupling to surface waves in a nanosized water pump.

We conducted a molecular dynamics simulation to investigate current inversions in a nanosized water pump based on a single-walled carbon nanotube powered by mechanical vibration. It was found that the water current depended sensitively on the frequency of mechanical vibration. Especially in the resonance region, the nanoscale pump underwent reversals of the water current. This phenomenon was attributed to the dynamics competition of the water molecules in the two sections (the left and right parts) divided by the vibrating atom and the differences in phase and decay between the two mechanical waves generated by mechanical vibration and propagating in opposite directions toward the two ends of the carbon nanotube. Our findings provide an insight into water transportation through nanosized pumps and have potential in the design of high-flux nanofluidic systems and nanoscale energy converters.

A transition between bistable ice when coupling electric field and nanoconfinement.

The effects of an electric field on the phase behavior of water confined inside a nanoscale space were studied using molecular dynamics simulations. It was found that the diffusion coefficient of water reaches its maximum when value of the surfaces' charge is at the threshold, qc = 0.5e. This unexpected phenomenon was attributed to the intermediate state between two stable ice states induced by nanoconfinement and the electric field generated by charged surfaces, respectively. Our finding is helpful to understand electromelting and electrofreezing of water under nanoconfinement with the electric field.

Electromanipulating water flow in nanochannels.

In sharp contrast to the prevailing view that a stationary charge outside a nanochannel impedes water permeation across the nanochannel, molecular dynamics simulations show that a vibrational charge outside the nanochannel can promote water flux. In the vibrational charge system, a decrease in the distance between the charge and the nanochannel leads to an increase in the water net flux, which is contrary to that of the fixed-charge system. The increase in net water flux is the result of the vibrational charge-induced disruption of hydrogen bonds when the net water flux is strongly affected by the vibrational frequency of the charge. In particular, the net flux is reaches a maximum when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.

Electricity resonance-induced fast transport of water through nanochannels.

We performed molecular dynamics simulations to study water permeation through a single-walled carbon nanotube with electrical interference. It was found that the water net flux across the nanochannel is greatly affected by the external electrical interference, with the maximal net flux occurred at an electrical interference frequency of 16670 GHz being about nine times as high as the net flux at the low or high frequency range of (<1000 GHz or >80,000 GHz). The above phenomena can be attributed to the breakage of hydrogen bonds as the electrical interference frequency approaches to the inherent resonant frequency of hydrogen bonds. The new mechanism of regulating water flux across nanochannels revealed in this study provides an insight into the water transportation through biological water channels and has tremendous potential in the design of high-flux nanofluidic systems.

Graphyne as the membrane for water desalination.

Permeation through membrane with pores is important in the choice of materials for filtration and separation techniques. Here, we report by the molecular dynamics simulations that a single-layer graphyne membrane can be impermeable to salt ions, while it allows the permeation of water molecules. The salt rejection and water permeability of graphyne are closely related to the hydrostatic pressure, type of graphyne membrane, and the salt concentration of solution, respectively. By analyzing hydration shell structure, we found that the average coordination number of ions plays a key role in water purification. Our calculation showed that the salt rejection of the graphyne-3 membrane is the best and it can keep an ideal rate of 100% in consideration cases. In comprehensive evaluation of both salt rejection and permeability, the graphyne-4 is a perfect purification membrane. To sum up, our results indicated that the graphynes (graphyne-3 and -4) not only have higher salt rejection but also possess higher water permeability which is several orders of magnitude higher than conventional reverse osmosis membranes. The single-layer graphyne membrane may have a great potential application as a membrane for water purification.

Vibrating-charge-driven water pump controlled by the deformation of the carbon nanotube.

The directed transport of water molecules in a single-walled carbon nanotube (SWNT) based on a ratchet effect is investigated by molecular dynamics simulations. The system is driven far away from thermal equilibrium by an additional deterministic perturbation of a vibrating charge, and the spatial inversion symmetry is broken by the continuous deformations of the SWNT. It is well-known that the water flux across a circular channel decreases when the channel is narrowed or deformed. However, our simulation results show that the water flux almost increases linearly within a deformation of 1.9 Å. There exists an optimized value of deformation at which the pumping capacity takes its maximum value. Moreover, the direction of transport even exhibits a change of sign with narrowing the carbon nanotube.

Water permeation through single-layer graphyne membrane.

We report the molecular dynamics simulations of spontaneous and continuous permeation of water molecules through a single-layer graphyne-3 membrane. We found that the graphyne-3 membrane is more permeable to water molecules than (5, 5) carbon nanotube membranes of similar pore diameter. The remarkable hydraulic permeability of the single-layer graphyne-3 membrane is attributed to the hydrogen bond formation, which connects the water molecules on both sides of the monolayer graphyne-3 membrane and aids to overcome the resistance of the nanopores, and to the relatively lower energy barrier at the pore entrance. Consequently, the single-layer graphyne-3 membrane has a great potential for application as membranes for desalination of sea water, filtration of polluted water, etc.

Unidirectional motion of a water nanodroplet subjected to a surface energy gradient.

We perform molecular dynamics simulations to demonstrate that when a nanodroplet is confined inside a carbon nanotube (CNT), unidirectional motion can be created by a nonzero surface energy gradient. It is found that the water nanodroplet moves along the direction of increasing surface energy. The transportation efficiency of the water nanodroplet is found to be dependent on the surface energy gradient; environmental temperature; and the flexibility, diameter, and defectiveness of the CNT. It is shown that higher surface energy gradient, the smaller diameter of the CNT, and fewer defects promote higher transportation efficiency. However, when the temperature is too high or too low, the water transport across the CNT is impeded. Except for the initial stage at the relatively low environmental temperature, higher flexibility of the CNT wall reduces the transportation efficiency. It is also found that the hydrogen bonds of water molecules play a role in the dynamic acceleration process with a wavelike feature. The present work provides insight for the development of CNT devices for applications such as drug delivery, nanopumps, chemical process control, and molecular medicine.