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Solid-liquid-gas reactions are ubiquitous and are encountered in both nature and industrial processes 1-4 . A comprehensive description of gas transport in liquid and following reactions at the solid-liquid-gas interface, which is substantial in regard to achieving enhanced triple-phase reactions, remains unavailable. Here, we report a real-time observation of the accelerated etching of gold nanorods with oxygen nanobubbles in aqueous hydrobromic acid using liquid-cell transmission electron microscopy. Our observations reveal that when an oxygen nanobubble is close to a nanorod below the critical distance (~1 nm), the local etching rate is significantly enhanced by over one order of magnitude. Molecular dynamics simulation results show that the strong attractive van der Waals interaction between the gold nanorod and oxygen molecules facilitates the transport of oxygen through the thin liquid layer to the gold surface and thus plays a crucial role in increasing the etching rate. This result sheds light on the rational design of solid-liquid-gas reactions for enhanced activities., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The optical properties of silicon quantum dots (Si QDs) are strongly influenced by circumjacent surface-adsorbed molecules, which would highly affect their applications; however, water, as the ubiquitous environment, has not received enough attention yet. We employed the time-dependent density functional calculations to investigate the water effect of photoluminescence (PL) spectra for Si QDs. In contrast with the absorption spectra, PL spectra exhibit distinct characteristics. For Si32H38, PL presents the single maximum in the dry and humid environment, while the emission spectrum displays a dual-band fluorescence spectroscopy in the low-humidity environment. This phenomenon is also observed in the larger Si QDs. The distinct character in spectroscopy is dominated by the stretching of the Si-Si bond, which could be explained by the self-trapped exciton model. Our results shed light on the Si-water interaction that is important for the development of optical devices based on Si-coated surfaces.

Using molecular dynamics simulations, we show that the evaporation of nanoscale water on hydrophobic-hydrophilic patterned surfaces is unexpectedly faster than that on any surfaces with uniform wettability. The key to this phenomenon is that, on the patterned surface, the evaporation rate from the hydrophilic region only slightly decreases due to the correspondingly increased water thickness; meanwhile, a considerable number of water molecules evaporate from the hydrophobic region despite the lack of water film. Most of the evaporated water from the hydrophobic region originates from the hydrophilic region by diffusing across the contact lines. Further analysis shows that the evaporation rate from the hydrophobic region is approximately proportional to the total length of the contact lines.

Using molecular dynamics simulations, we find a reversible transition between the dispersion and aggregation states of solute molecules in aqueous solutions confined in nanoscale geometry, which is not observed in macroscopic systems. The nanoscale confinement also leads to a significant increase of the critical aggregation concentration (CAC). A theoretical model based on Gibbs free energy calculation is developed to describe the simulation results. It indicates that the reversible state transition is attributed to the low free energy barrier (of order kBT) in between two energy minima corresponding to the dispersion and aggregation states, and the enhancement of the CAC results from the fact that at lower concentrations the number of solute molecules is not large enough to allow the formation of a stable cluster in the confined systems.

The dynamics and structure of the hydrogen-bond network in confined water are of importance in understanding biological and chemical processes. Recently, terahertz (THz) time domain spectroscopy was widely applied for studying the kinetics of molecules and the hydrogen-bond network in water. However, the characteristics of the THz spectroscopy varying with respect to the confinement and the mechanism underlying the variation are still unclear. Here, on the basis of molecular dynamics simulations, the relationship between the anisotropic dielectric relaxation and the structure of the water confined in a carbon nanotube (CNT) was investigated. The results show that there are two preferred hydrogen-bond orientations of the confined water in the nanotube: (1) parallel to the CNT axis and (2) perpendicular to the CNT axis, which are clearly different. Moreover, the response of the orientations to the increment of the CNT diameters is opposite, leading to the opposite variations of the dielectric relaxation times along the two directions. The anisotropy in the relaxation time can be presented by the anisotropic dielectric permittivity which is able to be observed through THz spectroscopy. The anormal behaviors above are attributed to the special structure of the water close to the nanotube wall due to the confinement and hydrophobicity of CNT. These studies contribute an important step in understanding the THz experiments of water in nanoscales, and designing a chamber for specific chemical and biological reactions by controlling the diameters and materials of the nanotube.

The evaporation of a tiny amount of water on the solid surface with different wettabilities has been studied by molecular dynamics simulations. From nonequilibrium MD simulations, we found that, as the surface changed from hydrophobic to hydrophilic, the evaporation speed did not show a monotonic decrease as intuitively expected, but increased first, and then decreased after it reached a maximum value. The analysis of the simulation trajectory and calculation of the surface water interaction illustrate that the competition between the number of water molecules on the water-gas surface from where the water molecules can evaporate and the potential barrier to prevent those water molecules from evaporating results in the unexpected behavior of the evaporation. This finding is helpful in understanding the evaporation on biological surfaces, designing artificial surfaces of ultrafast water evaporating, or preserving water in soil.

Nanoscale gas bubbles have surprising stability at water/solid surfaces. Herein, we summarize progress made on investigating gases at the water/solid interfaces on the nanometer scale. The gas states include nanobubbles, micropancakes, multiple gas layers and their coexistence; these were investigated from experimental and theoretical aspects. The stability of nanoscale gas bubbles may be attributed to high inner density, as observed in molecular dynamic simulations and theoretical analysis. Moreover, it was found that there were maximal length scales for stable nanobubbles, namely, 100 nm high and a curvature radius of 2 μm., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The base pair hybridization of a DNA segment was studied using molecular dynamics simulation. The results show the obvious correlation between the probability of successful hybridization and the accessible surface area to water of two successive base pairs, including the unpaired base pair adjacent to paired base pair and this adjacent paired base pair. Importantly, two metastable structures in an A-T base pair were discovered by the analysis of the free energy landscape. Both structures involved addition of a water molecule to the linkage between the two nucleobases in one base pair. The existence of the metastable structures provide potential barriers to the Watson-Crick base pair, and numerical simulations show that those potential barriers can be surmounted by thermal fluctuations at higher temperatures. These studies contribute an important step toward the understanding of the mechanism in DNA hybridization, particularly the effect of temperature on DNA hybridization and polymerase chain reaction. These observations are expected to be helpful for facilitating experimental bio/nanotechnology designs involving fast hybridization.

The kinetics of water filling narrow single-walled carbon nanotubes was studied using molecular dynamics simulations. The time required to fully fill a nanotube was linear with respect to the tube length. We observed that water molecules could enter into nanotubes of different lengths, either from one end or from both ends. The probability of having a nanotube filled completely from both ends increased exponentially with the tube length. For short tubes, filling usually proceeded from only one end. For long tubes, filling generally proceeded from both tube ends over three stages, i.e., filling from one end, filling from both ends, and filling from both ends with the dipole reorientation of water molecules to give a concerted ordering within the fully filled tube. The water molecules in the partially filled nanotube were hydrogen bonded similarly to those in the fully filled nanotube. Simulations for the reference Lennard-Jones fluid without hydrogen bonds were also performed and showed that the filling behavior of water molecules can be attributed to strong intermolecular hydrogen bonding.

The dynamics of water inside nanochannels is of great importance for biological activities as well as for the design of molecular sensors, devices, and machines, particularly for sea water desalination. When confined in specially sized nanochannels, water molecules form a single-file structure with concerted dipole orientations, which collectively flip between the directions along and against the nanotube axis. In this paper, by using molecular dynamics simulations, we observed a net flux along the dipole-orientation without any application of an external electric field or external pressure difference during the time period of the particular concerted dipole orientations of the molecules along or against the nanotube axis. We found that this unique special-directional water transportation resulted from the asymmetric potential of water-water interaction along the nanochannel, which originated from the concerted dipole orientation of the water molecules that breaks the symmetry of water orientation distribution along the channel within a finite time period. This finding suggests a new mechanism for achieving high-flux water transportation, which may be useful for nanotechnology and biological applications.

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.

Using molecular dynamics simulation, we show direct evidence of the unexpected phenomenon of "water that does not wet a water monolayer" at room temperature. This phenomenon is attributed to the structure of the water beneath the water droplet, which exhibits an ordered water monolayer. Remarkably, there remains a considerable number of dangling OH bonds in this room temperature water monolayer, in contrast with the absence of dangling OH bonds at cryogenic temperature.

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.

Recent experiments and numerical simulations have shown that the water molecules confined on the surfaces of some substrates, including the surfaces of cellular components in tissues and cells, form icelike ordered structures. If a protein folds in an environment with those icelike ordered water molecules, its behavior may be different from that in bulk water. Here, the effect of this ordered water environment on protein folding is studied by using an off-lattice Gō-like model. It is found that the ordered water environment significantly improves the native state stability and greatly speeds up the folding rate of the proteins.

We used molecular dynamics simulation to study the effect of the external structure on water permeation across a single-walled nanochannel. In contrast with the macroscopic scenario, the outside structure greatly affects the water transport across the nanochannel. Remarkably, the ratio of maximal to minimal flux reached a value of about two for different outside structures. These findings are expected to be helpful in design of high-flux nanochannels and provide an insight into the contribution of the lipid membrane to water permeation across biological water channels.

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 A 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 A 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.

We report a novel form of the gaseous state at the interface of water and highly oriented pyrolytic graphite (HOPG) that is induced by local supersaturation of gas. Such local supersaturation of gas next to the HOPG substrate can be achieved by (1) displacing an organic liquid with a gentle flow of water, (2) displacing cold water (approximately 0 degrees C) with a gentle flow of warm water (approximately 40 degrees C), or (3) preheating the HOPG substrate to approximately 80 degrees C before exposing it to water at room temperature. In addition to the spherical-cap-shaped nanobubbles reported by many researchers, flat (quasi-two-dimensional, pancake-like) gas layers and nanobubble-flat gas layer composites (spherical-cap-shaped nanobubbles sitting on top of the quasi-two-dimensional gas layers) were detected. These entities disappeared after the system was subjected to a moderate level of degassing (approximately 0.1 atm for 1.5 h), and they did not form when the liquids involved in the aforementioned displacing procedure (to induce local supersaturation of gas) had been predegassed (to approximately 0.1 atm). The stability and some physical properties of these newly found gaseous states are examined.

The dynamics of water molecules in a single-walled carbon nanotube (SWNT) under continuous deformations was studied with molecular dynamics simulations. The flux and occupancy remain almost fixed within a deformation of 2.0 A but decrease sharply for a further deformation of 0.6 A. The nanopore is an excellent on-off gate that is both effectively resistant to deformation noises and sensitive to available signals. Biological water channels are expected to share this advantage due to similar wavelike water distributions. The minimal external force required for triggering an open-close transition falls within the working range of many available experimental facilities, which provides the possibility of developing SWNT-based nanoscale devices.

The evaporation of water is essential in the macroscopic world. Recent researches show that, on solid surfaces, the evaporation of nanoscale water is quite different from that on bulk water surfaces. In this review, we show the theoretical progress in the study of nanoscale water evaporation on various solid surfaces: the evaporation rate of nanoscale water does not show a monotonic decrease when the solid surface changes from hydrophobic to hydrophilic; the evaporation of nanoscale water on hydrophobic–hydrophilic patterned surfaces is unexpectedly faster than that on uniform surface; the evaporation of nanoscale water on patterned graphene oxide is faster than that on homogeneous one; how temperature affects the evaporation of nanoscale water on solid surface; how ions affect the evaporation of nanoscale water on graphene oxide. [ABSTRACT FROM AUTHOR]

Whether thermal noise is coloredor white is of fundamental importance. In conventional theory, thermalnoise is usually treated as white noise so that there are no directionaltransportations in the asymmetrical systems without external inputs,since only the colored fluctuations with appropriate autocorrelationtime length can lead to directional transportations in the asymmetricalsystems. Here, on the basis of molecular dynamics simulations, weshow that the autocorrelation time length of thermal noise in wateris ∼10 ps at room temperature, which indicates that thermalnoise is not white in the molecular scale while thermal noise canbe reasonably assumed as white in macro- and meso-scale systems. Theautocorrelation time length of thermal noise is intrinsic, since thevalue is almost unchanged for different temperature coupling methods.Interestingly, the autocorrelation time of thermal noise is correlatedwith the lifetime of hydrogen bonds, suggesting that the finite autocorrelationtime length of thermal noise mainly comes from the finite lifetimeof the interactions between neighboring water molecules. [ABSTRACT FROM AUTHOR]