Hydrated cation-π interactions of π-electrons with hydrated Mg2+ and Ca2+ cations.

Hydrated cation-π interactions at liquid-solid interfaces between hydrated cations and aromatic ring structures of carbon-based materials are pivotal in many material, biological, and chemical processes, and water serves as a crucial mediator in these interactions. However, a full understanding of the hydrated cation-π interactions between hydrated alkaline earth cations and aromatic ring structures, such as graphene remains elusive. Here, we present a molecular picture of hydrated cation-π interactions for Mg2+ and Ca2+ by using the density functional theory methods. Theoretical results show that the graphene sheet can distort the hydration shell of the hydrated Ca2+ to interact with Ca2+ directly, which is water-cation-π interactions. In contrast, the hydration shell of the hydrated Mg2+ is quite stable and the graphene sheet interacts with Mg2+ indirectly, mediated by water molecules, which is the cation-water-π interactions. These results lead to the anomalous order of adsorption energies for these alkaline earth cations, with hydrated Mg2+-π < hydrated Ca2+-π when the number of water molecules is large (n ≥ 6), contrary to the order observed for cation-π interactions in the absence of water molecules (n = 0). The behavior of hydrated alkaline earth cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-π interactions and hydration effects. These findings provide valuable details of the structures and the adsorption energy of hydrated alkaline earth cations adsorbed onto the graphene surface.

Light-Boosting Highly Sensitive and Ultrafast Piezoelectric Sensor Based on Composite Membrane of Copper Phthalocyanine and Graphene Oxide.

Self-powered wearable pressure sensors based on flexible electronics have emerged as a new trend due to the increasing demand for intelligent and portable devices. Improvements in pressure-sensing performance, including in the output voltage, sensitivity and response time, can greatly expand their related applications; however, this remains challenging. Here, we report on a highly sensitive piezoelectric sensor with novel light-boosting pressure-sensing performance, based on a composite membrane of copper phthalocyanine (CuPC) and graphene oxide (GO) (CuPC@GO). Under light illumination, the CuPC@GO piezoelectric sensor demonstrates a remarkable increase in output voltage (381.17 mV, 50 kPa) and sensitivity (116.80 mV/kPa, <5 kPa), which are approximately twice and three times of that the sensor without light illumination, respectively. Furthermore, light exposure significantly improves the response speed of the sensor with a response time of 38.04 µs and recovery time of 58.48 µs, while maintaining excellent mechanical stability even after 2000 cycles. Density functional theory calculations reveal that increased electron transfer from graphene to CuPC can occur when the CuPC is in the excited state, which indicates that the light illumination promotes the electron excitation of CuPC, and thus brings about the high polarization of the sensor. Importantly, these sensors exhibit universal spatial non-contact adjustability, highlighting their versatility and applicability in various settings.

Solid-liquid-gas reaction accelerated by gas molecule tunnelling-like effect.

Solid-liquid-gas reactions are ubiquitous and are encountered in both nature and industrial processes1-4. 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.

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.

Thermally Reduced Graphene Oxide Membranes Revealed Selective Adsorption of Gold Ions from Mixed Ionic Solutions.

The recovery of gold from water is an important research area. Recent reports have highlighted the ultrahigh capacity and selective extraction of gold from electronic waste using reduced graphene oxide (rGO). Here, we made a further attempt with the thermal rGO membranes and found that the thermal rGO membranes also had a similarly high adsorption efficiency (1.79 g gold per gram of rGO membranes at 1000 ppm). Furthermore, we paid special attention to the detailed selectivity between Au3+ and other ions by rGO membranes. The maximum adsorption capacity for Au3+ ions was about 16 times that of Cu2+ ions and 10 times that of Fe3+ ions in a mixture solution with equal proportions of Au3+/Cu2+ and Au3+/Fe3+. In a mixed-ion solution containing Au3+:Cu2+:Na+:Fe3+:Mg2+ of printed circuit board (PCB), the mass of Au3+:Cu2+:Na+:Fe3+:Mg2+ in rGO membranes is four orders of magnitude higher than the initial mass ratio. A theoretical analysis indicates that this selectivity may be attributed to the difference in the adsorption energy between the metal ions and the rGO membrane. The results are conducive to the usage of rGO membranes as adsorbents for Au capture from secondary metal resources in the industrial sector.

Revisit the Hydrated Cation-π Interaction at the Interface: A New View of Dynamics and Statistics.

Carbon-based matter, such as biomolecules and graphitic structures, often form a liquid-solid/soft matter interface in salt solution and continuously affect the surrounding cations through hydrated cation-π interactions. In this Perspective, we revisit the effect of the hydrated cation-π interactions at the interface using statistical physics, which reveals how hydrated cation-π interactions affect every component dynamically and cause a time-dependent statistical effect at the liquid-solid/soft interface. We also highlight several pieces of experimental evidence from a statistical perspective and discuss the remarkable applications related to environmental protection, industrial manufacturing, and biological sciences.

Size-Dependent Spontaneous Separation of Colloidal Particles in Sub-Microliter Suspension by Cations.

Great efforts have been made to separate micro/nanoparticles in small-volume specimens, but it is a challenge to achieve the simple, maneuverable and low-cost separation of sub-microliter suspension with large separation distances. By simply adding trace amounts of cations (Mg2+/Ca2+/Na+), we experimentally achieved the size-dependent spontaneous separation of colloidal particles in an evaporating droplet with a volume down to 0.2 µL. The separation distance was at a millimeter level, benefiting the subsequent processing of the specimen. Within only three separating cycles, the mass ratio between particles with diameters of 1.0 µm and 0.1 µm can be effectively increased to 13 times of its initial value. A theoretical analysis indicates that this spontaneous separation is attributed to the size-dependent adsorption between the colloidal particles and the aromatic substrate due to the strong hydrated cation-π interactions.

Enhancement of the Water Affinity of Histidine by Zinc and Copper Ions.

Histidine (His) is widely involved in the structure and function of biomolecules. Transition-metal ions, such as Zn2+ and Cu2+, widely exist in biological environments, and they are crucial to many life-sustaining physiological processes. Herein, by employing density function calculations, we theoretically show that the water affinity of His can be enhanced by the strong cation-π interaction between His and Zn2+ and Cu2+. Further, the solubility of His is experimentally demonstrated to be greatly enhanced in ZnCl2 and CuCl2 solutions. The existence of cation-π interaction is demonstrated by fluorescence, ultraviolet (UV) spectroscopy and nuclear magnetic resonance (NMR) experiments. These findings are of great importance for the bioavailability of aromatic drugs and provide new insight for understanding the physiological functions of transition metal ions.

Hydrated cation-π interactions of π-electrons with hydrated Li+, Na+, and K+ cations.

Cation-π interactions are essential for many chemical, biological, and material processes, and these processes usually involve an aqueous salt solution. However, there is still a lack of a full understanding of the hydrated cation-π interactions between the hydrated cations and the aromatic ring structures on the molecular level. Here, we report a molecular picture of hydrated cation-π interactions, by using the calculations of density functional theory (DFT). Specifically, the graphene sheet can distort the hydration shell of the hydrated K+ to interact with K+ directly, which is hereafter called water-cation-π interactions. In contrast, the hydration shell of the hydrated Li+ is quite stable and the graphene sheet interacts with Li+ indirectly, mediated by water molecules, which we hereafter call the cation-water-π interactions. The behavior of hydrated cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-π interactions and hydration effects. These findings provide valuable details of the structures and the adsorption energy of hydrated cations adsorbed onto the graphene surface.

Ultrahigh Density of Gas Molecules Confined in Surface Nanobubbles in Ambient Water.

To understand the unexpected and puzzling long-term stability of nanoscale gas bubbles, it is crucial to probe their nature and intrinsic properties. We report herein synchrotron-based scanning transmission X-ray microscopy (STXM) evidence of highly condensed oxygen gas molecules trapped as surface nanobubbles. Remarkably, the analysis of absorption spectra of a single nanobubble revealed that the oxygen density inside was 1-2 orders of magnitude higher than that in atmospheric pressure, and these bubbles were found in a highly saturated liquid environment with the estimated oxygen concentration to be hundreds of times higher than the known oxygen solubility in equilibrium. Molecular dynamics simulations were performed to investigate the stability of surface nanobubbles on a heterogeneous substrate in gas-oversaturated water. These results indicated that gas molecules within confinement such as the nanobubbles could maintain a dense state instead of the ideal gas state, as long as their surrounding liquid is supersaturated. Our findings should help explain the surprisingly long lifetime of the nanobubbles and shed light on nanoscale gas aggregation behaviors.

Unexpected sequence adsorption features of polynucleotide ssDNA on graphene oxide.

The sequence features of single-stranded DNA (ssDNA) adsorbed on a graphene oxide (GO) surface are important for applications of the DNA/GO functional structure in biosensors, biomedicine, and materials science. In this study, molecular dynamics (MD) simulations were used to examine the adsorption of polynucleotide ssDNAs (A12, C12, G12, and T12) and single nucleotides (A, C, G, and T) on the GO surface. For the latter case, the nucleotide-GO interaction energy followed the trend G > A > C > T, even though it was influenced by specific adsorption sites. In the case of polynucleotides, unexpectedly polythymidine (T12) had the strongest interaction with the GO surface. The angle distributions of the adsorbed nucleobases indicated that T12 was more likely to form a quasi-parallel structure with GO compared to A12, C12, or G12. This was attributed to the weakest π-stacking interactions of thymine. Weaker intra-molecular base-stacking interactions made it easier to break the structures of pyrimidine bases relative to those of purine bases. Weaker inter-molecular base-stacking interactions between T12 and the GO surface enabled T12 to adjust its structure easily to a more stable one by slipping on the surface. This result provides a new understanding of polynucleotide ssDNA adsorption on GO surfaces, which will help in the design of functional DNA/GO structure-based platforms.

Unconventional Atomic Structure of Graphene Sheets on Solid Substrates.

The atomic structure of free-standing graphene comprises flat hexagonal rings with a 2.5 Å period, which is conventionally considered the only atomic period and determines the unique properties of graphene. Here, an unexpected highly ordered orthorhombic structure of graphene is directly observed with a lattice constant of ≈5 Å, spontaneously formed on various substrates. First-principles computations show that this unconventional structure can be attributed to the dipole between the graphene surface and substrates, which produces an interfacial electric field and induces atomic rearrangement on the graphene surface. Further, the formation of the orthorhombic structure can be controlled by an artificially generated interfacial electric field. Importantly, the 5 Å crystal can be manipulated and transformed in a continuous and reversible manner. Notably, the orthorhombic lattice can control the epitaxial self-assembly of amyloids. The findings reveal new insights about the atomic structure of graphene, and open up new avenues to manipulate graphene lattices.

Precise control of the interlayer spacing between graphene sheets by hydrated cations.

Recently, we have demonstrated that highly efficient ion rejection by graphene oxide membranes can be facilely achieved using hydrated cations to control the interlayer spacing in GO membranes. By using density functional theory calculations, we have shown that different hydrated cations can also precisely control the interlayer spacings between graphene sheets, which are smaller than graphene oxide sheets; this indicates ion sieving. The interlayer distances are 9.35, 8.96 and 8.82 Å for hydrated Li+, Na+ and K+, respectively. Since the radii of the hydrated Na+ and Li+ ions are larger than that of hydrated K+, graphene membranes controlled by the hydrated K+ ion can exclude K+ and the other two cations with larger hydrated volumes. Further analysis of charge transfer and orbit analysis showed that this type of control by the hydrated cations is attributed to the strong hydrated cation-π interactions; moreover, when soaked in a salt solution, graphene membranes adsorb hydrated Na+ and Li+ and form intercalation compounds. However, it is hard to find K-doped intercalation compounds in the inner part of graphene.

Janus effect of antifreeze proteins on ice nucleation.

The mechanism of ice nucleation at the molecular level remains largely unknown. Nature endows antifreeze proteins (AFPs) with the unique capability of controlling ice formation. However, the effect of AFPs on ice nucleation has been under debate. Here we report the observation of both depression and promotion effects of AFPs on ice nucleation via selectively binding the ice-binding face (IBF) and the non-ice-binding face (NIBF) of AFPs to solid substrates. Freezing temperature and delay time assays show that ice nucleation is depressed with the NIBF exposed to liquid water, whereas ice nucleation is facilitated with the IBF exposed to liquid water. The generality of this Janus effect is verified by investigating three representative AFPs. Molecular dynamics simulation analysis shows that the Janus effect can be established by the distinct structures of the hydration layer around IBF and NIBF. Our work greatly enhances the understanding of the mechanism of AFPs at the molecular level and brings insights to the fundamentals of heterogeneous ice nucleation.

Unexpectedly High Salt Accumulation inside Carbon Nanotubes Soaked in Dilute Salt Solutions.

We experimentally demonstrate the formation of salt aggregations with unexpectedly high concentration inside multiwalled carbon nanotubes (CNTs) soaked only in dilute salt solution sand even in solutions containing only traces of salts. This finding suggests the blocking of fluid across CNTs by the salt aggregations when CNTs are soaked in a dilute salt solution with the concentration of seawater or even lower, which may open new avenues for the development of novel CNT-based desalination techniques. The high salt accumulation of CNTs also provides a new CNT-based strategy for the collection or extraction of noble metal salts in solutions containing traces of noble metal salts. Theoretical analyses reveal that this high salt accumulation inside CNTs can be mainly attributed to the strong hydrated cation-π interactions of hydrated cations and π electrons in the aromatic rings of CNTs.

Tumor Cell-Specific Nuclear Targeting of Functionalized Graphene Quantum Dots In Vivo.

Specific targeting of tumor tissues is essential for tumor imaging and therapeutics but remains challenging. Here, we report an unprecedented method using synthetic sulfonic-graphene quantum dots (sulfonic-GQDs) to exactly target the cancer cell nuclei in vivo without any bio- ligand modification, with no intervention in cells of normal tissues. The key factor for such selectivity is the high interstitial fluid pressure (IFP) in tumor tissues, which allows the penetration of sulfonic-GQDs into the plasma membrane of tumor cells. In vitro, the sulfonic-GQDs are repelled out of the cell membrane because of the repulsive force between negatively charged sulfonic-GQDs and the cell membranes which contributes to the low distribution in normal tissues in vivo. However, the plasma membrane-crossing process can be activated by incubating cells in ultrathin film culture medium because of the attachment of sulfonic-GQDs on cell memebranes. Molecular dynamics simulations demonstrated that, once transported across the plasma membrane, the negatively charged functional groups of these GQDs will leave the membrane with a self-cleaning function retaining a small enough size to achieve penetration through the nuclear membrane into the nucleus. Our study showed that IFP is a previously unrecognized mechanism for specific targeting of tumor cell nuclei and suggested that sulfonic-GQDs may be developed into novel tools for tumor-specific imaging and therapeutics.

Dynamic Cooperation of Hydrogen Binding and π Stacking in ssDNA Adsorption on Graphene Oxide.

Functional nanoscale structures consisting of a DNA molecule coupled to graphene or graphene oxide (GO) have great potential for applications in biosensors, biomedicine, nanotechnology, and materials science. Extensive studies using the most sophisticated experimental techniques and theoretical methods have still not clarified the dynamic process of single-stranded DNA (ssDNA) adsorbed on GO surfaces. Based on a molecular dynamics simulation, this work shows that an ssDNA segment could be stably adsorbed on a GO surface through hydrogen bonding and π-π stacking interactions, with preferential binding to the oxidized rather than to the unoxidized region of the GO surface. The adsorption process shows a dynamic cooperation adsorption behavior; the ssDNA segment first captures the oxidized groups of the GO surface by hydrogen bonding interaction, and then the configuration relaxes to maximize the π-π stacking interactions between the aromatic rings of the nucleobases and those of the GO surface. We attributed this behavior to the faster forming hydrogen bonding interaction compared to π-π stacking; the π-π stacking interaction needs more relaxation time to regulate the configuration of the ssDNA segment to fit the aromatic rings on the GO surface.

Water flow in carbon-based nanoporous membranes impacted by interactions between hydrated ions and aromatic rings.

Carbon-based nanoporous membranes, such as carbon nanotubes (CNTs), graphene/graphene oxide and graphyne, have shown great potential in water desalination and purification, gas and ion separation, biosensors, and lithium-based batteries, etc. A deep understanding of the interaction between hydrated ions in an aqueous solution and the graphitic surface in systems composed of water, ions and a graphitic surface is essential for applications with carbon-based nanoporous membrane platforms. In this review, we describe the recent progress of the interaction between hydrated ions and aromatic ring structures on the carbon-based surface and its applications in the water flow in a carbon nanotube. We expect that these works can be extended to the understanding of water flow in other nanoporous membranes, such as nanoporous graphene, graphyne and stacked sheets of graphene oxide.

A nonmonotonic dependence of the contact angles on the surface polarity for a model solid surface.

Based on molecular dynamics simulations, we found a nonmonotonic relationship between the contact angle of water droplets and the surface polarity on a solid surface with specific hexagonal charge patterns at room temperature. The contact angle firstly decreases and then increases as polarity (denoted as charge q) increases from 0 e to 1.0 e with a vertex value of q = 0.5 e. We observed a different wetting behavior for a water droplet on a conventional nonwetted solid surface when q ≤ 0.5 e, and a water droplet on an ordered water monolayer adsorbed on a highly polar solid surface when q > 0.5 e. The solid-water interaction, density of water, hydrogen bonds, and water structures were analyzed. Remarkably, there was up to six times difference in the solid-water interactions despite the same value of the apparent contact angle values.

S-shaped velocity deformation induced by ionic hydration in aqueous salt solution flow.

Ionic hydration shells are the most noticeable microscopic feature in an aqueous salt solution, and have attracted attention due to their possible contribution to its flow behavior. In this paper, we find by molecular dynamic simulations that an S-shaped velocity profile is induced by the ionic hydration shells in the nano channel flow. Our theoretical analysis implies a linear relationship between the energy density inside the first hydration shell of the ions and the deformation strength of the velocity profiles of aqueous salt solutions, where the deformation strength is quantified by the curvature length defined by the linear deviation extended from the velocity profile. Our simulation results confirm that such a linear relationship holds for chloride salt solutions with monatomic cations, e.g., K, Na, Ca, Mg, Al and the Na/Ca models by varying the valence number of Na and Ca in the salt solutions. Furthermore, the influence of the flow velocity and the channel width upon the velocity deformation strength are also investigated. Our results indicate that the calculated curvature length provides a numerical evaluation for nano flow behavior and would be helpful in nanofluidic device design.