Combining classical molecular dynamics simulations and density functional theory calculations, we find that cations block water flow through narrow (6,6)-type carbon nanotubes (CNTs) because of interactions between cations and aromatic rings in CNTs. In wide CNTs, these interactions trap the cations in the interior of the CNT, inducing unexpected open or closed state switching of ion transfer under a strong electric field, which is consistent with experiments. These findings will help to develop new methods to facilitate water and ion transport across CNTs.

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Recent experiments have shown the coexistence of both large unoxidized and oxidized regions on graphene oxide (GO), but the underlying mechanism for the formation of the GO atomic structure remains unknown. Now, using density functional calculations, 52 oxidation pathways for local pyrene structures on GO were identified, and a kinetic profile for graphene oxidation with a high correlation between oxidation loci was proposed, which is different from the conventional view, which entails a random distribution of oxidation loci. The high correlation is an essential nature of graphene oxidation processes and can be attributed to three crucial effects: 1) breaking of delocalized π bonds, 2) steric hindrance, and 3) hydrogen-bond formation. This high correlation leads to the coexistence of both large unoxidized and oxidized regions on GO. Interestingly, even in oxidized regions on GO, some small areas of sp(2)-hybridized domains, similar to "islands", can persist because of steric effects.

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A source of missing sulfate production associated with high-level fine-particle pollution in the megacities of China is believed to stem from the oxidation of a notable fraction of sulfur dioxide (SO 2 ) by nitrogen dioxide (NO 2 ) in aqueous aerosol environments, suggesting that an unknown reaction pathway exists for aqueous sulfur oxidation. At weakly acidic aerosols, the dissolved SO 2 mainly exists in the form of HSO 3 − , whereas at neutral aerosols, SO 3 2− becomes the main form. Herein, by using both ab initio molecular metadynamics simulations and high-level quantum mechanical calculations, we show a hitherto unreported chemical mechanism for the formation of sulfate through the reaction between HSO 3 − /SO 3 2− anions at the surface/in the interior of a water nanodroplet and gas-phase NO 2 molecules. For weakly acidic aerosols, contrary to the conventional high-barrier electron-transfer pathway in the gas phase, HSO 3 − at the water nanodroplet surface can transfer an electron to NO 2 with a low free-energy barrier of 4.7 kcal/mol through a water bridge. For neutral aerosols, the electron-transfer pathway between SO 3 2− in the interior of the water nanodroplet and NO 2 needs to overcome a lower free-energy barrier of 3.6 kcal/mol to form SO 3 − , with the assistance of the hydrogen-bonding network of water molecules. This new reaction pathway for the sulfate formation from HSO 3 − /SO 3 2− via water nanodroplets and gaseous NO 2 provides a new perspective on the growth of haze particles from pre-existing aqueous aerosols and suggests that new control strategies are needed to address haze pollution.

By combining molecular dynamics simulations and quantum mechanics calculations, we show the formation of a composite structure composed of embedded water molecules and the COOH matrix on carboxyl-terminated self-assembled monolayers (COOH SAMs) with appropriate packing densities. This composite structure with an integrated hydrogen bond network inside reduces the hydrogen bonds with the water above. This explains the seeming contradiction on the stability of the surface water on COOH SAMs observed in experiments. The existence of the composite structure at appropriate packing densities results in the two-step distribution of contact angles of water droplets on COOH SAMs, around 0° and 35°, which compares favorably to the experimental measurements of contact angles collected from forty research articles over the past 25 years. These findings provide a molecular-level understanding of water on surfaces (including surfaces on biomolecules) with hydrophilic functional groups.

Recent experiments have shown the coexistence of both large unoxidized and oxidized regions on graphene oxide (GO), but the underlying mechanism for the formation of the GO atomic structure remains unknown. Now, using density functional calculations, 52 oxidation pathways for local pyrene structures on GO were identified, and a kinetic profile for graphene oxidation with a high correlation between oxidation loci was proposed, which is different from the conventional view, which entails a random distribution of oxidation loci. The high correlation is an essential nature of graphene oxidation processes and can be attributed to three crucial effects: 1) breaking of delocalized p bonds, 2) steric hindrance, and 3) hydrogen-bond formation. This high correlation leads to the coexistence of both large unoxidized and oxidized regions on GO. Interestingly, even in oxidized regions on GO, some small areas of sp 2 -hybridized domains, similar to "islands", can persist because of steric effects.

BackgroundNanomaterials hold great promise for applications in the delivery of various molecules with poor cell penetration, yet its potential for delivery of metal ions is rarely considered. Particularly, there is limited insight about the cytotoxicity triggered by nanoparticle-ion interactions. Oxidative stress is one of the major toxicological mechanisms for nanomaterials, and we propose that it may also contribute to nanoparticle-ion complexes induced cytotoxicity.MethodsTo explore the potential of nanodiamonds (NDs) as vehicles for metal ion delivery, we used a broad range of experimental techniques that aimed at getting a comprehensive assessment of cell responses after exposure of NDs, metal ions, or ND-ion mixture: 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, Trypan blue exclusion text, optical microscope observation, synchrotron-based scanning transmission X-ray microscopy (STXM) and micro X-ray fluorescence (μXRF) microscopy, inductively coupled plasma-mass spectrometry (ICP-MS), reactive oxygen species (ROS) assay and transmission electron microscopy (TEM) observation. In addition, theoretical calculation and molecular dynamics (MD) computation were used to illustrate the adsorption properties of different metal ion on NDs as well as release profile of ion from ND-ion complexes at different pH values.ResultsThe adsorption capacity of NDs for different metal ions was different, and the adsorption for Cu2+ was the most strong among divalent metal ions. These different ND-ion complexes then had different cytotoxicity by influencing the subsequent cellular responses. Detailed investigation of ND-Cu2+ interaction showed that the amount of released Cu2+ from ND-Cu2+ complexes at acidic lysosomal conditions was much higher than that at neutral conditions, leading to the elevation of intracellular ROS level, which triggered cytotoxicity. By theoretical approaches, we demonstrated that the functional carbon surface and cluster structures of NDs made them good vehicles for metal ions delivery.ConclusionsNDs played the Trojan horse role by allowing large amounts of metal ions accumulate into living cells followed by subsequent release of ions in the interior of cells, which then led to cytotoxicity. The present experimental and theoretical results provide useful insight into understanding of cytotoxicity triggered by nanoparticle-ion interactions, and open new ways in the interpretation of nanotoxicity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12989-014-0075-z) contains supplementary material, which is available to authorized users.