Quantum chemical molecular dynamics simulation of carbon nanotube–graphene fusion

Abstract

Here we report a quantum mechanical molecular dynamics (QM/MD) study of a fusion process of an open-ended carbon nanotube on a graphene hole, which results in the formation of a so-called pillared graphene structure – a three-dimensional nanomaterial consisting entirely of sp²-carbons. The self-consistent-charge density-functional tight-binding potential was adopted in this study. Two different sizes of graphene holes with 12 or 24 central carbon atoms removed from a graphene flake, and a (6,6) carbon nanotube with a compatible diameter were adopted. Formations of 6–7–6/5–8–5 defect structures were found on the fusion border between tube and graphene hole. The 6–7–6 structure was found to bear less curvature-induced strain energy and therefore to be more stable and much easier to form than the 5–8–5 structure.     

Simple benzene derivatives adsorption on defective single-walled carbon nanotubes: a first-principles van der Waals density functional study

We have investigated the interaction between open-ended zig-zag single-walled carbon nanotube (SWCNT) and a few benzene derivatives using the first-principles van der Waals density functional (vdW-DF) method, involving full geometry optimization. Such sp ²-like materials are typically investigated using conventional DFT methods, which significantly underestimate non-local dispersion forces (vdW interactions), therefore affecting interactions between respected molecules. Here, we considered the vdW forces for the interacting molecules that originate from the interacting π electrons of the two systems. The ?0.54 eV adsorption energy reveals that the interaction of benzene with the side wall of the SWCNT is typical of the strong physisorption and comparable with the experimental value for benzene adsorption onto the graphene sheet. It was found that aromatics are physisorbed on the sidewall of perfect SWCNTs, as well as at the edge site of the defective nanotube. Analysis of the electronic structures shows that no orbital hybridization between aromatics and nanotubes occurs in the adsorption process. The results are relevant in order to identify the potential applications of noncovalent functionalized systems.

Biocatalysts for fuel cells: efficient hydrogenase orientation for H<Subscript>2</Subscript> oxidation at electrodes modified with carbon nanotubes

We report the modification of gold and graphite electrodes with commercially available carbon nanotubes for immobilization of Desulfovibrio fructosovorans [NiFe] hydrogenase, for hydrogen evolution or consumption. Multiwalled carbon nanotubes, single-walled carbon nanotubes (SWCNs), and amine-modified and carboxyl-functionalized SWCNs were used and compared throughout. Two separate methods were performed: covalent attachment of oriented hydrogenase by controlled architecture of carbon nanotubes at gold electrodes, and adsorption of hydrogenase at carbon-nanotube-coated pyrolytic graphite electrodes. In the case of self-assembled carbon nanotubes at gold electrodes, hydrogenase orientation based on electrostatic interaction with the electrode surface was found to control the electrocatalytic process for H₂ oxidation. In the case of carbon nanotube coatings on pyrolytic graphite electrodes, catalysis was controlled more by the geometry of the nanotubes than by the orientation of the enzyme. Noticeably, shortened SWCNs were demonstrated to allow direct electron transfer and generate high and quite stable current densities for H₂ oxidation via adsorbed hydrogenase, despite having many carboxylic surface functions that could yield unfavorable hydrogenase orientation for direct electron transfer. This result is attributable to the high degree of oxygenated surface functions in addition to the length of shortened SWCNs that yields highly divided materials. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Fabrication of N‐doped Graphene–Carbon Nanotube Hybrids from Prussian Blue for Lithium–Sulfur Batteries

Hybrid nanostructures containing 1D carbon nanotubes and 2D graphene sheets have many promising applications due to their unique physical and chemical properties. In this study, the authors find Prussian blue (dehydrated sodium ferrocyanide) can be converted to N‐doped graphene–carbon nanotube hybrid materials through a simple one‐step pyrolysis process. Through field emission scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, Raman spectra, atomic force microscopy, and isothermal analyses, the authors identify that 2D graphene and 1D carbon nanotubes are bonded seamlessly during the growth stage. When used as the sulfur scaffold for lithium–sulfur batteries, it demonstrates outstanding electrochemical performance, including a high reversible capacity (1221 mA h g^?1 at 0.2 C rate), excellent rate capability (458 and 220 mA h g^?1 at 5 and 10 C rates, respectively), and excellent cycling stability (321 and 164 mA h g^?1 at 5 and 10 C (1 C = 1673 mA g^?1) after 1000 cycles). The enhancement of electrochemical performance can be attributed to the 3D architecture of the hybrid material, in which, additionally, the nitrogen doping generates defects and active sites for improved interfacial adsorption. Furthermore, the nitrogen doping enables the effective trapping of lithium polysulfides on electroactive sites within the cathode, leading to a much‐improved cycling performance. Therefore, the hybrid material functions as a redox shuttle to catenate and bind polysulfides, and convert them to insoluble lithium sulfide during reduction. The strategy reported in this paper could open a new avenue for low cost synthesis of N‐doped graphene–carbon nanotube hybrid materials for high performance lithium–sulfur batteries.     

Conjugates between photosystem I and a carbon nanotube for a photoresponse device

Photosystem I (PS I) is a large pigment–protein complex embedded in the thylakoid membranes that performs light-driven electron transfer across the thylakoid membrane. Carbon nanotubes exhibit excellent electrical conductivities and excellent strength and stiffness. In this study, we generated PSI–carbon nanotube conjugates dispersed in a solution aimed at application in artificial photosynthesis. PS I complexes in which a carbon nanotube binding peptide was introduced into the middle of the PsaE subunit were conjugated on a single-walled carbon nanotube, orienting the electron acceptor side to the nanotube. Spectral and photoluminescence analysis showed that the PS I is bound to a single-walled carbon nanotube, which was confirmed by transmission electron microscopy. Photocurrent observation proved that the photoexcited electron originated from PSI and transferred to the carbon nanotube with light irradiation, which also confirmed its orientated conjugation. The PS I–carbon nanotube conjugate will be a useful nano-optoelectronic device for the development of artificial systems.     

Molecular Dynamics Simulations of Carbon Nanotube Rolling and Sliding on Graphite

Abstract

Molecular dynamics simulations were carried out to investigate the origin of friction for carbon nanotubes on graphite substrates. In an initial simulation, a (10,10) nanotube was placed in an ‘in-registry’ starting position where the hexagonal lattice of the substrate matched that of the nanotube. In a second simulation, the substrate was oriented 90 degrees to the nanotube. A uniform force was applied to the nanotubes for 500 fs to set them into motion. The simulation was then run until the nanotubes stopped moving relative to the substrate. Only sliding was observed in the out-of-registry simulation, while periodic sliding and rolling was observed in the in-registry simulation. The latter is a result of the relatively larger surface corrugation for the in-registry case and occurs to avoid direct atomic collisions between nanotube and substrate atoms as the nanotube is moved along the substrate. Analysis of the kinetic energy suggests that the transition between sliding and rolling contributes to enhanced energy dissipation and higher net friction. These results are consistent with preliminary experimental observations by Superfine and coworkers.     

Molecular insight into adsorption affinities of Carmustine drug on boron and nitrogen doped functionalized single-walled carbon nanotubes using density functional theory including dispersion correction calculations and molecular dynamics simulation

Abstract

We report a quantum mechanics calculation and molecular dynamics simulation study of Carmustine drug (BNU) adsorption on the surface of nitrogen (N) and boron (B) doped-functionalized single-walled carbon nanotubes. The stability of the optimized complexes is determined on the basis of relative adsorption energy (ΔE_ads). The ΔE_ads results claim that drug molecule tends to adsorb on the nitrogen and boron doped functionalized tubes with the energy values in the range of ?61.177 to ?95.806?kJ/mol. Based on the obtained results, it is observed that N-doping compared with B-doping has improved more effectively drug absorption on the surface of functionalized nanotube. The results of Atoms in Molecule calculations indicate that drug adsorbs molecularly via hydrogen bonds interactions on the surface doped-functionalized carbon nanotubes. Moreover, molecular dynamics simulation is performed to investigate the dynamics behavior of the drug molecules on the nitrogen-doped functionalized carbon nanotube (f-NNT) and functionalized carbon nanotube (f-CNT). The higher average calculated electrostatic and van der Waals energies as well as higher number of intermolecular hydrogen bonds in BNU-f-NNT in comparison with BNU-f-CNT model suggest the more effectual interaction between drug molecules and nitrogen-doped functionalized carbon nanotube.

Communicated by Ramaswamy H. Sarma     

Modeling the interaction between anti-cancer drug penicillamine and pristine and functionalized carbon nanotubes for medical applications: density functional theory investigation and a molecular dynamics simulation

Abstract

The present study focuses on the prediction and investigation of binding properties of penicillamine with pure (5,5) single-walled carbon nanotube (SWCNT) and functionalized SWCNT (f-SWCNT) through the B3LYP and M06-2X functionals using the 6-31G** basis set. The electronic and structural properties, adsorption energy and frontier molecular orbitals of various configurations are examined. Our theoretical results indicated that the interaction of the nanotubes with penicillamine molecule is weak so that the drug adsorption process is typically physisorption. Also, results of theoretical calculations show that the adsorption of the drug molecule on f-SWCNT is stronger with shorter intermolecular distances in comparison to pure SWCNT. The natural bond orbital (NBO) analysis of studied systems demonstrates that the charge is transferred from penicillamine molecule to the nanotubes. Furthermore, molecular dynamics (MD) simulation is employed to evaluate the dynamic and diffusion behavior of drug molecule on SWCNT and f-SWCNT. Energy results show that drug molecule spontaneously moves toward the carriers, and the van der Waals energy contributions in drug adsorption are more than electrostatic interaction. The obtained results from MD simulation confirm that the functionalization of SWCNT leads to increase in the solubility of the carrier in aqueous solution.

Communicated by Ramaswamy H. Sarma     

Controlling activation barrier by carbon nanotubes as nano-chemical reactors

The oxidative addition of primary amine on a monocyclic phospholane was studied in confined conditions. This one-step chemical reaction has been investigated using the DFT technique to elucidate the role of confinement in carbon nanotubes on the reaction. Calculations were carried out by a progressive increase of the nanotube diameters from 10 Å to 15 Å in order to highlight the dependence of the reactivity on the nanotube diameter. First, single point investigations were dedicated to the study of reactants, transition states, and products placed in the different nanotubes while keeping their optimized structure as free compounds. Second, all studied compounds were relaxed inside nanotubes and their geometries were fully optimized. Within these approaches, we proved that the activation barrier could be controlled depending on the confinement, generating a well-controlled catalysis process.     

Controlling the Active Sites of Sulfur‐Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal‐free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal‐free catalysts, but the lack of understanding of active‐sites at the atomic‐level has slowed the design of highly active and stable metal‐free catalysts. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. This bidoping strategy introduces stable sulfur–carbon active‐sites. Fluorescence emission of the sulfur K‐edge by X‐ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM‐EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm^?2, but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube–graphene nanolobes behave like the state‐of‐the‐art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm^?2 at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light‐weight bidoped sulfur carbon nanotubes are potential candidates for next‐generation metal‐free regenerative fuel cells.     

A method for selective isolation and aggregation of epicuticular wax nanotubes in Picea pungens

A method for selectively isolating and aggregating intact nanotubes from leaf surfaces, in sample quantities useable for their characterisation in further experimental investigations, is presented here. It uses liquid sucrose, as a saturated solution, with its wetting behaviour modified by the addition of controlled amounts of ethanol, as a temporary entrapment and release medium, for removing nanotube material from the leaf surface, here of Picea pungens (Engelmann). This harvesting technique works by the slow, gentle transition of the medium from liquid to solid, capturing the nanotubes, and then back to liquid again, releasing them, to form an aggregate sample, with little compromise to the structural integrity of individual nanotubes. Scanning electron microscopy (SEM) images are presented rigorously illustrating the technique and its effectiveness. Comparison with other recent methods reveals its advantages, and potential applications are explored.     

Multifunctional Effect of p‐Doping,Antireflection, and Encapsulation by Polymeric Acid for High Efficiency and Stable Carbon Nanotube‐Based Silicon Solar Cells

Silicon solar cells among different types of solar energy harvesters have entered the commercial market owing to their high power conversion efficiency and stability. By replacing the electrode and the p‐type layer by a single layer of carbon nanotubes, the device can be further simplified. This greatly augments the attractiveness of silicon solar cells in the light of raw material shortages and the solar payback period, as well as lowering the fabrication costs. However, carbon nanotube‐based silicon solar cells still lack device efficiency and stability. These can be improved by chemical doping, antireflection coating, and encapsulation. In this work, the multifunctional effects of p‐doping, antireflection, and encapsulation are observed simultaneously, by applying a polymeric acid. This method increases the power conversion efficiency of single‐walled carbon nanotube‐based silicon solar cells from 9.5% to 14.4% and leads to unprecedented device stability of more than 120 d under severe conditions. In addition, the polymeric acid‐applied carbon nanotube‐based silicon solar cells show excellent chemical and mechanical robustness. The obtained stable efficiency stands the highest among the reported carbon nanotube‐based silicon solar cells.     

Predicting doxorubicin drug delivery by single-walled carbon nanotube through cell membrane in the absence and presence of nicotine molecules: a molecular dynamics simulation study

Abstract

In order to study the interaction of the anticancer agent Doxorubicin with the single-walled carbon nanotubes with different diameters as drug delivery systems, the molecular dynamics (MD) simulations have been used. Also, for design and development of intracellular Doxorubicin drug delivery systems, a series of steered MD simulations are applied to explore the possibility of encapsulated Doxorubicin–carbon nanotube penetration through a lipid bilayer in presence and absence of Nicotine molecules at different pulling rates. Our simulation results showed that in spite of the adsorption of drug molecules on the outer sidewall of the nanotubes, the spontaneous localization of one Doxorubicin molecule into the cavity of the nanovectors with larger diameters is observed. It is found that the presence of Nicotine molecules in extracellular medium increases the required force for pulling nanotube-encapsulated drug as well as the required time for penetration process, especially at higher velocity. Also, the entering process of the Nicotine molecules into the carbon nanotube causes that the encapsulated drug molecule is fully released in the hydrophobic phase of the lipid bilayer.

Communicated by Ramaswamy H. Sarma     

CD2 Promotes Human Natural Killer Cell Membrane Nanotube Formation

Membrane nanotubes are thin membranous projections that physically connect two cells. While nanotubes have been studied in human natural killer (NK) cells and are implicated in aiding NK cell cytotoxic function, requirements for their formation to susceptible target cells remain incompletely understood. Here we demonstrate that the CD2-CD58/48 receptor-ligand interaction promotes and is required for nanotube formation in human NK cells. In the CD2⁻ NK cell line YTS, a stable CD2 expression variant enabled effective nanotube formation, and was associated with better cytotoxic function. Importantly, only interactions between an NK cell and a susceptible target cell were associated with multiple nanotubes and the number of nanotubes was inversely correlated with their length. Quantitative live cell fluorescence microscopy of CD2 nanotubes revealed time-dependent enrichment and localization of CD2 to the nanotube tip, and blocking CD2 receptor-ligand interactions prevented nanotube formation. Increased nanotube formation was not simply a feature of receptor-ligand pairing, as a KIR-MHC interaction in the same cell line system failed to promote nanotube formation. Additionally, blocking LFA-1-ICAM and 2B4-CD48 receptor-ligand interactions failed to inhibit nanotube formation. Thus only specific receptor-ligand pairs promote nanotubes. CD2 also promoted nanotube formation in ex vivo NK cells suggesting that CD2 plays a crucial role in the generation of nanotubes between an NK cell and its target.     

Rapid microwave synthesis of chitosan modified carbon nanotube composites

Chitosan modified multi-walled carbon nanotube composites were synthesized under microwave irradiation. The resultant chitosan modified multi-walled carbon nanotube composites were purified by twice adjusting of pH value of the solution and centrifugating in succession. The surface functional groups of chitosan modified multi-walled carbon nanotube composites are confirmed by Fourier transform infrared spectroscopy and UV–vis spectroscopy. Transmission electron microscopic images further show the morphologic changes of the carbon nanotubes. Thermal gravimetric analysis shows that the chitosan content in the chitosan modified multi-walled carbon nanotube composites is much higher than conventional methods. The whole reaction procedure can be completed in 1 h under microwave irradiation.     

Protein immunosensor using single-wall carbon nanotube forests with electrochemical detection of enzyme labels

Vertically aligned arrays of single-wall carbon nanotubes (SWNT forests) on pyrolytic graphite surfaces were developed for amperometric enzyme-linked immunoassays. Improved fabrication of these SWNT forests utilizing aged nanotube dispersions provided higher nanotube density and conductivity. Biosensor performance enhancement was monitored using nanotube-bound peroxidase enzymes showing a 3.5-fold better sensitivity for H2O2 than when using fresh nanotubes to assemble the forests, and improved detection limits. Absence of improvements by electron mediation for detection of H2O2 suggested very efficient electron exchange between nanotubes and enzymes attached to their ends. Protein immunosensors were made by attaching antibodies to the carboxylated ends of nanotube forests. Utilizing casein/detergent blocking to minimize non-specific binding, a detection limit of 75 pmol mL(-1) (75 nM) was achieved for human serum albumin (HSA) in unmediated sandwich immunosensors using horseradish peroxidase (HRP) labels. Mediation of the immunosensors dramatically lowered the detection limit to 1 pmol mL(-1) (1 nM), providing significantly better performance than alternative methods. In the immunosensor case, the average distance between HRP labels and nanotube ends is presumably too large for efficient direct electron exchange, but this situation can be overcome by electron mediation.     

Evaluation of thermo-mechanical properties of graphene/carbon-nanotubes modified asphalt with molecular simulation

A comparative study was conducted to determine the effects of graphene and carbon nanotubes on the thermo-mechanical properties of asphalt binder using molecular simulations and experiments. Micro-morphology of graphene and carbon nanotubes was measured by scanning electron microscopy. Thermal stability and glass transition temperature were investigated by differential scanning calorimeter. Simulation results indicated that the T_g had slightly changed for graphene-modified asphalt (GMA) and carbon nanotubes-modified asphalt (CNsMA) and that the thermal expansion coefficients and thermal conductivity increased along with the adding amount of graphene or carbon nanotubes. The T_g calculated by density–temperature method was closer than the experimental T_g and the T_g decreased in the order of CNsMA, GMA and asphalt. Young’s modulus of asphalt, GMA and CNsMA were 9.2658, 25.7563 and 17.8249 GPa at 298 K, respectively, which indicated that thermo-mechanical properties of asphalt showed considerable improvements after the addition of graphene or carbon nanotubes, and carbon nanotubes-modified asphalt and GMA were promising candidates for the future modified asphalt.     

Synthetic Chloride-Selective Carbon Nanotubes Examined by Using Molecular and Stochastic Dynamics

Synthetic channels, such as nanotubes, offer the possibility of ion-selective nanoscale pores which can broadly mimic the functions of various biological ion channels, and may one day be used as antimicrobial agents, or for treatment of cystic fibrosis. We have designed a carbon nanotube that is selectively permeable to anions. The virtual nanotubes are constructed from a hexagonal array of carbon atoms (graphene) rolled up to form a tubular structure, with an effective radius of 4.53 Å and length of 34 Å. The pore ends are terminated with polar carbonyl groups. The nanotube thus formed is embedded in a lipid bilayer and a reservoir containing ionic solutions is added at each end of the pore. The conductance properties of these synthetic channels are then examined with molecular and stochastic dynamics simulations. Profiles of the potential of mean force at 0 mM reveal that a cation moving across the pore encounters an insurmountable free energy barrier of ∼25 kT in height. In contrast, for anions, there are two energy wells of ∼12 kT near each end of the tube, separated by a central free energy barrier of 4 kT. The conductance of the pore, with symmetrical 500 mM solutions in the reservoirs, is 72 pS at 100 mV. The current saturates with an increasing ionic concentration, obeying a Michaelis-Menten relationship. The pore is normally occupied by two ions, and the rate-limiting step in conduction is the time taken for the resident ion near the exit gate to move out of the energy well.     

A major locus involved in the formation of the radial oxygen loss barrier in adventitious roots of teosinte Zea nicaraguensis is located on the short‐arm of chromosome 3

A radial oxygen loss (ROL) barrier in roots of waterlogging‐tolerant plants promotes oxygen movement via aerenchyma to the root tip, and impedes soil phytotoxin entry. The molecular mechanism and genetic regulation of ROL barrier formation are largely unknown. Zea nicaraguensis, a waterlogging‐tolerant wild relative of maize (Zea mays ssp. mays), forms a tight ROL barrier in its roots when waterlogged. We used Z. nicaraguensis chromosome segment introgression lines (ILs) in maize (inbred line Mi29) to elucidate the chromosomal region involved in regulating root ROL barrier formation. A segment of the short‐arm of chromosome 3 of Z. nicaraguensis conferred ROL barrier formation in the genetic background of maize. This chromosome segment also decreased apoplastic solute permeability across the hypodermis/exodermis. However, the IL and maize were similar for suberin staining in the hypodermis/exodermis at 40 mm and further behind the root tip. Z. nicaraguensis contained suberin in the hypodermis/exodermis at 20 mm and lignin at the epidermis. The IL with ROL barrier, however, did not contain lignin in the epidermis. Discovery of the Z. nicaraguensis chromosomal region responsible for root ROL barrier formation has improved knowledge of this trait and is an important step towards improvement of waterlogging tolerance in maize.