Biogas upgrading using single-walled carbon nanotubes by molecular simulation

Abstract

Biogas from anaerobic digestion of biological wastes is a renewable energy resource that mainly contains CH₄, CO₂, trace amounts of H₂S and a fraction of H₂O vapour. In order to transfer biogas into biomethane to meet the standards for use as vehicle fuel or for injection in the natural gas grid, removing H₂S from biogas in advance is necessary. In addition, biogas is usually saturated with water vapour. It is significant to study the effect of the presence of H₂O on the biogas separation performance. Adsorption of H₂S/CO₂/CH₄ and H₂O/CO₂/CH₄ ternary mixtures using single-walled carbon nanotubes (SWCNT) were investigated via the Grand Canonical Monte Carlo (GCMC) method. We studied the effects of carbon nanotube diameter, –COOH modification, temperature and pressure on H₂S adsorption. The results indicate that the presence of hydrophilic –COOH groups does affect the separation of H₂S/CO₂/CH₄ mixtures. Temperature swing adsorption is more suitable than pressure swing adsorption for the separation of H₂S/CO₂/CH₄ mixtures. The effect of water vapour on the separation of CO₂/CH₄ was also investigated. The result shows that the presence of H₂O has little effect on the selectivity of CO₂/CH₄ in pristine CNT, but the selectivity of CO₂/CH₄ with the presence of H₂O is markedly enhanced after modification in –COOH modified SWCNT with specific modification degree. It is expected that this work could provide some useful information for biogas upgrading.     

Inhibitory effects of single-walled carbon nanotubes on biofilm formation from Bacillus anthracis spores

This study reports the inhibitory effect of single walled carbon nanotubes (SWCNTs) on biofilm formation from Bacillus anthracis spores. Although the presence of 50 to 100 μg ml^?1 of SWCNTs in the suspension increased spore attachment in the wells of 96-well plates, the presence of 200 μg ml^?1 of SWCNTs in the germination solution decreased the germination percentage of the attached spores by 93.14%, completely inhibiting subsequent biofilm formation. The inhibition kinetics of 50 μg ml^?1 SWCNTs on biofilm formation showed that this concentration inhibited biofilm formation by 81.2% after incubation for 48 h. SWCNT treatment in the earlier stages of biofilm formation was more effective compared to treatment at later stages. Mature biofilms were highly resistant to SWCNT treatment.     

Cytotoxicity of doxrubicin loaded single-walled carbon nanotubes

Carbon nanotube (CNTs) is a new alternative for efficient drug delivery and it has a great potential to change drug delivery system profile in pharmaceutical industry. One of the important advantage of CNTs is their needle-like, cylindrical shape. This shape provides a high surface area for multiple connections and adsorption onto for millions of therapeutic molecules. CNTs can be internalized by cells via endocytosis, passive diffusion and phagocytosis and release the drug with different effects like pH and temperature. The acidic nature of cancer cells and the susceptibility of CNTs to release the drug in the acidic environment have made it a promising area of research in cancer drug delivery. In this research, we investigated cell viability, cytotoxicity and drug delivery in breast cancer cell line by designing non-covalent single walled carbon nanotubes (SWNT)–doxorubicin (DOX) supramolecular complex that can be developed for cancer therapy. Applied high concentrations of DOX loaded SWNTs changed the actin structure of the cells and prevented the proliferation of the cells. It was showed that doxorubicin loaded SWNTs were more effective than free doxorubicin at relatively small concentrations. Once we applied same procedure for short and long (short: 1–1.3 µm; long: 2.5–4 µm) SWNTs and compared the results, more disrupted cell structure and reduction in cell proliferation were observed for long CNTs. DOX is bounded more to nanotubes in basic medium, less bound in acidic environment. Cancer cells were also examined for concentration at which they were effective by applying DOX and it was seen that 3.68 µM doxorubicin kills more than 55% of the cells.     

Controlled phospholipid functionalization of single-walled carbon nanotubes

These studies show that single-walled carbon nanotubes (SWNTs) can be effectively modified using phospholipids. Using a simple surface modification of SWNTs, followed by deposition of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine (DCPE) phosphipid, results in stable water-dispersible SWNTs with highly uniform thickness.     

Myeloperoxidase-induced biodegradation of single-walled carbon nanotubes is mediated by hypochlorite

Broadening prospects of using single-walled carbon nanotubes (SWNTs) in medicine and biotechnology raise the concerns about both their toxicity and the mechanisms of biodegradation and elimination from the body. SWNTs biodegradation as a result of catalytic activity of myeloperoxidase (MPO) was shown in the isolated MPO system as well as in the suspension of neutrophils [Kagan V.E. et al., 2010]. In the present study we analyzed the ability of different MPO-produced oxidants to oxidize and to degrade SWNTs. The comparison of the ability of various peroxidases to degrade SWNTs in vitro revealed that myeloperoxidase, due to its ability to produce hypochlorite, and lactoperoxidase, due to its ability to produce hypobromite, are extremely efficient in the degrading of carbon nanotubes. The biodegradation of SWNTs in the model system can also be induced by free radicals generated as a result of heme degradation and, to a lesser extent, by active oxoferryl intermediates of peroxidases. Our experiments showed that in the presence of blood plasma, peroxidase intermediates or free radical products of heme degradation were unable to initiate biodegradation of carbon nanotubes, only the generation of hypochlorite by MPO can cause the biodegradation of carbon nanotubes in vivo. At high concentrations, hypochlorite caused decrease in optical absorbance of plasma-containing SWNTs suspension, which is indicative of the nanotube degradation. Our results unambiguously suggest that hypochlorite can serve as a main oxidizing agent to modify and degrade nanotubes at the sites of inflammation and in phagosomes.     

Increasing amperometric biosensor sensitivity by length fractionated single-walled carbon nanotubes

In this work the sensitivity-increasing effect of single-walled carbon nanotubes (SWCNTs) in amperometric biosensors, depending on their average length distribution, was studied. For this purpose the SWCNTs were oxidatively shortened and subsequently length separated by size exclusion chromatography. Transmission electron micrographs of different fractions of SWCNTs were collected. Diaphorase "wired" to an osmium redox polymer was blended with the shortened SWCNTs of different lengths. Depending on the average length of the SWCNTs the sensitivity of the amperometric biosensor model system towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH) was increased by a factor of five. The best performance was achieved with SWCNTs of medium length. The linear range for NADH detection was between 5muM and 7mM, the maximum sensitivity was 47nAmuM(-1)cm(-2), and the detection limit was 1muM. The biosensor exhibited excellent electrocatalytic properties. Even at relatively high NADH concentrations the oxidative current was limited by the diffusion rate of NADH.     

Conformation of graphene folding around single-walled carbon nanotubes

The low bending rigidity of graphene facilitates the formation of folds into the structure. This curvature change affects the reactivity and electron transport of the sheet. One novel extension of this is the intercalation of small molecules into these folds. We construct a model incorporating a single-walled carbon nanotube into a sheet of folded graphene. Variational calculus techniques are employed to determine the minimum energy structure and the resulting curves are shown to agree well with molecular dynamics study.

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Graphical Abstract Using calculus of variations, the elastic bending energy and van der Waals energy are minimised giving rise to Euler-Lagrange equation for which analytical solutions are derived to determine the optimal curved sturctures of graphene wrapped around carbon nanotubes . Overall agreement between the analytical solutions (with different values of bending rigidities) and results from molecular dynamics simulations (grey) is shown here for (6,6), (8,8) and (10,10) armchair nanotubes, respectively.

     

Coordination chemistry on the surface of single-walled carbon nanotubes

A facile method to coordinate transition metal complexes (TMCs) on single-walled carbon nanotubes (SWNTs) has been developed. Reaction of Zn(OAc)₂ with carboxylic acid functionalized SWNTs (SWNT-COOH) affords SWNT complexes ‘zipped-together’ by zinc carboxylate units (termed SWNT-TMC-1 herein). Reactions of SWNT-TMC-1 with 2,2′-bipyridine or 4,4′-bipyridine gave two new SWNT-TMCs, the former being ‘unzipped’ (SWNT-TMC-2), and the latter involving an additional ligand bridge between the zinc ions (SWNT-TMC-3). Inclusion of 2,2′-bipyridine and 4,4′-bipyridine into the SWNT-TMCs was confirmed by IR spectroscopy. The microstructures of SWNT-TMC-2 and SWNT-TMC-3 were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (Xps), elemental mapping and linear profiles analysis.     

Amphiphilic helical peptide enhances the uptake of single-walled carbon nanotubes by living cells

The success of many projected applications of carbon nano-tubes (CNTs) to living cells, such as intracellular sensors and nanovectors, will depend on how many CNTs are taken up by cells. Here we report the enhanced uptake by HeLa cells of single-walled CNTs coated with a designed peptide termed nano-1. Atomic force microscopy showed that the dispersions were composed of individual and small bundles of nano-1 CNTs with 0.7- to 32-nm diameters and 100- to 400-nm lengths. Spectroscopic characterizations revealed that nano-1 disperses CNTs in a non-covalent fashion that preserves CNT optical properties. Elemental analyses indicated that our sample preparation protocol involving sonication and centrifugation effectively eliminated metal impurities associated with CNT manufacturing processes. We further showed that the purified CNT dispersions are taken up by HeLa cells in a time- and temperature-dependent fashion, and that they do not affect the HeLa cell growth rate, evidence that the CNTs inside cells are not toxic under these conditions. Finally, we discovered that approximately 6-fold more CNTs are taken up by cells in the presence of nano-1 compared with medium containing serum but no peptide. The fact that coating CNTs with a peptide enhances uptake offers a strategy for improving the performance of applications that require CNTs to be inside cells.     

Molecular dynamics simulation of single-walled carbon nanotubes inside liquid crystals

Molecular dynamic simulations of systems of single-walled carbon nanotubes (CNTs) in liquid crystalline solvents were performed, in order to investigate the effect of the molecular structure and phase of the liquid crystal (LC) on the interactions between the CNTs. Three different LC molecules (5CB, 8CB and 5CF) were considered in our study. Our results with 5CB and 8CB suggest that increasing the chain length of the hydrophobic part of the LC molecule by three carbon atoms is insufficient to decrease the tendency for the CNTs to aggregate in the LCs. Additionally, varying the phase of the LC is also insufficient to decrease the aggregation tendency of the CNTs. However, simulations with 5CF (which has fluorine atoms in the head group of the LC molecule) suggest that this LC solvent can decrease the tendency of the CNTs to aggregate. This study is relevant to assist experimentalists with the development of high-quality dispersions of large concentrations of CNTs in the LCs.     

Activity and stability comparison of immobilized NADH oxidase on multi-walled carbon nanotubes,carbon nanospheres,and single-walled carbon nanotubes

Nanomaterials have been studied widely as the supporting materials for enzyme immobilization because in theory, they can provide low diffusion resistance and high surface/volume ratio. Common immobilization methods, such as physical adsorption, covalent binding, crosslinking, and encapsulation, often cause problems in enzyme leaching, 3D structure change and strong mass transfer resistance. We have previously demonstrated a site-specific enzyme immobilization method, which is based on the specific interaction between a His-tagged enzyme and functionalized single-walled carbon nanotubes (SWCNTs), that can overcome the foresaid constraints. In this work, we broadened the use of this immobilization approach by applying it on other nanomaterials, including multi-walled carbon nanotubes and carbon nanospheres. Both supporting materials were modified with N_α,N_α-bis(carboxymethyl)-l-lysine hydrate prior to enzyme immobilization. The resulting nanomaterial–enzyme conjugates could maintain 78–87% of the native enzyme activity and showed significantly better stability than the free enzyme. When compared with the SWCNT–enzyme conjugate, we found that the size variance among these supporting nanomaterials may affect factors such as surface curvature, surface coverage and particle mobility, which in turn results in differences in the activity and stability among these immobilized biocatalysts.     

Adamantane/beta-cyclodextrin affinity biosensors based on single-walled carbon nanotubes

One challenging goal for the development of biosensors is the conception of three-dimensional biostructures on electrode surfaces. With the aim to develop 3D architectures based on single-walled carbon nanotubes (SWCNTs) frameworks a novel adamantane-pyrrole monomer was synthesized. After electrochemical polymerization at 0.95V in acetonitrile, the resulting polypyrrole film provided affinity interactions with beta-cyclodextrin. SWCNT coatings were thus functionalized with poly(adamantane-pyrrole) and applied to the anchoring of glucose oxidase (GOX), modified with beta-cyclodextrin. By using this affinity system adamantine-cyclodextrin, beta-cyclodextrin-modified gold nanoparticles were attached onto the functionalized SWCNT deposit as intermediate layer. This allows the immobilization of adamantane-tagged GOX. The responses of these biosensors to glucose were measured by potentiostating the modified electrodes at 0.7V versus saturated calomel electrode (SCE) in order to oxidize the enzymatically generated hydrogen peroxide in the presence of glucose and oxygen. The highest sensitivity and maximum current density were recorded for the configuration based on beta-cyclodextrin-modified gold particles as intermediate layer between adamantine-functionalized SWCNTs and GOX (31.02 mAM(-1)cm(-2) and 350 microAcm(-2), respectively). The similar configuration without SWCNTs exhibits a sensitivity and J(max) of 0.98 mAM(-1)cm(-2) and 75 microAcm(-2), respectively. The resulting supramolecular assemblies were characterized by scanning electron microscopy (SEM). Advantages and disadvantages of the different preparation methods and the performance of each affinity sensor setup are discussed in detail.     

Stabilization of unstable CGC+ triplex DNA by single-walled carbon nanotubes under physiological conditions

Triplex formation is a promising strategy for realizing artificially controlling of gene expression, reversible assembly of nanomaterials and DNA nanomachine and single-walled nanotubes (SWNTs) have been widely used as gene and drug delivery vector or as ‘building blocks’ in nano-/microelectronic devices. CGC⁺ triplex is not as stable as TAT triplex. The poor stability of CGC⁺ triplex limits its use in vitro and in vivo. There is no ligand that has been reported to selectively stabilize CGC⁺ triplets rather than TAT. Here, we report that SWNTs can cause d(CT)•d(AG) duplex disproportionation into triplex d(C⁺T)•d(AG)•d(CT) and single-strand d(AG) under physiological conditions. SWNTs can reduce the stringency of conditions for CGC⁺ triplex formation studied by UV–vis, CD, DNA melting, light scattering and atomic force microscopy. Further studies indicate that electrostatic interaction is crucial for d(CT)•d(AG) repartition into triplex d(C⁺T)•d(AG)•d(CT). Our findings may facilitate utilization of SWNTs–DNA complex in artificially controlling of gene expression, nanomaterials assembly and biosensing.     

Dispersion and bilayer interaction of single-walled carbon nanotubes modulated by covalent and noncovalent PEGylation

Single-walled carbon nanotubes (SWNTs) covalently functionalised with polyethylene glycol (PEG) or noncovalently coated with PEGylated lipids were simulated in water and in lipid bilayers at different PEG sizes and grafting densities using coarse-grained force fields. Starting with the random position of three SWNT–PEG complexes in water, larger PEGs at higher grafting densities more significantly inhibit the aggregation of SWNTs because of larger radii of gyration and hydrodynamic radii of the SWNT–PEG complex, which influence the thickness and the wrapping extent of PEG layer. In particular, PEG-functionalised SWNTs, where PEGs are evenly grafted along the SWNT, disperse, while PEG-coated SWNTs aggregate because SWNTs are less covered by randomly adsorbed PEGylated lipids. Simulations of SWNT–PEGs in lipid bilayers show that PEG (M_w = 550 and 2000)-functionalised SWNTs bind to the bilayer surface but do not insert into the bilayer, while PEG-coated SWNTs insert into the bilayer because PEGylated lipids detach from SWNTs and mix with bilayer lipids. These findings support recent experiments at the same PEG size and density, which suggested that PEG-coated SWNTs may form bundles and thus cannot be easily excreted through the renal route, while PEG-functionalised SWNTs may remain individual and thus show more renal excretion.     

Insertion of short amino-functionalized single-walled carbon nanotubes into phospholipid bilayer occurs by passive diffusion

Carbon nanotubes have been proposed to be efficient nanovectors able to deliver genetic or therapeutic cargo into living cells. However, a direct evidence of the molecular mechanism of their translocation across cell membranes is still needed. Here, we report on an extensive computational study of short (5 nm length) pristine and functionalized single-walled carbon nanotubes uptake by phospholipid bilayer models using all-atom molecular dynamics simulations. Our data support the hypothesis of a direct translocation of the nanotubes through the phospholipid membrane. We find that insertion of neat nanotubes within the bilayer is a "nanoneedle" like process, which can often be divided in three consecutive steps: landing and floating, penetration of the lipid headgroup area and finally sliding into the membrane core. The presence of functional groups at moderate concentrations does not modify the overall scheme of diffusion mechanism, provided that their deprotonated state favors translocation through the lipid bilayer.     

Adsorption mechanism of single guanine and thymine on single-walled carbon nanotubes

Bio-nano hybrids introduce magnificent applications of nanomaterials to various fields. The choice of carbon nanotube as well as sequence selection of the nucleic acid bases play a crucial role in shaping DNA–carbon nanotube hybrids. To come up with a clear vision for the choice of carbon nanotube and nucleic acid bases to create bio-nano hybrids, we studied the adsorption mechanism of the nucleic acid bases guanine and thymine on four different types of nanotubes based on density functional theory. Nucleic acid bases exhibit differential binding strengths according to their structural geometry, inter-molecular distances, the carbon nanotube diameter, and charge transfer. The π–π interaction mechanism between the adsorbent and adsorbate is discussed in terms of charge density profile and electronic band structure analysis.     

In situ synthesis of amylose/single-walled carbon nanotubes supramolecular assembly

A supramolecular assembly of amylose and single-walled carbon nanotubes (SWNTs) was synthesized in situ through vine-twining polymerization. Raman analysis indicated that the amylose-SWNTs supramolecular assembly was formed after the polymerization and SEM images displayed the twisted ribbons in the SWNTs wrapped by amylose. The dispersion stability of the SWNTs in aqueous solutions was improved by the wrapping of short-chain amylose molecules around the SWNTs.     

β-Carotene encapsulation into single-walled carbon nanotubes: a theoretical study

Recently, the encapsulation of β-carotene molecules into carbon nanotubes has been achieved. In this work, we report molecular dynamics simulations and tight-binding density functional-based results for a theoretical study of the encapsulation processes. Our results show that the molecules undergo geometrical deformations when encapsulated with significant changes in their electronic structure. Based on these results, we propose a new interpretation to the changes associated with the β-carotene absorption bands experimentally observed.     

Offset configurations for single- and double-strand DNA inside single-walled carbon nanotubes

Nanotechnology is a rapidly expanding research area, and it is believed that the unique properties of molecules at the nano-scale will prove to be of substantial benefit to mankind, especially so in medicine and electronics. Here we use applied mathematical modelling exploiting the basic principles of mechanics and the 6–12 Lennard-Jones potential function together with the continuum approximation, which assumes that intermolecular interactions can be approximated by average atomic surface densities. We consider the equilibrium offset positions for both single-strand and double-strand DNA molecules inside a single-walled carbon nanotube, and we predict offset positions with reference to the cross-section of the carbon nanotube. For the double-strand DNA, the potential energy is determined for the general case for any helical phase angle ?, but we also consider a special case when ? = π, which leads to a substantial simplification in the analytical expression for the energy. As might be expected, our results confirm that the global minimum energy positions for a single-strand DNA molecule and a double-strand DNA molecule will lie off axis and they become closer to the tube wall as the radius of the tube increases.