Antioxidant Potential of Aqueous Dispersions of Fullerenes C60, C70, and Gd@C82

The antioxidant potential (capacity and activity) of aqueous fullerene dispersions (AFD) of non-functionalized C₆₀, C₇₀, and Gd@C₈₂ endofullerene (in micromolar concentration range) was estimated based on chemiluminescence measurements of the model of luminol and generation of organic radicals by 2,2′-azobis(2-amidinopropane) dihydrochloride (ABAP). The antioxidant capacity was estimated by the TRAP method, from the concentration of half-suppression, and from the suppression area in the initial period. All three approaches agree and show that the antioxidant capacity of AFDs increased in the order Gd@C₈₂ < C₇₀ < C₆₀. Mathematical modeling of the long-term kinetics data was used for antioxidant activity estimation. The effect of C₆₀ and C₇₀ is found to be quenching of the excited product of luminol with ABAP-generated radical and not an actual antioxidant effect; quenching constants differ insignificantly. Apart from quenching with a similar constant, the AFD of Gd@C₈₂ exhibits actual antioxidant action. The antioxidant activity in Gd@C₈₂ is 300-fold higher than quenching constants.     

Polymeric fullerene oxide (fullerene ozopolymers) produced by prolonged ozonation of C60 and C70 fullerenes

The prolonged ozonation of C₆₀ and C₇₀ fullerene produces light brown to brown amorphous solids which are insoluble in chlorinated and hydrocarbon solvents but which are readily soluble in water, acetone and ethanol where they give dark-brown solutions. The polymeric and polyelectrolytic nature of these solids has been shown by viscosimetric and cryoscopic measurements. Due to the polymeric nature the solids have been called ‘ozopolymers’. The reactivity and the ozone uptake have been measured quantitatively during the ozonation of C₆₀ and C₇₀ fullerene in CCl₄. Three different C₆₀ ozopolymer samples have been produced at different degrees of ozonation: at O₃/C₆₀ molar ratio of 8, 14 and 26. The C₇₀ ozopolymer has been produced at an O₃/C₇₀ molar ratio of 30. All the samples have been studied by FT-IR spectroscopy and the C₆₀ ozopolymers have been easily reduced by the action of Zn dust and acetic acid or by the action of H₂S and studied by FT-IR. The action of oxidizing agents has been studied as well. C₆₀ ozopolymer is a polyelectrolyte rich in carboxyl groups and for this reason it possesses a high metal binding capacity similar to that of humic acids. The thermal stability of C₆₀ ozopolymer and its reduced derivatives as well as the thermal stability of C₇₀ ozopolymer has been checked by thermogravimetric analysis in nitrogen and air flow. Finally, the electrical conductivity of C₆₀ ozopolymer was found to be σ=2.8×10⁻⁸ S/cm which is three orders of magnitude lower than that of pure C₆₀.     

Separation of fullerenes C60 and C70 using a crystallization‐based process

A crystallization‐based process that separates pure fullerenes C₆₀ and C₇₀ from their mixture using o‐xylene as the solvent has been developed. Isothermal solid–liquid equilibrium phase diagrams of the C₆₀‐C₇₀‐o‐xylene ternary system for a number of temperatures were first determined at 1 atm. Taking advantage of the shift in solvent‐free composition of the C₆₀‐C₇₀ double saturation point with temperature and based on the solid solution‐forming phase behavior between C₆₀ and C₇₀, the flowsheet of a general crystallization process was then synthesized. It involved the fractionation of a C₆₀‐C₇₀ fullerene mixture into C₆₀‐rich and C₇₀‐rich solid solutions using temperature‐swing crystallization, followed by purification of the solid solutions with multistage crystallization into pure C₆₀ and C₇₀ solids. To demonstrate process feasibility, bench‐scale batch experiments were performed using a commercially available fullerene mixture that was pretreated by adsorption to remove higher fullerenes. C₆₀ and C₇₀ solids of purity higher than 99 wt % were obtained. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Fullerenes synthesis in combustion

The early suggestion in fullerenes research that fullerenes might be produced in flames was soon supported by the observation of polyhedral carbon ions in flames and in 1991 was confirmed by the recovery and identification of fullerenes C₆₀ and C₇₀ from benzene/oxygen flames. Recent research has determined the effects of pressure, carbon/oxygen ratio, temperature and the type and concentration of diluent gas, on the yields of C₆₀ and C₇₀ in subatmospheric pressure premixed laminar flames of benzene and oxygen. Similar flames but with acetylene as fuel have also been found to produce fullerenes, but in smaller yields than with benzene fuel. The largest observed yields of C₆₀ + C₇₀ from benzene/oxygen flames are substantial, being 20% of the soot produced and 0.5% of the carbon fed. The largest rate of production of C₆₀ + C₇₀ was observed at a pressure of 69 Torr, a C/O ratio of 0.989 and a dilution of 25% helium. Several striking differences between fullerenes formation in flames as compared to the widely used graphite vaporization method include, in the case of flames, an ability to vary the C₇₀/C₆₀ ratio from 0.26 to 8.8 (cf., 0.02 to 0.18 for graphite vaporization) by adjustment of flame conditions and production of several isomers each of fullerenes C ₆₀, C₇₀, C₆₀O and C₇₀O. Many of the apparent isomers are thermally metastable, one C₆₀ converting to the most stable form with a half-life of 1h at 111°C. The structures of the apparent C₆₀ and C₇₀ isomers necessarily must include abutting five-membered rings, previously assumed to be disallowed because of their high strain energy. The chemistry of fullerenes formation in flames is in some ways similar to that of soot formation, but important differences are seen and assumed to reflect the effects of the curved, strained structures of fullerenes and their precursors.     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

Transformation of C70 peapods into double walled carbon nanotubes

X-ray diffraction studies comparing the transformation of C₆₀ and C₇₀ peapods into double walled carbon nanotubes are presented. The structures of the as-formed DWCNTs are strikingly similar, showing that they are not dependent on the nature of the fullerene precursor. High temperature X-ray diffraction measurements of C₇₀ peapods below the coalescence temperature show that confined C₇₀ molecules in large tubes undergo an orientational transition to free rotations. Fast re-orientations of C₇₀ molecules allow cyclo-addition between adjacent fullerenes to form, in good agreement with the mechanism of coalescence proposed in the literature for C₆₀ molecules.     

In vivo evaluation of carbon fullerene toxicity using embryonic zebrafish

There is a pressing need to develop rapid whole animal-based testing assays to assess the potential toxicity of engineered nano-materials. To meet this challenge, the embryonic zebrafish model was employed to determine the toxicity of fullerenes. Embryonic zebrafish were exposed to graded concentrations of fullerenes [C₆₀, C₇₀, and C₆₀(OH)₂₄] during early embryogenesis and the resulting morphological and cellular responses were defined. Exposure to 200 μg/L C₆₀ and C₇₀ induced a significant increased in malformations, pericardial edema, and mortality; while the response to C₆₀(OH)₂₄ exposure was less pronounced at concentrations an order of magnitude higher. Exposure to C₆₀ induced both necrotic and apoptotic cellular death throughout the embryo. While C₆₀(OH)₂₄ induced an increase in embryonic cellular death, it did not induce apoptosis. Our findings concur with results obtained in other models indicating that C₆₀(OH)₂₄ is significantly less toxic than C₆₀. These studies also suggest that the embryonic zebrafish model is well-suited for the rapid assessment of nanomaterial toxicity.     

Deposition of C60, C70 and C84 fullerene molecules, in benzene via a change of the fluid state, from a gas–liquid two phase region to the critical point

We dissolve C₆₀, C₇₀ or C₈₄ molecules in benzene and change the fluid state from a gas–liquid two-phase region (25.0 °C) to the critical point (289.0 °C) and from the critical point to the original state (25.0 °C) along the gas–liquid coexistence curve. We find that particle-like and whisker-like nano/micro clusters, which are composed of C₆₀ molecules, deposit on the surface of a silicon substrate placed vertically in C₆₀/benzene solution during the temperature change, whereas no appreciable clusters are detected on the silicon substrate in either C₇₀/benzene or C₈₄/benzene solutions. The clusters, in which fcc lattice structures are formed by C₆₀ molecules, remain stable in the solution. The present result suggests that C₆₀ molecules can be separated and extracted from a mixture of C₆₀, C₇₀ and C₈₄ molecules dissolved in benzene.     

Effects of Aqueous Dispersions of C60, C70 and Gd@C82 Fullerenes on Genes Involved in Oxidative Stress and Anti-Inflammatory Pathways

Background: Fullerenes and metallofullerenes can be considered promising nanopharmaceuticals themselves and as a basis for chemical modification. As reactive oxygen species homeostasis plays a vital role in cells, the study of their effect on genes involved in oxidative stress and anti-inflammatory responses are of particular importance. Methods: Human fetal lung fibroblasts were incubated with aqueous dispersions of C₆₀, C₇₀, and Gd@C₈₂ in concentrations of 5 nM and 1.5 µM for 1, 3, 24, and 72 h. Cell viability, intracellular ROS, NOX4, NFκB, PRAR-γ, NRF2, heme oxygenase 1, and NAD(P)H quinone dehydrogenase 1 expression have been studied. Results & conclusion: The aqueous dispersions of C₆₀, C₇₀, and Gd@C₈₂ fullerenes are active participants in reactive oxygen species (ROS) homeostasis. Low and high concentrations of aqueous fullerene dispersions (AFD) have similar effects. C₇₀ was the most inert substance, C₆₀ was the most active substance. All AFDs have both “prooxidant” and “antioxidant” effects but with a different balance. Gd@C₈₂ was a substance with more pronounced antioxidant and anti-inflammatory properties, while C₇₀ had more pronounced “prooxidant” properties.     

Transformation of fullerene peapods to double-walled carbon nanotubes induced by UV radiation

Double-walled carbon nanotubes were prepared by XeCl-laser irradiation of fullerene (C₆₀ or C₇₀) peapods. Raman spectroscopy evidences less defect structure of outer tubes, as compared to those in double-walled carbon nanotubes grown by thermal treatment of peapods. The diameter distribution also differs from that of the thermally prepared nanotubes. At the given laser fluence, the conversion of C₇₀@SWCNT into double-walled carbon nanotubes was more efficient than the corresponding conversion of C₆₀@SWCNT.     

Snapshots of the Fragmentation for C70@Single-Walled Carbon Nanotube: Tight-Binding Molecular Dynamics Simulations

In previously reported experimental studies, a yield of double-walled carbon nanotubes (DWCNTs) at C₇₀@Single-walled carbon nanotubes (SWCNTs) is higher than C₆₀@SWCNTs due to the higher sensitivity to photolysis of the former. From the perspective of pyrolysis dynamics, we would like to understand whether C₇₀@SWCNT is more sensitive to thermal decomposition than C₆₀@SWCNT, and the starting point of DWCNT formation, which can be obtained through the decomposition fragmentation of the nanopeapods, which appears in the early stages. We have studied the fragmentation of C₇₀@SWCNT nanopeapods, using molecular dynamics simulations together with the empirical tight-binding total energy calculation method. We got the snapshots of the fragmentation structure of carbon nano-peapods (CNPs) composed of SWCNT and C₇₀ fullerene molecules and the geometric spatial positioning structure of C₇₀ within the SWCNT as a function of dynamics time (for 2 picoseconds) at the temperatures of 4000 K, 5000 K, and 6000 K. In conclusion, the scenario in which C₇₀@SWCNT transforms to a DWCNT would be followed by the fragmentation of C₇₀, after C₇₀, and the SWCNT have been chemically bonding in the early stages. The relative stability of fullerenes in CNPs could be reversed, compared to the ranking of the relative stability of the encapsulated molecules themselves.     

Supercritical Fluid Extraction of Fullerenes C60 and C70 from Carbon Soot

Abstract

A new branch of organic chemistry has begun to emerge following the development of a carbon-arc electric discharge process by Krätschmer et al. which produced a carbon soot containing a series of caged buckminsterfullerene C₆₀, C₇₀, and higher homologues. While these compounds are normally recovered either by sublimation in an inert atmosphere or by extraction with benzene, toluene, or a higher boiling aromatic, enhanced selectivity can be achieved by operating at supercritical conditions. The ability of supercritical fluid solvents to fractionate the extractable material of the carbon soot by pressure and/or temperature tuning of the solvent power appears to be possible due to the significant molecular weight and size difference between C₆₀ and C₇₀ which yield different threshold solubility densities. While CO₂ is unable to extract any fullerenes, selective extraction of C₆₀ is obtained when using a nonaromatic modifier such as cyclopentane. Quantitative recoveries are achieved by employing moderate concentrations of an aromatic solvent modifier such as toluene at pressures imparting solvent densities greater than 0.8 g/cm³.     

C60 affects DNA replication in vitro by decreasing the melting temperature of DNA templates

The effect of C₆₀ on DNA replication in vitro was studied by a quantitative real-time polymerase chain reaction (QPCR) system using a designed 110bp single-stranded DNA as the template. The results indicated that the efficiency of QPCR can be dramatically enhanced by C₆₀ at the beginning of the exponential phase, and that QPCR production can be significantly inhibited by C₆₀ at later cycles or at the plateau, which is the period represented by an interesting inverted U-shaped time response curve. Two different sized double-stranded DNA fragments (110bp and 60bp) were used to investigate the possible interaction between C₆₀ and DNA. The melting curve results showed that C₆₀ significantly decreased the melting temperatures (Tm) of the DNA fragments. By changing the concentrations of the initial DNA templates or C₆₀ in the QPCR system, we found that the decreased Tm value of DNA templates by C₆₀ is the primary reason for the increased QPCR efficiency. Our findings are consistent with the conclusions of other studies that C₆₀ can disrupt DNA replication in vitro by binding to DNA and changing the conformation of DNA templates.     

C60(Cg) and C70(g): A computational study of the pressure and temperature dependence of their populations

Equilibrium mixtures of pure carbon gas-phase aggregates, C_n(g). are treated combining all available observational as well as computational thermodynamical data for n = 1, 2, 3, 4, 5, 60 and 70. A considerable sensitivity to temperature and pressure is pointed out, showing that there are both regions of a higher relative population of C₆₀(g) as well as of C₇₀(g). There can be significant competition between the formation of small and large clusters. Relations between the full-equilibrium situation (i.e., including graphite), gas-phase equilibrium, and the nonequilibrium situation are discussed.     

Supramolecular Chromatographic Separation of C60 and C70 Fullerenes: Flash Column Chromatography vs. High Pressure Liquid Chromatography

A silica-bound C-butylpyrogallol[4]arene chromatographic stationary phase was prepared and characterised by thermogravimetric analysis, scanning electron microscopy, NMR and mass spectrometry. The chromatographic performance was investigated by using C₆₀ and C₇₀ fullerenes in reverse phase mode via flash column and high-pressure liquid chromatography (HPLC). The resulting new stationary phase was observed to demonstrate size-selective molecular recognition as postulated from our in-silico studies. The silica-bound C-butylpyrogallol[4]arene flash and HPLC stationary phases were able to separate a C₆₀- and C₇₀-fullerene mixture more effectively than an RP-C₁₈ stationary phase. The presence of toluene in the mobile phase plays a significant role in achieving symmetrical peaks in flash column chromatography.     

Redox potentials and binding enhancement of fullerene and fullerene-cyclodextrin systems in water and dimethylsulfoxide

The formal redox potentials of electron transfer reactions of fullerene, methanofullerene, fullerene-cyclodextrin complex and methanofullerene conjugates with cyclodextrins in aqueous and dimethylsulfoxide solutions are reported. These new compounds are surface active and retain the redox activity of C₆₀ even in aqueous medium. Compounds have been characterized by an electrochemical admittance technique, which offers an advantage of separating faradaic and capacitive properties. Observed difference of formal redox potentials of the free fullerene forms and their cyclodextrin-containing compounds were used to determine the binding enhancement. Results are interpreted in terms of inter-molecular host-guest interactions of C₆₀-cyclodextrin conjugates.     

Surface-enhanced Raman scattering studies on C60 fullerene self-assemblies

Surface-enhanced Raman scattering (SERS) was used to investigate C₆₀ self-assembling in solvents like pyrrolidine (Py) and N-methyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C₆₀ aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C₆₀ and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at ∼342 cm⁻¹. In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I_h C₆₀ symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800 cm⁻¹ is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C₆₀ in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C₆₀ cage. These complexes are noticed in the SERS spectrum by the replacement of the original H_g(1) band at ∼269 cm⁻¹ with two others at ∼255 and ∼246 cm⁻¹. In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118 cm⁻¹, which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C₆₀ clusters of different size.     

Mechanistic investigation and selectivity of the grafting onto C60 of macroradicals prepared by cobalt-mediated radical polymerization

The grafting mechanism of poly(vinyl acetate) macroradicals prepared by cobalt-mediated radical polymerization onto C₆₀ is investigated. The experimental conditions directly impact the nature and stability of the PVAc/C₆₀ adducts. In the presence of residual initiating radicals that can compete with PVAc° macroradicals for addition onto C₆₀, mixtures of PVAc/C₆₀ adducts having between one and eight polymer chains per C₆₀ are formed. PVAc/C₆₀ adducts prepared with low [PVAc]:[C₆₀] ratios may contain weak C₆₀–C₆₀ bonds that further dissociate and account for the instability of the products. The formation of such dimers can be lessened by increasing the temperature from 30 °C to 100 °C. The temperature increase also allows a complete dissociation of the PVAc-Co dormant species into PVAc° macroradicals and an almost quantitative grafting of eight PVAc chains onto C₆₀, leading to well-defined C₆₀(PVAc)₈ octa-adducts. These results might shed new light on the grafting onto C₆₀ of macroradicals prepared by other CRP techniques.     

Reversible photoreaction of C60-containing poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) was reacted with strong base NaH to convert its pendant hydroxy to oxy anions, followed with nucleophilic addition to buckminsterfullerene-C₆₀. The resulted PVA(C60−Na⁺)_n products were then converted to PVA(C₆₀H)_n by stirring with a strong acid cation exchanger of the H⁺ form. The reduced viscosities of PVA(C₆₀H)_n decreases with amount of C₆₀, and are between 0.38 and 0.66 dL/g compared to 1.97 dL/g of the original PVA. Repeated photocrosslinking under 300 nm light and photocleavage under 254 nm light of these C₆₀-containing PVAs were investigated in DMSO by tracing their UV absorption variations at 325 nm (maximum absorption of C₆₀H). It is confirmed that both PVA(C₆₀H)_n exhibit excellent reversibility at least under the observed three cycles. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 605–611, 1998     

Interfacial electronic structures between fullerene and multilayer graphene for n-type organic semiconducting devices

The interfacial electronic structure of fullerene (C₆₀) deposited on a multilayer graphene (MLG) film was measured using in situ ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The energy level alignment at the interface of C₆₀/MLG was estimated by the shifts in the highest occupied molecular orbital (HOMO) and the vacuum level during step-by-step deposition of C₆₀ on the MLG. The shift of the HOMO level indicates that there is a small band bending at the interface of C₆₀/MLG. The vacuum level was shifted 0.06 eV toward the low binding energy with additional C₆₀ on the MLG. The measurements reveal that the height of the electron injection barrier is 0.59 eV, while the hole injection barrier height is 2.01 eV. We present a complete interfacial energy level diagram for C₆₀/MLG.