Écotoxicité des nanotubes de carbone dans l’environnement : contexte et état de l’art

Parmi les nombreux nanomatériaux existants, les nanotubes de carbone (NTC), dont la production mondiale annuelle atteint plusieurs centaines de tonnes, font partie d’une catégorie de matériaux à part, tant leur potentiel d’application est vaste et leurs propriétés exceptionnelles. La toxicité des NTC est depuis peu étudiée en santé humaine, avec pour preuve de nombreuses publications disponibles, à ce jour, sur des études in vitro et indique que leur innocuité n’est pas démontrée. En revanche, la bibliographie dénote incroyablement par ses lacunes au niveau de leur impact potentiel sur les écosystèmes et sur les espèces qui les peuplent. Or, il paraît raisonnable d’envisager la dissémination des NTC dans l’environnement à chacune des étapes de leur cycle de vie (conception, production, utilisation et fin de vie de produits finis). Par conséquent, il paraît tout aussi raisonnable d’en étudier les risques sur la composante biologique du milieu réceptacle et « concentrateur » de pollution tel que le compartiment aquatique. Dans ce contexte, cette synthèse a pour objectif premier de réaliser un état de l’art des connaissances disponibles concernant les effets écotoxiques des NTC chez divers organismes, et notamment aquatiques. Le contexte actuel souligne l’importance de ce type de synthèse, qui répond à de réels besoins d’information de la société. Cette synthèse se veut motrice dans un domaine encore peu exploré, celui de l’écotoxicité des NTC chez les organismes aquatiques. Les études disponibles indiquent divers effets toxiques liés à l’ingestion par les organismes des NTC présents dans les milieux d’exposition

Structure in Nascent Carbon Nanotubes Revealed by Spatially Resolved Raman Spectroscopy

The understanding of carbon nanotubes (CNT) growth is crucial for the control of their production. In particular, the identification of structural changes of carbon possibly occurring near the catalyst particle in the very early stages of their formation is of high interest. In this study, samples of nascent CNT obtained during nucleation step and samples of vertically aligned CNT obtained during growth step are analysed by combined spatially resolved Raman spectroscopy and X-Ray diffraction measurements. Spatially resolved Raman spectroscopy reveals that iron-based phases and carbon phases are co-localised at the same position, and indicates that sp2 carbon nucleates preferentially on iron-based particles during this nucleation step. Depth scan Raman spectroscopy analysis, performed on nascent CNT, highlights that carbon structural organisation is significantly changing from defective graphene layers surrounding the iron-based particles at their base up to multi-walled nanotube structures in the upper part of iron-based particles

Buffer layers inhomogeneity and coupling with epitaxial graphene unravelled by Raman scattering and graphene peeling

The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during the sublimation of SiC (0001). The existence of covalent bonds between some of the carbon atoms in this layer and the underlying silicon atoms makes it different from epitaxial graphene. We report a systematical and statistical investigation of the BL signature and its coupling with epitaxial graphene by Raman spectroscopy. Three different kinds of BLs are studied: bare buffer layer obtained by direct growth (BL 0), interfacial buffer layer situated between graphene and SiC (c-BL 1) and the interfacial buffer layer without graphene above (u-BL 1). To obtain the latter, we develop a mechanical exfoliation of graphene by depositing and subsequently removing an epoxy-based resin or nickel layer. The observed BLs are ordered-like on the whole BL growth temperature range. BL 0 Raman signature may vary from sample to sample but also forms patches on the same terrace. u-BL 1 share similar properties with BL 0 , albeit with more variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. The signal onset on the high frequency side upshifts upon graphene coverage, that cannot be explained by a simple strain effect. Two fine peaks situated at 1235 and 1360 cm-1 are present for epitaxial monolayer while absent for BL and transferred graphene. These findings point to a coupling between graphene and BL

International amphibian micronucleus standardized procedure (ISO 21427-1) for in vivo evaluation of double-walled carbon nanotubes toxicity and genotoxicity in water

Considering the important production of carbon nanotubes (CNTs), it is likely that some of them will contaminate the environment during each step of their life cycle. Nevertheless, there is little known about their potential ecotoxicity. Consequently, the impact of CNTs on the environment must be taken into consideration. This work evaluates the potential impact of well characterized double-walled carbon nanotubes (DWNTs) in the amphibian larvae Xenopus laevis under normalized laboratory conditions according to the International Standard micronucleus assay ISO 21427-1:2006 for 12 days of half-static exposure to 0.1–1–10 and 50 mg L−1 of DWNTs in water. Two different endpoints were carried out: (i) toxicity (mortality and growth of larvae) and (ii) genotoxicity (induction of micronucleated erythrocytes). Moreover, intestine of larvae were analyzed using Raman spectroscopy. The DWNTs synthetized by catalytic chemical vapor deposition (CCVD) were used as produce (experiment I) and the addition of Gum Arabic (GA) was investigated to improve the stability of the aqueous suspensions (experiment II). The results show growth inhibition in larvae exposed to 10 and 50 mg L−1 of DWNTs with or without GA. No genotoxicity was evidenced in erythrocytes of larvae exposed to DWNTs, except to 1 mg L−1 of DWNTs with GA suggesting its potential effect in association with DWNTs at the first nonacutely toxic concentration. The Raman analysis confirmed the presence of DWNTs into the lumen of intestine but not in intestinal tissues and cells, nor in the circulating blood of exposed larvae

Assessment of the potential in vivo ecotoxicity of Double-Walled Carbon Nanotubes (DWNTs) in water, using the amphibian Ambystoma mexicanum

Because of their specific properties (mechanical, electrical, etc), carbon nanotubes (CNTs) are being assessed for inclusion in many manufactured products. Due to their massive production and number of potential applications, the impact of CNTs on the environment must be taken into consideration. The present investigation evaluates the ecotoxic potential of CNTs in the amphibian larvae (Ambystoma mexicanum). Acute toxicity and genotoxicity were analysed after 12 days of exposure in laboratory conditions. The genotoxic effects were analysed by scoring the micronucleated erythrocytes in the circulating blood of the larvae according to the French standard micronucleus assay. The results obtained in the present study demonstrated that CNTs are neither acutely toxic nor genotoxic to larvae whatever the CNTs concentration in the water, although black masses of CNTs were observed inside the gut. In the increasing economical context of CNTs, complementary studies must be undertaken, especially including mechanistic and environmental investigations

Nanofils de fer et nanotubesde carbone à haute température

Les matériaux à la lumière de SOLEILDes modifications des températures detransition voire l’apparition de nouvellesphases n’existant pas pour les matériauxmacroscopiques peuvent être trouvées.De premiers résultats, dans le casde nanoparticules et nanofils de ferconfinés dans des nanotubes de carbone, ont étéobtenus par les équipes du LPS (Orsay), du LFP (Saclay),et du CEMHTI (Orléans) sur la ligne DIFFABS

Carbon nanotube ecotoxicity in amphibians: assessment of ultiwalled carbon nanotubes and comparison with double-walled carbon nanotubes

The potential impact of industrial multiwalled carbon nanotubes (MWNTs) was investigated under normalized laboratory conditions according to the International Standard micronucleus assay ISO 21427–1 for 12 days of half-static exposure to 0.1, 1, 10 and 50 mg/l of MWNTs in water. Three different end points were carried out for 12 days of exposure: mortality, growth inhibition and micronuclei induction in erythrocytes of the circulating blood of larvae. Raman spectroscopy analysis was used to study the presence of carbon nanotubes in the biological samples. Considering the high diversity of carbon nanotubes according to their different characteristics, MWNTs were analyzed in Xenopus larvae, comparatively to double-walled carbon nanotubes used in a previous study in similar conditions. Growth inhibition in larvae exposed to 50 mg/l of MWNTs was evidenced; however, no genetoxicity (micronucleus assay) was noticed, at any concentration. Carbon nanotube localization in the larvae leads to different possible hypothesis of mechanisms explaining toxicity in Xenopus

In situ time resolved wide angle X-Ray diffraction study of nanotube carpet growth: nature of catalyst particles and progressive nanotube alignment

International audienceAbstract Catalytic Chemical Vapor Deposition is the most promising process to obtain Vertically Aligned Carbon Nanotube (VACNT) carpets. Live analysis of growing VACNT is crucial to reveal their nucleation and growth mechanisms. We present novel time resolved in situ X-ray diffraction (XRD) analysis on growing macroscopic VACNT carpets enabling us to get statistical information on catalytic phase together with nanotube progressive alignment. A specific synthesis set-up has been developed to perform such in situ synchrotron XRD experiments. Nucleation kinetics of the different phases are evidenced: first, orthorhombic Fe3C crystalline phase is formed, followed by the formation of CNTs and finally of γ-Fe, demonstrating that Fe3C particles are the nucleation seeds for CNT growth. The additional formation of Fe3C or γ-Fe nanowires inside CNTs is associated with capillary forces and mobility of them at 850°C. Experiments also reveal the progressive formation and alignment of VACNT carpets during the continuous precursor injection. Quantification of the alignment degree allows one to get a better understanding of the effect of precursor injection rate and CNT length on VACNT alignment. The overall results are key issues for the scaling-up of VACNT synthesis and their applications towards commercialization

Epitaxial growth of low doped monolayer graphene on 4H-SIC (0001) at low argon pressure

International audienceThermal decomposition of silicon carbide (SiC) provides transfer free and wafer-scale homogeneous graphene forming on a semi-insulating substrate. Large-area monolayer graphene growth was initially developed at the atmospheric pressure (around 900 mbar) in an argon ambient and at high temperature (>1650°C). However, it remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. In this work, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time of ~1h30. Here, we discuss the initial growth stages from buffer layer to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy (AFM) analyses show that a buffer layer, fully covering the Si-face of SiC, forms as the first step of growth (1600°C). Subsequently, 1LG starts to grow at step edges (1700°C) and continue to cover the buffer layer with a step-flow growth mechanism, as shown in the literature. Eventually, continued large-scale graphene films were achieved at 1750°C with a reproducible process. These graphene films were characterised by Raman, STM and electrical measurement. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map is very close to the experimental value reported for a 1LG. Atomically resolved scanning tunnelling microscopy evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer. Moreover, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film. The carrier density of pHall~1010 - 1011 cm-2 in 1LG films can be achieved at 1.7K in helium atmosphere. Thus, our as-grown 1LG graphene films with low p-type carrier density provide perspectives for further studies and applications on graphene

Epitaxial growth of low doped monolayer graphene on 4H-SIC (0001) at low argon pressure

International audienceThermal decomposition of silicon carbide (SiC) provides transfer free and wafer-scale homogeneous graphene forming on a semi-insulating substrate. Large-area monolayer graphene growth was initially developed at the atmospheric pressure (around 900 mbar) in an argon ambient and at high temperature (>1650°C). However, it remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. In this work, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time of ~1h30. Here, we discuss the initial growth stages from buffer layer to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy (AFM) analyses show that a buffer layer, fully covering the Si-face of SiC, forms as the first step of growth (1600°C). Subsequently, 1LG starts to grow at step edges (1700°C) and continue to cover the buffer layer with a step-flow growth mechanism, as shown in the literature. Eventually, continued large-scale graphene films were achieved at 1750°C with a reproducible process. These graphene films were characterised by Raman, STM and electrical measurement. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map is very close to the experimental value reported for a 1LG. Atomically resolved scanning tunnelling microscopy evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer. Moreover, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film. The carrier density of pHall~1010 - 1011 cm-2 in 1LG films can be achieved at 1.7K in helium atmosphere. Thus, our as-grown 1LG graphene films with low p-type carrier density provide perspectives for further studies and applications on graphene