Structural defects play a major role in the acute lung toxicity of multiwall carbon nanotubes: physicochemical aspects

Our plain-English summary of Structural defects play a major role in the acute lung toxicity of multiwall carbon nanotubes: physicochemical aspects, by I. Fenoglio, G. Greco, M. Tomatis, et al (Chemical Research in Toxicology, 2008).

The study

• Authors: I. Fenoglio, G. Greco, M. Tomatis, J. Muller, E. Raymundo-Piñero, F. Béguin, A. Fonseca, J.B. Nagy, D. Lison, B. Fubini
• Journal: Chemical Research in Toxicology
• Published: 2008;21(9):1690-7
• Cited by: 118 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carbon nanotubes (CNT) have been reported to elicit toxic responses in vitro and in vivo, ascribed so far to metal contamination, CNT length, degree of oxidation, or extent of hydrophilicity. To examine how structural properties may modulate the toxicity of CNT, one preparation of multiwall CNT has been modified (i) by grinding (introducing structural defects) and subsequently heating either in a vacuum at 600 degrees C (causing reduction of oxygenated carbon functionalities and reduction of metallic oxides) or in an inert atmosphere at 2400 degrees C (causing elimination of metals and annealing of defects) and (ii) by heating at 2400 degrees C in an inert atmosphere and subsequently grinding the thermally treated CNT (introducing defects in a metal-deprived carbon framework). The presence of framework and surface defects, metals, and oxygenated functionalities was monitored by means of a set of techniques, including micro-Raman spectroscopy, adsorption calorimetry, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and atomic emission spectroscopy. Contrary to traditional toxicants, such as asbestos, CNT may quench rather than generate oxygenated free radicals. The potential of the modified CNT to scavenge hydroxyl radicals was thus evaluated by means of electron spin resonance spectroscopy (spin trapping). The original ground material exhibited a scavenging activity toward hydroxyl radicals, which was eliminated by heating at 2400 degrees C but restored upon grinding. This scavenging activity, related to the presence of defects, appears to go paired with the genotoxic and inflammatory potential of CNT reported in the companion paper. Thus, defects may be one of the major factors governing the toxic potential of CNT.

In plain English

This was a laboratory, cell-free physicochemical study of one preparation of multiwall carbon nanotubes (CNT) made by catalytic decomposition of ethylene on a cobalt-iron catalyst on alumina. The authors modified the material by grinding for 6 h (CNTg), heating the ground material in vacuum at 600 °C (CNTg600) or in argon at 2400 °C (CNTg2400), and heating at 2400 °C followed by grinding (CNT2400g). Metals, surface oxygen, surface area, defects (Raman D/G intensity ratio), water adsorption energy and free-radical behaviour were measured. Heating at 2400 °C largely removed Fe and Co and lowered the defect ratio (ID/IG 0.58 plus or minus 0.14 versus 1.16 plus or minus 0.08 for CNTg); grinding the heated sample raised it again (0.93 plus or minus 0.09). No sample generated detectable hydroxyl radicals. CNT, CNTg and CNTg600 abolished the hydroxyl-radical ESR signal, CNTg2400 did not, and scavenging returned after grinding. The authors concluded that defects are one of the major physicochemical determinants of the toxic potential of CNT, with the lung toxicity data taken from a companion paper.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experimental study: physicochemical characterisation and cell-free radical (ESR spin-trapping) testing of deliberately modified multiwall carbon nanotubes
• Who was studied: Not people or animals: one preparation of multiwall carbon nanotubes, synthesised on a cobalt-iron catalyst and purified with NaOH, plus four samples derived from it by grinding and heating
• Exposure: Modification of multiwall CNT by grinding and by heating: grinding 6 h, heating in vacuum at 600 °C, heating at 2400 °C in argon, or heating at 2400 °C then grinding 6 h
• Main results:
  • Metal content (percent): CNTg Al 1.97, Co 0.49, Fe 0.48; CNTg600 Al 2.25, Co 0.54, Fe 0.51; CNTg2400 Al 0.37, Co 0.15 x 10-3, Fe 0.10 x 10-4; CNT2400g Al 0.06, Co 0.07, Fe 0.08. Surface oxygen: 1.95, 0.46, 0.10 and 0.73 percent respectively
  • Specific surface area and defect ratio: CNT 213 m2/g, ID/IG 1.05 plus or minus 0.05; CNTg 299 m2/g, 1.16 plus or minus 0.08; CNTg600 274 m2/g, 1.10 plus or minus 0.10; CNTg2400 190 m2/g, 0.58 plus or minus 0.14; CNT2400g 240 m2/g, 0.93 plus or minus 0.09
  • Molar enthalpy of water adsorption (kJ/mmol, at 5 Torr): CNT 11.8, CNTg 29.2, CNTg600 14.8, CNTg2400 0.4, CNT2400g 4.0, i.e. hydrophilicity highest for CNTg and lowest for CNTg2400
  • None of the samples generated detectable hydroxyl radicals in the ESR spin-trapping test
  • Hydroxyl-radical scavenging: the ESR signal disappeared with CNT, CNTg and CNTg600, remained unchanged with CNTg2400, and disappeared again with the heated-then-ground sample CNT2400g
  • In the authors' summary table aligning their measurements with the companion paper's lung toxicity results, CNTg2400 (defects annealed, no metals) showed no scavenging and no inflammation or genotoxicity, while CNT2400g (defects reintroduced, no metals) showed scavenging, inflammation and genotoxicity
• The authors' conclusion: The authors concluded that defects are one of the major physicochemical determinants of the toxic potential of CNT, and that the radical scavenging potential relates mainly to the amount and nature of defects rather than to residual metals
• Limitations the authors state:
  • The authors state that an additional role of metal impurities in the scavenging effect cannot be excluded
  • The authors state it remains unresolved whether toxicity is strictly linked to the scavenging activity or whether the two are unrelated consequences of a defective carbon framework
  • The sample preparation for electron microscopy itself caused the nanotubes to clump together
  • The Raman D band does not distinguish between different kinds of defects

Topics

Medical terms in this study, explained in our Lexicon: Hydroxyl Radical.

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