The iron-related molecular toxicity mechanism of synthetic asbestos nanofibres: a model study for high-aspect-ratio nanoparticles

Our plain-English summary of The iron-related molecular toxicity mechanism of synthetic asbestos nanofibres: a model study for high-aspect-ratio nanoparticles, by F. Turci, M. Tomatis, I.G. Lesci, et al (Chemistry, 2011).

The study

• Authors: F. Turci, M. Tomatis, I.G. Lesci, N. Roveri, B. Fubini
• Journal: Chemistry
• Published: 2011;17(1):350-8
• Cited by: 51 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos shares with carbon nanotubes some morphological and physico-chemical features. An asbestos-like behaviour has been recently reported by some authors, though the mechanism of toxicity may be very different. To identify at the atomic level the source of toxicity in asbestos, the effect of progressive iron loading on a synthetic iron-free model nanofibre previously found non-toxic in cellular tests was studied. A set of five synthetic chrysotile nanofibres [(Mg,Fe)3(Si2O5)(OH)4] has been prepared with Fe ranging from 0 to 1.78 wt %. The relationship between fibre-induced free-radical generation and the physico-chemical characteristics of iron active sites was investigated with spin-trapping techniques on an aqueous suspension of the fibres and Mössbauer and EPR spectroscopies on the solids, respectively. The fully iron-free fibre was inert, whereas radical activity arose with even the smallest amount of iron. Surprisingly, such activity decreased upon increasing iron loading. Mössbauer and EPR revealed isolated iron ions in octahedral sites that undergo both axial and rhombic distortion and the occurrence of aggregated iron ions and/or extra-framework clustering. The isolated ions largely prevailed at the lowest loadings. Upon increasing the loading, the amount of isolated iron was reduced and the aggregation increased. A linear relationship between the formation of carbon-centred radicals and the amount of rhombic-distorted isolated iron sites was found. Even the smallest iron contamination imparts radical reactivity, hence toxicity, to any chrysotile outcrop, thereby discouraging the search for non-toxic chrysotile. The use of model solids that only differ in one property at a time appears to be the most successful approach for a molecular understanding of the physico-chemical determinants of toxicity. Such findings could also be useful in the design of safer nanofibres.

In plain English

This was a laboratory chemistry study of synthetic fibres, not a study of people or animals. The authors made five synthetic chrysotile asbestos nanofibre samples containing 0, 0.67, 0.81, 1.67 and 1.78 percent iron by weight. They measured how many free radicals each produced in water containing hydrogen peroxide, or containing formate plus ascorbic acid. The authors treat these reactive molecules as relevant to toxicity. The iron-free fibre produced no significant radicals. The sample with the least iron (0.67 percent) produced the most radicals, and adding more iron lowered radical production. The authors read the spectroscopy as showing that, at higher loadings, iron most likely gathered into clusters within the fibre. Mössbauer measurement, which could be recorded only for the 1.78 percent sample, found no iron oxides. Carbon-centred radical production tracked one type of isolated iron site (adjusted R2 0.87), but hydroxyl radical production did not (adjusted R2 0.41). Drawing on this and earlier studies, the authors concluded that their results do not support the existence in nature of any non-toxic chrysotile form.

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 (cell-free) chemistry experiment on synthetic fibres. Free radicals were measured with spin-trapping and EPR, and iron sites with solid-state EPR and Mössbauer spectroscopy.
• Who was studied: Five synthetic chrysotile nanofibre samples made by hydrothermal synthesis, with iron loadings of 0, 0.67, 0.81, 1.67 and 1.78 wt%. No people or animals were studied.
• Exposure: Iron content (added as ferric chloride during synthesis) in synthetic chrysotile. The fibres were placed in hydrogen peroxide, or in sodium formate with and without ascorbic acid as a reducing agent.
• Main results:
  • The iron-free synthetic chrysotile did not generate a significant amount of free radicals from hydrogen peroxide. The iron-doped samples did, at both the lowest and the highest iron loading.
  • In hydrogen peroxide, the lowest iron loading (0.67 wt%) produced hydroxyl radicals. Each further increase in iron gave a linear decrease in hydroxyl radical yield.
  • With formate alone, no sample at any iron loading produced carbon-centred radicals.
  • With ascorbic acid added, the iron-free fibre stayed inert and all iron-doped samples produced carbon-centred radicals. The 0.67 wt% sample produced the most, and reactivity fell with more iron until it levelled off.
  • Specific surface area did not change with iron content: 55 m2 per gram for 0 wt% iron and 52 m2 per gram for 1.78 wt% iron.
  • In the 1.78 wt% sample, Mössbauer spectroscopy estimated 85% of iron in octahedral sites and 15% in tetrahedral sites, and suggested no iron oxides were present.
  • The authors interpreted the EPR changes at higher iron loadings as most likely reflecting iron clusters forming within the fibre framework, in which iron may partly lose reactivity.
  • Carbon-centred radical yield correlated with the EPR signal attributed to isolated, rhombically distorted iron sites (adjusted R2 = 0.87).
  • Hydroxyl radical yield did not correlate with that EPR signal (adjusted R2 of 0.41).
• The authors' conclusion: The authors concluded, on the basis of this and their earlier studies, that even a very small amount of iron gives chrysotile radical reactivity. They also concluded that their results do not support the existence in nature of any non-toxic chrysotile form, since iron contamination always occurs in minerals.
• Limitations the authors state:
  • Mössbauer spectroscopy could be recorded only for the 1.78 wt% sample.
  • The Mössbauer estimates carry a 5% instrumental error.
  • The ascorbic acid concentration used was higher than the concentration normally found in the lung lining layer.
  • The EPR data were interpreted with a qualitative approach rather than by calculating the full spin-Hamiltonian parameters.
  • Intensity values for the EPR signal at g = 2 were not reported because measuring them could give biased results.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Octahedral Site.

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