Potential Role of Soluble Metal Impurities in the Acute Lung Inflammogenicity of Multi-Walled Carbon Nanotubes.

Dong-Keun Lee,Wan Soo Yun,Wan-Seob Cho,Kyung Seuk Song,Jong Sung Kim,Aeyeon Kang,Il Je Yu,Jiyoung Jeong,Soyeon Jeon

doi:10.3390/nano10020379

Abstract

Multi-walled carbon nanotubes (MWCNTs) have variable metal impurities, but little is known about the impact of soluble metal impurities on the toxicity of MWCNTs. Here, we evaluated the role of soluble metal impurities to the acute inflammogenic potential of MWCNTs, using five types of high purity MWCNTs (>95%). MWCNTs and their soluble fractions collected at 24 h after incubation in phosphate-buffered saline showed diverse metal impurities with variable concentrations. The fiber-free soluble fractions produced variable levels of reactive oxygen species (ROS), and the iron level was the key determinant for ROS production. The acute inflammation at 24 h after intratracheal instillation of MWCNTs to rats at 0.19, 0.63, and 1.91 mg MWCNT/kg body weight (bw) or fiber-free supernatants from MWCNT suspensions at 1.91 and 7.64 mg MWCNT/kg bw showed that the number of granulocytes, a marker for acute inflammation, was significantly increased with a good dose-dependency. The correlation study showed that neither the levels of iron nor the ROS generation potential of the soluble fractions showed any correlations with the inflammogenic potential. However, the total concentration of transition metals in the soluble fractions showed a good correlation with the acute lung inflammogenic potential. These results implied that metal impurities, especially transitional metals, can contribute to the acute inflammogenic potential of MWCNTs, although the major parameter for the toxicity of MWCNTs is size and shape.

Highlights

The toxicity of multi-walled carbon nanotubes (MWCNTs) is closely related to their physicochemical properties, such as size, shape, solubility, and rigidity [1,2]
We found that the iron is the critical element producing reactive oxygen species (ROS), but the acute inflammogenic potential of soluble fractions was not well correlated with either the concentration of iron or the ROS generation potentials, suggesting that none was the main factor responsible for lung inflammation in rats induced by MWCNTs
We identified the hazard of soluble metal impurities and suggested that the iron level is closely related to the ROS generation potential, but it is not consistent with the magnitude of inflammation of the soluble fraction of MWCNT

Summary

Introduction

The toxicity of multi-walled carbon nanotubes (MWCNTs) is closely related to their physicochemical properties, such as size, shape, solubility, and rigidity [1,2]. As these properties depend on the methods of manufacture, the toxicity study of specific MWCNTs cannot represent all MWCNTs. the read-across study to evaluate the physicochemical parameters related to the toxicity endpoints of MWCNTs represents a promising approach to help establish the regulatory requirements [3,4,5]. Pure hexagonal graphite-based materials without defects are generally less reactive and have a high potential for quenching free radicals than metal/metal oxide nanoparticles; pure carbon materials can be classified as a low toxicity group if their physical properties are not considered [11,12,13]. 100% pure MWCNTs are hardly prepared as a final product because of the addition of metal catalysts during synthesis processes [14]

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Results

Discussion

Conclusion