Structural Defects Play a Major Role in the Acute Lung Toxicity of Multiwall Carbon Nanotubes: Physicochemical Aspects

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.

Aerosolization System for Experimental Inhalation Studies of Carbon-Based Nanomaterials

“…Grinding, heating, and purification can have an effect on the surface chemistry and size distribution of SW-CNTs (Shen et al 2005;Fenoglio et al 2008). For example, acid treatment can remove amorphous carbon, reduce lengths of entangled bundles, preferentially remove metal impurities, and introduce functional groups on the tube sidewalls (Shen et al 2005).…”

Assessing the impact of the physical properties of industrially produced carbon nanotubes on their interaction with human primary macrophages in vitro

“…Whilst this is not the case with the NTL sample (due to the increased ROS formation shown), the intensity and duration of the sonication period, which has been shown to be optimal in suspending these CNT samples [24], could cause structural surface defects that in turn do not elucidate to true hazard potential of these CNTs. Structural defects to the surface of CNTs remains an important issue [28,29], as previously shown by Kagan et al [42], and requires further, in-depth investigation to understand how they may contribute to the potential adverse health effects of CNTs.…”

“…Yet, despite such an exhaustive review of the literature, no clear correlation between CNT physical characteristics and a specific biological response -apart from those CNTs fitting the fiber paradigm [i.e., long ( > 15 µm), high aspect ratio ( ≥ 1:3) and biopersistent] [24-27] -was noted. The inability to show any clear findings in regard to the physical properties of bundled, tangled and short CNTs was attributed to discrepancies in (i) the production method (e.g., chemical vapor deposition vs. arc discharge) of CNTs [3], and thus (ii) levels of contaminant metals in the CNT sample [28][29][30], (iii) the dose and exposure method used [31] and (iv) the biological system employed (i.e., in vitro vs. in vivo) [8]. By controlling such variables, and in an attempt to only consider the sole biological impact of CNT physical properties [32], the aim of this study was to focus upon the cellular response of five physically different and currently industrially produced CNTs (Table 1).…”

Carbon nanotubes-properties and applications: a review

“…Such approaches are essential for the further development and safe use of CNTs at the occupational level as well as at the level of the general user. From various studies of the toxicity of CNTs, it is clear that surface modifications are important to minimize CNT toxicity [163][164][165][166][167]. A coating could be a useful tool, as it allows modification of the external surface environment of CNTs without affecting their intrinsic structure or basic properties.…”