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Industrial multi-walled carbon nanotubes (ig-MWCNTs) make up the majority of carbon nanomaterials, and human contact with them is the most probable. At the same time, the biodegradation of ig-MWCNTs by phagocytes has not been studied—existing articles consider mainly laboratory-grade/functionalized MWCNTs (l-MWCNTs), in contrast to which ig-MWCNTs are a highly heterogeneous nanomaterial in terms of morphological and physicochemical characteristics. The aim of the present study was to analyze ig-MWCNTs' biodegradation by proinflammatory macrophages. We focused on both extra- and intracellular ig-MWCNTs' degradation. We analyzed biodegradation of two different types of ig-MWCNTs by human (THP-1) and murine (RAW264.7) macrophages. After 10 days of incubation, we studied nanoparticle localization within cells; isolated intra- and extracellular ig-MWCNTs were used for quantitative analysis. Ultrastructural and morphometric analysis were performed using transmission electron microscopy; electron diffraction was used for nanotube identification. To estimate chemical alterations, energy-dispersive X-ray spectroscopy and Raman spectroscopy were used. The study showed that both intra- and extracellular ig-MWCNTs undergo almost complete biodegradation, but in different ways: intracellular nanotubes become perforated and reduce to graphene flakes, while extracellular become thinner. We believe that the demonstrated variability in the destruction of ig-MWCNTs by cells suggests the possibility of creating nanomaterials with controlled biodegradation properties. [ABSTRACT FROM AUTHOR]

The propensity of multi-walled carbon nanotubes (MWCNTs) for biodegradation is important for their safe use in medical and technological applications. Here, we compared the oxidative degradation of two samples of industrial-grade MWCNTs-we called them MWCNT-d and MWCNT-t-upon their treatment with sodium hypochlorite (NaOCl). The MWCNTs had a similar inner diameter but they differed about 2-fold in the outer diameter. Electron microscopy combined with morphometric analysis revealed the different degradation of the two types of MWCNTs after their incubation with NaOCl-the thicker MWCNT-d were damaged more significantly than the thinner MWCNT-t. The both types of MWCNTs degraded at the inner side, but only MWCNT-d lost a significant number of the outer graphitic layers. Raman spectroscopy demonstrated that both MWCNTs had a similar high defectiveness. Using energy-dispersive X-ray spectroscopy, we have shown that the more degradable MWCNT-d contained the same level of oxygen as MWCNT-t, but more metal impurities. The obtained results suggest that the biodegradability of MWCNTs depends not only on the wall thickness but also on the defects and impurities. Thus, the biodegradability can be regulated by the synthesis conditions or the post-synthesis modifications. Such degradation flexibility may be important for both medical and industrial applications.