Given the increasing use of carbon nanotubes (CNT) in composite materials and their possible expansion to new areas such as nanomedicine which will both lead to higher human exposure, a better understanding of their potential to cause adverse effects on human health is needed. Like other nanomaterials, the biological reactivity and toxicity of CNT were shown to depend on various physicochemical characteristics, and length has been suggested to play a critical role. We therefore designed a comprehensive study that aimed at comparing the effects on murine macrophages of two samples of multi-walled CNT (MWCNT) specifically synthesized following a similar production process (aerosol-assisted CVD), and used a soft ultrasonic treatment in water to modify the length of one of them. We showed that modification of the length of MWCNT leads, unavoidably, to accompanying structural (i.e. defects) and chemical (i.e. oxidation) modifications that affect both surface and residual catalyst iron nanoparticle content of CNT. The biological response of murine macrophages to the two different MWCNT samples was evaluated in terms of cell viability, pro-inflammatory cytokines secretion and oxidative stress. We showed that structural defects and oxidation both induced by the length reduction process are at least as responsible as the length reduction itself for the enhanced pro-inflammatory and pro-oxidative response observed with short (oxidized) compared to long (pristine) MWCNT. In conclusion, our results stress that surface properties should be considered, alongside the length, as essential parameters in CNT-induced inflammation, especially when dealing with a safe design of CNT, for application in nanomedicine for example.

Vertically aligned carbon nanotube (VACNT) forests are promising for supercapacitor electrodes, but their industrialisation requires a large-scale cost-effective synthesis process suitable to commercial aluminium (Al) foils, namely by operating at a low temperature (<660 °C). We show that Aerosol-Assisted Catalytic Chemical Vapour Deposition (CCVD), a single-step roll-to-roll compatible process, can be optimised to meet this industrial requirement. With ferrocene as a catalyst precursor, acetylene as a carbon source and Ar/H₂ as a carrier gas, clean and dense forests of VACNTs of about 10 nm in diameter are obtained at 615 °C with a growth rate up to 5 µm/min. Such novel potentiality of this one-step CCVD process is at the state-of-the-art of the multi-step assisted CCVD processes. To produce thick samples, long synthesis durations are required, but growth saturation occurs that is not associated with a diffusion phenomenon of iron in aluminium substrate. Sequential syntheses show that the saturation trend fits a model of catalytic nanoparticle deactivation that can be limited by decreasing acetylene flow, thus obtaining sample thickness up to 200 µm. Cyclic voltammetry measurements on binder-free VACNT/Al electrodes show that the CNT surface is fully accessible to the ionic liquid electrolyte, even in these dense VACNT forests.

Potentially, the toxicity of multiwalled carbon nanotubes (MWCNTs) can be reduced in a safe-by-design strategy. We investigated if genotoxicity and pulmonary inflammation of MWCNTs from the same batch were lowered by a) reducing length and b) introducing COOH-groups into the structure. Mice were administered: 1) long and pristine MWCNT (CNT-long) (3.9 μm); 2) short and pristine CNT (CNT-short) (1 μm); 3) CNT modified with high ratio COOH-groups (CNT-COOH-high); 4) CNT modified with low ratio COOH-groups (CNT-COOH-low). MWCNTs were dosed by intratracheal instillation at 18 or 54 μg/mouse (∼0.9 and 2.7 mg/kg bw). Neutrophils numbers were highest after CNT-long exposure, and both shortening the MWCNT and addition of COOH-groups lowered pulmonary inflammation (day 1 and 28). Likewise, CNT-long induced genotoxicity, which was absent with CNT-short and after introduction of COOH groups. In conclusion, genotoxicity and pulmonary inflammation of MWCNTs were lowered, but not eliminated, by shortening the fibres or introducing COOH-groups.

The present work explores the role of the carbon source content and the Fe/C ratio on the synthesis of vertically aligned carbon nanotubes (VACNTs) by one-step aerosol-assisted CCVD operated at a medium temperature (615 °C) on aluminum substrates. The main objective was to overcome the limitations of VACNT growth, constituting a drawback for applications requiring thick VACNTs. By using acetylene as carbon feedstock and ferrocene as a catalyst precursor, we demonstrate that when acetylene content is reduced to 1.5 vol%, it is possible to grow VACNT carpets up to 700 µm thick while maintaining constant VACNT growth for a long duration (up to 160 min). The carbon conversion yield is significantly improved when the acetylene content reaches 1.5 vol%. The Al surface roughness also influences VACNT growth. An optimum Fe/C ratio of 0.8 wt.% coupled with a low acetylene content gives the highest growth rate (5.4 µm/min) ever reported for a thermal aerosol-assisted CCVD process operated at such a low temperature. The CNT number density can be controlled by varying the Fe/C ratio, enabling high density growth (e.g., 1.3 × 10¹¹ CNT/cm²).

Carbon nanotubes (CNT) can interact with the biological environment, which could participate in their associated toxicity. We recently demonstrated that pH is an important player of CNT fate inside macrophages. We wanted to further characterize such process, and therefore designed a study dedicated to decipher CNT biodegradation by macrophages, as a function of two major physico-chemical properties in regard with nanotoxicology; length and degree of functionalization. To achieve our aim, we synthesized, following a single initial production process, four MWCNT differing in length and/or surface chemistry: S-CNT (short), SF-CNT (short functionalized), L-CNT (long) and LF-CNT (long functionalized). Raman spectroscopy analysis performed on CNT recovered after exposure of RAW 264.7 macrophages for 6, 24, or 48 h demonstrate that CNT show early signs of biodegradation over time inside macrophages. The modulation of CNT length and functionalization, resulting in the modification of iron accessibility, both represent critical determinants of the biodegradation process; short pristine CNT were more prone to biodegradation than long CNT (pristine or functionalized), while short functionalized CNT were protected. Incubation of cells with Concanamycin completely prevents CNT from being modified, demonstrating that this biodegradation process is dependent on an intracellular pH-dependent mechanism. Interestingly, and despite evidence of degradation via Raman spectroscopy, the CNT length and diameter were not altered during the course of the study. In conclusion, our results identify a new mechanism of CNT biodegradation inside macrophages. This could give new insights for the understanding of CNT-associated toxicity, and represent important tools to develop safe(r)-by-design nanomaterials.

Vertically aligned carbon nanotubes (VACNT) are manufactured nanomaterials with excellent properties and great potential for numerous applications. Recently, research has intensified toward achieving VACNT synthesis on different planar and non-planar substrates of various natures, mainly dependent on the user-defined application. Indeed, VACNT growth has to be adjusted and optimized according to the substrate nature and shape to reach the requirements for the application envisaged. To date, different substrates have been decorated with VACNT, involving the use of diffusion barrier layers (DBLs) that are often insulating, such as SiO₂ or Al₂O₃. These commonly used DBLs limit the conducting and other vital physico-chemical properties of the final nanomaterial composite. One interesting route to improve the contact resistance of VACNT on a substrate surface and the deficient composite properties is the development of semi-/conducting interlayers. The present review summarizes different methods and techniques for the deposition of suitable conducting interfaces and controlled growth of VACNT on diverse flat and 3-D fibrous substrates. Apart from exhibiting a catalytic efficiency, the DBL can generate a conducting and adhesive interface involving performance enhancements in VACNT composites. The abilities of different conducting interlayers are compared for VACNT growth and subsequent composite properties. A conducting interface is also emphasized for the synthesis of VACNT on carbonaceous substrates in order to produce cost-effective and high-performance nano-engineered carbon composites.