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Preparation of novel materials by filling carbon nanotubes

Final Report Summary - NANO-RC (Preparation of novel materials by filling carbon nanotubes)

Carbon nanotubes (CNTs), an allotrope form of carbon, combine excellent mechanical, electronic, and thermal properties. Their cylindrical nanostructure can be understood as rolled up graphene layers forming molecular-scale tubes with diameters in the nanometer range. Since the report by Sumio Iijima in the early 90's on the preparation of carbon nanotubes, a large amount of research has been done to exploit their properties. The NANO-RC proposal has focused on taking advantage of their tubular structure which allows the confinement of materials in their interior. The project has been developed through the achievement of four main objectives which focused on:

(i) the purification of as-received carbon nanotubes and the subsequent;
(ii) encapsulation of chosen materials for application in two areas of emerging interest;
(iii) formation of nanomaterials and
(iv) biomedical fields.

The formation of nanowires using CNTs as nanotemplates will allow the synthesis of novel materials with useful and exciting physical and chemical properties, whereas filled nanotubes are envisaged as promising agents for medical applications including in vivo imaging, tumour targeting and drug delivery.

The purification of CNTs is essential since various types of unwanted products result from their synthesis (graphitic nanoparticles, amorphous carbon, fullerenes, catalyst material). In order to obtain pure CNTs, we have implemented a steam purification treatment. CNTs have been purified by treating the as-synthesised material with steam at 900 °C. During this process, the amorphous carbon and the graphitic shells coating the catalytic metal particles are removed. Next, the samples were treated with HCl to dissolve the now exposed catalytic impurities, rinsed with water and dried. The resulting samples contain a low metal content, which is desired for biomedical applications. Furthermore, no damage to the carbon nanotube tubular structure was observed. Single-walled carbon nanotubes (SWCNTs) have been purified following our previous work (J. Phys. Chem. B 110, 22318, 2006; Small 4, 1501, 2008) whereas the conditions to achieve an efficient purification of MWCNTs have been further investigated (manuscript in preparation). The post-treated samples have been characterised by combination of thermogravimetric analysis (TGA), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM).

The second step is the filling of purified CNTs with a variety of materials. The hollow structure of the purified carbon nanotubes allows their internal filling. The encapsulation of compounds into the purified carbon nanotubes was achieved by solution, vapour and molten phase capillary filling. Using these procedures, the filling of the nanotubes with species like metal halides, nitrates and acetates was realised (Phys. Status Solidi C 7, 2739, 2010). Both carbon and WS2 nanotubes have been used as templates for the formation of nanostructures (Nano Res. 3, 170, 2010) and also nanocomposites with nanoparticles with diameters in the range 5 - 20 nm have been obtained covering the entire length of the nanotubes. TGA, TEM and diffraction methods were used to characterise these nanomaterials.

Within the frame of the NANO-RC project, a methodology that allows the containment and controlled release of the encapsulated cargoes has also been developed. We have shown that a pH-triggered release of the filled materials can be achieved in aqueous media by the use of functionalised fullerenes (Carbon 48, 1912, 2010). These act as removable corks at the openings of carbon nanotubes. The containment / discharge of materials was monitored by means of both TEM and ultraviolet-visible (UV-Vis) spectroscopy. Whereas UV-Vis spectroscopy provides information of the bulk material, electron microscopy techniques provide local information of the sample and allow direct imaging of materials down to the atomic scale. The developed nano-corks have potential for the development of 'smart' drug delivery systems. CNTs are attractive as multifunctional carrier systems because a chosen payload can be encapsulated in the internal space, whereas outer surfaces can be chemically modified to match specific needs. For example, the functionalisation of nanotubes with carbene derivatives was achieved, with the final materials presenting higher dispersibilities than the purified samples (J. Mater. Chem. 21, 19080, 2011). These functionalised nanotubes are prone to further derivatisation and could have potential application not only in the biomedical field but also in the areas of composite materials and molecular electronics. In this respect, we have recently shown that is possible to transfer functionalised CNTs onto substrates by means of matrix assisted pulsed laser evapouration (Carbon, doi: 10.1016/j.carbon.2012.05.023).

A major achievement has been the development of simultaneously filled and functionalised CNTs with potential application in diagnosis and therapy (Nature Materials 9, 485, 2010). The constructs were prepared by first filling the SWCNTs with Na125I, followed by their sidewall functionalisation with carbohydrates. Steam treated short SWCNTs were employed in this study to further improve the biocompatibility of the materials. Effectively, the nanotubes guaranteed essentially zero leakage of the radionuclide and remained stable for extended periods. The sealing of iodide within single-walled carbon nanotubes enabled its biodistribution to be completely redirected from tissue with innate affinity (thyroid) to lung. Surface functionalisation of these nanocapsules offers versatility towards modulation of biodistribution of the radioemitting crystals in a manner determined by the nanotube that delivers them.

In conclusion, during this NANO-RC project, new insights into the carbon nanotube chemistry have been obtained, investigating the possible routes for the purification and filling of these materials, their surface functionalisation and the controlled release of materials from their interior using fullerenes as removable corks. The potential of filled CNTs for useful application in many areas is evident, as represented for instance by the work carried out in the use of nanotubes for in the areas of diagnosis and therapy.