Solubilization and Dispersion of Carbon Nanotubes

The carbon nanotubes (CNTs), one of the best novel nanostructures [1] and classic objects in nanotechnology, form bundle-like structures with very complex morphologies with a high number of Van der Waals interactions, causing extremely poor solubility in water or organic solvents. Due to their exceptional combination of mechanical, thermal, chemical, and electronic properties, single-walled (SWNTs or SWCNTs) and multiwalled carbon nanotubes (MWNTs or MWCNTs) are considered as unique materials, with very promising future applications, especially in the field of nanotechnology, nanoelectronics, and composite materials. Additionally, CNTs are becoming highly attractive molecules for applications in medicinal chemistry. At present, potential biological and medical applications [2] of CNTs have been little explored, in particular for drug delivery purposes [3]. The main difficulty to integrate such materials into biological systems derives from their lack of solubility in physiological solutions. Functionalization of CNTs with the assistance of biological molecules remarkably improves the solubility of nanotubes in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine, and bioengineering. Many of these applications require an increased “solubility” of CNTs in solvents, first of all in water, especially for biological applications. This could be reached by their functionalization, which is a very actively discussed topic in contemporary literature because the planned modification of CNT properties is believed to open the road toward real nanotechnology applications [4]. It is difficult to prepare an aqueous dispersion of CNTs stable for months; their insolubility has been a limitation for the practical applications of this unique material. Proper dispersion of CNT materials is important to retaining the electronic properties of nanotubes. The redissoluble functional compound/CNT composites are needed for post-processing because CNT dispersions usually easily aggregate and therefore make additional processing very difficult.

Ozone can be used alone or with combination with UV light. Thus, chemical functionalization of MWCNTs was carried out [1, 2] by UV/ozone treatment. The presence of oxygen-containing groups (such as carboxylic, quinine, and hydroxyl groups) on the MWCNT surfaces by UV/ozone treatment was confirmed resulting in dispersion stability better than for pristine MWCNTs in polar solvents. Authors predicted the dispersion states of MWCNTs treated with UV/ozone in 32 different solvents in terms of a Hansen solubility parameter sphere, which increased as UV/ozone exposure time increased. The solubility of UV/ozone-treated MWCNTs in polar organic solvents can be increased by as much as 320%, maintaining almost the same thermal stability, compared to raw MWCNTs [3]. The dispersed nanotubes suspension (MWCNTs functionalized by oxygen- and hydrogen-containing groups) can be stable for a few months after treatment [4].

During more than 20-year period of investigations in the field of carbon nanotubes and their dispersions in water and organic solvents, a series of important observations, effects, and conclusions have been made, in particular after the CNT dispersions have been studied within solution. As it will be seen below, influence of solvent and external influence (i.e., UV light or the presence of ions), free radicals, and a row of other factors can improve or affect the CNT dispersibility, which could differ depending on CNT type and their purity.

In this section, we present several selected experimental procedures, directly borrowed from original articles. We hope this material will be very useful as educational material for M.Sc. and Ph.D. students, working in the areas of biochemistry and drug delivery, fabrication of CNT thin films from dispersion, application of CNT composites with polymers, etc.

The field of carbon nanotubes continues to be one of the hottest topics in the nanoscience and nanotechnology, so the development of their functionalization and dispersion methods is and will be an important research area, at least for the next 10 years. A series of contemporary techniques are being used for CNT solubilization, from physical (classic ultrasound, plasma treatment, UV light, dielectrophoresis [1, 2], gel electrophoresis [3, 4], density-gradient ultracentrifugation [5, 6], irradiation techniques, and chromatography [7], among others) to chemical and biological, applying inorganic (other carbon allotropes, iodine, bromine, metallic sodium in liquid ammonia, CO₂, peroxides, ozone, metal salts, and mineral acids) and organic (acids, salts, polymers, dyes, natural products, in particular diazonium salts [8]; functionalization with porphyrins, amines, pyrene, polymers, sugars, etc.) compounds and biomolecules (DNA, peptides, amino acids, etc.), as well as micelles on their basis and some metal complexes. CNTs can be functionalized by oxidation (peroxy acids; metal oxidants, such as osmium tetroxide, potassium permanganate, chromates; ozone; oxygen; superoxides) or reduction reactions (interactions with thiols, carbenes, dienes, etc.).