Introduction

The carbon nanotubes (CNTs), one of the best novel nanostructures 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 (SWCNTs) and multi-walled carbon nanotubes (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 of CNTs have been little explored, in particular for drug delivery purposes. 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 common 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 CNTs properties is believed to open the road towards real nanotechnology applications. 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 CNTs materials is important to retaining the electronic properties of nanotubes. The redissoluble functional compound/CNTs composites are needed for post processing because CNTs dispersions usually easy aggregate and therefore make additional processing very difficult. 

Available Solubilization Methods 

A series of contemporary techniques are being used for CNTs solubilization, from physical (classic ultrasound, plasma treatment or UV-light) to chemical and biological, applying inorganic (other carbon allotropes, iodine, metallic sodium in liquid ammonia, CO2, peroxides, metal salts and mineral acids) and organic (acids, salts, polymers, dyes, natural products and biomolecules) compounds, as well as well as micelles on their basis and some metal complexes. Frequently, physical action (more frequently ultrasound, more rarely hydrothermal technique) is combined with chemical/biological treatment. In some cases, successive steps can be applied, for instance use of low- and high-weight surfactants, mineral acid treatment for creation of –OH and –COOH groups and their further interaction with organic molecules. Carbon nanotube dispersion in nematic liquid crystals is also known. 

Important Observations and Considerations 

It has been suggested that van der Waals interaction, π-π stacking interactions between aromatic rings in organic compounds and CNTs, and hydrophobic interaction are major factors that are responsible for the CNTs dispersion. Choosing surfactants, to be able to stabilize carbon nanotubes in water, it is necessary to employ such dispersing agents that

  • strongly adsorb on the nanotube surface,
  • present hydrophilic groups, better if rigid, that extend toward the aqueous phase,
  • are not very mobile on the nanotube surface, and
  • show aggregates with structure dependent on nanotube diameter and chirality. 

For appropriate solvent selection, it was established for SWCNTs case that (1) heavier solvents (and small in size) most probably are better solvents for SWCNTs, (2) higher polarizability of the solvent molecule increases the dispersibility. Among solvents, water is most preferable due to much more existing and potential CNTs applications, in particular more for medical and biological purposes. N-methyl-2-pyrrolidinone (NMP), N-dodecyl-pyrrolidinone (N12P), acetone, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), cyclohexyl-pyrrolidinone (CHP) have been considered as suitable solvents for CNTs dispersion, meanwhile, in contrast, much precipitation can be obviously observed for systems of the CNTs in water, ethanol and toluene. 

Each discussed method leads to an improvement of CNTs “solubility”, frequently considerable. The formed dispersions could be stable for long periods of time, from several weeks to some months, even sometimes remained stable after centrifugation. Sometimes, unpredictable results could occur, such, for example, in case of action of humic acid to effectively disperse MWCNTs, but not SWCNTs, into stable suspensions under the studied conditions, or when structures of amorphous carbon and carbon particles of MWCNTs were completely eliminated and the tips of nanotubes opened applying planetary ball mill. 

Some curious, but naturally determined and explained observations were emphasized by researchers, in particular on better dispersibility of the CNTs with bigger diameters (treatment with plasma or graphene oxide) or more favorable suspension of CNTs using negatively charged nanodiamonds as compared to positively charged particles, possibly caused by electrostatic interactions. The presence of π-conjugated systems that can potentially interact with CNTs to induce its dispersion is considered as a contributing factor, as well as both hydrophilic and hydrophobic moieties in amphiphilic bifunctional molecules. Additionally, a huge number of polyaromatic compounds (especially pyrene derivatives) and a variety of polymers and biomolecules are noted to be used as surfactants or functionalizing agents. As an example, bifunctional molecules with pyrene groups exhibited high CNTs solubility in common organic solvents with very different polarities such as tetrahydrofuran, toluene, and n-hexane. Several special studies have been carried out in the areas of influence of solvent and light on CNTs dispersibility, combinations and abilities of surfactants, CNTs cytotoxicity, etc. 

In addition to CNTs, there are, although considerable lesser information, on the solubilization of other carbon allotropes and nanocarbons, for example graphite, nanoonions, fullerenes, graphene, nanodiamonds, nanodots, etc.

Bibliography

  • Solubilization and Dispersion of Carbon Nanotubes. Springer-Nature, 2017, 258 pp. ISBN 978-3-319-62950-6.

"This article is authored by Prof. Oxana V. Kharissova and Prof. Boris I. Kharissov, Universidad Autónoma de Nuevo León, Monterrey, México"

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