The unique properties, such as high electrical conductivity, mechanical strength and chemical stability, of carbon nanotubes (CNT) make them extremely suitable for developing electro-chemical sensors and biosensors. Potential applications using CNTs are, however, often limited due to their insolubility in many solvents due to strong intertube van der Waals interactions. Therefore, strategic approaches are important toward the solubilization of CNTs by debundling for the applications of CNTs in biochemistry, biology, pharmaceutics and medicine. Two approaches have been reported toward the solubilization/dispersion of CNTs: one is a chemical modification of CNTs via covalent bonding and the other one is a physical modification via noncovalent-bonding. As per review articles describing recent developments on soluble nanotubes, solubilization of CNTs in solution is possible with solubilizers which are functional compounds composed of an aromatic moiety and a solvophilic moiety. Solubilizers carrying a hydrophilic or hydrophobic moiety can dissolve CNTs in dipolar solvents like water and alcohols and in non-polar solvents, respectively. CNTs have also been reported to be soluble in aqueous micellar solutions of commercially available surfactants.

For developing sensors, their electrodes are made using CNT dissolved in suitable solutions. Electrodes made by CNT modification have shown excellent catalytic properties toward the electrochemical processes of many compounds. In most cases, for casting the electrodes CNTs are temporarily dispersed in solvents such as dimethyl formamide and acetone, but the problem is CNTs were insoluble in most solvents. Hence this method of electrode preparation is complicated, and its application in biosensor systems is limited due to the usage of organic solvents.

To dissolve CNTs in solvents, especially in water, several strategies have been proposed, which involved the covalent modification of CNTs with hydrophilic groups and the non-covalent functionalization of CNTs with surfactants or polymers. Since the covalent modification will impair the physical and chemical properties of CNTs, and the usage of surfactants may cause denaturation of biomolecules, the polymer-based solubilization of CNTs is the promising approach. CNTs are soluble in solution, where the solubilizers are functional compounds that are composed of an aromatic moiety and a solvophilic moiety. In this concept, solubilizers carrying a hydrophilic or hydrophobic moiety are expected to dissolve CNTs in dipolar solvents like water and alcohols and in non-polar solvents, respectively. CNTs have been reported to be soluble in aqueous micellar solutions of suitable commercially available surfactants.

Matteo Pasquali and colleagues from Rice University in Texas, US, used chlorosulfonic acid to dissolve single-walled carbon nanotubes (SWNTs). The tubes dissolve spontaneously, forming true solutions at low concentrations and liquid crystals at higher concentrations, but Pasquali explains that because the tubes are actually dissolved, they can be processed using standard techniques that are impossible with more usual suspensions of the tubes.
Polymers

The number of polymers that render CNTs soluble in solutions is very limited. Researchers have reported the dissolution of CNTs in Nafion1 solutions and the construction of a glucose biosensor based on the Nafion1-solubilized CNTs. In addition, also a dehydrogenase biosensor has been fabricated based on the solubilization of CNTs in chitosan solutions. However, in both systems, the modification of electrodes can be achieved by casting CNT solutions on the electrode surfaces only with uncontrollable thickness of the resulting CNT–polymer films which results in irreproducible sensors. Moreover, the enzyme immobilization method in these two biosensors was cross-linking with glutaraldehyde, which was complicated and not biocompatible for enzymes.

Researchers report a simple and controllable method for the modification of electrodes with a chitosan–CNT nanocomposite through electrodeposition. As the nanocomposite exhibits excellent electro catalytic ability in the reduction and oxidation of hydrogen peroxide, and chitosan is a biocompatible polymer, an enzyme–chitosan–CNT composite based biosensor can be developed through a simple one-step electrodeposition method.

Chitosan stock solution is prepared and MWCNT, (95%) are solubilized in chitosan solutions with the help of ultrasonication. Two polished and cleaned gold electrodes kept at a very narrow distance apart are connected to a low voltage DC power supply and dipped into the CNT + chitosan solution. H in the solution is reduced to H2 at the cathode, and the pH near the cathode surface gradually increases. As the solubility of chitosan is pH-dependent, when the pH exceeds the pKa of chitosan (about 6.3), chitosan becomes insoluble and the chitosan entrapped CNT will get deposited onto the cathode surface. The electrodeposited chitosan–CNT nanocomposite film has wire-like substances homogeneously distributed within the film. This can be attributed to the wrapping of CNTs with chitosan chains and the electrodeposited nanocomposite is mainly composed of chitosan-wrapped CNTs.

Copolymer/CNT dispersions

The aqueous solubilization of carbon nanotubes (CNTs) with the aid of a block copolymer possessing one polyelectrolyte block (namely polystyrene-b-sodium (sulfamate/carboxylate polyisoprene)) is reported in literature. The solubilization protocol, based on the co-dissolution of the copolymer and the CNTs, leads to the formation of supramolecular assemblies on the side walls of the tubes. Viscosity measurements on solutions of the copolymer decorated CNTs indicate that the polyelectrolyte effect, which is observed in the case of net polymers is preserved in a lesser extent in the case of the copolymer/CNTs dispersions.