Functionalization of carbon nanotubes (CNTs) is of importance for fundamental studies and the applications of CNTs since as-produced CNTs form aggregated/bundle structures and their dispersion in solution requires CNT functionalization. Especially, biological applications, one of the most promising applications for CNTs, strongly require a stable functionalization since the aggregation of CNTs has the risk of toxicity in vivo. As a functionalization method, chemical functionalization based on the covalent bond provides a stable modification of the CNT surfaces. However, the introduction of the structural defects decreases the unique CNT features such as large area-hydrophobicity surfaces useful for hydrophobic drug loading on the surfaces. On the other hand, physical (noncovalent) modification based on the physical adsorption of the functional molecules onto the CNT surfaces is able to retain the structure and intrinsic properties of the CNTs. For CNT biological applications, we need to recognize that the physical adsorption of small molecules is a dynamic process and the exchange of the molecules on the CNTs with the molecules in the bulk solution is always occurring, which causes the removal of the adsorbed molecules from the CNT surfaces inside the body. The use of a polymeric dispersant dramatically improves the stability of the CNTs through a multipoint physical adsorption, but it has been revealed that the wrapped polymer is unwrapped when a large number of molecules having a strong affinity with the CNTs are added. Indeed, Cherukuri et al. reported that the injection of polymer-wrapped Single-walled carbon nanotubes (SWNTs) into blood sera resulted in rapid displacement by endogenous proteins since some proteins very strongly adsorb onto the SWNT surface. Such a risk of aggregation and the replacement in vivo requires a new functionalization method to enhance the dispersion stability.
In 2003, Taton et al. reported a pioneering study to stabilize polymer-wrapped SWNTs based on the cross-linking of the polymers around the CNTs. Similarly, the cross-linking of surfactant micelles has been studied to improve the stability of the CNT dispersion states. Such cross-linking strategies provide a promising approach to achieve a highly stable and non-destructive CNT coating because the wrapping layers are stabilized based on the covalent bonding of polymers around the CNT surfaces. However, this approach has only been applicable to molecules that are able to disperse the CNTs, which prevents the versatility of the concept.
Here, we demonstrate a new strategy to synthesize a cross-linked polymer network with an ultrathin layer (~nm-size) around the surface of the CNTs using the inner nanospace of a surfactant micelle as the regulated polymerization site. It has been recognized that in the CNT solutions dispersed by surfactant micelles, the CNTs are encapsulated by the surfactant micelles to provide a stable dispersion of the CNTs. Recently, the molecular penetration inside the micelles due to the hydrophobicity of the interior space was reported. Therefore, the monomer or the polymer together with the cross-linker is expected to penetrate inside the interior of the micelles due to the relatively hydrophobic nature of the space and form a cross-linked polymer network around the CNTs. One of great advantages of our strategy is that there is no need for the monomers to disperse the CNTs since the CNT dispersion is carried out by the surfactant micelles, thus a variety of monomers are applicable to the present system.