“Carbon nanotubes have numerous potential applications as a result of their outstanding mechanical and electrical properties,” Ravi Kane of Rensselaer told nanotechweb.org. “Strategies for functionalizing carbon nanotubes are critical for the pursuit of these applications. It is particularly desirable to develop methods for functionalizing nanotubes non-covalently, in order to retain their attractive electronic and mechanical properties.”

To achieve this non-covalent functionalization, Kane and colleagues placed the nanotubes in an aqueous solution of hydrolyzed-poly(styrene-alt-maleic anhydride) (h-PSMA) polymer. Hydrophobic interactions caused the polymer to adsorb non-covalently onto the nanotube surface. Then the team used the carboxylic acid groups in the h-PSMA to form covalent bonds to the polymer polyethyleneimine (PEI). This created a cross-linked polymer bilayer. According to Kane, the cross-linking increased the stability of the polymer layer. Repeating these steps built up a multilayered polymer film, containing alternating layers of polyanions and polycations. Finally, the team topped off the coating with a layer of poly(acrylic acid) (PAA).

“The thickness of each individual layer depends on the choice of the polymer, and may be further tuned by varying the pH and ionic strength of the solution used for the deposition,” added Kane.

In this way, the researchers coated graphite layers, single-walled carbon nanotubes and multi-walled carbon nanotubes with polymer multilayers. They also attached 40 nm diameter nanoparticles of gold to both single-walled and multi-walled carbon nanotubes functionalized with a multilayer polymer with the structure (h-PSMA-c-PEI)2-PAA-PEI.

“Functionalization of carbon nanotubes with polymer multilayers allows the introduction of reactive groups (for example, carboxylic acid or amine groups) onto the nanotube surface,” said Kane. “These reactive groups may be used to attach biospecific ligands, as well as groups that resist the non-specific adsorption of biomolecules, for applications in biosensors. The polymer multilayers can also be used to mediate the attachment of nanoparticles to single-walled and multi-walled nanotubes, creating hybrid nanostructures.”

Such hybrid nanostructures could have electronic, magnetic or catalytic applications. For example, scientists could incorporate metal or semiconductor nanoparticles to alter the nanotube’s electronic properties.

The researchers reported their work in Nano Letters.