Synthesis of nickel-nitrilotriacetic acid coupled single-walled carbon nanotubes for directed self-assembly with polyhistidine-tagged proteins

Nickel-nitrilotriacetic acid-functionalized single-walled carbon nanotubes have been synthesized for the directed, reversible self-assembly of polyhistidine-tagged macromolecules onto the nanotube surface. Carbon nanotubes were first covalently functionalized with 4-carboxybenzene diazonium salt, which rendered them water-soluble. The acid moieties on the carbon nanotube were covalently reacted with N alpha,N alpha-bis(carboxymethyl)-L-lysine hydrate, forming amide bonds to the nanotube complex. The nitrilotriacetic acid (NTA) moiety of the N alpha,N alpha-bis(carboxymethyl)-L-lysine was complexed with Ni2+ and used to specifically bind a polyhistidine-tagged photosynthetic reaction center (RC) from Rhodobacter sphaeroides as a model system. We demonstrate that the histidine-tagged RC protein (RC-His) specifically binds to SWNT-NTA-Ni in a reversible manner. The free RC-His in solution can be removed through histidine binding to Ni-NTA-agarose resin and purified by filtration. This approach allows for both positional and orientational control over protein binding to carbon nanotubes.