S-Layer Protein Coated Carbon Nanotubes

Carbon nanotubes (CNTs) have already been considered for medical applications due to their small diameter and ability to penetrate cells and tissues. However, since CNTs are chemically inert and non-dispersible in water, they have to be chemically functionalized or coated with biomolecules to carry payloads or interact with the environment. Proteins, although often only randomly bound to the CNT surface, are preferred because they provide a better biocompatibility and present functional groups for binding additional molecules. A new approach to functionalize CNTs with a closed and precisely ordered protein layer is offered by bacterial surface layer (S-layer) proteins, which have already attracted much attention in the functionalization of surfaces. We could demonstrate that bacterial S-layer proteins (SbpA of Lysinibacillus sphaericus CCM 2177 and the recombinant fusion protein rSbpA(31-1068)GG comprising the S-layer protein and two copies of the IgG binding region of Protein G) can be used to disperse and functionalize oxidized multi walled CNTs. Following a simple protocol, a complete surface coverage with a long-range crystalline S-layer lattice can be obtained. When rSbpA(31-1068)GG was used for coating, the introduced functionality could be confirmed by binding gold labeled antibodies via the IgG binding domain of the fusion protein. Since a great variety of functional S-layer fusion proteins has already been described, our new technology has the potential for a broad spectrum of functionalized CNTs.