Lipid bilayer functionalization of multiwalled carbon nanotubes

Abstract

Integration of the technologically favorable mechanical and electrical properties of carbon nanotubes (CNTs) with the specific recognition properties of proteins could enable the development of novel bioelectronic, in particular biosensing, applications. The hydrophobic graphene surface of CNTs, however, is not a biological substrate and as-synthesized CNTs aggregate in aqueous solution. CNTs can be easily dispersed by non-covalent binding of surfactants like sodium dodecyl sulfate, but the use of such detergents is undesirable because they unfold proteins and degrade cell membranes. We show here that carbon nanotubes can also be dispersed by coating them with biocompatible surfactant analogs. Incubation of multiwalled CNTs with sonicated vesicles of synthetic phospholipids resulted in a stable aqueous suspension of the nanotubes, also after removal of the vesicles by centrifugation. When the vesicles were doped with a fluorescently labelled lipid, the washed CNTs could be observed by fluorescence microscopy. Additionally, atomic force microscopy indicated that the nanotubes were coated by a smooth layer, with occasional defects or transitions to a second layer. These discontinuities were consistently 4-5 nm deep, the typical thickness of a lipid bilayer. It can thus be concluded that vesicle fusion results in the formation of lipid bilayers on the surface of multiwalled CNTs. We addressed the influence of vesicle size, lipid acyl chain saturation, lipid head group charge, CNT surface modification, and CNT diameter on the efficiency of lipid coating. Significantly, it proved possible to include a fluorescently labelled transmembrane peptide in nanotube-supported bilayers, and we are currently investigating whether this can also be achieved for membrane protein