Supported lipid bilayer/carbon nanotube hybrids XINJIAN ZHOU 1 * , JOSE M. MORAN-MIRABAL 2 , HAROLD G. CRAIGHEAD 2 AND PAUL L. McEUEN 1 * 1 Laboratory of Atomic and Solid-State Physics, Cornell University, Ithaca, New York 14853, USA 2 Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA *e-mail: xz58@cornell.edu; mceuen@ccmr.cornell.edu Published online: 25 February 2007; doi:10.1038/nnano.2007.34 Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube—with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes. Single-walled carbon nanotubes (SWNTs) 1 offer unique opportunities for chemical 2 and biological sensing 3,4 . SWNT transistors 5 have mobilities 6 that exceed those of silicon and have transverse dimensions comparable to a strand of DNA. They also work efficiently in aqueous environments 7 and, unlike silicon- based biosensors, they do not require an insulating layer to separate the ions from the conducting channel. An exciting possibility is to use a nanotube to probe the properties of lipid membranes and their functional constituents. Supported lipid bilayers (SLBs) 8–10 self-assemble from phospholipids on flat hydrophilic substrates like glass and mimic many properties of cell membranes. The lipids in an SLB are laterally mobile, and many reconstituted membrane proteins embedded in them remain functional 11,12 . Although it has been shown that lipid bilayers can form on top of multiwalled carbon nanotubes 13 or SWNTs coated with polyelectrolytes 14 , the nanotubes in these experiments were not used as detection elements and the bilayers were not supported on flat substrates. Another group has placed membrane patches on SWNT field-effect transistors (FETs) 15 , but no continuous SLBs were formed and the measurements were carried out in dry conditions. Here, we show the integration of SLBs with SWNT FETs (Fig. 1). We first demonstrate membrane continuity and lipid diffusion over the tube. However, we also show that the nanotube acts as a diffusion barrier for a membrane-bound tetanus toxin protein, and that the strength of the barrier depends on the diameter of the nanotube. Finally, we present results on the electrical detection of specific binding of streptavidin to biotinylated lipids. The formation of fluidic SLBs on SWNTs will allow the study of lipid–SWNT interactions and sensing of analytes binding to specific receptors embedded in the SLBs. These studies should have an impact on our understanding of model and biological membranes. PHYSICAL PROPERTIES OF SLB/SWNT HYBRID STRUCTURES We first formed 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) SLBs on chips containing SWNTs (see Methods). Before examining the regions around the nanotubes, we tested the overall quality of the SLBs on SiO 2 using two criteria. First, SLBs should be spatially uniform, as measured by fluorescence microscopy. Second, the lipid molecules in SLBs should be laterally mobile, as observed using fluorescence recovery after photobleaching (FRAP) 16 . For this, the fluorophores inside a region are bleached with long exposure to light and, once the bleaching is finished, the fluorescence in this region is allowed to recover by diffusion. The diffusion coefficients can be measured using FRAP or fluorescence correlation spectroscopy (FCS) 17 (see Sections I and II of Supplementary Information). Both methods rely on the lateral diffusion of fluorescent lipids into a probed volume, which only occurs if the bilayer is continuous and laterally fluid. The diffusion coefficients, D, for the DOPC bilayers were extracted from fits to the FCS autocorrelation curves (see Methods), giving a value of D ¼ 5.4 + 0.1  10 28 cm 2 s 21 . This value compares favourably with previously published values 18 . With proper surface preparation, both criteria can routinely be met. To test the continuity and fluidity of the formed SLB over a nanotube, we used FRAP on devices with the geometry shown in Fig. 2a. We created two square SLB patches 19 connected by a 2-mm-wide, 12-mm-long channel. Nanotubes emerging from the black catalyst islands (visible in the optical micrograph) cross the channel. We photobleached the fluorophores in the right square and observed whether the fluorescence could recover by the diffusion of fluorophores from the left box across the nanotube to the right box. Figure 2b–d shows fluorescence images of the recovery process. It may be observed that fluorescent lipids can diffuse across the nanotube. No discernible ARTICLES nature nanotechnology | VOL 2 | MARCH 2007 | www.nature.com/naturenanotechnology 185