Resolving the various interactions of lipids and proteins in the plasma membrane of living cells with high spatiotemporal resolution is of upmost interest . Here we introduce an innovative design of plasmonic nanogap antennas to monitor single-molecule events on model biological membranes at physiological relevant concentrations by means of fluorescence correlation spectroscopy. Our design involves the fabrication of in-plane plasmonic nanogap antennas arrays embedded in nanometric-size boxes to provide full surface accessibility of the hotspot-confined region. Using these antennas we recently reported fluorescence enhancement factors of 104-105 times on individual molecules diffusing in solution, together with nanoscale detection volumes in the zeptoliter range . In principle, the planarity of these antennas should enable similar studies on biological membranes without unwanted membrane curvature effects.
To show their applicability, we recorded the diffusion of individual molecules inserted in multi-component lipid bilayers as a simple mimetic system that recapitulates some of the most important features of cell membranes. We prepared membranes of different compositions: saturated phospholipids, sphingolipids and cholesterol and used antennas of different gap sizes (10-45 nm). The diffusion of individual molecules on membranes consisting of phospholipids and/or in a mixture with sphingolipids resulted Brownian, confirming homogenous lipid distribution. Interestingly, the strong confinement of antennas revealed the formation of transient (<1ms lifetime) nanoscopic domains of ~11 nm in size upon cholesterol addition. These results indicate that in-plane antennas represent a highly promising non-invasive tool to investigate the nanoscale dynamic organization of biological membranes and its impact in biological function.
 D. Lingwood, K. Simons, Science 327, 46 (2010).
 V. Flauraud et al, submitted.
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