We introduce an innovative design of planar plasmonic nanogap antenna arrays and demonstrate its potential to study the spatiotemporal organization of mimetic biological membranes at the nanoscale. We exploit our novel nanogap antenna platform with different nanogap sizes (10-45 nm) combined with fluorescence correlation spectroscopy to reveal the existence of nanoscopic domains in mimetic biological membranes. Our approach takes advantage of the highly enhanced and confined excitation light provided by the antennas together with their extreme planarity to investigate membrane regions as small as 10 nm in size with microsecond temporal resolution. We first demonstrate the ultra-high confinement of photonic antennas on biological membranes. Moreover, we show that cholesterol slows down the diffusion of individual fluorescent molecules embedded in the lipid bilayer, consistent with the formation of nanoscopic domains enriched by cholesterol. Incorporation of hyaluronic acid (HA) to the ternary lipid mixture further slows down molecular diffusion, suggesting a synergistic effect of cholesterol and HA on the dynamic partitioning of mimetic biological membranes.
KEYWORDS: Silicon, Molecules, Near field scanning optical microscopy, Nanoantennas, Solid state physics, Antennas, Luminescence, Near field optics, Optical resonators, Resonators
Broadband subwavelength optical resonators have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters. Recently, high-index dielectrics have been proposed as an alternative to plasmonic materials to design optical resonators with low ohmic losses. In this study, the interaction between a silicon nanoantenna and solid-state emitters is characterized by tuning the position of a 100 nm diameter fluorescent sphere in the vicinity of an e-beam fabricated silicon disk using scanning-probe microscopy. If the nanodisk resonance matches the emission wavelength of the fluorescent molecules, we observe enhanced decay rates at short distances; while, for an out-of-resonance antenna, the fluorescence lifetime is locally increased. Furthermore, our experiments highlight the ability of silicon antennas to increase far-field collection efficiencies, in agreement with numerical simulations (D. Bouchet et al, Phys. Rev. Applied 6, 064016 (2016)).
The intensity of spontaneous emission from fluorescent dye molecules can be further enhanced, by more than two orders of magnitude, in the nanoscale gap between silicon nanodisks. This is evidenced at the single molecule level using fluorescence correlation spectroscopy with freely diffusing emitters (R. Regmi et al, Nano Lett. 16, 5143 (2016)).
These results demonstrate the potential of silicon antennas for the manipulation of solid-state emitters at the nanoscale and at room temperature.
Resolving the various interactions of lipids and proteins in the plasma membrane of living cells with high spatiotemporal resolution is of upmost interest [1]. 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 [2]. 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.
References:
[1] D. Lingwood, K. Simons, Science 327, 46 (2010).
[2] V. Flauraud et al, submitted.
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