Antenna-Enhanced Fluorescence Correlation Spectroscopy Resolves Calcium-Mediated Lipid-Lipid Interactions

Fluorescence correlation spectroscopy (FCS) has provided a wealth of information on the composition, structure, and dynamics of cell membranes. However, it has proved challenging to reach the spatial resolution required to resolve biophysical interactions at the nanometer scale relevant to many crucial membrane processes. In this work, we form artificial cell membranes on dimeric, nanoplasmonic antennas, which shrink the FCS probe volume down to the ∼20 nm length scale. By analyzing the autocorrelation functions associated with the fluorescence bursts from individual fluorescently tagged lipids moving through the antenna "hotspots", we show that the confinement of the optical readout volume below the diffraction limit allows the temporal resolution of FCS to be increased by up to 3 orders of magnitude. Employing this high spatial and temporal resolution to probe diffusion dynamics of individual dye-conjugated lipids, we further show that lipid molecules diffuse either as single entities or as pairs in the presence of calcium ions. Removal of calcium ions by addition of the chelator EDTA almost completely removes the complex contribution, in agreement with previous theoretical predications on the role of calcium ions in mediating transient interactions between zwitterionic lipids. We envision that antenna-enhanced FCS with single-molecule burst analysis will enable resolving a broad range of challenging membrane biophysics questions, such as stimuli-induced lipid clustering and membrane protein dynamics.