2D fluorescence correlation to visualize influence of size curvature and phase structure of silica nanoparticle-supported small unilamellar vesicle membrane

Abstract

Single-molecule fluorescence correlation spectroscopy (SM-FCS) was utilized to probe the impact of size curvature and phase structure on membrane dynamics in silica nanoparticles-supported small unilamellar vesicles (Si-SUVs). DOPC and DPPC Si-SUVs were prepared with three different sizes (∼50, 25, and 15 nm in radius), in which DiD dye as a fluorescence probe was implanted in each Si-SUV. DOPC and DPPC membranes have liquid disordered (Lα) and gel phase (Lβ) structures at room temperature, respectively. The detailed spatial dynamics of the lipid membrane in such smaller SUVs (below 50 nm) have never been clarified due to the low spatial resolution limit of available optical microscopes. This work visualizes the membrane's curvature- and phase-dependent spatial dynamics by using a 2D correlation plot that characterizes the interplay between the diffusion motion and the triplet-state lifetime of embedded DiD in a membrane. Irrespective of the membrane phase, the DiD diffusion motion in the membrane slows down as increasing membrane curvature. The homogeneous nature of DOPC membranes reflects into a single block of correlation distribution for all three studied curvature sizes. In contrast, a rigid structure of DPPC membrane appears to have three blocks in correlation plots corresponding to the outer and inner leaflets, and bouquet clustering. These blocks became more distinct with an increase in membrane curvature size. The hydrophilic attraction between the inner leaflet lipids and silica nanoparticles restricts the motion of inner leaflet and also plays a role in relaxing the tense conjugation between inner and outer leaflets.