Two-dimensional rotational manipulation of nanoparticles on lipid bilayers with an optical vortex beam

Optical trapping is a non-invasive technique for manipulating nano- and microscopic objects and is widely used to investigate biological processes, such as membrane viscosity, membrane-cytoskeleton interactions, and the regulation of cellular functions. Optical vortex beams can maintain orbital angular momentum (OAM) and have recently been used for optical manipulation. When nanoparticles in aqueous solutions are rotated at the laser focus owing to the optical forces derived from the optical vortex beam, their subsequent motion is governed by the OAM. The dynamics of nanoparticles attached to the biological membrane may be further affected by the viscoelasticity of the membrane and hydrodynamic coupling; however, it is unclear whether such rotational motion on lipid bilayers can be controlled. In this study, we applied an optical vortex beam to the two-dimensional rotational manipulation of fluorescent nanoparticles attached to a supporting lipid bilayer (SLB) and investigated their rotational behavior. We revealed that the single nanoparticles attached to the SLB rotated more slowly than those in an aqueous solution, but their orbital motion was still clearly driven by the OAM of the beam. The orbital radius of rotation was tuned according to the magnitude of the topological charge, and an angle velocity that changed linearly in proportion to both the laser power and nanoparticle diffusion coefficient was identified, which was consistent with theoretical calculations. These results suggest that optical vortex beams can manipulate nanoparticles attached to SLB with controllable rotational dynamics. Such rotational manipulation of nanoparticles on lipid bilayers can provide a platform for studying the effects of nanoparticle rotation on the local organization of membrane components and can be useful for developing methods to regulate their dynamic properties.