Biased Diffusion of Quantum Dots on Lipid Bilayers in an Optical Trap

Optical trapping provides a noncontact and nondestructive technique for manipulating micro- and nanoparticles with a focused laser beam. When the optical forces acting on individual nanoparticles are insufficient to achieve stable trapping, they induce a bias in Brownian diffusion toward the laser focus, resulting in the local accumulation of the nanoparticles. This optically biased diffusion presents a novel mechanism for regulating molecular assembly in two-dimensional membrane systems and provides a means to probe the lateral dynamics of membrane-associated biomolecules. In this study, we investigated the particle dynamics of quantum-dot-labeled streptavidin (QD-SA) bound to a supported lipid bilayer (SLB) in an optical trap. Single-particle tracking analysis of fluorescence images revealed that the diffusion coefficient of individual QD-SA particles on the SLB showed no systematic increase or decrease with increasing laser power, indicating that the optical forces were insufficient for the stable trapping of single nanoparticles on a two-dimensional membrane. In contrast, both the average transit time and the number of QD-SA particles within the laser focal spot tended to increase with increasing laser power. These results indicate that optical forces induce the biased diffusion of QD-SA particles toward the focal spot, leading to the local accumulation of multiple particles rather than the stable trapping of a single particle on the SLB. These findings demonstrate that the biased diffusion induced by optical forces can be exploited to control the spatiotemporal organization of membrane-associated molecules on the lipid bilayers.