Rotational Dynamics of Laterally Frozen Nanoparticles Specifically Attached to Biomembranes

The strongly polarized light scattering of gold nanorods at their longitudinal plasmon frequency allows for the tracking of single gold nanorod lateral positions and orientations via optical dark-field microscopy. We monitor both lateral and rotational diffusion of polymer-coated gold nanorods attached to artificial biomembranes on solid supports. The attachment is mediated by the biotin-streptavidin receptor-ligand system, but weak interaction is also observed in the absence of streptavidin. In the latter case, we observe a two-dimensional lateral diffusion of the nanorods with a diffusion coefficient of 0.5 mu m(2) s(-1). This lateral motion is strongly reduced with the addition of streptavidin. However, the particles are still able to rotate, and we study their rotational motion using polarization contrast microscopy. The rotational diffusion time in the range of 100 ms depends on the biotin concentration in the membrane, hence the number of anchor points, and oil the temperature. Cooling the membrane beyond its gel-fluid transition point leads to a reduction of the rotational motion. The experimental results can be understood in terms of dragging forces introduced by. the surface viscosity of the membrane. A quantitative analysis shows that entire patches of the membrane move with the particles.