Translational Anisotropy and Rotational Diffusion of Gold Nanorods in Colloidal Sphere Solutions

We studied the translational and rotational diffusion of gold nanorods within a rod/sphere composite liquid using fluctuation correlation spectroscopy (FCS). The nanorods of length L similar to 60 nm and diameter d similar to 17 nm were used at a fixed concentration of similar to 1 pM. The concentration of colloidal Ludox spheres (size similar to 26 nm) was varied up to a volume fraction of phi = 0.3 or approximately 7 spheres/L-3. Our experiments showed significant translational anisotropy as the sphere concentration was increased. The translational diffusion parallel to the rod axis (DT parallel to) followed very close to the bulk viscosity of the solution. However, the diffusion normal to the rod axis (DT perpendicular to) experienced a significantly higher frictional force. For volume fraction phi > 0.1 a slightly modified caging theory by Pecora and Deutch could explain the rotational diffusion of the rods very well. At low volume fraction the agreement is poor, which we interpreted as modification of the local ordering of the spheres, which can affect the rod rotation.