The dynamical properties of spherical objects, such as random coil polymers, proteins, and colloidal particles, are well understood and have been studied by a variety of experimental techniques. [1±4] Spheres in the sub-micrometer and micrometer size range have the attractive feature of allowing one to visualize crystallization, melting, and other processes that are difficult to observe directly with smaller objects. [5±8] Studies of this type have not yet been conducted with non-spherical particles, despite the fact that many interesting nanoscale objects such as bacteriophage viruses, nanowires, and nanotubes have high aspect ratios. The assembly of nanowires and nanotubes on surfaces is often accomplished by using chemical or fluidic methods. [9±12] Therefore, understanding the dynamical properties of suspensions of high aspect ratio particles in contact with surfaces is an important step towards realizing self-assembling circuits, nanoscale machines, and other functional assemblies. Further, there is a wealth of theoretical work that motivates experimental studies of the dynamics and collective behavior of rod-like particles. [13±17] We report here the visualization of two-dimensionally (2D) confined suspensions of metal nanorods, and determine their diffusion coefficients by particle tracking measurements. We find that the diffusion coefficient is strongly affected by the surface chemistry of the nanowires and the substrates, and by the dimensions of the nanowires themselves.Diffusion coefficients may be obtained from observations of either ensemble or trajectory diffusion. [18] Ensemble diffusion is usually measured when individual particles are not observable (e.g., molecules in solution) by spectroscopic or electrochemical techniques. Trajectory diffusion is commonly associated with the Brownian motion of larger particles and may be observed directly by microscopy. Ensemble and trajectory diffusion experiments should yield the same results with the diffusion coefficient, D, defined in the one-dimensional (1D) case aswhere áx 2 ñ is the average of the square of the displacement and t is the time spanned by each displacement measurement. By measuring many values of particle displacement at regular time intervals we were able to calculate the diffusion coeffi-cients for different sizes of particles on surfaces and were able to modulate D by altering the surface chemistry of the particles and the surfaces. Gold nanorods were made by an electrochemical technique described previously. [19] Aqueous suspensions of rods were diluted to a concentration of approximately 10 7 rods mL ±1 , and 10 lL was applied to a microscope slide. This provided sufficiently few rods that diffusion lengths were small compared to the average interparticle spacing. A spacer (120 lm thick) was used to seal the droplet under a transparent cover glass. The rods, which sank quickly to the surface of the microscope slide, were observed in the brightfield mode of Olympus BX60M metallurgical optical microscope. We used a simple particle ...