Colloidal dispersion of gold nanorods: Historical background, optical properties, seed-mediated synthesis, shape separation and self-assembly

The color of colloidal dispersions of gold particles in a fluid, typically water, varies from red to blue, depending upon the shape and size of particles. The color and optical properties of gold nanoparticles originate from localized surface plasmons, and are sensitive to their local dielectric environment. Unlike nanospheres, the optical properties, hydrodynamic behavior as well as phase behavior of nanorods are influenced by their shape anisotropy. Thus, rods have an additional absorption peak, possess very different dynamics (affects sedimentation) and their concentrated dispersions form liquid crystalline phases. In this review, we focus on presenting the essential shape dependent optics, as well as the hydrodynamics and phase behavior of rod-like gold nanoparticles. We reveal our methodology for making less polydisperse nanorods sols by using an optimized seed-mediated synthesis (controlled chemistry), followed by shape separation by centrifugation (based on our hydrodynamics arguments). We elucidate the role of Brownian motion in determining colloidal stability and sedimentation behavior, and describe patterns formed by drying mediated assembly on glass slides and TEM grids. We outline early studies (before 1930) of gold sols that are not only instructive in learning about synthesis and physical properties of gold nanoparticles, but show how the study of colloidal gold established many key principles in colloidal science.