Citrate-reduced Au nanoparticles vs. monodisperse spheres: extinction and dynamic light scattering measurements

The most common protocol for the synthesis of Au nanospheres is the reduction of gold salts by sodium citrate. However, the particles obtained by this method are not quite perfectly spherical and their optical and physical properties suffer from polydispersity and nonuniformity. In this work, we synthesized sets of quasispherical Au nanoparticles with sizes from 20 to 70 nm by citrate-based methods as well as single-crystal Au nanospheres with uniform diameters ranging from 20 to 130 nm. Particles were characterized by TEM, UV-VIS and Dynamic Light Scattering (DLS). We found that the extinction spectra of monodisperse Au nanospheres are equal to those calculated by using Mie theory (both in the position and width of the plasmonic band). The deviation of shape from a sphere that is typical of citrate-stabilized nanoparticles leads to a significant departure of the optical properties from those predicted by Mie theory. The non-sphericity of these particles also affects the measurement of their size by DLS. In particular, citrate-stabilized nanoparticles display a bimodal size distribution associated to rotational diffusion.