Colloidal dispersions of gold rods:: Synthesis and optical properties

Colloidal dispersions of rod-shaped gold particles are obtained by electrodeposition of gold in the nanopores of anodized alumina attached to a conductive support followed by dissolution of the alumina and release of the rods from the support. Coagulation is prevented by steric repulsion due to adsorption of poly(vinylpyrrolidone) on the gold surface. The synthesis method presented is suitable for the preparation of dispersions of nearly monodisperse colloidal gold rods with adjustable aspect ratio Lid in the range 1.8 < L/d < 49. The length, L, is tuned between 39 and 729 nm, and the diameter, d, is between 12 and 22 nm. The absorbance spectra of randomly oriented colloidal gold rods display a transverse and a longitudinal resonance contribution. With increasing aspect ratio, the transverse resonance shows a small shift to shorter wavelengths, while the longitudinal resonance shifts into the near-infrared regime. The positions of the absorbance maxima agree with the predictions of the Gans theory.