Preparation and optical scattering characterization of gold nanorods and their application to a dot-immunogold assay

We describe optical monitoring of the synthesis of gold nanorods (NRs) based on seed-mediated growth in the presence of the soft surfactant template cetyltrimethyilammonium bromide. To separate NRs from spheres and surfactants we fractionated samples in the density gradient of glycerol. The optical properties of NRs were characterized by extinction and differential light-scattering spectra (at 90°, 450-800 nm) and by the depolarization light-scattering ratio, I_vh/I_vv, measured at 90° with a helium-neon laser. Theoretical spectra and the I_vh/I_vv ratios were calculated by the T-matrix method as applied to randomly oriented NRs, which were modeled by right-circular cylinders with semispherical ends. The simulated data were fitted to experimental observations by use of particle length and width as adjustable parameters, which were close to the data yielded by transmission electron microscopy. The sensitivity of the longwavelength resonance of NRs to the dielectric surroundings was examined both experimentally and theoretically by comparison of the extinction spectra of NRs in water and in a 25% glycerol solution. Finally, we discuss the application of NR-protein A conjugates to a dot-immunogold assay with the example of biospecific staining of human IgG molecules adsorbed onto small membrane spots.