Fabrication, stabilization, and optical properties of gold nanorods with silver shells

We describe experimental results on the synthesis and optical properties of gold nanorods with silver coating. The nanoparticles were fabricated by the seed-mediated growth of gold nanorods in a growth solution with ionic surfactant CTAB (cetyltrimethylammonium bromide); then the particles were separated in the density gradient of glycerol and the silver ions were reduced by ascorbic acid in the presence of polyvinylpirrolidone at alkalic conditions. The formation of a silver nanoshell was controlled by the shift of plasmon resonances (PR) of extinction and differential light scattering, by the appearance of characteristic Ag peaks in the EDX spectra of samples, by TEM data, and by visually inspecting changes in colloid colors. Theoretical calculations with a two-layered spheroid model are in agreement with spectral measurements. To evaluate the silver nanoshell thickness, we calculate a calibration curve for the relative PR shift. It is noted that the technology described gives particles that are instable in water environment, leading to notable drift of PRs and optical spectra in whole. We have found that the optical properties of gold-silver nanorods can be stabilized by transferring in ethanol or by the addition of polyacrilic acid. The basic advantage of the described protocol is the fine controlled tuning of the extinction and light scattering PRs of two-layered nanorods from NIR to 550 nm with an accuracy of about 10 nm. Such particles may find promising applications in biophotonics, bioimaging of cellular structures, contrasting OCT tissue images, etc.