Silica-coated gold nanorods with a gold overcoat: controlling optical properties by controlling the dimensions of a gold-silica-gold layered nanoparticle.

Silica shells were directly coated onto surfactant-capped gold nanorods by a simple one-step method. The procedure required no intermediate coating of the gold nanorod prior to the formation of the smooth silica shell, the thickness of which could be accurately controlled over the range 60-150 nm. These silica-encased gold nanorods were then covered with a gold overcoat to yield nanoparticles with unique optical properties that varied with the thicknesses of both the silica layer and the gold overcoat. Using these bulk solution-phase techniques, homogeneous distributions of gold-silica-gold layered nanoparticles with a pronounced plasmon extinction band in the near-IR (i.e., approximately 900-1700 nm) are readily and reproducibly prepared. More specifically, when using a core gold nanorod whose dimensions yield a plasmon band in the visible region of the spectrum (e.g., approximately 685 nm), the effect of the gold overcoat is to produce a broad plasmon band that is red-shifted by as much as approximately 1000 nm. As such, these multilaminate particles should be of interest as a convenient tool to enhance weak near-IR radiative transitions (e.g., singlet oxygen, O(2)(a(1)Delta(g)), phosphorescence at 1270 nm).