Engineering sub-100 nm multi-layer nanoshells

Nanoshells are a novel class of optically tunable nanoparticles that consist of alternating dielectric and metal layers. They can potentially be used as contrast agents for multi-label molecular imaging, provided that the shell thicknesses are tuned to specific ratios. Sub-100 nm multi-layer nanoshells can potentially have improved tissue penetration, generate a strong surface plasmon resonance, and may exhibit absorption peaks in the visible–near-infrared (NIR) spectrum. Herein we describe the synthesis and characterization of bilayered concentric nanoshells with an overall diameter of around 50 nm consisting of a gold core, a tunable silica spacer layer and an outermost gold shell, which is approximately 16 times smaller than previously described multi-layered nanoparticles. The structured nanoshells were visualized by transmission electron microscopy (TEM) at each step of preparation. The absorption spectra of the gold–silica bilayered nanoshells are in good agreement with Mie's prediction and their resonance peak position is a function of the relative thickness of silica and gold layers.