Gold nanorods and magnetic nanocrystals are two materials that are being studied for their suitability as imaging contrast agents in biological systems. Magnetic nanocrystals can be used to enhance magnetic resonance imaging (MRI) contrast, for labeling of cancer cells, and also for intracellular labeling. The magnetic response of a nanocrystal can be tuned by varying the size of the nanocrystal as well as the composition.

Gold nanorods are being explored for biological and medical use as optical contrast agents for dark field and two-photon luminescence diagnostic imaging and photothermal therapy of cancer cells. They are attractive candidates for medical imaging because their optical response can be tuned to near-infrared wavelengths, which penetrate deep into cells and tissue; furthermore, they do not photobleach or blink, and are chemically inert and biologically compatible.

Gold nanorods have been produced in many laboratories by a colloidal seed-mediated, surfactant-assisted approach that has been widely studied and improved. Nonetheless, the reproducibility of the synthesis (i.e., the nanorod size, shape, and yield) has been a persistent challenge facing the technique. Many studies have addressed this issue, attributing differences in reproducibility to a wide variety of factors including seed aging time, the method of mixing the seed and growth solutions, variations in salt concentration and temperature of the growth solution, as well as nanorod growth time. We discovered that one significant factor in nanorod synthetic reproducibility is the supplier of the surfactant, hexadecylcetyltrimethylammonium bromide (CTAB), and that CTAB obtained from some suppliers does not yield gold nanorods and produces only spherical gold nanoparticles instead. It turns out that an impurity in the CTAB surfactant greatly affects nanorod formation.

Though as-synthesized gold nanorods are soluble in water, the bilayer of CTAB on their surface may render these particles harmful to living systems. As-synthesized magnetic nanocrystals are not soluble in water because they are capped with organic ligands. By coating both of these materials with silica, they are made water soluble and can thus be introduced into biological systems. Furthermore, when a fluorescent dye is embedded into the silica coating of the nanocrystals, an additional imaging modality is added. Thus, the final fluorescent dye doped silica coated nanocrystal is a heterostructure with dual imaging modality.

This presentation will reveal the importance of CTAB in the synthesis of gold nanorods and describe their subsequent coating with fluorescent dye doped silica. A similar coating strategy which has been applied to magnetic nanocrystals will also be discussed.