Biological applications of gold nanorods

Gold nanoparticles have been used as an additive for aesthetic purposes for centuries. However, only in the last decade scientists have begun to understand the fundamental concepts that explain the fascinating optical properties nanoscale particles possess. Gold nanoparticles may be tuned to absorb and scatter light in the visible to near-infrared region of the electromagnetic spectrum. These properties result from the collective oscillations of electrons along the metal surface and can be altered by changing the particles' shape, size, or environment. Gold nanoparticles having a rod-like morphology are of particular interest because of their anisotropic shape. The absorption profile of gold nanorods includes two absorption bands: one due to light absorbed along the short axis (transverse) and the other due to absorption along the long axis (longitudinal). As the rod length increases, so does the longitudinal band red shift together with an increase in the extinction coefficient. As a result of this optical control and sensitivity to changes in local environment, gold nanorods are useful materials for sensing, photothermal therapy, and imaging. This article highlights established and emerging applications of gold nanorods as a platform for viable biological tools.