Rapid purification of gold nanorods for biomedical applications

Small gold nanorods (GNRs) with longitudinal plasmon absorption in the near-infrared window (700–900 nm) are of great interest for in vivo optical applications (e.g., photothermal therapy) and for their high-payload-to-carrier ratio for drug delivery. Common synthetic strategies for GNR production afford spherical and cubical nanoparticles in addition to the desired GNRs. Thus, several methods have been proposed for the selective separation of GNRs from the reaction by-products. For example, centrifugation has been used to separate the high aspect ratio (AR) GNRs (AR > 4). However, it is difficult to separate small sized GNRs with low AR (AR ≤ 4) that are particularly promising for biomedical applications. Here, we describe a simple and fast procedure for the separation of small GNRs with AR of 4, and length of 28 nm from reaction by-products. The shape separation is achieved through centrifugation according to the following steps: • Isolation of all gold products of the reaction from the excess of cetyltrimethylammonium bromide through a first cycle of centrifugation. • Optimization of the speed and the time of centrifugation for the separation of GNRs from the reaction by-products. • Shape separation of GNRs through a second cycle of centrifugation. The effectiveness of this procedure is documented.