Biotin-streptavidin-induced aggregation of gold nanorods: tuning rod-rod orientation.

We report herein biotin-streptavidin-mediated aggregation studies of long gold nanorods. We have previously demonstrated end-to-end linkages of gold nanorods driven by the biotin-streptavidin interaction (Caswell et al. J. Am. Chem. Soc. 2003, 125, 13914). In that report, the specific binding of biotin disulfide to the gold nanorod edges was achieved due to the preferred binding of thiol molecules to the Au[111] surface (gold nanorod ends) as opposed to the gold nanorod side faces. This led to the end-end linkage of gold nanorods upon subsequent addition of streptavidin. In this report we demonstrate a simple procedure to biotinylate the entire gold nanorod surface and subsequently form a 3-D assembly by addition of streptavidin. Gold nanorods were synthesized by the three-step seeding protocol documented in our previous articles. The surface of gold nanorods was further modified by a layer of a weak polyelectrolyte, poly(acrylic acid), PAA. A biotin molecule which has an amine group at one end (biotin-PEO-amine) was anchored to the carboxylic acid group of the polyelectrolyte using the well-known carbodiimide chemistry. This process biotinylates the entire gold nanorod surface. Addition of streptavidin further leads to aggregation of gold nanorods. A closer look at the aggregates reveals a preferential side-to-side assembly of gold nanorods. The gold nanorods were characterized at each stage by UV-vis spectroscopy, light scattering, and transmission electron microscopy (TEM) measurements.