DNA-gold triangular nanoprism conjugates.

Over the past two decades, there has been an enormous effort to understand, control, and use the relationship between the morphology of nanomaterials and their physical and chemical properties. Indeed, a great deal of effort has focused on controlling the size, shape, and composition of nanostructures. However, equally important is the surface chemistry of such structures, especially when one considers that the ratio of surface to bulk atoms is much higher than that in microor macroscopic systems. For example, the surface plasmon resonances (SPRs) of nanoparticles are highly dependent upon the dielectric medium that surrounds them, including adlayers of molecules and solvent. Catalytic activity of a nanoparticle is also highly dependent upon adsorbate. Indeed, the development of ways to control the structure and chemical functionalization of a nanoparticle surface is a major area of research, and has led to the discovery of new properties and applications for this important class of materials. In the case of isotropic metal nanoparticles, functionalization has led to a variety of exciting scientific insights and applications, including the use of nanoparticles for spectroscopic labels, gene-regulation agents, and ultrasensitive biodiagnostic tools. In general, less has been done with anisotropic structures, but there have been some important observations and advances made for nanorod and nanoshell structures. For example, Gole and Murphy have reported that gold nanorods functionalized with biotin-disulfide will assemble in an end-to-end manner when interconnecting streptavidin proteins are added to a colloid containing them. The authors concluded that such assembly behavior was indicative of endselective functionalization. Mann and co-workers described a similar example with oligonucleotide-modified nanorods, but in contrast with the Murphy system, they observe sheetlike assembly of nanorods along their long axes. These results present a challenge to both understand and establish control over the face-selective functionalization of anisotropic nanoparticles.