Plasmonic properties of regiospecific core-satellite assemblies of gold nanostars and nanospheres.

Solution-based molecularly-mediated bottom-up assembly of gold nanostars and nanospheres in regiospecific core-satellite nanoarchitectures is reported. The controlled assembly is driven by coupling reactions in solution between small, rigid, Raman-active organic molecules bound to the surface of the nanoparticles, and leads to much narrower interparticle gaps than achievable with DNA-based assembly methods. In the described system, gold nanostars with multiple sharp spikes, ideal for electromagnetic field enhancement, are used as the core particle onto which spherical satellites are assembled. Transmission electron micrographs show that the core-satellite structures assemble with <2 nm interparticle gaps and regiospecific binding of only one sphere per spike, and the process can be followed by monitoring changes in the surface enhanced Raman scattering (SERS) spectra of the Raman active linkers. The assembled structures give rise on average to two orders of magnitude SERS signal enhancement per nanoparticle in comparison to their constituents, which can be attributed to the creation of SERS "hot spots" between the nanostar tip and the satellite sphere. Two dimensional finite element electromagnetic models show strongly confined electromagnetic field intensity in the narrow interparticle gaps of core-satellite assemblies, which is significantly enhanced in comparison to the constituent nanoparticles, thus corroborating the experimental findings. Thus, the assemblies reported here can be envisioned as SERS-tags for imaging purposes as well as a model system for SERS-based chemical sensing with improved sensitivity.