Tuning gold nanostar morphology for the SERS detection of uranyl.

The impact of tunable morphologies and plasmonic properties of gold nanostars are evaluated for the surface enhanced Raman scattering (SERS) detection of uranyl. To do so, gold nanostars are synthesized with varying concentrations of the Good's buffer reagent, 2-[4-(2-hydroxyethyl)-1-piperazinyl]propanesulfonic acid (EPPS). EPPS plays three roles including as a reducing agent for nanostar nucleation and growth, as a nanostar-stabilizing agent for solution phase stability, and as a coordinating ligand for the capture of uranyl. The resulting nanostructures exhibit localized surface plasmon resonance (LSPR) spectra that contain two visible and one near-infrared plasmonic modes. All three optical features arise from synergistic coupling between the nanostar core and branches. The tunability of these optical resonances are correlated with nanostar morphology through careful transmission electron microscopy (TEM) analysis. As the EPPS concentration used during synthesis increases, both the length and aspect ratio of the branches increase. This causes the two lower energy extinction features to grow in magnitude and become ideal for the SERS detection of uranyl. Finally, uranyl binds to the gold nanostar surface directly and via sulfonate coordination. Changes in the uranyl signal are directly correlated to the plasmonic properties associated with the nanostar branches. Overall, this work highlights the synergistic importance of nanostar morphology and plasmonic properties for the SERS detection of small molecules.