Analysis of Nanostar Reshaping Kinetics for Optimal Substrate Fabrication.

Abstract

Gold nanostars (NS) are emerging as a versatile tool in surface-enhanced Raman scattering (SERS) applications because of their wide localized surface plasmon resonance (LSPR) tunability, simple synthesis procedure, and high SERS enhancement. These particles are commonly used in solutions with a stabilizing coating shell (e.g., thiolated molecules or silver shell). However, coatings cannot be used for the fabrication of SERS substrates as the NS have to interact with the substrate planar surface. Without coating, NS have been observed to change over time, leading to a hypochromic shift of the LSPR. To understand this shift, we synthesized surfactant-free gold NS with different spike morphologies and investigated their reshaping morphology and kinetics. Using TEM, the NS sharp spike features were observed to reshape over time. The kinetics of this process were analyzed and determined by monitoring the LSPR, which was observed to follow an exponential decay over time. We used an empirical fit for the LSPR-shift data as a function of time, which permits to predict the LSPR at a specific time based only on the initial LSPR (independently of the initial spike morphology). We show the effect of the LSPR on the SERS signal for the NS and how the SERS signal correlated to our prediction. Finally, we evaluated our approach by fabricating SERS substrates with immobilized NS and collecting the reflectance spectra. We were able to predict the substrate LSPR and aim for an optimal LSPR with an average 3% deviation. These new insights on NS reshaping can permit the fabrication of NS-based substrates with desirable optical/plasmonic properties.