Shaping Gold Nanostar Electric Fields for Surface-Enhanced Raman Spectroscopy Enhancement via Silica Coating and Selective Etching

The application of gold nanostars in direct and indirect surface-enhanced raman spectroscopy (SERS) sensing has significantly grown in the past few years, mainly because of the particles' excellent field enhancement properties. However, experimental demonstrations correlating SERS signal enhancements to specific morphology features of the nanostars are still scarce, primarily because of the complexity of the nanostar morphology itself. Herein, we have addressed this fundamental issue by synthesizing surfactant-free gold nanostars, coating them with a uniform silica layer, and then etching the silica away selectively with NaBH₄ to expose increasing amounts of the metallic surface. We have then functionalized the nanoparticles with a Raman active molecule, aminothiophenol (ATP), and compared the resulting SERS spectra with those obtained on surfactant-free stars functionalized with ATP. Through comparison of the experimental results with the electric field intensities and distributions calculated via finite element simulations, we have observed a strong correlation between the Raman signal enhancements obtained experimentally and the heat losses calculated on three-dimensional representations of the same nanostructures. We believe that our model could be used to predict the effectiveness of nanostars at enhancing SERS signals based on their overall morphology, even when thorough experimental characterization is lacking.