Bimetallic Core–Shell Nanostars with Tunable Surface Plasmon Resonance for Surface-Enhanced Raman Scattering

Abstract

Noble metal complex nanomaterials were widely investigated in bioscientific systems for surface-enhanced Raman scattering (SERS) imaging. Silver (Ag) has higher enhancement factors, arising from better optical properties, in comparison to gold (Au), which has better chemical stability and a broader localized surface plasmon resonance (LSPR) region extending to red-NIR, and both should be taken into account to enhance SERS performance. Among them, nanostars (NSs) have emerged as one promising geometry because of their strong enhanced electromagnetic fields induced by the lightning-rod effect from their branches. Therefore, core–shell NSs have been employed as efficient SERS substrates for trace detection because bimetallic nanoparticles provide richer plasmonic modes from their integrated material-dependent and size/shape-correlated plasmonics. Herein, we prepared two uniform and tunable core–shell NSs with different cores (Ag or Au) and the same Au shell, tailoring size and branch morphology to investigate their induced LSPR properties. As a result, their extinction spectra nearly coincide when the size and shape of two NSs become similar. A parallel electric field distribution and response wavelength were obtained as the shells are alike. Thus, it strongly suggests that the shell dominates its LSPR region in the core–shell NS. Meanwhile, an improved SERS activity with an enhancement factor of 1.5 × 107 was achieved in Ag@Au compared to Au@Au NSs because of the addition of Ag. This work offered certain theoretical and experimental references for the plasmonic metal NS with extensively enhanced electromagnetic fields, providing an effective SERS substrate for imaging and detection in life sciences.