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

Abstract

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