Monodisperse\nColloidal Gold Nanorings: Synthesis and Utility for Surface-Enhanced\nRaman Scattering

Abstract

Highly monodisperse colloidal Au\nnanorings (Au NRs) were synthesized via galvanic replacement reaction\nof 1D chain assembly of Co nanoparticles (Co NPs) in HAuCl₄. The Au NRs exhibited tunable localized surface plasmon resonance\n(LSPR) wavelengths ranging from 651 to 760 nm as the aspect ratio\nincreased from 3.8 ± 0.5 to 7.1 ± 1.8, consistent with calculations\nusing the finite-difference time domain (FDTD) method. The FDTD calculations\nindicated intense field enhancement within the cavity of well-separated\nAu NRs as well as the presence of hot spots between closely situated\nAu NRs. The Au NRs are individually active for surface-enhanced Raman\nscattering (SERS). We show that the steric hindrance of densely packed\npoly­(vinylpyrrolidone) (PVP) stabilizer at the particle junctions\nof the 1D chain assembly of Co NPs to AuCl₄^– diffusion is of critical importance for Au NR formation. We further\ndemonstrate that whether an analyte experiences the hot-spot effect\nor not depends strongly on its ability to enter the junction points\nand thus its affinity to PVP on the surface of Au NRs in our system\nof study.