Monodisperse Colloidal Gold Nanorings: Synthesis and Utility for Surface-Enhanced Raman Scattering

Highly monodisperse colloidal Au nanorings (Au NRs) were synthesized via galvanic replacement reaction of ID chain assembly of Co nanoparticles (Co NPs) in HAuCl4. The Au NRs exhibited tunable localized surface plasmon resonance (LSPR) wavelengths ranging from 651 to 760 nm as the aspect ratio increased from 3.8 +/- 0.5 to 7.1 +/- 1.8, consistent with calculations using the finite-difference time domain (FDTD) method. The FDTD calculations indicated intense field enhancement within the cavity of well-separated Au NRs as well as the presence of hot spots between closely situated Au NRs. The Au NRs are individually active for surface-enhanced Raman scattering (SERS). We show that the steric hindrance of densely packed poly(vinylpyrrolidone) (PVP) stabilizer at the particle junctions of the ID chain assembly of Co NPs to AuCl4- diffusion is of critical importance for Au NR formation. We further demonstrate that whether an analyte experiences the hot-spot effect or not depends strongly on its ability to enter the junction points and thus its affinity to PVP on the surface of Au NRs in our system of study.