Optimal Size of Silver Nanoparticles for Surface-Enhanced Raman Spectroscopy

The optimal size of spherical silver nanoparticles (AgNPs) for off-resonance surface-enhanced Raman scattering (SERS) was found to be ∼50 nm based on the equivalent Ag content in AgNP colloids. It is understood that the SERS intensity of adsorbates on the surface of metal nanoparticles is dependent on the size and shape of the particles of interest. Herein, we report a seeded growth mechanism for the formation of silver nanoparticles that allows superior control over the size of the resultant nanoparticles with relatively low polydispersity. The high degree of size control allows for a better understanding of the study of the effect of particle size on SERS intensity. The Raman study performed here employed a long-wavelength excitation (785 nm) so as to avoid photochemical degradation of adsorbed species and photochemical transformation under intense excitation. Under these experimental conditions, it was found that the optimal size of AgNPs for providing a maximum SERS intensity of adsorbed R6G is ∼50−60 nm, a result that is expected to extend to other adsorbates as well.