Optical properties and SERS efficiency of tunable gold/silver nanoshells

Tunable hollow Au/Ag nanospheres with an outer particle diameter of 55 nm and extinction maxima in the range of 600–700 nm were synthesized. An increasing amount of gold in the preparation of the colloids leads to a red shift of the plasmon band. A absorption, scattering and extinction spectra of these nanoshells were calculated using Mie theory. The corresponding dielectric functions of the alloy are determined by the gold/silver content of the particles. For particles with a thin shell (thickness <20 nm) also a damping factor must be considered.The formation of an alloy was experimentally confirmed by the characteristic surface plasmon band in the extinction spectrum. A comparison with theoretical extinction spectra demonstrates that the experimentally observed red shift of the plasmon band is due to a thinner shell. Specifically, the Mie calculations suggest a shell thickness of 3.5–5.5 nm for the synthesized nanoshells.The relative SERS efficiency of the Au/Ag nanoshells was determined with 4-mercaptobenzoic acid (MBA) as a probe molecule. Experimental SERS spectra of nanoshells covered with a self-assembled monolayer of MBA were recorded. A qualitative analysis of their relative SERS efficiency was performed using two dominant Raman bands of MBA. Additionally, quantitative predictions of SERS efficiencies based on calculated Mie scattering intensities are presented.