Improving surface enhanced Raman signal reproducibility using gold-coated silver nanospheres encapsulated in silica membranes

Solution-phase nanoparticles are widely used in surface-enhanced Raman scattering (SERS) but provide limited quantitative information because of their dynamic optical properties. To overcome this problem, silica membrane stabilized solution-phase nanoparticles composed of silver cores and gold shells (Ag@Au@SiO2) are synthesized, characterized, and used for predictable and quantitative detection of 4-aminothiophenol using SERS. Two key parameters including local effective refractive index and void volume near the metal cores are correlated to SERS activity. First, the effective local refractive index and void volumes formed near the metal surface are characterized using the localized surface plasmon resonance of the Ag@Au nanoparticles and semi-empirical dielectric modeling for the porous silica membrane. The characteristic electromagnetic field decay length is estimated at 11 nm while both linear and nonlinear refractive index sensitivities are found to be 170 and 360 nm/RIU, respectively. The internally etched silica membrane stabilized Ag@Au@SiO2 nanoparticles can be engineered to exhibit effective local refractive indices ranging from 1.366 to 1.458. Second, SERS signals associated with these nanomaterials and 4-aminothiophenol are shown to indirectly depend on the effective local refractive index, which directly correlates to increasing void volume in the silica near the metal particle. This effect is attributed to well-controlled molecule-accessible volumes near the metal surface where the local electric field strength is largest. Finally, small variations in the effective refractive index (+/- 0.01), nanoparticle concentration, and nanoparticle to molecule concentrations influence the magnitude of the SERS signal. As such, these findings are expected to improve the design and surface modification of solution-phase SERS-active substrates for quantitative and reproducible SERS detection.