Interactions between Silica-Coated Gold Nanorod Substrates and Hydrophobic Analytes in Colloidal Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman scattering (SERS)-based detection of suspension-phase analytes holds great promise for a variety of applications; however, plasmonic colloidal SERS substrates are not stable in many solution conditions unless they are protected by a stabilizing agent. Mesoporous silica shells on plasmonic nanoparticle cores have been demonstrated to perform well in a variety of liquid matrices. However, this silica shell can be seen as barrier from the perspective of the analyte, as the analyte molecules need to reach the plasmonic core after they pass through the shell. In this work, mesoporous silica-coated gold nanorods have been synthesized and characterized as aqueous colloidal SERS substrates systematically considering how SERS performance is impacted by three different factors: adsorbed molecules, the silica shell, and bulk solvent media. The results show that SERS signal intensities from the model hydrophobic analyte, trans-1,2-bis(4-pyridyl) ethylene (BPE), are enhanced when the pore size, hydrophobicity of the shell, and ionic strength are increased, indicating more favorable interaction between the substrates and the analyte. The silica shell presented herein facilitates efficient adsorption of the analyte to the gold core and enhanced sensitivity to environmental refractive index changes. This efficient adsorption can be further enhanced by controlling the incubation temperature. Overall, this work reveals how substrate exposure conditions can be tuned to maximize analyte SERS signals without compromising the silica shell that protects the plasmonic properties of the SERS-enhancing core.