Mesoporous silica-coated gold nanorods (AuNR@MS) act as a colloidally stable Raman sensing platform with a built-in analyte size cutoff. Herein, these core-shell plasmonic nanostructures were presented with a range of thiolated Raman-active molecules to probe the limits of this platform for SERS sensing. The experimental results show generally, that the transport of molecules through the mesopores is highly dependent on the size of the molecule and specifically, that AuNR@MS with pores of ∼4 nm diameter are able to sense analytes with molecular dimensions smaller than 1.5 nm. This sensing platform will likely find broad use, performing well even in complex media based on the high colloidal stability imbued by the mesoporous silica shell.