Here, we report an innovative facile polymer-templated synthesis of Ag/Au bimetallic nanoparticles (BNPs) on large surface for sensing. By controlling the reaction kinetics, the bimetallic nanoparticles have been successfully prepared with a variety of structures: heterostructures, eccentric core-shells, and physical mixtures of two metals. The amount and initial shape of the Au seeds determined the final architectures of bimetallic nanostructures. The underlying synthesis mechanism of BNPs was regulated by a seed-mediated growth (SMG) on the surface. The newly formed Ag atoms were directed to selectively nucleate and then epitaxially grow on specific facets of cubic/hexagonal Au seeds. Comprehensive results demonstrating an optical response of two metallic/poly (methyl methacrylate) (M⁺/PMMA) layers on opaque surfaces by micro-extinction measurements were obtained. Sensors based on anisotropic bimetallic substrates were ideal for sensitive 4,4′ -bipyridine (4,4’-BP) detection. The polymer surface-induced accumulation of high electric fields on nanoscale sensing volumes of anisotropic core/shell BNPs allowed more analyte molecules to access their high-index facets. This sample possessed a high surface-enhanced Raman scattering (SERS) sensitivity despite only being validated on very small densities of surfaces; thanks to the synergistic effects between the two coupled metals. The present findings suggest that a surfactant-free synthesis can be used as a powerful mean of defining growth strategies based on silicon substrate platforms.