Ag@Au yolk shells decorated onto silica spheres for enhanced plasmonic applications

Silica surfaces covered with a homogenous layer of metal nanoparticles exhibit superior plasmonic properties when compared with pristine colloidal counterparts, as they have higher scattering probability with enhanced potential utility. Precise galvanic replacement reactions performed on such sacrificial templates are of great interest. Herein, we demonstrate the robust formation of Ag@Au yolk shells decorated onto the dielectric silica spheres via a direct one-step galvanic replacement procedure, evidently initiating the deposition of Au onto low surface energy Ag facets (prefixed onto the silica spheres), resulting in a disjointed framework of compact dealloyed metal lobes, wherein their facets remain smooth enough with a high degree of crystallinity and exhibit maximal porosity. Our combined spectroscopic data analyses corroborate the plausible mechanism governing solely the formation of Ag@Au core-shell onto the silica spheres rather than their hollow counterparts. Furthermore, we made the respective monolayer films using the well-known air-water interface self-assembly technique, which allowed us to easily decorate different substrates with a uniform array of these surface-coupled metal nanoparticles. Enhanced refractive index (RI) sensing applications based on the sensitivity and FOM (figure of merit) calculations demonstrate the versatile nature of our synthetic protocol for direct synthesis of optical metamaterials along with their intriguing light-matter interactions, in general. Also, the plasmonic vector field of 2-D assembled nanoparticle monolayer displays intrigued SERS enhancement of crystal violet dye when compared with their respective drop-casted films, thus establishing a novel method for producing diverse meta-structures based efficient SERS substrates with promising applications in ultrasensitive chemical and biological sensing.