Enlarge the biologic coating-induced absorbance enhancement of Au-Ag bimetallic nanoshells by tuning the metal composition.

Surface plasmon resonance (SPR) absorption properties of biologic layer-coated Au-Ag core-shell nanoparticle have been studied theoretically. By comparing with pure Au and Ag nanoparticles, the absorption peak intensity of Au-Ag bimetallic nanoparticle is more dependent on the dielectric coating, and the longer wavelength peak corresponding to the Au-Ag interface is more sensitive to the coating thickness. As the Au-Ag composition ratio is increased, the dielectric coating-dependent peak intensity change is always non-monotonous, and the critical Au-Ag composition ratio corresponding to the greatest peak intensity increase could be further tuned by changing the dielectric constant of the coated dielectric layer. The physical mechanism of the non-monotonous sensitivity change has been attributed to the Au-Ag composition ratio-dependent separation and overlap of the two SPR peaks from Au-Ag interface and outer Ag surface. These results indicate that the plasmonic absorption intensity of Au-Ag core-shell nanoparticle could be used in refractive index sensing and biologic detection.