Improve the plasmonic optical tunability of Au nanorod by Pt coating: the application in refractive index sensing

Plasmonic light absorption properties of bimetallic Au–Pt core–shell nanorod are investigated theoretically. The plasmonic absorption intensity comparison between the longitudinal peak corresponding to outer Pt surface (denoted as band $$\hbox {Pt}_{\mathrm {L}})$$ and the transverse peak corresponding to Au–Pt interface (denoted as band $$\hbox {Au}_{\mathrm {T}})$$ is sensitive to the environmental dielectric constant. By increasing the environmental dielectric constant, the band $$\hbox {Pt}_{\mathrm {L}}$$ fades down, whereas the band $$\hbox {Au}_{\mathrm {T}}$$ gets intense. So the absorption discrepancy between $$\hbox {Pt}_{\mathrm {L}}$$ and $$\hbox {Au}_{\mathrm {T}}$$ bands could be greatly enhanced by increasing the environmental dielectric constant, which is more sensitive to the environmental refractive index than single plasmonic band. This refractive index sensing based on two bands’ absorption discrepancy could be further improved by increasing the Pt coating thickness or the aspect ratio of inner Au nanorod. The refractive index sensing based on red shift of the longitudinal peak corresponding to Au–Pt interface is also competitive, which can also be improved by increasing the Pt coating thickness or aspect ratio of inner Au nanorod. A mechanism based on media polarization-related electric field discontinuity and distribution of surface charge density was investigated to illuminate the absorption intensity-dependent refractive index sensing. In this Au–Pt core–shell nanorod, the $$\hbox {Pt}_{\mathrm {L}}$$ band fades down, whereas the $$\hbox {Au}_{\mathrm {T}}$$ band gets intense as the environmental dielectric constant is increased. So the absorption discrepancy between $$\hbox {Pt}_{\mathrm {L}}$$ and $$\hbox {Au}_{\mathrm {T}}$$ bands could be greatly enhanced by increasing the environmental dielectric constant, which is more sensitive to the environmental refractive index than single plasmonic band.