RI-dependent extinction spectra. Near the (a, c) dipole resonance mode of nanorods of types A and C and (b, d) quadruple resonance mode of nanorods of types B and D, respectively, with all the structures in a surrounding medium of RI varying from 1.33 to 1.37. The black arrows represent the shifting direction of the resonance peak from the case RI = 1.33 to RI = 1.37. The red double arrows denote the linewidth of each peak. Insets are schematics of nanoparticle geometries and their electric near-field amplitude distributions at the corresponding LSPR wavelengths. (e) Peak wavelengths λsp as a function of the surrounding RI for different LSPR modes/shapes corresponding to (a) to (d). The RI sensitivities dλsp/dn of the four curves are 712.2, 722.1, 689.3, and 676.9, in the unit of nm/RIU, respectively.