Optical responses of Ag_xIn_1-x (left) and Ag_xAl_1-x (right) for x = 0.75 (light blue), x = 0.50 (green) and x = 0.25 (deep purple). (c, d): experimental spectra of BNPs embedded in silica. Ag_xAl_1-x BNPs were size-selected, while Ag_xIn_1-x BNPs were not. For both compositions, the higher the proportion of Al or In, the more the resonance is blue-shifted. (a, b) simulations performed within the multi-shell model structure. The dotted lines correspond to absorption spectra of Ag_XoIn_1-Xo@In and Ag_XoAl_1-Xo@Al BNPs for the three nominal proportions of silver x = 0.25, 0.50 and 0.75, with X_o = 0.75. In such configurations, the LSPRs are always blue-shifted compared to the experimental spectra. The full lines correspond to absorption spectra taking into account the oxidation of the indium or aluminium shell, resulting in multi-shell structures Ag_XcIn_1-Xc@In@In₂O₃ or Ag_XcAl_1-Xc@Al@Al₂O₃ where Xc (≠ X_o) is the silver atomic proportion in the core. For Ag-In and for the nominal proportions of silver x = 0.25, 0.50 and 0.75, the values of X_c are 90%, 92.5% and 95% and the volume percentages of indium oxidation V_ox are 30%, 50% and 85%, respectively. For Ag-Al, the spectra in full line correspond to silver atomic proportions in the core X_c = 90%, 92.5% and 100% and volume percentages of aluminium oxidation V_ox = 97.5%, 100% and 95%, for nominal concentrations x = 0.25, 0.50 and 0.75, respectively. These amounts of In₂O₃ and Al₂O₃ were adjusted to obtain the best agreement of the spectral position of the resonance with experimental spectra.