a–c, The resonance energy of the same Au nanoprism as shown in Fig. 3a, measured by EELS at P1 (a), P2 (b) and P3 (c) in vacuum (Vi, blue solid line), CO (pCO = 110 Pa, turquoise dash-dotted line) and after CO evacuation (Vf, red short-dashed line), displaying the blueshifts of LSP resonance in the presence of CO by 0.087±0.002 eV, 0.040±0.002 eV and 0.078±0.002 eV, respectively, and the recovery to the original energies. d, Relative energy shifts of LSP resonance, ΔE (%) = (ELSP(PCO)–ELSP,0)/ELSP,0×100%, measured by EELS at P1 as a function of pCO, show ΔE increases with increasing pCO. Inset, ΔE at Vi and pCO = 0.01 Pa, respectively. Error bars indicate single standard deviation uncertainties of determining the LSP resonance energy by peak fitting.