Fig. 2. XPS characterization. (A) XPS spectra of pure Au NPs, Au NPs modified with 4-Mpy, and Au@4-Mpy@AgNPs. (B–D) The corresponding high-resolution spectra of Ag 3d (indicated by the green bar in A), Au 4f (indicated by the magenta bar in (A)) and S 2p (indicated by the blue bar in (A)).