Place exchange Place exchange reactions between tiopronin-MPCs and thiolate-containing peptides, HA and FLAG, followed a method similar to Hostetler et al. [18, 38, 39] and assumes that epitope-tiopronin S N 2 place exchange is one for one. While it has been shown by 1 H NMR that the interchange of ligands is 1:1, the final ratio of the original ligand to exchanged ligand depends on ligand length, concentration of entering and exiting ligands, as well as the placement of the ligand (vertice, edge, or terrace) on the surface of the cluster [18, 38] . It should also be noted that the reported ratio will be an average exchanged ratio calculated from 1 H NMR, as the dispersity of MPC size will cause variations in the exact number of exchanged ligands on each cluster [18] . With the X-ray diffraction identification [40] of staple motifs, which are composed of one Au(I) and two thiolates, as part of the capping structure on gold-thiolate nanoparticles, place exchange must insert the new thiolate into the existing staples at the same time as the previous thiolate is lost in the S N 2 mechanism. Tiopronin MPCs were co-dissolved in DI water with free thiolated epitope (1:10). If necessary to aid the solvation of the epitopes into water, the epitopes were first dissolved into ethanol. The place exchange reaction took only one night for the exchange of a single epitope, while requiring 3 days for the exchange of two epitopes simultaneously. Solutions were dialyzed at room temperature for approximately 3 days as previously described and then dried under air [29] . The epitope composition of the MPCs was determined by TGA and 1 H NMR analysis (Electronic Supplementary Material Figs. S2 and S3).