In the development of a synthetic positive control for this influenza immunoassay, functionalized gold MPCs were synthesized and characterized. TEM analysis of the tiopronin-gold nanoparticles (Au-tiop) resulted in an average cluster size of 2.6±0.6 nm, with a range from 1.5 to 4 nm (Electronic Supplementary Material Fig. S1 ). The TGA tiopronin MPC mass loss equaled 37.9% and yielded a 2.67 Au:Tiop stoichiometric ratio for the nanoparticles assuming the loss of the Au(I)-thiolate staples (Electronic Supplementary Material Figs. S2 and S4 ). The assumption of the loss of the Au(I)-thiolate staples is justified by the observation of gold-thiolate compounds upon thermal decomposition in a mass spectrometer [46] . The average number of gold atoms calculated per cluster equaled 544 atoms. Combining the TEM and TGA yields a calculated empirical formula Au 544 Tiop 204 for the 2.6-nm diameter particle. Following place exchange of epitope(s) for tiopronin, the epitope loading was determined through 1 H NMR (Electronic Supplementary Material Fig. S3 ). The epitopes HA and FLAG each contain an exclusive amino acid whose proton signal occurs at a unique location in the 1 H NMR spectrum. Specifically, valine (V) is unique to the FLAG epitope and lysine (K) to the HA epitope. Tyrosine (Y) is common to both epitopes, but occurs in different stoichiometric ratios. The integrated resonance for each functionalized nanoparticle can be compared with that for nanoparticles protected solely with tiopronin to determine the average epitope stoichiometry of the nanoparticles. Peaks used for quantification consisted of signals at 1.2 ppm (V, 2 CH 3 ), 1.45 ppm (tiopronin, CH 3 ), 2.90 ppm (K, 2 ε-CH 2 ), 6.75 ppm (Y, δ-CH), and 7.05 ppm (Y, ε-CH Material Fig. S4) .