MPC synthesis and characterization Gold-tiopronin-protected MPCs were synthesized as previously described [29] [30] [31] . Briefly, tiopronin-protected gold nanoparticles were synthesized by dissolving tiopronin and HAuCl 4 ·3H 2 O (3:1) into a MeOH/acetic acid solution (6:1). The reaction was stirred for 30 min and then cooled in an ice bath. NaBH 4 was then added in 10× molar excess of gold. Reaction product was stirred for 1 h. The solvent was removed under vacuum, and the pH was lowered to 1 with concentrated HCl. Reaction product purification by dialysis with cellulose ester membranes (Spectra/Por CE, MWCO= 10,000) removed any excess tiopronin. Average particle diameter was determined by transmission electron microscopy (TEM). TEM images were taken on a Phillips CM20 instrument after applying aqueous MPC samples to Formvar-coated 200-mesh copper grids (Ted Pella). The microscope operated at 200 keV with magnification in the range ×150-750,000. Thermogravimetric analysis (TGA) was performed with a TGA 1000 (Instrument Specialists, Inc.) to calculate Au-tiopronin stoichiometry. From TEM and TGA data, the nanoparticle size and Au-tiopronin stoichiometry were obtained. MPCs used in the following experiments had an average diameter of 2.6±0.6 nm and a base composition of Au 544 Tiop 204 (140.4 kDa), calculated from TEM and TGA data (at 650°C), respectively (Electronic Supplementary Material Figs. S1, S2, and S4) [32] [33] [34] .