GNS synthesis and characterization

GNS were synthesized following published methods^32,33. All glassware and stir bars were cleaned by Aqua Regia prior to synthesis. All chemicals were purchased from Sigma-Aldrich and used without further purification. The 16 nm GNS were produced by adding 0.5 mL 3% (W/V) sodium citrate into boiled 50 mL 0.25 mM HAuCl₄ under vigorous stirring. The resulting 16 nm GNS were used both for aggregate study and as seeds for 30 nm GNS. The 30 nm GNS were synthesized by adding 0.875 mL 25 mM HAuCl₄, 15 mM sodium citrate, 12.5 mL as-made 16 nm GNS and 25 mM hydroquinone successively into vigorously stirred MilliQ water with continued stirring for 30 min at room temperature. The resulting GNS were characterized by dynamic light scattering (DLS, Brookhaven Zeta PALS instrument), transmission electron microscopy (TEM, FEI Tecnai T12), and UV–Vis spectroscopy (Ocean Optics, Dunedin). We also synthesized 5.5 nm CTAB capped GNS following a previously published protocol³⁴. This synthesis method requires excess amount of CTAB to stabilize GNS and the CTAB precipitate affects DLS reading. In addition, upon purification irreversible aggregation of 5.5 nm GNS occurred making further experiments impossible.