Gold Nanoparticle Synthesis

The 20 nm particles were prepared according to a combination of the Turkevich and Frens’ methods.^49,50 In short, 25 mL of a 1 mM chloroauric acid (HAuCl₄) solution was brought to a gentle boil while stirring. Five mL of a 1% trisodium citrate dehydrate solution was added to the boiling solution, and a condenser was attached to the flask. The mixture was boiled for 10 min before the sample was cooled to room temperature. The size of the particles was confirmed using DLS.

A seed-mediated growth was used to obtain particles of 40 and 100 nm diameter.¹⁵ Seeds were prepared by heating 15 mL of a 2.2 mM sodium citrate solution to boiling. 50 μL of 50 mM HAuCl₄ was added, and the mixture boiled for 10 min, before the temperature of the solution was maintained at 90 °C. To perform controlled overgrowth of the seeds, the sequential addition of small amounts (1–3 mL) of 10 mM citrate solution was alternated with that of a 1 mM HAuCl₄ solution (0.35–1 mL). It is critical to keep the molar ratio of citrate:HAuCl₄ at 10:1. This manipulation was repeated as many times as required to obtain particles of the desired size, which was monitored by DLS. The sizes of the GNPs were confirmed by TEM and DLS (Figure S14).