Synthesis of Gold Nanoparticles and Nanostar

The synthesis of AuNPs was achieved by reducing HAuCl₄ with citric acid [37 (link)]. Specifically, AuNPs were synthesized by refluxing a 0.01 wt.% HAuCl₄ aqueous solution at 100 °C for 30 min, followed by the addition of a 1 wt.% aqueous solution of trisodium citrate (0.9 mL) and continuing the reflux for an additional 1 h. AuNSs were synthesized using a modified seed-mediated method [38 (link)]. The gold nanoparticle seeds were synthesized using the citrate reduction method mentioned above. The volume of a 1 wt.% trisodium citrate solution added to the HAuCl₄ aqueous solution was adjusted to 4 mL to synthesize the gold nanoparticle seeds with a diameter of 12 nm. A 1 mL colloidal aqueous solution of gold nanoparticle seeds was mixed with 100 mL of 0.01 wt.% HAuCl₄ aqueous solution, 700 μL of 1 mM hydrochloric acid, and 1 mL of 2 mM silver nitrate. Upon addition of 500 μL of an aqueous solution containing 0.1 M L-ascorbic acid to this mixed solution, AuNSs were synthesized. To prevent aggregation of the AuNSs, 0.7 mL of a 1 wt.% trisodium citrate aqueous solution and 3 mL of a 1 M sodium hydroxide aqueous solution were added, followed by stirring the mixture for approximately 10 min.