Preparation of Colloidal Gold Solution
The principle of using 1% tri-sodium citrate to reduce hydrogen tetrachloroaurate hydrate (HAuCl4) to prepare colloidal gold particles of different sizes was used (Liu et al., 2012;). One milliliter of 1% chloroauric acid was added to 100 mL of ultrapure water, mixed well, and heated until bubbles appeared uniformly and started rising upward, after which 2.0 mL of 1% trisodium citrate solution was added immediately, and heating continued for 8–10 min. The Erlenmeyer flask was carefully removed and allowed to cool to room temperature. Ultrapure water was added to a volume of 100 mL.