Citrate-capped AuNPs were prepared according to the previous report. Briefly, 300 mL HAuCl4 aqueous solution (0.3 mM) was heated to vigorous boiling in a round-bottom flask under stirring. Then, 15 mL sodium citrate (1 %, w/v) was quickly put into the solution and heated for another
10 min under stirring. The solution changed from pale yellow to deep red. Finally, the solution was cooled to room temperature under stirring. The concentrations of the AuNPs were measured by UV–vis spectroscopy using the molar adsorption coefficient at the maximum absorption band of the colloidal gold solution.