The 14.2 - nm spherical Au NPs were prepared through citrate mediated reduction of HAuCl4 . Aqueous 1 mM HAuCl4 (250 mL) was brought to a vigorous boil while stirring in a round bottom flask fitted with a reflux condenser ;38.8 mM trisodium citrate (25mL) was added rapidly to the solution , which was heated for another 15 min , during which time its color changed from pale yellow to deep red . The solution was cooled to room temperature with continuous stirring