The schematic diagram for the synthetic procedure is show in Fig. 1 . The method used for the synthesis of the Au NPs was similar to one described previously [28] . Briefly, a 50 mL aqueous solution containing 0.01% HAuCl 4 and 0.01% tri -sodium citrate was prepared in a conical flask. Next, 1.5 mL of ice cold 0.38% NaBH 4 solution was added to the solution all at once while stirring. The solution turned pink immediately after adding NaBH 4 , indicating particle formation.