n brief, 100 mL of 0.01g HAuCl4 solution was brought to reflux with stirring, and 2.5 ml of 1% sodium citrate solution was introduced into this HAuCl4 solution. The mixture solution was kept boiling for another 30 min and left to cool to room temperature. As trans-mission electron microscopy(TEM) images show, the diameter of Au nanopatricles was about 16 nm(Fig.S1) The obtained Au nanoparticles were then stored in brown glass bottle at 4 ◦C for further use.