*4.2. Preparation of the Nanoparticles*

The GNPs were prepared using the reducing HAuCl<sup>4</sup> method with trisodium citrate. Briefly, one milliliter of 1% HAuCl<sup>4</sup> aqueous solution was added to 99 mL of boiling threefold-distilled water. Then, 1.5 mL of 1% trisodium citrate was quickly added to the above solution under the boiling and stirring condition. The mixed reaction solution was kept boiling for 5 min in order to let the GNPs develop after the color of solution changed from deep black into brilliant wine red. Then, the prepared GNPs were cooled down and stored at 4 ◦C. Finally, the diameters and dispersion of the GNPs were evaluated through a transmission electron microscope.
