AgNPs were prepared by the reduction of AgNO3 with NaBH4. Under vigorously magnetic stirring, 0.010 g NaBH4 was added into 100 mL aqueous solution of AgNO3 with final concentration of 10−4 M, producing a light yellow colored solution. After being stirred for 5 min, 0.1 M HCl was used for adjusting the pH of the solution. Then 0.2 mL 10−3 M 4-MBA was added to the prepared AgNPs and left to stir for about 1 h to make sure self-assembly of the 4-MBA onto the surface of silver particles. All of the above procedures are prepared in the ice bath. When the reaction was over, the ice bath was taken away and the solution was left to reach the room temperature.