The gold salt solution (HAuCl 4 .3H 2 O) was prepared at a concentration of 1 mM after the sodium citrate reducing solution (C 6 H 5 Na 3 O 7 ) was added with vigorous stirring and heated to 100 °C. under reflux. The color of the solution changed from light yellow, dark black, and finally to burgundy red which indicates the formation of gold nanoparticles. The boiling and stirring were maintained for 25 min, and then the medium was left to cool to room temperature under stirring. Citrates play both the role of reducing agent and stabilizer for NPs, due to their negative charge which prevents agglomeration of particles by electrostatic repulsion.