Anionic gold nanospheres of 12 nm in size were synthesized according to the Turkevich-Frens method. Briefly, 20 mL of a 1mM HAuCl4 aqueous solution was brought to boiling temperature under constant and vigorous magnetic stirring. 2 mL of 1% citric acid solution was then added and the solution was further boiled for 20 min until it acquired a deep red color.