The seed solution was prepared as fol-
lows: HAuCl4 (0.5 mM, 5 mL) was added to CTAB (0.2 M, 5 mL)
solution under constant stirring at 25 ◦C; then freshly prepared ice-
cold NaBH4 (0.01 M, 0.06 mL) was injected quickly into the solution,
resulting in a brownish yellow solution; vigorous stirring of the
seed solution was continued for 3 min for complete decomposi-
tion of excess NaBH4; it was then kept at 25 ◦C for 2 h to obtain
stable seed solution. NaBH4 should not expose in atmosphere for
a long time because of its easy decomposition after absorption of
moisture.