bromide (CTAB, 99%), gold chloride trihydrate (HAuCl4·3H2O, 99%), 4-mercaptobenzoic acid (4-MBA, 99%), sodium borohydride
(NaBH4, 99%), poly(vinylpyrrolidone) (PVP ≈ 55,000),
and hexadecyl trimethyl ammonium chloride (CTAC, 99%) were purchased
from Sigma-Aldrich. Ascorbic acid (AA, 99.99%), ethanol (99%), and
silver nitrate (AgNO3, 99%) were purchased from Xilong
Scientific. Melamine was obtained from Shanghai Aladdin Biochemical
Technology Co., Ltd. Cyclohexane (99%), dichloromethane (99%), octane
(99%), and methanol were purchased from Sinopharm Group Chemical Reagent
Co., Ltd., Shanghai. The experimental solutions were prepared with
ultra-pure Milli-Q water (18.2 MΩ) or ethanol. The glassware
was soaked in freshly prepared aqua regia and then rinsed with plenty
of ultra-pure Milli-Q water.