Depletion sphere: Explaining the number of Ag islands on Au nanoparticles† †Electronic supplementary information (ESI) available: Details of the synthesis; UV-Vis and SERS spectra; and large-view TEM images. See DOI: 10.1039/c6sc02276f Click here for additional data file.

Materials and Instruments: All chemicals were used as received without further purification. Hydrogen tetrachloroaurate(III) hydrate (HAuCl4·3H2O), 99.9% (metal basis Au 49%) was purchased from Alfa Aesar; sodium citrate tribasic dihydrate (99.0%), hydroquinone (99.5%) and AgNO3 (99.9999%) were purchased from Sigma-Aldrich; 2-mercaptobenzoimidazole-5-carboxylic acid was purchased from Tokyo Chemical Industry Co., Ltd. (TCI); 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (Sodium salt, abbreviated as PSH) was purchased from Avanti Polar Lipids, Inc.; amphiphilic diblock copolymer polystyrene-block-poly(acrylic acid) (PS154-b-PAA49, Mn = 16000 for the polystyrene block and Mn = 3500 for the poly(acrylic acid) block, Mw/Mn = 1.15) was purchased from Polymer Source, Inc.; Deionized water (resistance > 18.2 MΩ·cm) was used in all reactions. Copper specimen grids (300 mesh) with formvar/carbon support film (referred to as TEM grids in the text) were purchased from Beijing XXBR Technology Co.