The Ag NPs and Au NPs were prepared by reduction of
AgNO3 and HAuCl4 (5104 M) with 1.6103M
sodium citrate after boiling [30,31]. To deposit a shell
from silica on the Ag NPs and Au NPs cores, 2.5mL of
103M 3-aminopropyl trimethoxysilane was added to
500mL of the Ag NPs and Au NPs under vigorous
magnetic stirring.