The core–shell structures were synthesized and characterized
by using a sequential synthesis technique provided
by reverse micelles in a cetyltrimethylammonium bromide
(CTAB), n-butanol and water system.17,18,20–23 The Fe nanoparticles
were synthesized by two microemulsion solutions
([water]:[detergent] = 2.54). Briefly, the size of the particles
was controlled by the molar ratio of the water to surfactant
(ω = [H2
O]/[CTAB]). The reaction process was carried out
under flowing argon gas. In the first step, two microemulsion
solutions were prepared using 0.2 M FeSO4
(aqueous
[aq]) (catalog number F8633, Sigma-Aldrich, St Louis,
MO, USA) and 0.5 M NaBH4
(aq) (catalog number 480886,
Sigma-Aldrich), respectively, in a ratio of ω = 2.54. These
solutions were mixed together using a magnetic stirrer at a
speed of 260 rpm, and the solution turned black upon the
reduction of the iron. In the second step, the shell of gold was
synthesized. Two additional micellar solutions were prepared
using 0.05 M HAuCl4
(aq) (catalog number 254169, SigmaAldrich)
and 0.8 M NaBH4
(aq), respectively, with ω = 3.8.
These solutions were purged by argon to minimize the oxygen
contamination caused during oxidation of the iron core.
These solutions were added to the reaction mixture from Step
1. In this case, the size of micelles in Step 2 solutions was
increased slightly (ω = 3.24), within the reaction mixture to
allow for the growth of gold shell on the core. The reaction
was stirred under flowing argon for 2 hours. The nanoparticles
were ultrasonically washed in argon-purged ethanol
three times and then magnetically collected. The particles
were then dried under vacuum and sealed in argon. The Fe
nanoparticles were synthesized in a similar way; however,
only two microemulsion solutions with 0.05 M of FeSO4
(aq) and 0.5 M of NaBH4
(aq) were combined to form Fe
nanoparticles, as described in Step 1.24,25 The Fe nanoparticles
were then washed and recovered in the argon-purged absolute
ethanol solutions for the following experiments.