Laser-Assisted Synthesis of Superparamagnetic Fe@Au Core−Shell Nanoparticles

A novel method combining wet chemistry for synthesis of an Fe core, 532 nm laser irradiation of Fe nanoparticles and Au powder in liquid medium for deposition of an Au shell, and sequential magnetic extraction/acid washing for purification has been developed to fabricate oxidation-resistant Fe@Au magnetic core−shell nanoparticles. The nanoparticles have been extensively characterized at various stages during and up to several months after completion of the synthesis by a suite of electron microscopy techniques (HRTEM, HAADF STEM, EDX), X-ray diffraction (XRD), UV−vis spectroscopy, inductively coupled plasma atomic emission spectroscopy, and magnetometry. The surface plasmon resonance of the Fe@Au nanoparticles is red shifted and much broadened as compared with that of pure colloidal nano-gold, which is explained to be predominantly a shell-thickness effect. The Au shell consists of partially fused ∼3-nm-diameter fcc Au nanoparticles (lattice interplanar distance, d = 2.36 Å). The 18-nm-diameter magnetic core is bcc Fe single domain (d = 2.03 Å). The nanoparticles are superparamagnetic at room temperature (300 K) with a blocking temperature, T b, of ≈170 K. After 4 months of shelf storage in normal laboratory conditions, their mass magnetization per Fe content was measured to be 210 emu/g, ∼96% of the Fe bulk value.