Hybrid Au–Fe₃O₄ Nanoparticles: Plasmonic, Surface Enhanced Raman Scattering, and Phase Transition Properties

In this work, we report on the synthesis of hybrid Au–Fe₃O₄ nanoparticles (NPs) using a novel one-pot synthesis method that makes use of triethylene glycol as a solvent, a reducing agent, and a stabilizing layer. The produced nanoparticles consist of Au cores decorated with magnetite Fe₃O₄ nanoparticles. Optical absorption measurements combined with numerical simulations showed that the Au–Fe₃O₄ nanoparticles exhibit a localized surface plasmon resonance clearly red-shifted with respect to that of bare Au nanoparticles. This strong plasmonic resonance is exploited to produce surface-enhanced Raman scattering (SERS) from both the organic molecules and the iron oxide surrounding the Au cores. We found that the SERS signal exhibits strong temporal fluctuations which are used to identify the origin of the observed Raman lines. In particular, we clearly point out the presence of an iron hydroxide (γ-FeOOH) layer at the surface of the Fe₃O₄ nanoparticles forming the shell. This result is supported by numerical simulations of the plasmonic near field generated by the Raman probe. Moreover, we investigate the light-induced phase transition from magnetite to hematite (α-Fe₂O₃). Owing to the strong SERS effect we were able to detect the formation of diiron-oxo bonds during the phase transition. These bonds are ascribed to the presence of a mixed magnetite/maghemite phase. We thus propose a new scheme where the phase transition is triggered by the iron hydroxide surface layer. Such a transition is here studied for the first time in Au–Fe₃O₄ hybrid nanoparticles where the gold cores act as plasmonic nanoheaters responsible for the thermally induced phase transition.