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

Abstract

In this work, we report on the synthesis of hybrid Au−Fe 3 O 4 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 Fe 3 O 4 nanoparticles. Optical absorption measurements combined with numerical simulations showed that the Au− Fe 3 O 4 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 Fe 3 O 4 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-Fe 2 O 3 ). 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−Fe 3 O 4 hybrid nanoparticles where the gold cores act as plasmonic nanoheaters responsible for the thermally induced phase transition.

Encapsulating Au‐Fe3O4 Nanodots into AIE‐active Supramolecular Assemblies: Ambient Visible‐light Harvesting “Dip‐Strip” Photocatalyst for C−C/C−N Bond Formation Reactions

“…In the absorption studies, the simultaneous addition of Au 3+ and Fe 3+ ions (15 equiv) to the solution of derivative 6 , SPR band of Au NPs appeared at 550 nm (after 25 mins) which was gradually shifted to 570 nm. The intensity of the level‐off tail was also increased, thus, signifying the preparation of Au‐Fe 3 O 4 magnetic nanocomposites (Figure S7) . The color of the solution was also changed from colorless to yellow to purple and finally turned to reddish brown (Figure S8).…”

“…Additionally, peaks were detected for S2p, C1s, O1s and N1s which were due to organic residue (Figure S12). The appearance of band at 630 cm −1 in the Raman spectrum relates to the A1g vibration mode of Fe 3 O 4 NPs (Figure S13) . The DT‐TGA analysis of PTh‐ co ‐PANI‐6: Au‐Fe 3 O 4 nanomaterials suggested a nonlinearly weight loss from 200 to 600 °C.…”

“…The intensity of the level-off tail wasa lso increased, thus, signifying the preparation of Au-Fe 3 O 4 magnetic nanocomposites ( Figure S7). [11] The color of the solution was also changed from colorless to yellow to purple and finally turned to reddish brown( Figure S8). The entirep rocess was complete within 45 mins.…”

“…As ignificant overlap between absorption spectra of Au-Fe 3 O 4 NPsa nd emission spectra of the oxidized speciesw as observedw hich indicated the possibility of good energy transfer between them. To check the validity of our hypothesis, we examined the model reactionb etween aniline (10) and methyl propiolate (11) in the presence of CO source in the presence of PTh-co-PANI-6: Au-Fe 3 O 4 NPs as catalyst. The reaction was complete in 5h to furnish the desired product in 87 %y ield.…”

Magnetic gold nanotriangles by microwave-assisted polyol synthesis

“…Au-SPIONs nanostructures [26][27][28][29][30][31] have been obtained by one-pot 29,32 or seed mediated synthesis resulting in dumbbells 31,33 , core-shell 26 , nanoflowers 34 , star-shape 28 or clusters 35 . Still, another strategy involves the separate fabrication of Au and iron oxide nanoparticles and the subsequent assemble by a third component 27,36 .…”