Formation of bifunctional conglomerates composed of magnetic γ-Fe2O3 nanoparticles and various noble metal nanostructures

Abstract

Bifunctional conglomerates made of magnetic and noble metal nanoparticles have many applications in the fields of catalysis, chemical analysis, biology, and medicine. Therefore, it is very important to find a way to synthesize such conglomerates that makes it possible to easily control the size and shape of the attached noble metal nanoparticles. In this contribution, we propose a method of synthesizing such conglomerates in which the noble metal nanoparticles are prepared before being bound to the magnetic nanostructures, and in which the initial size and shape of the noble metal nanoparticles does not change during the process of the formation of the conglomerates with magnetic γ-Fe2O3 nanoparticles. The proposed synthesis is based on directly binding the metal and γ-Fe2O3 nanoparticles by means of molecules of (3-aminopropyl)trimethoxysilane (APTMS). Functionalization with APTMS leads to the formation of nanostructures (methoxy groups of APTMS react with the surface of the Fe2O3 nanoparticles, forming strong FeOSi bonds) with a terminal amine group. The noble metal nanoparticles are then attached to the amine-terminated γ-Fe2O3 nanoparticles. This binding process has been successfully tested for decahedral Ag, cubic Ag, hollow Au, and hollow Pt nanoparticles. As far as we know synthesized composites are the first examples of magnetic composites containing such anisotropic metallic nanostructures. The magnetic-plasmonic conglomerates obtained have been used as materials for easily producing very homogeneous substrates for surface-enhanced Raman scattering (SERS) measurements. Conglomerates with Pt nanoparticles have been tested as catalysts for reducing 2-nitrophenol. We found that, after a relatively quick decomposition of the weakly bonded γ-Fe2O3-Pt nanostructures in the reaction mixture, the stability of the remaining conglomerates was large enough for them to be recycled in a magnetic field after the reaction and reused. This process may be repeated effectively many times.