Homocysteine-protected gold-coated magnetic nanoparticles: Synthesis and characterisation

An easy and simple two-step reaction is employed to synthesise a new type of ligand-protected (homocysteine-protected) gold-coated iron oxide nanoparticle (homocys-Au-Fe₃O₄). Fe₃O₄ nanoparticles are used as the central core to prepare homocys-Au-Fe ₃O₄ in aqueous state without precipitation and aggregation of nanoparticles. Fe₃O₄ nanoparticles are initially prepared and subsequently coated with Au layers under hot citrate reduction of HAuCl₄. The citrate monolayer of the nanoparticles is then ready for place-exchange with homocysteine molecules to produce the well dispersed homocys-Au-Fe₃O₄ nanoparticles. These homocys-Au-Fe ₃O₄ nanoparticles have been fully characterised by X-ray photoelectron spectroscopy, visible absorption spectroscopy, magnetic susceptibility measurements, Fourier transform infrared spectroscopy, thermogravimetric analysis, atomic absorption spectroscopy, energy dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The homocys-Au-Fe₃O₄ nanoparticles show good paramagnetic properties and are coated with ultra-thin layers of gold atoms (∼0.5 nm) having an average diameter of ca. 12 nm. These magnetic nanoparticles are well dispersed in water and stable at physiological pH without precipitation. The visible absorption spectrum of homocys-Au-Fe ₃O₄ can be altered by pH. These nanoparticles are aggregated in an acidic environment but dissociated at high pH conditions in a reversible manner. This article has provided important insights into the design of new water-soluble magnetic nanoparticles for biomedical, analytical and catalytic applications.