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-Fe3O4 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-Fe3O4 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-Fe3O4 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.