The preparation of nearly monodisperse and highly crystalline nanoscale spinel-type transition metal ferrites by two novel non-hydrolytic approaches will be reviewed. These synthetic methodologies are based on the complexation of the transition metal ions with polyol molecules and the thermolysis of metal carboxylates in high boiling point organic solvents, respectively. Due to the surface layer of the absorbed organic molecules, the as-prepared nanoparticles are stable against agglomeration and can be easily dispersed in various solvents, either polar and non-polar. The size of the nanocrystals can be varied in a wide range by changing the reaction conditions, such as the nature of the reaction medium, reaction temperature and reaction time. A detailed characterization of the nanoparticles by using a wide variety of techniques, including powder X-Ray diffraction, IR spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), 1H-NMR spectrometry and X-ray fluorescence spectroscopy (XPS) was performed in order to elucidate the morphology and the properties of the variable-sized Fe₃O₄ nanoparticles. Additionally, the magnetic properties of the variable-sized Fe₃O₄ nanoparticles were investigated at various temperatures and under different magnetic fields. Both finite size and interparticle interaction effects were identified to influence the magnetic behavior of the nanosized particles. Furthermore, the high surface reactivity of the nanocrystals enables the atttachment of gold nanograins to their surface with formation of colloidal Fe₃O₄/Au nanocomposites which are highly stable against separation and exhibit magnetic properties similar to those of the parent Fe₃O₄ nanocrystals. By chemically bonding bioactive molecules to the attached Au nanoparticles these novel nano-architectures open up new opportunities for the use of the magnetic nanoparticles as a platform for various biomedical applications.

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