Synthesis, Characterization, and Application of Extremely Small-sized Iron Oxide Nanoparticles

Abstract

Extremely small-sized nanoparticles, of size < 3 nm, have become important because their optical, magnetic, and catalytic properties are distinguished from those of large sized nanoparticles or molecules. Their properties are strongly dependent on their dimension, so the size-controlled synthesis of extremely small-sized nanoparticles is essential for further applications. Iron oxide nanoparticles are very important materials because of their interesting size-dependent magnetic properties. Extremely small-sized iron oxide nanoparticles exhibit weak magnetization, and can be applied to biomedical applications. The present work focuses on the large scale synthesis, characterization, and application of extremely small-sized iron oxide nanoparticles (ESIONs). The ESIONs were synthesized by thermal decomposition of iron-oleate complexes, and were applied as T1 magnetic resonance imaging (MRI) contrast agents. Sizes and size distribution of the nanoparticles were characterized in an easy and precise way through matrix-assisted laser desorption/ionization - time-of-flight (MALDI-TOF) mass spectrometry. Firstly, gram-scale synthesis of (ESIONs) was achieved by thermal decomposition of iron oleate complex in the presence of oleyl alcohol. Magnetization of small iron oxide nanoparticles was much less than that of large-sized nanoparticles due to their small magnetic moment and the spin canting effect. The small magnetic moment of the ESIONs enables them to be used as T1 MRI contrast agents. ESION-enhanced in vivo MR imaging showed bright T1 image which is maintained for a long time attributed to their moderate size. Secondly, a rapid and reliable method to determine the sizes and size distributions of extremely small-sized iron oxide nanoparticles is presented using mass spectrometrometry. The mass spectra obtained from MALDI-TOF mass spectrometry could readily provide size information using a simple equation. The size distribution obtained from the mass spectrum is well-matched with the data acquired from transmission electron microscope (TEM) which requires long and tedious analysis work. The size distribution from mass spectrum is highly resolved and is capable of detecting a difference in size of even few Angstroms. The mass spectrum technique was used for the investigation of formation mechanism of iron oxide nanoparticles. From ex situ measurements, it was observed that iron-oxo clusters were produced from the iron precursor, and eventually 3 nm iron oxide nanoparticles were achieved. The mass-to-size estimation will be found as easy and accurate analytical tool for various purposes including the formation mechanism studies to develop new synthetic methods for various kinds of nanoparticles with desired characteristics.