Monocrystalline iron oxide nanocompounds (MION): Physicochemical properties

Abstract

We have previously described a novel monocrystalline iron oxide nanocompound (MION), a stable colloid that enables target specific MR imaging. In this study, the physicochemical properties of MION are reported using a variety of analytical techniques. High resolution electron microscopy indicates that a MION consists of hexagonal shaped electron‐dense cores of 4.6 ± 1.2 nm in diameter. This iron oxide core has an inverse spinel crystal structure which was confirmed by x‐ray powder diffraction. Chemical analysis showed that each core has 25 ± 6 dextran molecules (10 kD) attached, resulting in a unimodal hydrodynamic radius of 20 nm by laser light scattering. Because of the flexibillity of the dextran layer, the radius is only 8 nm in nonaqueous reverse micelles. At room temperature, MION exhibit superparamagnetic behavior with an induced magnetization of 68 emu/g Fe at 1.5 T. Mössbauer studies show that the saturation internal magnetic field is 505 KOe, and blocking temperature is at 100 K. The R1 relaxivity of MION is 16.5 (mM‐sec)‐¹ and the R2 relaxivity is 34.8 (mM‐sec)‐¹ in aqueous solution at 37°C and 0.47 T. In vitro phantom studies show that the detectability of MION in liver tissue is less than 50 nmol Fe/g tissue using gradient echo imaging techniques.