Rhodamine B Iron Oxide Nanoparticles IRBs are water soluble iron oxide nanoparticles with fluorescence dye Rhodamine B on their surfaces. Their zeta potential is near neutral. The overall thickness of the organic layers is about 6 nm. The hydrodynamic size established using a Zeta Trac (DLS) is about 12-14 nm larger than the inorganic core size that is measured with TEM. IRBs are excellent reagents for cell labeling and tracking. There are at least 40 Rhodamine B molecules on each nanoparticle. IRBs can be visualized by both MRI and fluorescence (ex. 555nm; em. 580 nm). IRBs show stability and resistance to aggregation in commonly used biocompatible buffers such as borate, Tris, HEPES, PBS and MES buffers.