Absolute Mag™ Rhodamine B Iron Oxide Nanoparticles have fluorescence dye Rhodamine B on their surfaces. The nanoparticles are water soluble. They have a near neutral zeta potential and an organic layers with about 6 nm thickness. The hydrodynamic size established using a Zeta Trac (DLS) is about 12-14 nm larger than the inorganic core size measured with TEM. Absolute Mag™ Rhodamine B Iron Oxide Nanoparticles are excellent reagents for cell labeling or tracking and are visualized by both MRI and fluorescence. Also, they are stable and resistant to aggregation in general buffers like borate, Tris, HEPES, PBS, and MES buffers.