Absolute Mag™ PEI Coated Iron Oxide Nanoparticles, which have no linkable reactive groups on the surface and coated with amphiphilic polymer and polyethylenimine (PEI), are water soluble iron oxide nanoparticles. Their zeta potential is about +10-20 mV. Their organic layers contain a monolayer of oleic acid, a monolayer of amphiphilic polymer and a monolayer of PEI, whose thickness is about 10 nm. The hydrodynamic size of the nanoparticles is about 20 nm larger than their inorganic core size measured by TEM. Absolute Mag™ PEI Coated Iron Oxide Nanoparticles are very stable in most buffer solutions in the pH range of 5-10 and can survive autoclaving process (121 °C for 30 min). Also the negatively charged molecules, such as DNA and RNA, can be captured by Absolute Mag™ PEI Coated Iron Oxide Nanoparticles through charge-charge interaction.