Absolute Mag™ PDDA Coated Iron Oxide Nanoparticles, which are coated by amphiphilic polymer and polydiallydimethylammounium chloride (PDDA), are water soluble iron oxide nanoparticles. There''s no linkable reactive groups on the surface. Their zeta potential is more than +50 mV. Their organic layers contain a monolayer of oleic acid, a monolayer of amphiphilic polymer and a monolayer of PDDA, whose thickness is about 8 nm. The hydrodynamic size is about 14-16 nm larger than their inorganic core size measured by TEM. Absolute Mag™ PDDA Coated Iron Oxide Nanoparticles are very stable in most buffer solutions with a pH ranges from 3 to 14. Also negatively charged molecules, such as DNA and RNA, can be captured through charge-charge interaction.