PEG Carboxyl Iron Oxide Nanoparticles SPP is a group of water soluble iron oxide nanocrystals with amphiphilic polymer coating and PEG-COOH coating. Their reactive group is carboxylic acid and their zeta potential is from -15 mV to -5 mV. Their organic layer consists of a monolayer of oleic acid, a monolayer of amphiphilic polymer and a monolayer of PEG-COOH. The overall thickness of the organic layers is about 6 nm. SPP is very stable in most buffer solutions in the pH range of 4-10. SPP can be conjugated to amine containing molecules with low non-specific binding.