Water soluble, superparamagnetic iron oxide nanoparticles with a core diameter of 24 – 26 nm and a low size dispersity (≤7%) as measured by Small Angle X-ray Scattering (SAXS). Nanoparticle cores are coated with a monolayer of oleic acid and a monolayer of an amphiphilic polymer with carboxylic acid functional groups displayed on the surface. Carboxylic acid functionality allows conjugation to molecules containing amine groups, such as proteins and peptides. The hydrodynamic diameter of the particles is approximately 30 nm larger than the core size measured by SAXS, and their zeta potential ranges from -60 to -70 mV. These particles are stable in most buffer solutions in the range of pH 4 – 10.