Absolute Mag™ Protein G Iron Oxide Nanoparticles are coated with amphiphilic polymer and water soluble. Protein G on the particle surface is conjugated to amphiphilic polymer layer. Their zeta potential is from -20mV to -40mV. The organic layers contain a monolayer of oleic acid and a monolayer of amphiphilic polymer. The total thickness is about 4 nm. The hydrodynamic size is about 8-10 nm larger than their inorganic core size measured by TEM. Absolute Mag™ Protein G Iron Oxide Nanoparticles are very stable in most buffer solutions and can bind to the Fc region of IgG, thus present Ab in a directional manner for enhanced signal-to-noise ratio and reproducibility.