Iron oxide nanoparticles are magnetic single crystals and can be used as magnetic resonance imaging contrast reagents for in-vivo imaing , drug delivery vehicle, hyperthermia generator for therapy, adjuvant for vaccine and magnetic sensing probe for in-vitro diagnostics. The PEG-Carboxylic acid functionalized iron oxide nanoparticles are coated with a monolayer of PEG polymer. They offer compact size and low non-specific binding. The reaction group is carboxyl acid which can react with amine containing molecules such as an antibody, peptide, DNA or other small molecules. The available iron oxide core sizes are 5, 10, 20 and 30 nm as namely OCNIAC05, OCNIAC10, OCNIAC20 and OCNIAC30, respectively.