Ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) coated with polyacrylic acid (PAA) were synthesized by a hydrothermal method in gram-scale quantity and extensively characterized. Only the nanoparticles subjected to an additional centrifugation step showed narrow size distribution, high polymeric coverage, and ideal superparamagnetism. In addition to improved physico-chemical properties, these nanoparticles feature high stability in vitro as well as dual T-1-T-2 performance as contrast agents (CAs) for magnetic resonance imaging (MRI), highlighting the importance of the additional separation step in obtaining material with the desired properties. (C) 2016 Elsevier Inc. All rights reserved.