[en] Although water-dispersible engineered nanoparticles (ENPs) have a wide range of applications, the ENPs used in many nanotoxicological studies tend to form micron-sized aggregates in the exposure media and thus cannot reflect the toxicity of real nanoparticles. Here we described the synthesis of bare hematite nanoparticles (HNPs-0) and two poly(acrylic acid) (PAA)-coated forms (HNPs-1 and HNPs-2). All three HNPs were well dispersed in deionized water, but HNPs-0 quickly aggregated in the three culture media tested. By contrast, the suspensions of HNPs-1 and HNPs-2 remained stable, with negligible amounts of PAA and Fe³⁺ liberated from either one under the investigated conditions. To better quantify the accumulation of the coated HNPs, a relatively innocuous ⁵⁵Fe-labeled form of HNPs-2 was synthesized as an example and its accumulation in three phytoplankton species was tested. Consistent with the uptake kinetics model for conventional pollutants, the cellular accumulation of HNPs-2 increased linearly with exposure time for two of the three phytoplankton species. These results demonstrate the utility of ⁵⁵Fe-labeled well-dispersible HNPs as a model material for nanoparticle bioaccumulation studies in nanotoxicology. - Highlights: • The PAA-coated hematite nanoparticles were synthesized in two ways. • They had good dispersibility and were innocuous to several organisms. • Negligible amount of PAA and Fe³⁺ was liberated from the nanoparticles. • The method for ⁵⁵Fe-labeled hematite nanoparticle quantification was established. • Their bioaccumulation followed the conventional biokinetics model. - ⁵⁵Fe-labeled well-dispersible hematite nanoparticles can serve as a model material for bioaccumulation studies in nanotoxicology.