A facile one-pot method has been developed to prepare poly(amino acid) functionalized, water-stable, biocompatible, and superparamagnetic iron oxide nanoparticles (NPs) with small diameters of similar to 10 nm. The obtained biocompatible magnetic nanoparticles capped with polyaspartic acid (PASP) exhibit a relatively high saturation magnetization (57.1 emu/g) and a much strong magnetic resonance (MR) T-2 relaxation effect with the transverse relaxivity coefficient (r(2)) as high as 302.6 s(-1) mM(-1). Interestingly, the as-prepared Fe3O4@PASP NPs are highly stable in aqueous solution and demonstrate the property of magnetic nanofluids. The high T-2 effect, good water-stability, superparamagnetization, biocompatibility and bioconjugatability render the as-synthesized Fe3O4@PASP NPs great desirable for bioapplications such as magnetic resonance imaging (MRI), bioseparation, targeted drug delivery, and so on. (C) 2013 Elsevier Ltd. All rights reserved.