Synthetic approaches that vigorously control the microstructures of water-dispersible gamma-Fe2O3 nanoparticles such as size and size uniformity are of importance to the potential biological applications of these nanomaterials. In the present paper, water-dispersible y-Fe2O3 nanocrystals with narrow size distributions (bipy-Fe2O3) were prepared via a site-exchange reaction. These particular materials are superparamagnetic and stable within a wide range of pH. Introduction of the biotin functionality onto the surfaces of bipy-Fe2O3 enabled the affinity isolation of the protein avidin from its incubation solution magnetically with 96% efficiency. (C) 2003 Elsevier Inc. All rights reserved.