Amino-functionalized Fe3O4 magnetic nanoparticles with high peroxidase-like activity (MNPs-NH2) are reported in this work. The peroxidase-like activity is approximately the same as naked Fe3O4 magnetic nanoparticles (MNPs). MNPs-NH2 displayed much better thermal stability and pH tolerance than horse-radish peroxidase (HRP). The optimal pH and temperature are approximately pH 4 and 40 degrees C, respectively. We showed that the MNPs-NH2 were successfully applied in a double-antibody sandwich "enzyme"-linked immunosorbent assay to replace of HRP, in which MNPs-NH2 were not only separation carriers, but also detection indicator. (C) 2013 Elsevier Inc. All rights reserved.