Dumbbell-like Pt48Pd52-Fe3O4 nanoparticles are synthesized and functionalized with oleylamine-polyethyleneglycol to serve as an efficient catalyst for H2O2 reduction and tetramethylbenzidine (TMB) oxidation in biological solutions. The Pt48Pd52-Fe3O4/TMB kit is even more active than the natural enzyme for H2O2 detection with a detection limit reaching 2 mu M, and is successfully used to quantitatively monitor the extracellular H2O2 generated by neutrophils.