Superoxide anion (O-2(-)) is implicated in a wide variety of biological phenomena and oxidative stress-related diseases. The electrochemical detection of O-2(-) is very attractive but relies on superoxide dismutase enzymes, thus suffering from high cost and low durability. The advances of nanoscience allows architecting while functionalizing a biomimetic sensing platform in nanoscales for high sensitivity and specificity. In this work, manganous phosphate (Mn-3(PO4)(2)) nanosheets, a biomimetic enzyme, are template-synthesized with DNA and further assembled on carbon nanotubes (CNTs) to form unique DNA-Mn-3(PO4)(2)-CNT nanocomposite sheets, of which the Mn-3(PO4)(2) sheets efficiently catalyze the dismutation of O-2(-) while CNTs enable fast electron transfer, thus achieving highly sensitive and specific detection of O-2(-) with long-term stability. The biomimetic O-2(-) sensor is further used to monitor O(2)(-)in situ released from mouse cancer cell and normal skin cell under drug stimulation, showing excellent real time quantitative detection capability. This work demonstrates a nanoscale approach to not only synthesize but also design a biomimetic enzyme for comparable performance with the natural enzyme-based biosensor while rendering much higher durability than the natural one and thus holding a great promise for broad applications in fundamental research, clinic diagnostics and screening for drug therapy effects.