Dealloyed-AuNi dendrite anchored on carboxylic acid groups of a conducting polymer is prepared and demonstrated for the catalysis of the oxygen reduction reaction (ORR) and detection of hydrogen peroxide (H2O2) released from living cells. The dendrite formation is initiated on a poly(benzoic acid-2,2':5',2 ''-terthiophene) (pTBA) layer, where the polymer layer acts as a stable substrate to improve the long-term stability and catalytic activity of the alloy electrode. A co-deposition of Au and Ni is performed to produce a Ni-rich Au surface at first; subsequent removal of the surface Ni atoms through electrochemical dealloying enhances the performance of the catalyst because of an increase in the electrochemically active area by 12 times. The hydrodynamic voltammetry of dealloyed-AuNi@pTBA shows a half-wave potential at -0.08 V, which is a large shift towards more positive potential when compared to those on AuNi@pTBA (-0.14 V) and commercial Pt/C (-0.12 V) electrodes. The proposed catalytic electrode achieved a superior analytical performance for the detection of trace H2O2 (at -0.15 V) released from cancer and normal cells with a very low detection limit (ca. 5 nM). In addition, the in vitro studies suggest no signifi cant cytotoxicity effect for the dealloyed sample and the viability of the cells are more than 85% even after 48 h of incubation.