NiO/ZnO p–n heterojunction electrochemical immunosensor for label-free detection of the mycobacterium tuberculosis Ag85 complex

Tuberculosis remains the leading cause of mortality attributable to a single infectious agent, with 10.7 million cases and 1.23 million deaths reported worldwide in 2024. The development of rapid, sensitive, and cost-effective diagnostic platforms is therefore essential for effective disease control. Herein, we report a label-free electrochemical immunosensor based on a NiO/ZnO composite electrode for the detection of the Mycobacterium tuberculosis Ag85 complex antigen. XPS and advanced scanning transmission electron microscopy analyses reveal intimate interfacial contact and local electronic contrast between p -type NiO and n -type ZnO. Mott–Schottky analysis and electrochemical impedance spectroscopy further indicate enhanced carrier density and reduced interfacial charge-transfer resistance in the composite electrode. Together, these results are consistent with the formation of a p – n heterojunction at the NiO/ZnO interface and support improved electrochemical transduction. Under optimized conditions, the immunosensor exhibited a linear response over a concentration range of 5–300 ng mL −1 , with a limit of detection of 0.77 ng mL −1 and a limit of quantification of 2.34 ng mL −1 . High analytical selectivity was observed, with negligible cross-reactivity toward antigens associated with other infectious diseases. The practical applicability of the platform was evaluated using Ag85-spiked rabbit serum samples, yielding recoveries between 92% and 102%. The device further demonstrated good reproducibility and satisfactory storage stability. These findings establish the NiO/ZnO composite electrode as a robust electrochemical transducer for Ag85 antigen detection and support its use as a proof-of-concept platform for tuberculosis diagnostics.