The eco-friendly synthesis of metal oxides pn junction composite with high visible light absorption and its photoelectrochemical monitoring on antibiotics is reported. The In O -CuO pn heterojunction composite was successfully prepared by in-situ hydrothermal decoration of CuO on the prepared In O using a simple reflux method. The obtained nanorods like In O -CuO pn heterojunction exhibited high conductivity with excellent stability for the facilitated photoelectrochemical detection of ornidazole (ONZ) that plays a role in aquatic toxicology. The photo-stability and optical characteristics of the In O -CuO heterojunction composite were analyzed through photocurrent and UV-visible studies. Mechanism of ONZ signaling has been proposed with appropriate band levels derived by Mott-Schottky analysis. An optimized In O -CuO heterojunction detects ONZ in the range 0.05-65.3 nM with 0.0092 nM as the limit of detection at - 0.45 V (vs. Ag/AgCl) working potential. The practical applicability of the sensor device was tested in chicken meat, human urine, and lake water samples containing ONZ. The recoveries of real samples were above 95% and results obtained were compared with electrochemical methods.