Bio-nanotechnology based cancer therapeutics exponentially increase every year. A therapeutic strategy to induce intracellular reactive oxygen species (ROS) has received promising success in oncotherapy. In this study, the new strategy has been exploited by the treatment of iridium (Ir) and Fe2+ ions with cancer cells to biosynthesize the biocompatible fluorescent iridium oxide (IrO2) and iron oxide nanoclusters (NCs) under the specific redox heterogeneous microenvironment of these diseased cells and tumors. The hydroxyl radical produced by the presence of Fe2+ and H2O2 in cancer cells apparently increased the ROS level in cancer cells during the process of biosynthesized NCs and, hence, simultaneously instigated apoptosis of relevant cells. Therefore, intracellular ROS-mediated in situ biosynthesis of IrO2 and iron oxide NCs may also act as anticancer agents and provide a promising pathway for targeted cancer therapy.