Green synthesis of silver and zinc oxide nanoparticles using Justicia adhatoda leaf extract for antimicrobial and anticancer applications

The clinical performance of standard chemotherapeutics is often limited by systemic toxicity and the rapid development of multidrug resistance. To address these challenges, we used an aqueous extract of Justicia adhatoda leaves to develop a sustainable, plant-based method for synthesizing Zinc Oxide (ZnO) and Silver (Ag) nanoparticles. This green approach provides a biocompatible alternative to traditional chemical methods, reducing hazardous waste. We characterized the resulting materials using XRD, FTIR, UV–Vis, FE-SEM, EDX, and Zeta potential analysis. Structural analysis confirmed that the synthesized ZnO nanoparticles formed a crystalline hexagonal wurtzite structure with an average particle size of 72.0 nm. In contrast, the Ag nanoparticles exhibited a face-centered cubic geometry with a mean particle size of 39.46 nm. Zeta potential measurements revealed high surface stability for both nanostructures, showing negative surface charges that prevent agglomeration. In the biological assay, ZnO NPs exhibited broader and more consistent antibacterial efficacy against Staphylococcus aureus and Klebsiella pneumoniae than Ag NPs in standard well diffusion. Time-kill kinetics further substantiated that ZnO NPs demonstrated more rapid, concentration-dependent bactericidal action over a 24-h window compared to Ag NPs. Conversely, MIC and MBC analyses showed that Ag NPs were highly effective against specific strains, such as S. aureus (1.56/3.125 µg/mL). Both nanostructures demonstrated potent, dose-dependent cytotoxicity against HeLa and A549 cancer cell lines. To the best of our knowledge, this is the first comparative study between ZnO and Ag nanoparticles synthesized from Justicia adhatoda under identical conditions. These findings highlight Justicia adhatoda as a powerful reducing and stabilizing agent, supporting the development of a scalable platform for multifunctional nanomaterials in targeted therapeutics.