Bioactive potential of Allamanda cathartica derived zinc oxide nanoparticles: antimicrobial, antioxidant, and cytotoxic properties with computational mechanistic insights

Abstract Green synthesis of metal oxide nanoparticles using medicinal plants provides a sustainable approach for developing bioactive nanomaterials. In this study, zinc oxide nanoparticles (ZnO-NPs) were biosynthesized using Allamanda cathartica leaf extract and evaluated through an integrated experimental and computational approach. The synthesized ZnO-NPs were characterized by UV–visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). UV–visible analysis showed a characteristic absorption maximum at 374 nm, while FTIR confirmed Zn–O bonding and the presence of surface-associated hydroxyl and organic functionalities. XRD analysis indicated a crystalline hexagonal wurtzite ZnO phase, and TEM revealed predominantly spherical nanoparticles approximately 20–60 nm in size. EDX confirmed Zn and O as the major elemental constituents. The biosynthesized ZnO-NPs exhibited broad-spectrum antibacterial activity against Escherichia coli , Bacillus cereus , Staphylococcus aureus , and Pseudomonas aeruginosa . DPPH radical-scavenging activity increased with nanoparticle concentration, with an IC₅₀ of 655.5 µg mL⁻¹. MTT analysis demonstrated concentration-dependent cytotoxicity against A549, MCF-7, and KB-3-1 cancer cells, with IC₅₀ values of 243.81, 175.92, and 189.62 µg mL⁻¹, respectively, whereas L929 fibroblasts showed a higher IC₅₀ of 640.16 µg mL⁻¹. The corresponding selectivity indices were 2.63, 3.64, and 3.38 for A549, MCF-7, and KB-3-1 cells, respectively, indicating preferential in-vitro cytotoxicity toward the tested cancer cells relative to L929 fibroblasts. DFT calculations of selected A. cathartica phytoconstituents yielded electronic reactivity descriptors that support their potential involvement in nanoparticle formation. At the same time, Monte Carlo simulations indicated energetically favorable adsorption of phytochemical species on the zinc-containing surface. Molecular docking further predicted favorable interactions with the selected biological target (PDB ID: 1FDW), providing complementary mechanistic insight into the observed bioactivity. Overall, A. cathartica -mediated ZnO-NPs exhibited combined antibacterial and differential anticancer activity, supporting their potential as bioactive nanomaterials for further biomedical investigation.