Green synthesis and characterization of zinc oxide nanoparticles using Saussurea lappa and Saussurea costus root extracts and investigation of their antibacterial activities

Bacterial pathogens cause infections that pose significant threats to human health, and antibiotic resistance is an increasing global challenge. Metal nanoparticles and their oxides have shown promise in overcoming microbial resistance. This study investigates the antibacterial activity of zinc oxide (ZnO) nanoparticles synthesized using root extracts of Saussurea lappa and Saussurea costus . Unlike most previous studies that predominantly utilize leaf extracts, this work focuses on root-derived phytochemicals, which are typically richer in stable secondary metabolites such as sesquiterpene lactones, phenolics, and alkaloids that can enhance nanoparticle reduction efficiency and stability. The biosynthesized nanoparticles were characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). SEM analysis revealed predominantly rod-shaped nanoparticles, while XRD confirmed the formation of crystalline ZnO. The average particle sizes were ~ 8 nm and ~ 10 nm for S. lappa and S. costus , respectively. FTIR analysis identified functional groups involved in nanoparticle stabilization and reduction, including C–C, C–H, C–N, O–H, and Zn–O. A key aspect of this study is the comparative evaluation of two closely related medicinal species, S. lappa and S. costus , providing insight into how species-specific phytochemical variations influence nanoparticle size, morphology, and antibacterial performance. The antibacterial activity of the ZnO nanoparticles was evaluated against Gram-positive bacteria ( Staphylococcus aureus and methicillin-resistant Staphylococcus aureus ) and Gram-negative bacteria ( Escherichia coli ). All ZnO nanoparticle samples demonstrated notable antibacterial activity against both Gram-positive and Gram-negative strains. This comparative, root-based approach distinguishes the present work from existing ZnO–plant synthesis studies and highlights the importance of plant-part selection and species variation in optimizing nanoparticle functionality. In conclusion, this study demonstrates that root extracts of S. lappa and S. costus can serve as effective agents for the green synthesis of ZnO nanoparticles with antibacterial properties, providing a sustainable and biocompatible approach for the development of novel antimicrobial agents.