Biomimetic green synthesis of ZnO nanoparticles using Cheilocostus speciosus and Gardenia gummifera with comprehensive characterization and bioactivity assessment

This study presents a green, cost-effective, and eco-friendly approach for synthesizing zinc oxide nanoparticles (ZnO NPs) using leaf extracts of Cheilocostus speciosus (CS) and Gardenia gummifera (GG) as bio-reducing, capping, and stabilizing agents. Zinc nitrate hexahydrate served as the precursor. The synthesized ZnO NPs were thoroughly characterized using XRD, FT-IR, UV-Vis-IR, XPS, SEM-EDS, and TEM-SAED techniques. XRD analysis confirmed a hexagonal zincite phase, with average crystallite sizes of 22 nm (CS-ZnO) and 25 nm (GG-ZnO). FT-IR spectra confirmed the presence of Zn-O bonds along with phytochemical-derived functional groups. UV-Vis-NIR spectroscopy revealed strong absorption around 320-348 nm, with band gap energies of 3.07 eV (CS-ZnO) and 2.74 eV (GG-ZnO), indicating size-dependent quantum confinement. SEM and TEM analyses showed spherical to conical morphology, with particle sizes ranging from 8-50 nm (CS-ZnO) and 18-90 nm (GG-ZnO). XPS confirmed Zn²⁺ oxidation state, and SAED indicated polycrystalline nature. Bioactivity assays demonstrated significant cytotoxic activity against A549 cell line, antimicrobial activity against E. coli, S. aureus, and Alternaria alternata, with CS-ZnO showing slightly higher inhibition zones. Antioxidant potential, assessed via DPPH assay, revealed higher activity than ascorbic acid at comparable concentrations. These results underscore the potential of phytosynthesized ZnO NPs in biomedical, pharmaceutical, and food preservation applications, highlighting the promise of green synthesis strategies in nanomaterials development.