Morphological, structural, optical and antimicrobial performance of Indian Knowledge System-based growth-time engineered green-synthesized ZnO nanostructures using Justicia adhatoda

Flower-like zinc oxide (ZnO) nanostructures were successfully synthesized using a green, wet-chemical protocol that employed Justicia adhatoda leaf extract. The growth time was tuned from 1 h to 4 h to investigate its impact on structural, optical, and antimicrobial properties. Morphological investigation revealed the formation of flower-like nanostructures and subsequent changes due to nucleation and Ostwald ripening. FTIR spectra confirmed the formation of Zn–O bonds. Furthermore, XRD analysis confirms the formation of highly crystalline nanostructures with crystallite sizes in the range of 15.06–24.42 nm. Lattice strain influenced both lattice parameters and lattice volume. The synthesized ZnO nanostructures exhibit high transparency in the visible window. An increase in band gap (3.05–3.19 eV) was observed, owing to decreased crystallite size resulting in weak quantum confinement and lattice strain in the crystal. Photoluminescence spectra exhibit intense UV emission around 327 nm, with increasing intensity at higher growth durations, indicating improved crystallinity. Furthermore, the extremely low intensity PL emission in the visible range indicates a very negligible amount of defects present in the samples. The green-synthesized ZnO nanostructures exhibit potent antibacterial activity against S. aureus and K. pneumoniae , with greater efficacy against S. aureus with minimum inhibitory concentration of 1.4502 mg-L − 1 and 3.07127mg-L − 1 for S. aureus and K. pneumoniae respectively. The antimicrobial mechanism is due to cell membrane disruption and cytoplasmic ionic imbalance caused by the penetration of the nanoparticles. This study highlights the potential of J. adhatoda -mediated ZnO nanostructures for eco-friendly antimicrobial and optoelectronic applications.