Green synthesis approach for flavonoids and protein-rich Plectranthus amboinicus leaf extract-coated ZnO nanoparticles for diverse biological applications

This study demonstrates the eco-friendly fabrication of zinc oxide nanoparticles (ZnO NPs) using a flavonoid- and protein-rich extract from Plectranthus amboinicus, a medicinal herb renowned for its therapeutic properties. UV–vis spectroscopy showed distinct absorbance peaks at 288 nm for the leaf extract and 358 nm for the synthesized ZnO NPs. X-ray diffraction (JCPDF file no. 00–036-1451) verified the presence of the hexagonal wurtzite crystalline phase in the nanoparticles. FTIR analysis revealed that flavonoids and proteins act as bio-reductants and facilitate the electron transfer to the Zn²⁺ ions, thereby converting them to their zero-valent (Zn⁰) state, leading to the formation of ZnO NPs. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses revealed spherical ZnO NPs with an average size of 58.24 nm. These nanoparticles exhibited potent antimicrobial effects, successfully inhibiting Shigella flexneri and Enterococcus faecalis, with biofilm inhibition rates surpassing 93 % at a concentration of 100 μg/mL. Antioxidant evaluations indicated a total antioxidant capacity of 75.5 ± 0.24 μg AAE/mg, a reducing power of 65.24 ± 0.73 μg AAE/mg, and 68.30 ± 0.16 % DPPH radicals scavenging activity at 200 μg/mL. Cytotoxic assessments showed the most significant activity at 200 μg/mL. The IC₅₀ values were recorded as 58.53 μg/mL, 40 μg/mL, and 21.69 μg/mL for treatments at 24 h, 48 h, and 72 h. Additionally, the fluorescence staining assay confirms significant apoptotic changes. The findings highlight the multifaceted potential of P. amboinicus-coated ZnO nanoparticles as viable candidates for functional foods and therapies, addressing antimicrobial resistance, oxidative stress, and cancer.