Biogenic zinc oxide nanoparticles synthesized from <i>Chenopodium quinoa</i> extract: Characterization and antibacterial activity against multidrug-resistant pathogens

Background: Metal oxide nanoparticles, particularly zinc oxide nanoparticles (ZnO NPs), are emerging as promising antimicrobial agents due to their broad-spectrum activity and low risk of resistance development. However, the mechanistic role of plant-derived biomacromolecules in nanoparticle stabilization and antibacterial performance remains insufficiently understood, largely because most studies evaluate bulk nanoparticle systems without distinguishing organic capping effects from the inorganic core. Green synthesis using plant-derived biomolecules offers a safer and more sustainable alternative to conventional chemical methods, which often rely on toxic reagents. Objective: This study aimed to develop a facile one-pot green synthesis of ZnO NPs using Chenopodium quinoa seed extract as both reducing and capping agents, and to dissect the relative contribution of quinoa-derived biomacromolecules versus the inorganic ZnO core to antibacterial performance by comparing nanoparticle-enriched (sediment) and biomolecule-enriched (supernatant) fractions. Methods: ZnO NPs were synthesized using C. quinoa extract and characterized by UV–visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDX). Antibacterial activity was evaluated against representative Gram-negative and Gram-positive bacterial strains. Results: UV–visible spectroscopy confirmed nanoparticle formation with a characteristic absorption peak at 356 nm, attributed to the intrinsic bandgap excitation of ZnO. XRD analysis revealed a highly crystalline hexagonal wurtzite structure with an average crystallite size of 46 nm. SEM and TEM images demonstrated uniform hexagonal morphology. FTIR results indicated that hydroxyl and carbonyl groups from quinoa biomacromolecules contributed to nanoparticle capping and stabilization. Biological assays showed strong, dose-dependent antibacterial activity against Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Micrococcus luteus, Streptococcus mutans, Staphylococcus aureus), with the biomolecule-enriched supernatant fraction consistently outperforming the nanoparticle-enriched sediment, reaching a maximum zone of inhibition of 45 mm. Conclusion: The study demonstrates that quinoa-derived biomacromolecules enable the green synthesis of stable and bioactive ZnO NPs, and that residual capping biomolecules contribute substantially to the antibacterial performance beyond the inorganic ZnO core alone. Novelty of the Study: Unlike conventional plant-mediated ZnO NP studies focused on fabrication and bulk antimicrobial activity, this work employs a fractionation approach separating nanoparticle-enriched (sediment) and biomolecule-enriched (supernatant) phases. This enables direct assessment of ZnO NPs and C. quinoa–derived capping biomacromolecules, demonstrating that antibacterial activity arises not only from the inorganic core but also from synergistic effects of residual capping phytochemicals, providing new mechanistic insight into green nanoparticle systems. Keywords: Green synthesis; Chenopodium quinoa; zinc oxide nanoparticles; biomolecule fractionation