Phytochemical Fraction-Mediated Synthesis of Ag and ZnO Nanoparticles: Exploring Reactivity Limits and Broad-Spectrum Antimicrobial Properties

Abstract The common generalization of plant extracts as universal, nonspecific reducing agents in metal nanoparticle (NP) synthesis is re-examined. We provide evidence that green synthesis relies on selective reactions between specific phytochemicals and metal precursors. Silver (Ag) and zinc oxide (ZnO) nanoparticles were successfully synthesized using the n-butanol fraction of Plantago major leaf extract, while identical protocols failed to yield copper phases. Characterized via Fourier-transform infrared, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, UV–vis, dynamic light scattering, and zeta potential, the annealed Ag and ZnO NPs exhibited average grain sizes of 20–37 nm (λmax = 428 nm) and 6–22 nm (λmax = 338 nm) and band gaps at 2.90 and 3.23 eV, respectively. Both Ag and ZnO NPs displayed broad-spectrum antibacterial activity against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, alongside strong antifungal efficacy against Candida albicans and Aspergillus brasiliensis. Notably, Ag NPs showed potent antifungal activity against A. brasiliensis (MIC: 15.625 μg/mL), while ZnO inhibited P. aeruginosa (MIC: 62.50 μg/mL). Conversely, the unreduced copper complex showed negligible bioactivity possibly due to residual dissolved ions. While the n-butanol fraction outperforms both crude extracts and aqueous/ethyl acetate fractions, this study challenges universal green synthesis dogmas, demonstrating metal-specific phytochemical divergence under identical parameters.