Phytochemical-Mediated Green Synthesis of Silver Nanoparticles from Eleusine indica and Amaranthus spinosus and Their Gram-Selective Antibacterial Activity

Antimicrobial resistance is escalating into a global health crisis, outpacing conventional antibiotics and demanding new bactericidal materials. Silver nanoparticles (AgNPs) are potent broad-spectrum antimicrobials, but conventional chemical and physical synthesis routes rely on toxic reductants and high energy inputs, generating hazardous byproducts. Plant-mediated green synthesis offers an eco-friendly alternative in which phytochemicals act simultaneously as reducing and capping agents; however, the molecular basis linking extract chemistry to nanoparticle behavior remains poorly defined. This study comparatively synthesized AgNPs using phenolic extracts of goosegrass (Eleusine indica) and spiny amaranth (Amaranthus spinosus) and dissected how each extract's phytochemical fingerprint governs particle formation and antibacterial selectivity. AgNPs were characterized by UV-Vis, FTIR, and SEM-EDX, and tested against Escherichia coli and Staphylococcus aureus. FTIR confirmed that hydroxyl, amine, and carbonyl groups mediated bioreduction and surface stabilization, while surface plasmon resonance and EDX (up to 82.7 wt% Ag) verified nanoparticle formation. Although spiny amaranth had markedly higher total phenolic content (167.1 vs 121.4 mg GAE/g), it produced larger, more agglomerated particles (1058 vs 305 nm) and a lower yield (21.9% vs 49.6%), demonstrating that phenolic abundance alone does not dictate colloidal outcome. Critically, antibacterial activity was Gram-selective: goosegrass AgNPs were more effective against E. coli and spiny amaranth AgNPs against S. aureus, a divergence attributed to differences in peptidoglycan architecture interacting with particle size and capping chemistry. These findings establish that the qualitative phytochemical identity of the reducing extract, rather than its phenolic quantity, controls AgNP size, stability, and bacterial selectivity, offering rational design principles for tailoring plant-based antimicrobials.