Green synthesis of silver nanoparticles using Houttuynia cordata Thunb rhizome extracts and their antibacterial potential against common foodborne pathogens

The biological synthesis of functionalised nanoparticles using plant materials is considered more cost-effective and eco-friendly than chemical and physical methods. Hence, this study demonstrated the synthesis of silver nanoparticles (AgNPs) using Houttuynia cordata Thunb rhizome (HCR). The extract, prepared with distilled water, provided the required phytochemical components, such as flavonoids, terpenoids, phenols, proteins and other essential bio-reducing agents. The AgNPs were exposed to reaction conditions, such as temperatures, pH values and AgNO3 concentrations, to identify the optimum synthesis condition and explore their impacts on particle size and antibacterial action against Gram-positive (Staphylococcus aureus) and Gram-negative (Salmonella enterica and Shigella dysenteriae) foodborne pathogens. The formation of AgNPs was indicated by specific surface plasmon resonance (SPR) intensity at 440-450 nm and colour changes under different reaction conditions. The structure and size distribution characterised by SEM and TEM revealed that the AgNPs exhibited a spherical-shaped structure with irregular contours, regular distribution and aggregation. Although the average particle size ranged from 2 to 100 nm, relatively smaller sizes were detected at 10 mM (54.30), 80 degrees C (49.50 nm) and pH 6 (60.16 nm) conditions. The FTIR spectrum and EDX spectral signals affirmed the presence of reducing and stabilising agents in HCR extract, following the reduction of AgNPs from Ag+ to Ag0. AgNPs also exhibited excellent antibacterial activity against S. aureus, S. enterica and Sh. dysenteriae, indicating a broad spectrum for various applications in the food and pharmaceutical industries.