Silver Nanoparticles Mediated by Natural Extracts Recovered from Wastes and By-Products ^†

Introduction: Current environmental problems demand an eco-friendlier approach to the chemical synthesis of metal nanoparticles (NPs) [1]. By using plant extracts obtained from waste materials and by-products resulting from the essential oil and textile industry, the resulting green nanoparticles represent a sustainable alternative for the classical synthetic route [2,3]. In our study, we obtained silver nanoparticles (AgNPs) by using aqueous plant extracts from Cannabis sativa, Thymus vulgaris, Lavandula angustifolia, and Origanum vulgare. The chemical composition of the extracts was determined by chromatographic and spectroscopic methods. AgNPs with less than 70 nm were obtained and characterized by UV-VIS, FT-IR spectroscopy, and SEM. The antioxidant activity (DPPH and ABTS assays) and the antibacterial properties against Gram-positive and Gram-negative bacteria of some of the samples were evaluated. Materials and methods: For the green NPs synthesis, the plant extracts were freshly prepared and filtered through a 0.45 mm PVDF, then 0.5 mL extract was mixed with 0.5 mL 5 mM silver nitrate solution and 0.7 mL purified water. The total reaction time was 8 min at 90 °C. Results: The bio-reduction of silver ions was mediated by the phenolic compounds present in the aqueous extracts. This reaction was easily observed by a visual colour change from pale gold to a reddish-brown and confirmed by UV-Vis spectral analysis, with an average particle size of 35–70 nm. Nanoparticle morphology was observed using SEM and average spherical shapes were noticed. Conclusions: The preparation of silver nanoparticles was successfully performed using aqueous extracts recovered from industrial wastes and by-products. Obtaining NPs through green synthesis is a fast, easy, and eco-friendly reaction that reduced Ag⁺ to Ag⁰ to spherical AgNPs with small average particle size.