Preparation of Alginate/AgNP Nanocomposite Hydrogels Incorporating Olive Leaf Extracts in Natural Deep Eutectic Solvents

In the present study, alginate-silver nanoparticle (Alg-AgNP) nanocomposite hydrogels possessing antibacterial activity were synthesized via an innovative route. A task-specific designed Natural Deep Eutectic Solvent (NADES), composed of glucose, lactic acid, and water, was utilized as a green extraction solvent of bioactive compounds from olive leaves (OLs). The NADES-olive leaf extract (NADES-OLE) was used as obtained for the preparation of the Alg-AgNP nanocomposite hydrogel as a multiple-role component. The NADES-OLE acts (a) as a crosslinking agent for the preparation of the alginate hydrogels, (b) as a reducing agent for the in situ synthesis of AgNPs during hydrogel formation, and (c) as a bioactivity enhancement agent due to the presence of compounds obtained from the olive leaves. The Alg-AgNP hydrogel preparation process was optimized through a Box-Behnken experimental design. The resulting nanocomposite hydrogels were characterized for their swelling capacity and water retention in phosphate buffer (pH 5.5), achieving 538% swelling capacity within 180 min and 90% water retention after 250 min. The AgNPs formed within the hydrogels were found to have an average size of 103.2 ± 5.6 nm, with a concentration of 1.2 10⁸ ± 2.2 ∗ 10⁷ particles/mL. Antibacterial testing of the nanocomposite hydrogels against foodborne pathogens, including Gram-negative (<i>Escherichia coli</i>, <i>Salmonella Typhimurium</i>, <i>Yersinia enterocolitica</i>) and Gram-positive bacteria (<i>Listeria monocytogenes</i>, <i>Staphylococcus aureus</i>, <i>Bacillus cereus</i>), revealed significant antibacterial activity, particularly against <i>E. coli</i> (64.9%), <i>Y. enterocolitica</i> (60.6%), <i>S. aureus</i> (79.1%), and <i>B. cereus</i> (55.3%), at a concentration of 1 mg/mL.