Silver nanoparticles on hydrolyzed spent coffee grounds (HSCG) for green antibacterial devices

Green and alternative strategies for the preparation of silver nanoparticles (AgNP) are the focus of considerable attention, given their wide range of applications. A clean, eco-friendly, low-cost, scalable and straightforward protocol for the production of AgNP is reported, using hydrolyzed spent coffee grounds (HSCG) both as a reducing agent and a stabilizing matrix. The sample was easily recovered in 50-60% yield by centrifugation and lyophilization. Size and morphological analyses combined with X-ray diffraction analysis indicated ca. 6% w/w incorporation of AgNP of about 20 nm size into the HSCG matrix. AgNP-HSCG exhibited efficient antioxidant properties in several chemical assays and antimicrobial activities (MIC< 0.1 mg/mL) against Gram-negative bacteria such as E. coli and P. aeruginosa. Antibacterial devices made of chitosan films containing AgNP-HSCG were also prepared that showed significant inhibition zones against the same bacterial strains, even at the lowest loading (0.3% w/w) of AgNP, and remarkable antioxidant properties in aqueous media. An economic evaluation of the whole process was carried out, demonstrating that manufacturing of both HSCG and chitosan-based antibacterial devices is a convenient and easy-to-perform process, with the potential of full industrial scalability. Overall, the adopted circular approach, designed to reuse a cheap and abundant agri-food waste while avoiding the use of toxic or noxious chemicals, makes HSCG a practical and easily accessible multifunctional material for the clean and sustainable synthesis of AgNP with adequate size and stability for incorporation in efficient antioxidants and antimicrobial devices.