Polysaccharidic spent coffee grounds for silver nanoparticle immobilization as a green and highly efficient biocide.

Spent coffee grounds (SCGs) contain abundant polysaccharides and several components with bioactivities. Despite many bio-functionalities, their bioactivities are not always satisfactory. Modifications of SCGs may overcome this issue. This work describes the method for reusing the SCGs as biological macromolecular supports and reducing agents to prepare silver nanoparticle (AgNP)/SCGS composites (AgNPs@SCGs) by biogenic synthesis. The AgNPs anchored on the surface of SCGs were synthesized by mixing the SCGs in AgNO3 solution with various pH conditions at room temperature. Scanning electron microscopy (SEM) and X-ray diffractometer (XRD) analysis confirmed the reduction of silver ions to AgNPs, and showed that the pH 4.5 condition could generate uniform and impurity-free AgNPs on the surface of SCGs. Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), and thermal gravimetric analysis (TGA) showed that the reducing process of AgNPs was mild and could preserve the original nature of the SCGs. The AgNPs@SCGs composites exhibited an excellent antimicrobial ability against S. aureus and E. coli compared to SCGs. The transformation of the polysaccharidic SCGs to AgNPs@SCGs composites by the green and sustainable method makes them highly valuable for developing the applications on antimicrobial products.