Green synthesis of cuprous oxide nanoparticles using spent coffee ground and its antifungal activity against Phytophthora palmivora in vitro

Spent coffee grounds (SCG), a byproduct generated in large quantities during the production of instant coffee, pose a potential environmental threat. Chemical analysis revealed that SCG contains approximately 67.66% lignocellulosic compounds and a high content of polyphenols (58.89 mg gallic acid equivalents/g), making it suitable as both a bio-based reductant and carrier for the green synthesis of cuprous oxide nanoparticles (Cu2O NPs). The resulting Cu2O NPs/SCG, containing ~2% Cu (w/w), exhibits an average particle size of 30.4 ± 4.5 nm as determined by scanning electron microscopy (SEM). Structural characterization was determined using X-ray diffraction and Fourier-transform infrared spectroscopy measurements. Elemental composition was analyzed semi-quantitatively using SEM-energy dispersive X-ray spectroscopy. The oxidation states of Cu species (Cu0, Cu+, and Cu2+) were quantified via atomic absorption spectroscopy and iodometric titration, showing that Cu+ was the predominant species, accounting for 93.50%. Notably, the synthesis process also removed approximately 98% of caffeine, a known phytotoxin, from the SCG. The Cu2O NPs/SCG exhibited 100% antifungal activity against Phytophthora palmivora in vitro at a Cu content of 50 mg/L, with a half maximal inhibitory concentration of 32.77 mg/L Cu. These results highlight the potential of Cu2O NPs/SCG as a sustainable agricultural material for disease control and plant growth promotion, aligning with principles of the circular economy.