Green synthesis of copper oxide nanoparticles using spent green tea extract and their dosage-dependent bioactivity and mango preservation performance

Spent green tea extract (SGTE) represents an underutilised post-consumption bioresource for the sustainable synthesis of functional nanomaterials. In this study, copper oxide nanoparticles (CuO NPs) were synthesised via a green route using SGTE as a dual reducing and stabilising agent, and the effect of extract dosage (0.02-0.05 g/mL) on structural characteristics, bioactivity, and preliminary mango preservation performance was systematically investigated. All samples exhibited phase-pure monoclinic CuO with crystallite sizes of 14.1-18.8 nm. Increasing SGTE dosage reduced the apparent optical absorption edge (3.32-3.05 eV) and influenced nanoparticle aggregation behaviour. Antioxidant activity increased with extract dosage, whereas antibacterial activity showed a non-monotonic trend, with optimal performance observed at 0.03-0.04 g/mL against representative Gram-positive and Gram-negative bacteria. A preliminary storage study demonstrated that CuO NPs treatment delayed visible spoilage, reduced weight loss and colour variation, maintained firmness, and lowered decay rates in mango fruits under ambient conditions. These findings reveal a dosage-dependent trade-off between antioxidant and antibacterial performance governed by surface functionality and aggregation effects. Overall, this work establishes a simple strategy for tuning structure-bioactivity relationships in green-synthesised CuO NPs while promoting waste valorisation, and provides a foundation for future integration into postharvest preservation strategies and active food-packaging systems.