From Agricultural Waste to Food Safeguard: Coffee Shell-Derived Carbon Dots with Antibacterial Activity for Fruit Freshness Extension

Postharvest bacterial spoilage poses a major threat to global food safety, which is particularly prominent for fruits with high moisture and nutrient contents. Hence, environmentally friendly and efficient antibacterial solutions are urgently needed. Carbon dots (CDs) exhibit promising antibacterial potential in fruit preservation. Upon screening, coffee-husk-derived carbon dots (CS-CDs) were identified to possess highly efficient antibacterial activity. In this work, CS-CDs were synthesized via a one-step hydrothermal method. The as-prepared CS-CDs have an average particle size of 2.0 nm, and their surfaces are rich in carboxyl, hydroxyl and amino groups, delivering distinct photoresponsive capacity. Antibacterial assays reveal that the inhibitory rates of CS-CDs against Escherichia coli and Staphylococcus aureus both exceed 95%. The bactericidal effect is mainly realized by reactive oxygen species and physical contact, which disrupt bacterial cell membranes and trigger cellular damage. Meanwhile, CS-CDs are verified to possess favorable biosafety for practical application. Finally, CS-CDs were successfully incorporated into carboxymethyl cellulose (CMC) to fabricate CS-CDs/CMC composite films with superior mechanical properties compared with pure CMC films. Film-property analyses demonstrate their suitability for fresh-fruit preservation packaging. Continuous monitoring and analysis of fruit quality during litchi and mango storage confirm the preservation and anti-spoilage performance of the composite film. This study provides a promising alternative to conventional synthetic preservatives and metal-ion-based antibacterial agents, offering advantages of environmental friendliness, high antibacterial efficiency, favorable biocompatibility, and reliable biosafety. Meanwhile, it provides a feasible strategy for the design and application of biomass-based antibacterial preservation-packaging materials.