Carbon Quantum Dots from Horticultural Waste: Sustainable Nanotechnology for Food Preservation

Losses of highly perishable fruits and vegetables remain a serious worldwide issue and generate large volumes of carbon-rich waste. This review examines how horticultural by-products, such as peels, pomace, seeds, and stems, can be converted into carbon quantum dots (CQDs) using green, bottom-up synthesis approaches, with particular emphasis on hydrothermal techniques. These methods produce small (2–10 nm), water-dispersible, fluorescent, and low-toxicity nanomaterials bearing abundant surface functional groups that confer strong antioxidant and antimicrobial activities. CQDs exert their effects through mechanisms including reactive oxygen species scavenging, inhibition of free radicals, disruption of microbial membranes, and interference with biofilm formation; these properties can be further tailored by heteroatom or metal doping. When incorporated into edible coatings and biopolymer films, CQDs improve UV shielding, barrier properties, and oxidative stability, which in turn suppresses microbial growth, slows quality deterioration, and extends the shelf life of fresh produce. CQDs also support intelligent packaging by serving as fluorescence-based nano-sensors capable of real-time detection of freshness markers, ethylene, gas composition, and contaminants. By relating the physicochemical features of CQDs to their functional performance, this review underscores their potential role in circular bioeconomy strategies that valorise food and horticultural waste. It also outlines key challenges, such as scale-up, standardisation, safety evaluation, and migration behaviour, that must be addressed for their broader adoption in sustainable food packaging.