Carrageenan-based washable coatings integrated with sulfur quantum dots for extending banana shelf life

Multifunctional sulfur quantum dots (SQDs) passivated with ethylenediamine (EDA) were prepared using sublimed sulfur by a facile hydrothermal method. The as-prepared SQDs were quasi-spherical with a mean particle size of 4.7 nm and showed excitation-dependent emission characteristics with potent antioxidant and antibacterial activity. The SQDs were used as multifunctional fillers to create κ-carrageenan-based active films and coatings for fresh produce. The addition of SQDs hardly changed the color and transparency of the carrageenan films but greatly increased their UV-A and UV-B blocking properties by 77 % and 47 %, respectively. The tensile strength of the SQD-added films was significantly increased (p < 0.05) without compromising flexibility and rigidity. In addition, the SQD-added film exhibited excellent antioxidant activity against ABTS free radicals and exhibited remarkable antibacterial properties against foodborne pathogenic bacteria such as E. coli and L. monocytogenes. In the case of the film added with 2 % (w/w) SQD, the number of viable cells decreased by 6.5 log CFU mL⁻¹ and 8 log CFU mL⁻¹, respectively, compared to the neat carrageenan film. The sustainable carrageenan/SQD formulations were applied as washable coatings on the surface of bananas, which substantially reduced the browning of the fruit, extending their shelf life for 8 d.