Effect of nitrogen and phosphorus co-doped carbon dots containing layer-by-layer self-assembled coating on UV resistance and thermal stability of cotton fabric

Nitrogen and phosphorus co-doped carbon dots (N, P co-doped CDs) were synthesized using a one-pot hydrothermal carbonization method. The resulting CDs were then applied to fabricate a multilayered coating on cotton fabric through a layer-by-layer self-assembly method, in combination with alginate and polyethyleneimine. When exposed to a 365 nm UV lamp, the coating containing N, P co-doped CDs emitted visible blue light. The cotton treated with these CDs was subsequently evaluated for UV-protection, thermal stability, and flame retardancy. Cotton-1 fabric (110 g/m²) treated with a 6-bilayer CD-containing coating had a significantly higher ultraviolet protection factor (UPF) of 148.4 compared to the pure cotton fabric which had a UPF of 6.65. The cotton-1 fabric treated with CD coating exhibited more char residues at 700 °C under a nitrogen atmosphere in the TGA test. The study also discovered that the CD-containing multilayered coating significantly decreased the peak heat release rate and total heat release rate of cotton-1/(PEI/ACD)₆ by 52.8% and 53.8% respectively, in comparison to cotton-1 in the microscale combustion calorimeter test. These findings indicate that N, P co-doped CDs, which have both UV absorption and environmental friendliness, could be potential utilized as a flame retardant on the surface of cotton fibers.