Transforming waste rubber gloves into value-added fluorescent carbon quantum dots for direct ink writing

About 10 million tons of waste rubber are generated annually in China, which has caused potential environmental pollution and resource waste due to improper disposal. In the circular economy context, there is a need for methods to make rubber waste into valuable carbon materials. Here, we report a simple acid-assisted hydrothermal process for the first time to convert nitrile gloves into value-added carbon quantum dots (CQDs), and their use in ink-writing hydrogels. The product yield of CQDs is 36 % under optimized conditions, surpassing previously reported results of CQDs derived from plastics. Moreover, the CQDs exhibit high fluorescence stability from pH 2–7, under ultraviolet irradiation during 3 h, and treated by H₂O₂ at concentrations of 0–500 μmol/L. CQDs were near-spherical with an average particle size of 2.45 nm and the lattice fringe spacing measured at 0.21 nm. The oxygen-containing groups on the surface of CQDs endowed them high hydrophilicity and excitation-dependent fluorescence behavior with a quantum yield of 9.7 % and a lifetime of 6.94 ns. Given their favorable fluorescence and hydrophilic properties, the direct ink-writing hydrogels prepared with CQDs and polyvinyl alcohol display unique patterns on polyethylene terephthalate (PET) substrates, exhibiting high dimensional accuracy and the ability to show fluorescent colors.