Green synthesis of carbon dots by microflow method and their application as sunscreen agent

Sunscreen agents derived from plants have been regarded as promising alternatives to artificial compounds. In this work, carbon dots (CDs) were prepared from carrot juice via a continuous microflow-based approach, where the influence of process parameters was studied and optimized. Complimentary characterization revealed the CDs not only have small size, narrow size distribution, and good water solubility, but also have abundant functional groups as well as excellent UV absorption performance. Relying on these properties, the CDs were used as UV absorbers, suggesting they have strong long-term UV absorption ability over a broad pH range. The UV-absorption properties of the CDs were confirmed by incorporating the CDs in polyvinyl alcohol (PVA) to get C-CDs@PVA films of different thickness, in which significantly enhanced UV absorption performance was observed. Besides, the sun protection performance is also related to the film thickness. Afterwards, the practical application of the CDs was evaluated by adding them in a typical skin cream. With the addition of the CDs, the cream has drastically reduced UV transmittance in both UVA and UVB regions, and exhibits better UV absorption performance than commercial sunscreen agents. The CDs also demonstrated low cytotoxicity and high DPPH radical scavenging activity, making them promising as green sunscreen absorbers. This work is expected to provide a guidance for the development of green and effective natural sunscreen agents via microflow-based method.