Nitrogen and sulfur Co-doped carbon dots with excellent fluorescent thermal stability for anti-counterfeiting and information encryption

In this work, a series of nitrogen and sulfur co-doped carbon dots (CDs) were synthesized with high photoluminescence quantum yields (PLQYs) and outstanding thermal stability. With the reaction solvent changing from water, ethanol, methanol, and dimethylformamide (DMF), a significant fluorescence emission with PLQY improvement from 42 %, 33 %, 25 %, and 18 %, respectively, accompanied by a red shift from 430 to 590 nm, (the color changes from blue, green, yellow, and red respectively). Simultaneously, excellent fluorescence stability can also be obtained across temperatures ranging from 15 to 95 °C. Combined with the density functional calculations (DFT) results, the underlying mechanism investigation reveals that the color change of fluorescence emission was probably induced by the increased particle size of CDs and increased graphitic N content. The enhanced thermal stability is induced by the presence of stable surface functional groups, including C=O, C=N, C=S, and -NH, among others, contributing to improved hydrophilicity, regulated particle aggregation, and mitigated thermal oxidation by limiting oxygen diffusion to fluorescent hubs. Notably, the obtained outstanding optical properties finally render these multicolor CDs suitable for information encryption and anti-counterfeiting applications.