Study on preparation of high yield lignin-based carbon dots by hydrothermal oxidation process and optical regulation

To improve the yield and fluorescence characteristic of lignin-based carbon dots, a two-step process of hydrothermal and oxidation was proposed to achieve efficient preparation of intense fluorescent lignin-based carbon dots (LCDs) in this study. N, S co-doped LCDs (NS-LCDs) and hydrothermal char were prepared by using mild sulfanilic acid as additive, then hydrothermal char as carbon source was oxidized and exfoliated to prepare high-yield oxidized LCDs (O-LCDs) via using NaOH and H 2 O 2 . It was found that the yields of NS-LCDs initially increased and then decreased with hydrothermal temperature increasing, reaching the maximum value of 5.71 % at 200 °C. And more than 50 % of the hydrothermal char was converted to O-LCDs by oxidative stripping, which resulted in the total yield of LCDs reached to 17.13 % at 200 °C. Meanwhile, the fluorescence quantum yield of NS-LCDs showed a similar trend, obtained a maximum value of 18.00 % at 200 °C, which was the same as content of C sp2 and C-N in XPS results. And the average particle size of NS-LCDs was less than O-LCDs, the average particle size 180-NS-LCDs was 3.61 nm, while that of 180-O-LCDs was increased to 3.92 nm. In addition, when the temperature was higher than 180 °C, the fluorescence lifetime of NS-LCDs was prolonged, which might be related to N and S doping, and 200-NS-LCDs showed good acid stability, temperature stability and photobleaching resistance. This study provides guidance for the application and preparation of high-quality LCDs.