Synthesis, solution and solid-state fluorescence of nitrogen self-doped carbon dots derived from Chlorella pyrenoidosa

In this work, we report a simple and green approach for the synthesis of nitrogen-doped carbon dots from Chlorella pyrenoidosa, hereafter referred to as CP-CDs, via microwave-assisted hydrothermal carbonization without any additives. The effects of temperature (170–210 °C) and time (1–3 h) on the structure, functional groups, optical properties and formation route of the CP-CDs were assessed in detail. FTIR, XPS, and XRD results showed that high temperatures promoted the nitrogen doping level, resulting in good fluorescence intensity and high quantum yields (QYs). As the reaction temperature increased from 170 °C to 210 °C, the amount of nitrogen doped into the CP-CDs increased significantly from 4.1% to 11.9% and the absolute QY improved from 1.79% to 2.31%. The CP-CDs solutions exhibited excitation-dependent blue fluorescence from 360 to 527 nm with the excitation from 280 to 460 nm. The optimal preparation temperature and time for CP-CDs aqueous solution were 210 °C and 2 h, respectively. The solid-state CP-CDs without any further processing exhibited self-quenching resistant performance. The obtained CP-CDs as fluorescence inks could be used in security printing.