Surface engineering and concentration-dependent emission activated flexible tunable fluorescence from lignin-based N-doped carbon dots

Achieving the flexible and tunable fluorescence of lignin-based N-doped carbon dots (LCDs) remains a win-win yet challenging strategy. In this study, combining the surface engineering and concentration regulation strategy enabled the flexible adjustment of LCDs fluorescence emission from blue to yellow regions (345 to 560 nm). The surface electron-donor -OH groups not only enhanced the photoluminescence quantum yield (PLQY) to 14.75 % but activated the spectral shift of reduced LCDs (RLCDs). These arose from the reinforced it-electron conjugation and generated new -OH-related extrinsic defects with elevating concentration. Conversely, the electronwithdrawing C--O groups participated in the blue-to-green fluorescence modulation. Still, they drastically reduced the PLQY of oxidized LCDs (OLCDs) to 2.13 %, due to the larger particle size and more non-radiative transfer induced by C--O groups. Based on these, CDs/PVA films and printed fluorescent patterns with multicolor fluorescence were realized for anti-counterfeiting, and the RLCDs@Al2O3 hybrid facilitated the precise fluorescence fingerprint tracking and recognition. This study provided a facile strategy for enriching and developing novel LCDs with flexible fluorescence for their advanced applications.