Green upcycling of spent liquor from nanocellulose production into multicolor-emissive carbon dots for Fe3+ ion detection.

The economical synthesis of carbon dots (CDs) with high quantum yields and long emission wavelengths from cellulose still remains a challenge due to the high crystallinity and molecular weight of cellulose. Herein, an acidic spent liquor generated during nanocellulose preparation was employed as an unconventional precursor to overcome these limitations, enabling the synthesis of sulfur-doped CDs. Through simple temperature modulation, the prepared CDs exhibited blue, green and orange fluorescence emissions. As the reaction temperature decreased, enhanced graphitization and increased CO/SO content on the amorphous surface were observed, leading to a systematic red shift in photoluminescence emission. The quantum yields of these three types of CDs were 10.4%, 15.5% and 11.0%, respectively. Notably, the orange CDs exhibited sensitive and selective detection of Fe3+ over a wide concentration range, achieving a low limit of detection of 14 nM. This work demonstrated a sustainable and efficient pathway for producing full-color CDs from low-cost, renewable, and environmentally benign cellulose-based biomass waste.