Sustainable carbon quantum dots from forestry and agricultural biomass with amplified photoluminescence by simple NH4OH passivation

High quality blue fluorescent carbon quantum dots (CDs) were fabricated from a dilute NH4OH/water solution of waste biomass via one-step hydrothermal carbonization. Different from conventional synthesis methods, NH4OH was used as the solvent as well as passivation agent rather than organic amine compounds. The best quantum yield (QY) of the nitrogen-doped biomass CDs was up to 16%, which was higher than the non-doped CDs (only 2%) and those passivated by organic molecules (7%). Further investigations revealed the unique emission behaviours of the NH4OH-passivated CDs dependent on excitation, pH values and solvents and interesting up-conversion photoluminescence. In addition, an in vitro cytotoxicity test demonstrated the negligible toxicity of NH4OH-passivated CDs, and they were successfully applied in bio-imaging of murine embryonic stem cells. This low-cost and green chemistry method provides a feasible route for value-added and sustainable utilization of waste biomass.