Soybean-derived blue photoluminescent carbon dots

Biomass-derived carbon dots (CDs) are biocompatible and have the potential in a variety of applications, including bio-imaging and bio-sensing. In this work, we use ground soybean residuals to synthesize carbon nanoparticles by hydrothermal carbonization (HTC), annealing at high temperatures, and laser ablation (LA) in a NH4OH solution. The carbon nanoparticles synthesized with the HTC process (HTC-CDs) exhibit photoluminescent characteristics with strong blue emission. The annealing of the HTC-CDs at temperatures in a range of 250 to 850 ⁰C causes the loss of the photoluminescent characteristics without any significant changes in the microstructures (amorphous structure) of carbon particles. The LA processing of the annealed HTC-CDs introduces nitrogen-contained surface-functional groups and leads to the recovery of the photoluminescent features that are different from those of the HTC-CDs and dependent on the fraction of nitrogen in the surface-functional groups. The photoluminescence of both the HTC-CDs and LA-CDs is largely due to the presence of N-contained surface-functional groups. The quantum yield of the LA-CDs is more stable than that of the HTC-CDs under continuous UV excitation and does not exhibit significant reduction over 150 min excitation. The methods used in this work provide a simple and green strategy to introduce surface-functional groups to carbon nanoparticles from biomass and biowaste and to produce stable photoluminescent CDs with excellent water-wettability.