Red-Emitting Carbon Quantum Dots for Biomedical Applications: Synthesis and Purification Issues of the Hydrothermal Approach

The possibility of performing the synthesis of red-emitting carbon quantum dots (r-CDs), ina well-controllable, large scale and environmentally sustainable way is undoubtedly of fundamentalimportance, as it will pave the way to their employment in advanced medical large-scale applications.Knowledge of the difficulties involved in producing r-CDs with reproducible optical, structural, andchemical characteristics, might help in their large-scale production, making the process standardizable.In this work, we present an experimental study, also supported by results reported in the literature,on the issues encountered during the synthesis and post-synthesis purification treatments of r-CDS.We focused on the hydrothermal approach as it was found to be more suitable for future largescale industrial applications. We propose three synthetic strategies and observed that employingp-phenylenediamine (p-PDA), as a precursor, the synthetic process showed low efficiency with lowyields of r-CDs, large amounts of unreacted precursor, and reaction intermediates. Changing reactionparameters does not improve performance. The r-CDs obtained using citric acid (CA) and urea,as precursors, resulted to be sensitive to pH and difficult to separate from the reaction mixture.Furthermore, the proposed synthetic strategies show that the hydrothermal preparation of r-CDSrequires approaches that are not fully sustainable.