Citric Acid Derived Carbon Dots, the Challenge of Understanding the Synthesis-Structure Relationship

Carbon dots (CDs) are highly-emissive nanoparticles obtained through fast and cheap
syntheses. The understanding of CDs’ luminescence, however, is still far from being comprehensive.
The intense photoluminescence can have different origins: molecular mechanisms, oxidation of
polyaromatic graphene-like layers, and core-shell interactions of carbonaceous nanoparticles. The
citric acid (CA) is one of the most common precursors for CD preparation because of its high
biocompatibility, and this review is mainly focused on CA-based CDs. The different parameters that
control the synthesis, such as the temperature, the reaction time, and the choice of solvents, were
critically described. Particular attention was devoted to the CDs’ optical properties, such as tunable
emission and quantum yields, in light of functional applications. The survey of the literature allowed
correlating the preparation methods with the structures and the properties of CA-based CDs. Some
basic rules to fabricate highly luminescent nanoparticles were selected by the metanalysis of the
current literature in the field. In some cases, these findings can be generalized to other types of CDs
prepared via liquid phase.