Unravelling the Effect of Reagent Concentration on the Intermediate Products Obtained in the First Stages of Carbon Dots Synthesis From the Hydrothermal Reaction of Citric Acid and Ethylenediamine

Fluorescent carbon dots (CDs) have attracted significant interest due to their excellent properties, making them excellent candidates for use as components in various optoelectronic devices or even biosensors. However, the origin of their photoluminescence is still the subject of debate, and their fabrication lacks the desired control at the molecular level and precise structural regulation to obtain materials with specific properties. One of the most widely used synthesis procedures is the hydrothermal method which uses citric acid (CA) and ethylenediamine (EDA) as reagents. Previous results show some inconsistencies in the Photoluminescent (PL) properties of the materials obtained by this procedure and the presence of small and highly fluorescent molecules produced in the initial stages of the reaction. However, the role of these molecules on the PL properties of these materials is unknown. In this work, we studied the effect of these products on the PL properties of these materials designing experiments in which the EDA/CA molar ratio was modified. The chemical structure of products was analyzed by 1 H NMR, FTIR, Raman and X-ray photoelectron (XPS) spectroscopies, Mass Spectrometry (MS) and X-ray diffraction (XRD). To interpret our results, we present a complex reaction mechanism with two competing processes of polymerization and formation of small molecules. Our results demonstrated that EDA/CA molar ratio determines pH of the reaction and the predominant process. We also demonstrate that the fluorescence of these materials is dominated by the emission of 5-oxo-1,2,2,3,5-tetrahydroimidazo [1,2-α] pyridine-7-carboxylic acid (IPCA) monomers and two types of IPCA aggregates.