ИСТИНА

Hydrothermal treatment of an aqueous solution of citric acid (CA) and ethylenediamine (EDA) leads to their polymerization, dehydration, carbonization, and possible graphitization [1,2]. Sufficiently large structures synthesized in this way are called carbon dots (CDs). Under certain conditions, such CDs – classified as polymer nanostructures – are known to produce ultra-bright luminescence in the violet-blue region with a quantum yield up to ∼100%. Our study [2] of 392 CA and EDA samples subjected to hydrothermal synthesis with varying EDA:CA ratio in the range of 0–20:1, temperature in 80–200 °C, and reaction time in 0.5–6 h, showed that the formation of samples’ main luminophores starts with the polymerization of precursors, accelerates with the initial carbonization of the samples, while the stage of graphitizing carbonization – formation of CDs’ cores – brings their partially destruction. Most publications suggest that this main – and only – luminophore is IPCA (5-oxo-1,2,3, 5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid or 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid) [3]. The present study points to a much more complex picture. The analysis of luminescence maps of 392 samples of CA and EDA reaction (examples in Fig. 1) allowed to identify 8+ different fluorescence signatures and track the stages of their appearance and extinction at different stages of CD synthesis. Some of these signatures merge or become absorbed by photoluminescence in the main maximum in the λex/λem 350/440 nm/nm region. To check whether different fluorescence signatures belong to separate molecular systems, choice samples were subjected to gel electrophoresis. The separation indicates the existence of up to 7 different fractions in the samples, differing in charge, luminescence, its quantum yield, and composition of functional groups. Fig. 1. Excitation-emission maps of EDA-CA samples’ luminescence. The study was supported by the grant from the Russian Science Foundation No. 22-12-00138, https://rscf.ru/en/project/22-12-00138/. [1] N. Papaioannou, M.-M. Titirici, A. Sapelkin, Investigating the effect of reaction time on carbon dot formation, structure, and optical properties, ACS Omega, 4, pp. 21658-21665, (2019). [2] A. M. Vervald, K. A. Laptinskiy, M. Yu. Khmeleva, T. A. Dolenko, Toward carbon dots from citric acid and ethylenediamine, part 1: Structure, optical properties, main luminophore at different stages of synthesis, Carbon Trends, 19, 100452, (2025). [3] P. Duan, B. Zhi, L. Coburn, C. L. Haynes, K. Schmidt-Rohr, A Molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid-state nuclear magnetic resonance, Magn. Reson. Chem., 58, pp. 1130-1138, (2020).