A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid‐state nuclear magnetic resonance

The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave‐assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid‐state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5‐oxo‐1,2,3,5‐tetrahydroimidazo[1,2‐a]pyridine‐7‐carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross‐peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical‐shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. The composition of fluorescent “carbon dots” synthesized by microwave‐assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid‐state NMR, proving the presence of the molecular fluorophore 5‐oxo‐1,2,3,5‐tetrahydroimidazo[1,2‐a]pyridine‐7‐carboxylic acid (IPCA) or its derivatives and their dispersal in the polyamide matrix. Chemical shift perturbation indicates at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. From quantitative 13C NMR, a high IPCA concentration (18 ± 2 wt%) was determined.