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

Abstract 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 13 C, 13 C{ 1 H}, 1 H─ 13 C, 13 C{ 14 N}, and 15 N solid‐state nuclear magnetic resonance (NMR) experiments. 13 C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15 N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13 C NMR peaks, including an unusual ═CH signal at 84 ppm ( 1 H chemical shift of 5.8 ppm) and ═CN 2 at 155 ppm, and two distinctive 15 N 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 1 H─ 13 C HETCOR spectrum with brief 1 H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13 C and 15 N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13 C 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.