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.

Molecular, Aromatic, and Amorphous Domains of N-Carbon Dots: Leading toward the Competitive Photoluminescence and Photocatalytic Properties

“…15,19 As the reaction progresses, the carbon cores are formed by the dehydration of the initially formed molecules such as fluorophore molecules. We further studied the nuclear magnetic resonance (NMR) spectroscopy ( 1 H NMR and 13 C NMR) for the as-synthesized N-CDs to distinguish sp 3 hybridized carbon atoms from the sp 2 carbon atoms, and this is depicted in Figure S1. Three different kinds of chemical environments in different regions of the 1 H NMR spectra of N 1 −CDs have been observed.…”

“…4,9−12 In this connection, several groups have already put their efforts to explain the intriguing photophysical properties of CDs based on the intricate structural features. 6,9,13,14 However, establishing a general model is extremely difficult as the fundamental properties usually change quite extensively by varying the synthesis parameters, precursors molecules, amount of the precursors, etc. 12,15,16 Therefore, the structure−property correlation is still one of the major concerns to be addressed in this new class of materials for development in multimodal applicabilities.…”

Effect of heteroatoms on the optical properties and enzymatic activity of N-doped carbon dots

“…The peak of N graphite (401 eV) located within the sp 2carbon domain appeared in the spectra of N-CD_2, 3, 4, and 5 [35][36][37]. Furthermore, the O 1s spectra consist of two peaks corresponding to C-O and C]O bonds (532.6 and 531.1 eV, respectively), with the O C]O peak being predominant in all N-CD samples, owing to the polymerization of the material through amide bond formation 38,39. On the other hand, O C-O peak is dominant in the spectrum of the non-doped CD.…”

Carbon Dots: Classification, Properties, Synthesis, Characterization, and Applications in Health Care—An Updated Review (2018–2021)

“…Furthermore, using NMR for CDs' characterization is quite easy, because of its non-destructive nature. Despite these advantages, NMR also exhibits considerable demerits such as lower sensitivity, greater time-consumption, and higher costs, when compared to the mass spectrometric methods [155,[189][190][191]. Figure 10 embodies the illustrative NMR spectra for CDs' characterization [192].…”