A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid‐state nuclear magnetic resonance. (23rd January 2020)

• Record Type: Journal Article
• Authors: Duan, Pu; Zhi, Bo; Coburn, Luke; Haynes, Christy L.; Schmidt‐Rohr, Klaus
• Other Names: Abraham Anuji guestEditor.; Krishnan Damodaran guestEditor.; Salager Elodie guestEditor.; Su Yongchao guestEditor.
• Is Part Of: Magnetic resonance in chemistry. Volume 58:Number 11(2020)
• Page Start: 1130
• Page End: 1138
• Publication Date: 2020-01-23
• Subjects: Quantitative solid‐state 13C NMR -- spectral editing -- 13C{14N} NMR -- fluorescent carbon dots -- molecular fluorophores -- photoluminescent polymer dots -- quantitative solid‐state 13C NMR -- spectral editing; Nuclear magnetic resonance spectroscopy -- Periodicals; Chemistry, Organic -- Periodicals; Magnetic resonance -- Periodicals; 538.36
• DOI: 10.1002/mrc.4985
• Languages: English
• ISSNs: 0749-1581

Abstract

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 ═CN2 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. Abstract : The composition of fluorescent "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 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 13 C NMR, a high IPCA concentration (18 ± 2 wt%) was determined.