Investigating the Synthesis, Structure, and Composition of Carbon Dots Using Solution-Phase NMR

Since their discovery, the photoluminescent properties and potential applications of carbon dots (CDs) have been studied extensively. However, some aspects of their structure and synthesis remain ambiguous because of limitations inherent to the analytical techniques used to study their structure. Herein, we demonstrate that high-concentration solution-phase 13C and 1H NMR enable quantitative investigation of the structural properties of two distinct CDs synthesized with a citric acid precursor, combined with either ethylenediamine or phenylalanine as a nitrogen source. Solution-phase NMR experiments reveal key insights into fluorophore binding, while saturation transfer (ST) NMR experiments conducted at different stages of the CACD synthesis provide indirect evidence for the formation of an aromatic CD core whose formation follows the creation of the polymeric segment. Additionally, advanced two-dimensional NMR techniques often used to investigate protein structure and dynamics were used for structural characterization of CDs at various stages of synthesis: reagents, prefiltration, dialysate, and purified CDs. Diffusion-ordered spectroscopy (DOSY) was used to probe CD hydrodynamic size and supported the presence of a single, multicomponent structure, while heteronuclear single-quantum coherence (HSQC) and heteronuclear multiple-bond correlation (HMBC) spectra revealed bonding between the polymeric component and the small-molecule fluorophore species of these CDs. For the citric acid/ethylenediamine CDs, distinct spectral signatures were observed for free and bound fluorophore. By combining these techniques, quantitative estimates of the relative contributions of the aromatic core, polymeric component, and fluorophore in citric acid/ethylenediamine CDs are reported as 60%, 36%, and 4%, respectively. Additionally, quantitative estimates of the polymeric segment and aromatic core in citric acid/phenylalanine CDs are 72% and 28%, respectively. Collectively, the analytical techniques outlined herein provide a generalizable framework for resolving CD structure and composition using solution-phase NMR, information that is important for determining structure-property relationships.