Carbon Dots (C dots) are an environmentally friendly class of fluorescent nanoparticles synthesized from earth-abundant elements. One of the biggest challenges in developing C dots for a broader range of applications is the lack of current understanding regarding their structure and fluorescent properties. In principle NMR has the molecular specificity and sensitivity to answer many of these questions, although to-date its application to C dots has been limited to qualitative insights. This limitation can be traced to the relatively low concentration of C dots used in NMR studies (typically < 10mg C dots/600 uL D2O) which have precluded the acquisition of detailed NMR, particularly ¹³C. However, we have shown that much higher concentrations of C dots (» 400mg/ 600 uL D₂O) remain colloidally stable. This has enabled the acquisition of quantitative ¹³C NMR and allowed the full range of solution phase NMR techniques (e.g. DOSY, HSQC) to be used for structural analysis of C dots synthesized from citric acid and ethylene diamine. For example, saturation transfer experiments have identified the presence of extended graphenic sheets that contain carbon and hydrogen atoms, not directly observable in NMR. Collective NMR analysis has enabled us to determine that the molecular IPCA fluorophore is embedded within a carbon core that contain both aliphatic and aromatic components. The relative concentration of these species can be quantified based on elemental analysis of the C dots and the use of internal NMR standards. In separate, but related experiments we have also used ¹³C NMR in conjunction with photobleaching to probe the fluorescence properties of C dots. These experiments reveal that there is a linear correlation between the C dot fluorescence and the IPCA concentration and that this molecular chromophore is responsible for the vast majority (> 80%) of the fluorescence. The residual contribution arising from a more photostable moiety which exhibits similar fluorescence properties to the IPCA. Overall, these studies highlight the potential for NMR to uncover the structure and properties of C dots, including the molecular identity of unknown chromophores.

Leslie Sigmon

Appearances