Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots

Abstract

Carbon dots (CDs) are a stable and highly biocompatible fluorescent material offering great application potential in cell labeling, optical imaging, LED diodes, and optoelectronic technologies. Because their emission wavelengths provide the best tissue penetration, red-emitting CDs are of particular interest for applications in biomedical technologies. Current synthetic strategies enabling red-shifted emission include increasing the CD particle size (sp domain) by a proper synthetic strategy and tuning the surface chemistry of CDs with suitable functional groups (e.g., carboxyl). Here we present an elegant route for preparing full-color CDs with well-controllable fluorescence at blue, green, yellow, or red wavelengths. The two-step procedure involves the synthesis of a full-color-emitting mixture of CDs from citric acid and urea in formamide followed by separation of the individual fluorescent fractions by column chromatography based on differences in CD charge. Red-emitting CDs, which had the most negative charge, were separated as the last fraction. The trend in the separation, surface charge, and red-shift of photoluminescence was caused by increasing amount of graphitic nitrogen in the CD structure, as was clearly proved by XPS, FT-IR, Raman spectroscopy, and DFT calculations. Importantly, graphitic nitrogen generates midgap states within the HOMO-LUMO gap of the undoped systems, resulting in significantly red-shifted light absorption that in turn gives rise to fluorescence at the low-energy end of the visible spectrum. The presented findings identify graphitic nitrogen as another crucial factor that can red-shift the CD photoluminescence.

Solvent‐Controlled Synthesis of Highly Luminescent Carbon Dots with a Wide Color Gamut and Narrowed Emission Peak Widths

“…The larger π‐electron system or the higher graphitic nitrogen content will result in the narrower energy gap, so as to shift the PL emission to the red region. As another proof, the theoretical calculations based on density functional theory note that the band gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital narrows when the particle size increases or the graphitic nitrogen content improves . Based on these results and conclusions, we believe that the PL emission of our CDs is based on surface‐states, and that their energy gaps are modulated by both the π‐electron system and graphitic nitrogen content.…”

Sustainable Synthesis of Bright Green Fluorescent Nitrogen‐Doped Carbon Quantum Dots from Alkali Lignin

“…The phenomenon may be caused by the presence of graphite nitrogen in the CQDs (Figure d), which did not appear in other relevant reports . Graphite nitrogen in CQDs provides an excess of electrons for the unoccupied π* orbitals of the conjugated system, making the HOMO→LUMO gap small (only 2.07 eV) and resulting in a PL redshift of the CQDs …”

Customizing the Electrochemical Properties of Carbon Nanodots by Using Quinones in Bottom‐Up Synthesis

“…Achieving such control is essential in order to produce high-quality nanomaterials with desired energy levels for applications.T his effort becomes even more relevant if we compare how the control of the redox activity is currently achieved for other types of materials. [32][33][34] Since CNDs could play an important role in applications such as energy conversion and photocatalysis, [10] for example,h ydrogen evolution, [35][36][37][38][39] several improvements have been proposed, among which are graphitization and N-doping. [28][29][30][31] Unfortunately,f or CNDs,itisstill necessary to tediously purify the as-prepared mixtures of carbon dots,w hich contain various sizes,d oping ratios,and surfaces.…”