Graphitic nitrogen induced identical fluorescent emission of carbon dots for scalable anti-counterfeiting applications

Identical fluorescent emission in carbon dots (CDs) is crucial for applications such as anti-counterfeiting which requires precise spectrum control. However, the optical properties of CDs are highly sensitive to reaction conditions, which complicate their consistency. In this study, nitrogen-doped CDs (N-CDs) were synthesized using a single precursor (2,4-dihydroxypyridine) under different conditions, all exhibiting identical blue fluorescence, regardless of size and reaction parameters. X-ray photoelectron spectroscopy (XPS) and elemental analysis confirmed a high concentration of graphitic and pyridinic nitrogen in the N-CDs. Density functional theory (DFT) calculations were employed to explore the influence of these nitrogen species on the optical properties of the N-CDs. The results demonstrated that graphitic nitrogen plays a pivotal role in size-independent emission by disrupting the conjugated sp²-carbon domains, effectively eliminating the quantum confinement effect. This study offers new insights into the role of graphitic nitrogen in band gap engineering and the fluorescence mechanism of CDs, revealing a path to achieve CDs with identical fluorescent emission. The stable and reproducible emission under various reaction conditions positions these N-CDs as ideal candidates for large-scale production of blue CDs, minimizing the need for complex purification processes. Furthermore, N-CDs were modified with urea to enhance hydrophilicity, making them suitable for use as markers in aqueous products. The resulting hydrogen-bond-enhanced reticulated structure also endowed the material with room-temperature phosphorescence (RTP) properties. This innovation grants N-CDs substantial potential for anti-counterfeiting applications across various media.