Solvent-free pyrolysis synthesis of solid-state fluorescent carbon dots: Optical properties and latent fingerprint imaging

Carbon dots (CDs) have garnered significant attention due to their remarkable optical properties and low cytotoxicity. However, most studies on CDs have predominantly focused on their fluorescence behavior in solution, because their π-π stacking interactions, fluorescence resonance energy transfer, and surface electron transitions can induce fluorescence quenching through aggregation. In this investigation, three variants of solid-state fluorescent CDs were synthesized using a solvent-free pyrolysis technique under atmospheric pressure at 200 °C. In the solvent-free CDs syntheses, urea as a dispersant, while the carbon precursors (m-phenylenediamine, DL-aspartic acid, and p-phenylenediamine) were diversified. The photoluminescence quantum yields of the resultant blue-, yellow-, and red-emitting CDs powders were measured to be 11.62, 3.59, and 0.57 %, respectively. Radiative transitions resulting from the eigenstates of CDs dominate. Fluorescence decay studies suggested that radiative transitions resulting from the eigenstates of the CDs dominate as the emission maximum shifts to the red region. These CDs exhibit outstanding solid-state fluorescence, enabling their successful utilization in latent fingerprinting imaging. Remarkably, the red-emitting CDs also exhibited blue room-temperature phosphorescence, observable for ~3 s.