Rational synthesis of red carbon dots with ultra-narrow bandwidth emission for precise live cell imaging

Fluorescence imaging is crucial for both fundamental research and biomedical applications, but the scarcity of low-cost, stable, and biocompatible fluorescence probes limits its effectiveness. Carbon dots (CDs), a class of nanomaterials with a size below 10 nm, offer advantages for bioimaging such as simple preparation, low-cost synthesis, biocompatibility, photostability, and tunable emission spectra. However, the broad fluorescence emission width reduces their imaging accuracy and color purity, affecting their suitability as bioimaging agents. In this study, the synthesis of narrow-emission bandwidth CDs and the effect of the precursor on their full width at half maximum (FWHM) were investigated. Four types of CDs with narrow red emission were prepared, CD1 from o-phenylenediamine (OPD), CD2 from OPD and urea, CD3 from OPD and L-phenylalanine (Phe), and finally CD4 from OPD, Phe, and urea. The synthesis temperature and time for CD1-CD4 were 180 °C and 8 h. The results showed excellent biocompatibility for CDs so that up to the concentration of 0.5 mg/mL, cell viability remains above 80%. Finally, CD3 with the narrowest FWHM (28.9 nm) showed the best capability for red staining of MCF-7 cells, especially for nucleolus imaging. The synthesized CDs represent a promising step in the research and development of fluorescence probes for cellular imaging applications.