Efficient red-emitting carbon dots for bioimaging and spectral engineering by polyetherimide modification

A novel type of red-emitting carbon dots was synthesized using urea and neutral red via the solvothermal method at a temperature of 200 °C for 4 h. The resulting CDs, named NR-CDs, display a distinct red emission at 620 nm when excited at a wavelength of 470 nm, achieving a quantum yield of 61.11% in acetone. Interestingly, NR-CDs exhibit strict solvent polarity dependence, i.e. the emission wavelength of NR-CDs red-shifts with an increase in solvent polarity. Besides, when incubated with NR-CDs and excited by an excitation wavelength of 470 nm, 4-T1 cells emit a clear red fluorescence. Meanwhile, high-quality in-vivo fluorescent images of zebrafish embryos and larvae were obtained by incubating with NR-CDs. Moreover, by modifying with polyetherimide (PEI), the peak value of PEI-NR-CDs shifts from 620 nm to 596 nm, and the emission is increased by 264 %. Co-localization experiment with commercial dye (Lyso-Tracker Green) shows that PEI-NR-CDs are localized in lysosomes and could be used as a lysosomal tracer for cells. Furthermore, PEI-NR-CDs were prepared into fluorescent ink, which exhibited orange fluorescence under UV light. All results demonstrate that as-obtained NR-CDs and PEI-NR-CDs can be used as reagent for fluorescent-imaging application.