Hydrothermal Synthesis of Sulfuric Acid-Mediated Yellow Carbon Dots and It Applications in Fluorescent Ink and Cell Imaging.

In this work, a series of sulfuric acid-mediated carbon dots (S-CDs) with different sulfur content were facile synthesized by the hydrothermal reaction from contaminant reactive brilliant blue (RB5). The S-CDs (S-CD-4) are selected due to their excellent optical properties in water, and the detailed structure information were characterized by XRD, Raman spectrum, FTIR spectra, and XPS. The morphology and thickness were further characterized by TEM (HRTEM, average diameter around 3.45 nm), and AFM (7-9 layers). Optical properties, photo stability and solvent effect of this S-CDs are investigated by absorption and fluorescence emission spectra. Notably, the photoluminescence of S-CDs is excitation-wavelength dependent, with emission wavelengths tunable from 670 nm to 707 nm. The maximum fluorescent emission peak is 707 nm under 680 nm excitation wavelengths with a large Stokes shift of up to 227 nm in DMF, exhibiting a moderate fluorescence intensity of 19.0%. The relationship between the amount of sulfur in CDs and the emission wavelength is further discussed by DFT calculation. Finally, S-CDs are successfully applied in fluorescent ink and cell imaging.