Fig. 1

The characterization of N-GQDs. a The TEM images and size distribution of N-GQDs; b, c the AFM topography and height distribution of N-GQDs; d the FT-IR spectrum of N-GQDs; e, f the hydrous particle diameter of N-GQDs in deionized (DI) water and culture medium; g the UV–Vis absorption spectra of N-GQDs in DI water and culture medium; h, i the excitation and emission spectra of N-GQDs in DI water and culture medium at λem = 420 nm and 480 nm, or λex = 350 nm and 380 nm