Graphene Quantum Dots: Efficient Mechanosynthesis, White-Light and Broad Linearly Excitation-Dependent Photoluminescence and Growth Inhibition on Bladder Cancer Cells

Heteroatom-doped graphene quantum dots (GQDs) have attracted considerable attention due to their potential applications in luminescent materials and biology. In this work, we developed a solvent-free gram-scale mechanochemical method for the preparation of nitrogen-doped graphene quantum dots (N-GQDs) with the highest solubility (31 mg/mL) in water reported to date. Commercial graphite was sheared and cut through grinding with solid melamine, then ground with solid KOH to get sub-5-nm-sized, 1-3-layered N-GQDs. Notably, these N-GQDs exhibit white-light emission and a broad excitation-dependent full-color photoluminescence from 463 nm to 672 nm. When the excitation light ranged from 325 nm to 485 nm, these mechanochemically-obtained N-GQDs exhibited bright white-light emission. Intriguingly, the change of emission wavelength has two-stage linear relationships with the change of the excitation, and the inflection point is at 580 nm (excited at 550 nm). The difference between the emission and excitation wavelength decreases from 138 to 12 nm, which also shows two-stage linear relationships with the change of the excitation wavelength. It is notable that the PL quantum yields of them are high, up to 26.6%. Furthermore, we studied the inhabitation of as-obtained N-GQDs on bladder cancer cells (UMUC-3); as a result, with the increase of the concentration of N-GQDs, the proliferation of cancer cells was obviously prohibited.