▎ 摘　　要

A facile one-step approach is developed for the large-scale synthesis of nitrogen doped graphene quantum dot (N-GQDs) at low temperature by using citric acid (CA) as the precursor. The emission wavelength of the N-GQDs is nearly excitation-dependent, which may mirror the effects from particles of different sizes and distribution of different emissive sites on nanoparticles. Moreover, the products are then tested with live Hela cells for the extraordinary biocompatibility. The cellular viability is remained to be 100% even after a 48 h exposure with the high N-GQDs concentration of 200 mu g mL(-1) and this survival rate is really much higher than that have been repotted previously. The as-prepared N-GQDs with negligible inherent toxicity exhibit excellent biological applications such as bio-imaging and cell tracking.