▎ 摘　　要

Doping graphene quantum dots (GQDs) with heteroatoms can provide new or improved structural and optical properties. Therefore, heteroatom doped GQDs have promising applications in environmental and biological fields. In this work, we present a novel nitrogen and phosphorus co-doped graphene quantum dots (N,P-GQDs), which were directly prepared via a facile hydrothermal method using tetrakis (hydroxymethyl) phosphonium chloride and ethylenediamine endcapped polyethylenimine as precursors. N,P-GQDs with two different doping ratios of nitrogen and phosphorus were prepared at different temperatures (230 and 250 degrees C). This N,P-GQDs showed good dispersity, thickness of 1-3 atomic layers and sizes of less than 20 nm. The N,P-GQDs were water dispersible, highly luminescent (with an absolute quantum yield of 9.4%), and showed excitation and concentration-dependent behavior. Moreover, the N,P-GQDs exhibited a sensitive response to NO2- in the concentration range of 5-30 nM with the detection limit of 2.5 nM. Significantly, cell toxicity experiment showed that the N,P-GQDs had negligible cytotoxicity and excellent biocompatibility. Thus, the N,P-GQDs could serve as a powerful fluorescent nano-sensor for imaging detection of nitrite in live cell. (C) 2016 Elsevier B.V. All rights reserved.