To investigate the effect of nitrogen on the photoluminescence properties of carbon quantum dots (CQDs), N-doped carbon quantum dots (N-CQDs) were synthesized by one-step hydrothermal treatment using biomass tar as the carbon precursor. As an inevitable organic pollutant, the unsaturated bonds in biomass tar, such as carboxylic acids, aldehydes, and aromatics, are favorable for formation of the graphitic carbon lattice. The obtained N-CQDs are spherical with an average particle size of 2.64 nm and the crystal lattice spacing is 0.25 nm, corresponding to the (100) facet of graphitic carbon. The N-CQDs emit bright blue photoluminescence under 365 nm ultraviolet light, and they have excellent water solubility and stability with a high quantum yield of 26.1%. Coordination between the functional groups on the N-CQD surface and Fe3+ ions is promoted because of the improved electronic properties and surface chemical reactivity caused by N atoms, leading to a significant fluorescence quenching effect of the N-CQDs in the presence of Fe3+ ions with high selectivity and sensitivity. There is a linear relationship between In (F-0/F) and the Fe3+ concentration in the N-CQD concentration range 0.06-1400 mu mol/L with a detection limit of 60 nmol/L, showing that the N-CQDs have great potential as a fluorescent probe for Fe3+ detection. (C) 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.