In this work, we invented a new method to synthesize boron- and nitrogen-co-doped graphene quantum dots (B, N-GQDs). This synthesis method was more reliable, simple, and environmentally friendly than the bottom-up methods reported in the current literature that use citric acid or other organic materials as the carbon source. Instead, B, N-GQDs were synthesized from graphene oxide (GO) by a top-down method. The obtained B, N-GQDs showed a bright blue luminescence under a UV lamp at 365 nm, and the absolute photoluminescence quantum yield (PLQY) was 5.13%. Because mercury(II) ions (Hg2+) are highly toxic and can seriously affect the ecological environment and human health, the detection of low-concentration Hg2+ and the establishment of rapid, accurate and sensitive methods for detecting Hg2+. are of great significance to the activities of life, including those of humans. Therefore, using the good luminescence properties of B, N-GQDs, we invented a new type of fluorescence sensor that can detect Hg2+. The sensor was very sensitive to Hg2+, had a good linear relationship in the concentration range of 0.2-1 mu M, and exhibited a minimum limit of detection of 6.4 nM. Furthermore, this approach was successfully applied to the detection of Hg2+ in real environmental water samples. (C) 2020 Elsevier Inc. All rights reserved.