Nitrogen and boron doped carbon quantum dots (NB-CQDs) with yellow-green emission were prepared via hydrothermal method using 2-hydroxyphenylboronic acid and ethylenediamine in aqueous solution. The as-prepared NB-CQDs exhibited bright yellow-green emission, excellent stability in aqueous solution with high ionic strength and good biocompatibility. We further explored the applications of NB-CQDs for sensing metal ions in aqueous solution. We found that the NB-CQDs could be utilized to sense Cr2O72− in aqueous solution due to the Cr2O72−-induced fluorescence quenching. In the sensing system, as low as 0.5 μM Cr2O72− can be successfully detected and a good linear relationship in the range of 0–250 μM has been obtained. And the NB-CQDs can be also used as ink for writing on paper, revealing the NB-CQDs can be applied for anti-counterfeiting. Moreover, the NB-CQDs exhibit negligible cytotoxicity and good biocompatibility, which is favorable for bioimaging applications. And the NB-CQDs were successfully applied for fluorescence imaging in HeLa cells.
