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Scheme 1 The luminescence mechanism of the red-emitting CDs and R-CDs as a highly efficient "on–off" fluorescent probe for the detection of Cr 2 O 7 2- and imaging in the living-cells. [Display omitted] • N/S-doped red emitting carbon dots (R-CDs) were prepared by a convenient one-step solvothermal method using o-PDA and p-ABSA as precursors. • These R-CDs could be used for the selective and sensitive detection of Cr 2 O 7 2- with excellent detection limit (80 nM). • The R-CDs possessed low cytotoxicity and had been successfully applied to cell imaging. Cr(VI) as a toxic heavy metal ion can easily enter into the body through drinking or eating and cause liver and kidney diseases as well as cancer. Considering its high biological toxicity and adverse effects on human body, it is desirable to develop a probe to monitor its level in the environment. Herein, a high-efficiency fluorescent nanoprobe based on red emissive carbon dots (R-CDs) was established through a convenient solvothermal strategy. The as-prepared CDs with excitation-independency had the fixed emission wavelength at 627 nm when the excitation wavelength was 560 nm. Further study manifested that the new surface state formed by nitrogen and sulfur doping and the increased conjugated system established through dehydration and carbonization were the main reasons for the fluorescence redshift. In this system, these R-CDs as a fluorescent probe exhibited high specificity and sensitivity to Cr 2 O 7 2- with the linear range of 4–40 μΜ and the limit of detection could reach 80.00 nM. The quenching of these CDs by Cr 2 O 7 2- was efficiently induced through a static quenching process. Meanwhile, the obtained CDs could enter into HeLa cells through endocytosis and exhibit bright red fluorescence in cells under a confocal laser scanning microscope. Thus, this work provided a promising probe not only for detecting Cr(VI) in natural environment but also for imaging in cells. [ABSTRACT FROM AUTHOR]