Fluorescent detections of cobalt, chromium, and ascorbic acid by carbon dots are of importance for human health and environment. Green synthesis of fluorescent carbon dots from biomass is essential for their sustainable applications. Herein, carbon dots were successfully prepared by a simple hydrothermal method using flax straw as carbon source. The obtained carbon dots possess a much higher quantum yield (20.7%) and excitation-dependent photoluminescence behavior. The "on-off" fluorescence principle, quenching of the fluorescence intensity of the carbon dots in the presence of Co2+ or Cr6+ based on static quenching effect and inner filter effect, is extended to "on-off-on" principle for detection of ascorbic acid based on the reduction of Cr6+ to Cr3+ by ascorbic acid. The linear ranges for detections of Co2+, Cr6+, and ascorbic acid are 0-500, 0.5-80, and 0-200 mu M, and the limits of the corresponding detections are 0.38, 0.19, and 0.35 mu M, respectively. Compared with most reported fluorescence detections, our linear ranges are significantly wider and our detection limits are much lower for the detections of Co2+, Cr6+, and ascorbic acid. This sensing platform is highly sensitive and selective to monitor Co2+ and Cr6+ in real water and ascorbic acid in vitamin C tablets with remarkable practicality. (C) 2020 Elsevier Inc. All rights reserved.