Reduced glutathione (GSH) is a crucial endogenous antioxidant that plays a central role in maintaining cell activity by involving in cellular defense against reactive oxygen species (ROS). Therefore, the detection of intracellular GSH is essential to monitor the cellular state. Herein, graphene oxide-MnO2-fluorescein (GO-MnO2-FL) nanocomposite was synthesized and used for GSH detection. GSH could reduce MnO2 to Mn2+ and block the energy transfer between FL and GO-MnO2, leading to the significant fluorescence enhancement of FL. The fluorescence-based GSH sensing has a limit of detection of 1.53 mu M and a wide detection range of 10 mu M-2 mM. Moreover, the GO-MnO2-FL nanocomposite was found to selectively image the cancer cells since the GSH content of cancer cells is higher than that of normal cells. In vivo experiments also confirmed the normal/cancer cell differentiation capability and excellent tumor-targeting ability of the GO-MnO2-FL nanocomposite. These properties make the GO-MnO2-FL nanocomposite an excellent fluorescent probe for GSH detection both in vitro and in vivo, and we believe it may hold great potential for GSH-mediated cancer diagnosis. (C) 2018 Elsevier Inc. All rights reserved.