Water-soluble carbon dots (CDs) have been successfully synthesized by green-one-pot hydrothermal method. The CDs with small particle size exhibit fairly good water solubility, tunable photoluminescence, up-converted photoluminescence, and low toxicity. What is more, the as-prepared CDs were successfully applied in the field of cell image. (C) 2015 Elsevier B.V. All rights reserved.