Simultaneously enhancing up-conversion fluorescence and red-shifting down-conversion luminescence of carbon dots by a simple hydrothermal process.

Fluorescent carbon dots (CDs) have attracted great attention in the biomedical field owing to their remarkable fluorescence property, low toxicity, and excellent water solubility. Because of the "water window" effect, the CDs with upconversion fluorescence or longer wavelength downconversion fluorescence are particularly more suitable for bioimaging because they can weaken the auto-fluorescence interferences and penetrate the cell or tissue more deeply. In this study, we reported a facile method to simultaneously enhance the upconversion fluorescence and tune the downconversion luminescence of CDs from blue to green light by simply adding H2O2 in a hydrothermal process. The H2O2-treated green fluorescent CDs with excellent upconversion property showed a high yield of 34% and a quantum yield of 16.5%, and low cell cytotoxicity, which could be directly used for live Hela cell imaging.