"Light on" fluorescence carbon dots with intramolecular hydrogen bond-regulated co-planarization for cell imaging and temperature sensing

Turn-on thermosensitive carbon dots (CDs) with dual functions of imaging and sensing are desirable for biological research and clinical diagnosis at the cellular level. Herein, we synthesized eight types of novel CDs by solvothermal treatment of selected carbon sources and chromatographic purification for turn-on fluorescence measurement of temperature and cell imaging. Comprehensive experimental characterization and theoretical calculations were conducted to reveal the structure and photoluminescence (PL) of the as-prepared CDs. The results elucidate that the co-planarization and electron cloud density distribution of the fluorophores can be regulated by the intramolecular hydrogen bonds, resulting in the temperature turn-on fluorescence response of the CDs by the effective suppression of nonradiative decay and enhancement of the radiative decay process during heating. The fluorescence responses are reversible to temperature variations in the tested range from 15 to 85 degrees C. The highest temperature sensitivity up to 5.3%/degrees C with a resolution of 0.09 degrees C is observed among all the prepared CDs, which is probably the most sensitive fluorescence thermometer reported so far. Moreover, the CDs exhibit excellent water solubility, minimal toxicity and superb photostability. Thus they have been successfully used as a dual functional turn-on nanothermometer with intracellular imaging probing features, demonstrating a promising prospect in the study of cellular states and functions with respect to temperature changes.