Toward High-Efficient Red Emissive Carbon Dots: Facile Preparation, Unique Properties, and Applications as Multifunctional Theranostic Agents

The achievement of high-efficient pure red emissive carbon dots (CDs) is still a great challenge as well as one of the most critical issues that hinders widespread applications of CDs. Herein, a facile approach for the preparation of high-efficient red emissive CDs (R-CDs) is reported, and they exhibit numerous unique features including pure red emission (λmax ≈ 640 nm), respectable quantum yield (22.9%), low cytotoxicity, two-photon excited fluorescence (TPEF), and high photothermal conversion efficiency (43.9% under irradiation of 671 nm laser). Moreover, the chemical composition and photophysical properties of the R-CDs are detailed characterized and analyzed, and from which their photoluminescence mechanism is proposed. Interestingly, the R-CDs are found to particularly light up RNA-rich nucleolus both in one-photon and two-photon modes as well as show excellent counterstain compatibilities with other classical subcellular dyes. The localization of the R-CDs in nucleolus is supported by ribonuclease digestion testing, and the stronger emission is further verified to be due to an accumulation process. In addition, the R-CDs are confirmed to be facilely conjugated with fluorescein isothiocyanate (FITC) and then bring it into living cells, which reveals their potentials to perform as carriers for delivery of drugs that cannot (or hardly) enter into living cells directly. Finally, the R-CDs are shown to be excellent in photothermal cancer therapy in vitro due to their high photothermal conversion efficiency. This study represents not only a facile method for the preparation of high-efficient R-CDs, but also opens many possibilities for applications, such as in biomedicine (multifunctional theranostic agents) and emitting/display devices, thanks to their unique and superior properties.