Strongly tricolor-emitting carbon dots synthesized by a combined aging–annealing route and their bio-application

Abstract

Compared with the traditional semiconductor quantum dots and organic dyes, fluorescent carbon dots (FCDs) are superior because of their high hydrophilicity, robust chemical inertness, facile functionalization, high resistance to photobleaching, low toxicity, and good biocompatibility. However, to get FCDs with a high quantum yield (QY) in a large quantity has still been challenge recently. Here we report a novel and simple method, the combined aging–annealing way, by which the CA–N–FCDs with specific characters were prepared using citric acid (CA) and urea (N source) as the raw materials. Compared with the use of a single hydrothermal treatment or annealing at relatively low temperature, the combination of the two methods can make excellent CDs. The QY of 78.8% of the as-prepared CA–N–FCDs is one of the highest yields reported till now. The optimal conditions for preparing the CA–N–FCDs were obtained including the hydrothermal aging temperature, time and the annealing conditions. The aging time was 1.5 h at around 170 °C, far lower than that used in the reported annealing or calcinating processes. The brightest sample was produced by annealing at 250 °C under the excitation of 365 nm light. The resulting uniform carbon nanoparticles with an average size of less than 10 nm were tricolor-emitting and blue was dominant, but including red and green colors. The CA–N–FCDs were of extremely low cell cytotoxicity applicable for being a kind of safe bio-material. Based on their high fluorescence quantum efficiency and the advantages mentioned above, these FCDs were then used for cell imaging and exhibited a perfect performance under 3 kinds of excitation bands (UV, blue, and green lights). Thus, these novel carbon dots will be able to be practically applied to immune labeling and imaging in vivo in the near future.