Ultrastable green fluorescence carbon dots with a high quantum yield for bioimaging and use as theranostic carriers

Carbon dots (Cdots) have recently emerged as a novel platform of fluorescent nanomaterials. These carbon nanoparticles have great potential in biomedical applications such as bioimaging as they exhibit excellent photoluminescence properties, chemical inertness and low cytotoxicity in comparison to widely used semiconductor quantum dots. However, it remains a great challenge to prepare highly stable, water-soluble green luminescent Cdots with a high quantum yield. Herein we report a new synthesis route for green luminescent Cdots imbuing these desirable properties and demonstrate their potential in biomedical applications. Oligoethylenimine (OEI)-beta-cyclodextrin (beta CD) Cdots were synthesised using a simple and fast heating method in phosphoric acid. The synthesised Cdots showed strong green fluorescence under UV excitation with a 30% quantum yield and exhibited superior stability over a wide pH range. We further assembled the Cdots into nanocomplexes with hyaluronic acid for potential use as theranostic carriers. After confirming that the Cdot nanocomplexes exhibited negligible cytotoxicity with H1299 lung cancer cells, in vitro bioimaging of the Cdots and nanocomplexes was carried out. Doxorubicin (Dox), an anticancer drug, was also loaded into the nanocomplexes and the cytotoxicity effect of Dox loaded nanocomplexes with H1299 lung cancer cells was evaluated. Thus, this work demonstrates the great potential of the novel OEI-beta CD Cdots in bioimaging and as theranostic carriers.