Antioxidant activity and in vitro fluorescence imaging application of N-, O- functionalized carbon dots

Nanomaterials with dual-functions integrating diagnostic and therapeutic abilities have attracted the interest in biomedical applications, and low-dimensional carbon dots have shown their potentialities in the field owing to their versatile optical and physicochemical properties. Yet the link between the surface emissive states and their structure and composition is not well understood, and their stability and biocompatibility needs to be further investigated. We have prepared a series of N- and O-doped carbon dots from a commercial commodity with a high surface functionalization, and performed a deep analysis to rationalize the structure-performance indicators that control their fluorescence, cytotoxicity and antioxidant properties. The synthesized carbon dots exhibited broad multiple surface emissive states: a bright blue emission at 430 nm governed by electronic transitions involving pyridones and carbonyl moieties, and a greenish emission at 500 nm due to transitions involving C-N and C-O groups or trap states. The carbon dots displayed good photostability with negligible photobleaching over continuous excitation during 2 h. The carbon dots also displayed good antioxidant activity correlated to the electron storage capacity of the aromatic core and O- and N- groups with proton exchange capacity. The carbon dots showed excellent cytotoxicity on human gingival fibroblast cell lines, and good response in in vitro fluorescence imaging over a wide concentrations range (0.05-1 mg/mL), similar to other contrast agents, demonstrating the potential of these N-O- doped carbon dots in imaging applications.