Red Emissive Sulfur, Nitrogen Codoped Carbon Dots and Their Application in Ion Detection and Theraonostics

It is highly desirable and a great challenge for red light emission of carbon dots under long wavelength excitation. Here, we developed a facile route to synthesize carbon dots with red emission due to the doping effect of S and N elements, borrowing from the concept of the semiconductor. The maximum emission locates at 594 nm under 560 nm excitation. The absolute photoluminescence (PL) quantum yield (QY) is as high as 29% and 22% in ethanol and water, respectively. XPS and FTIR spectra illustrated that there exist -SCN and -COOH groups on the surface of the carbon dots. They endow the carbon dots with high sensitivity for ion detection of Fe³⁺. The quenched PL emission of Fe³⁺-S,N-CDs can be recovered by adding ascorbic acid to release the -COOH and -SCN group due to Fe²⁺ formation in the presence of ascorbic acid. High PL QY of red emission is beneficial to application in bioimaging. Doxorubicin was loaded onto carbon dots through π-π stacking to form a theranostic agent. When the CD-Dox was injected into the tumor site, a strong PL emission was observed. The PL intensity indicates the concentration of the theranostic agent. After 7 times injection, both the tumor size and weight clearly decrease. The results demonstrate that the S,N-CDs are a potentially excellent bioimaging component in the theranostic field.