Interaction and energy transfer between carbon dots and serum human transferrin

Human serum transferrin has been used as a potential drug carrier to transport anticancer drugs to tumor cells, where transferrin receptors are overexpressed on the surface. Carbon dots are biocompatible, and have potential application to drug delivery. The interaction mechanism of human serum transferrin with carbon dots was investigated by fluorescence spectra, ultraviolet–visible absorption spectroscopy, circular dichroism, at extracellular pH 7.4 and endosomal pH 4.8, at varying temperatures. We observed a quenching of fluorescence of human serum transferrin in the presence of carbon dots by a static quenching mechanism and also analyzed the quenching results using the Stern–Volmer equation and obtained high-affinity binding to human serum transferrin dominated by hydrogen bonds or van der Waals forces. An isoemissive point was seen, indicating that quenching of human serum transferrin depends on the formation of human serum transferrin–carbon dots complex. Binding distance between carbon dots and human serum transferrin substantiated that the photoinduced electron transfer and Föster nonradiative resonance energy transfer mechanism were also involved in the quenching of protein. Furthermore, the circular dichroism also showed that carbon dots changed the secondary structure of human serum transferrin to some extent with decreased α-helix content or peak wavelength red-shifted. This study provides some theoretical foundation for carbon dots as a promising probe to monitor the process of delivering drugs to human serum transferrin in the future.