Gadolinium-doped carbon dots as nano-theranostic agents for MR/FL diagnosis and gene delivery.

Nano-theranostic agents play important roles in the development of therapeutic methods for serious diseases. In this study, novel carbon dots (CDs) L-CD/C-CD were prepared from Gd(iii) salt/complexes, cationic polymers and citric acid in the hope that they would combine the abilities of gene delivery and multi-modal (MR/FL) imaging. The CDs inherited the properties of good water-solubility and positive charge from their precursor polymers. In vitro gene transfection results showed that the CDs have good transfection efficiency and anti-serum ability, especially for L-CD, which has 74 times higher transfection efficiency than PEI 25 kDa in the presence of 10% serum. The CDs exhibited bright fluorescence, which was stable for several days under various pH. Confocal laser scanning microscopy revealed that the CDs could image HeLa cells with blue or green fluorescence well, and realize the monitoring of the gene delivery process. Besides, the CDs showed favorable biocompatibility with excellent performance in longitudinal relaxivity rates (r1) of 11.4 mM-1 s-1 for L-CD and 57.6 mM-1 s-1 for C-CD, which were about 3-15 times higher than that of the clinical Gd reagent Gd-DTPA (3.75 mM-1 s-1). Furthermore, the CDs could perform in vivo tumor-specific MR-imaging more clearly than Gd-DTPA, which is attributed to their suitable particle size and their resulting greater accumulation at tumor site via the EPR effect. This study provides a promising strategy for constructing multi-functional CDs for tumor theranostics.