PEI–folic acid modified carbon nanodots for cancer cell-targeted delivery and two-photon excitation imaging

Polyethyleneimine conjugated folic acid (PEI–FA) was coated onto the surfaces of fluorescent carbon nanodots (CDots) under neutral conditions through electrostatic interactions between the partially charged amino groups of PEI–FA and the carboxyl (–COOH) groups of the CDot surfaces. Because of the high abundance of folate receptors (FR) in cancer cells, folic acid (FA) was used as the targeting ligand to enhance the CDots' binding capability and penetration into the target cancer cells. We evaluated the amount of particles internalized by cancer cell lines displaying various levels of folate receptors. Two-photon excitation microscopy images revealed that the uptake of CDot–PEI–FA by FR-positive KB cancer cells was much higher than that by FR-negative A549 cancer cells. Moreover, quantitative flow cytometry analysis confirmed the receptor-mediated targeted delivery of the nanoparticles into different FR-expressing cells. The internalization of particles was ten-fold higher in the FR-positive KB cancer cells compared to that in the FR-negative A549 cancer cells; the difference was significant enough to be of biological importance. These preliminary results demonstrate the promising potential of CDot–PEI–FA for FR-positive cancer cell line diagnosis.