Influence of carbonization conditions on luminescence and gene delivery properties of nitrogen-doped carbon dots

Carbon dots (CDs) have been intensively investigated due to their unique photoluminescence (PL) properties that are improved through surface passivation with nitrogen-containing groups. Recently, gene delivery applications emerged as passivation of CDs may yield positively charged nanoparticles that can interact with negatively charged nucleic acids. However previous work in the field focused on the use of high molecular weight polyamines for CD passivation, posing the problem of the separation of nanoparticles from residual polymer that is harmful to cells. In this work, cationic CDs were prepared by pyrolysis of citric acid/bPEI600 (1/4, w/w) so unreacted low molecular weight reagents could be conveniently eliminated by extensive dialysis. Various reaction conditions and activation modes were evaluated and eleven CDs that exhibited superior solubility in water were produced. All the nanoparticles were characterized with respect to their physical, optical and PL properties and their ability to deliver plasmid DNA to mammal cells was evaluated. Despite their similar physical properties, the CDs displayed marked differences in their gene delivery efficiency. CDs produced under microwave irradiation in a domestic oven were revealed to be superior to all the other nanoparticles produced in this study and compared to the gold standard transfection reagent bPEI25k, with an optimal CD/pDNA w/w ratio that was significantly down shifted, as was the associated cytotoxicity.