Superior photodynamic effect of carbon quantum dots through both type I and type II pathways: Detailed comparison study of top-down-synthesized and bottom-up-synthesized carbon quantum dots

Carbon quantum dots (CQDs) have been extensively studied for bioimaging and photodynamic applications due to their low cost, excellent biocompatibility, rich surface chemistry and controllable optical properties. However, the detailed mechanism of the photodynamic activity has been rarely reported. To the best of our knowledge, this is the first report of (i) a systematic comparison of differently synthesized CQDs to unveil a relationship between chemical structure and photodynamic effect, and (ii) their detailed mechanism of action in photodynamic effects. CQDs prepared by top-down and bottom-up methods and their post-synthesis modification were compared in this study. CQDs prepared by a top-down method exhibited superior light-activated cell cytotoxicity compared to those by a bottom-up approach. The photodynamic index of CQDs was found to be 40–150 times larger than commercial photodynamic agents. It was concluded that both structural defects in sp2-carbon domains and oxygen-containing chemical groups have a crucial role in the excellent photodynamic performance. Measurements with selective quenchers of 1O2 and radical species indicated that the photodynamic mechanism of CQDs is through the combination of both type I (radical species production) and type II (singlet oxygen production) pathways.