Carboxy-Like Structure Determines Radical Scavenging Activity of Carbon Dots for Rice Resistance to Salt Stress

Abstract

Carbon dots (CDs) have an advancement in scavenging radicals. However, the potential struc-ture-function relationship is still unclear. Here, four kinds of CDs rich in carboxyl (CDs-c), hy-droxyl (CDs-h), and different contents of amino structures (CDs-a_0.2, CDs-a_2), respectively, were prepared by regulating the raw reagent using microwave assisted method. In vitro assays indicated that CDs-c had the highest scavenging activities against KMnO4, DPPH, ·OH, and O2- radicals. The changes in the morphology and chemical structure of these CDs after the reaction with ·OH suggested that the carboxy-like structures play significant roles in radical scavenging and antioxidant activity. Additionally, the calculation of reaction energy barriers using Density Functional Theory (DFT) revealed that the interaction between the carboxyl group and free radi-cals occurred in a barrier-free manner, resulting in the highest radical scavenging activity. In subsequent hydroponic experiments, rice seedlings pretreated with CDs-c showed the highest activity in their antioxidant defense system (SOD: 13.13%; POD: 40.55%; CAT: 133.33%; flavo-noid: 6.93%) and a significant enhancement in resistance to salt stress (fresh weight: 14.16%; height: 26.90%; chlorophyll content: 12.74%). This study uncovered the key active structures and mechanisms of CDs to scavenge oxidative radicals for plant antioxidant capacity under stress conditions and contributed to the management of environmental challenges faced by agriculture.