Wood-Based Carbon Quantum Dots Regulate Photosynthetic Carbon Fixation and Enhance Mechanical Properties of Soybean Straw

In order to achieve the goal of ‘double carbon’ and promote the high-value utilization of biomass materials, nitrogen-doped carbon quantum dots (N-CQDs) with high crystallinity were successfully prepared by one-step hydrothermal method using renewable Salix powder as a precursor, and citric acid and urea as an activator and nitrogen source. The research focused on the regulation mechanism of N-CQDs as nano-modulators on photosynthetic carbon fixation of soybean plants, accumulation of micro-structural carbohydrates (cellulose/lignin) in the body, and finally the macro-mechanical strength of soybean straw. The results showed that N-CQDs had a significant ‘low promotion and high inhibition’ effect on the physiological metabolism of soybean. A 5 mg/L low concentration treatment could effectively expand the blue-violet light capture area, accelerate the Hill reaction electron transfer on the thylakoid membrane, and significantly increase the cellulose content in soybean stem by 15.4% compared with the control group. In terms of macroscopic mechanical properties, the shear force of soybean straw in the best treatment group reached 134.76 N, the maximum tensile stress reached 4.25 MPa, and the average increase was 58%. However, the high concentration treatment of 50 mg/L resulted in a sharp decrease of 17.7% in cellulose content and a decrease in tensile stress to 2.21 MPa due to nanotoxicity or light saturation inhibition. In this study, a closed loop of ‘nanodot synthesis-microchemical composition evolution-macromaterial mechanical mapping’ was constructed, which provided a new strategy for source regulation and performance enhancement of biomass functional materials.