To develop nitrogen-doped carbon quantum dots (NCQDs) with antioxidant and immunomodulatory functions and evaluate their effects on peripheral nerve injury repair using a sciatic nerve injury model.

(1) In vitro experiment: NCQDs were synthesized using diallyldimethylammonium chloride and anhydrous citric acid. NCQDs were set to 4 concentration groups of 0, 10, 20, and 30 μg/mL, named CON group, NCQDs-10 group, NCQDs-20 group, and NCQDs-30 group. Mouse monocyte macrophage leukemia cell line (RAW 264.7) was inoculated into a culture plate according to the experimental design. After the cells adhered to the wall and the cell status was confirmed to be good, corresponding concentrations of NCQDs were added and cultured for 1 and 4 d, respectively, to detect cytotoxicity. After 24 h of culture, the antioxidant performance was detected. Five dorsal root ganglia (DRG) neurons from C57 mice were taken, and DAPI/NF-200 immunofluorescence staining was used to observe the effect of NCQDs on neuronal growth. NCQDs were used to treat DRG neurons for 12 and 24 h to verify their cytotoxicity and immunomodulatory properties on neurons. (2) In vivo experiment: A sciatic nerve injury model was established in 6-week-old C57 mice, which were randomly divided into a control group and a NCQDs group. Normal saline and NCQDs were locally injected into the injury site, respectively. At 7 d post-injury, nerve regeneration was assessed via nerve histology and spinal cord pathological sections. At 28 d post-injury, functional recovery of the sciatic nerve and ipsilateral hindlimb muscles were further evaluated using electrophysiology, footprint analysis, and gastrocnemius muscle histological staining. Serum biochemical parameters and H&E staining of major organs were performed to assess the in vivo biosafety of NCQDs.

(1) In vitro experiment: NCQDs exhibited excellent biocompatibility and can significantly reduce RAW264.7 cell activity, decrease ROS expression levels, and maintain a stable ROS clearance rate within 30 d. Zeta potential detection showed that the Zeta potential of NCQDs was +6.98 mV. Enzyme-linked immunosorbent assay detection showed that compared with the CON group, the expression levels of tumor necrosis factor-α, interleukin (IL)-6, IL-8, and IL-1β in the NCQDs-10 group, NCQDs-20 group, and NCQDs-30 group were all reduced, and the differences were statistically significant (P<0.05). (2) In vivo experiment: Compared with the control group, the NCQDs group showed a significant increase in the number of positive neurons, an increase in the number and distance of regenerated axons at the injury site, a significant reduction in action potential latency, and an increase in conduction velocity, with statistically significant differences (P<0.05); Behavioral analysis showed significant improvements in footprint area, contact density, and plantar pressure, while the average diameter of gastrocnemius muscle fibers increased significantly, with statistically significant differences (P<0.05). Hematological and serum biochemical indicators, as well as histological examinations of major organs such as heart, liver, spleen, lungs, and kidneys in two groups of mice, showed no significant abnormalities.

NCQDs possess favorable biosafety both in vitro and in vivo, they significantly enhance sciatic nerve repair through combined anti-inflammatory and antioxidant mechanisms.