Electrostatic regulated modification on the surface of carbon dots and its influence on photoluminescence

Blue-fluorescent carbon dots (CDs) were synthesized via a one-step hydrothermal method using 3,4-diaminobenzoic acid and citric acid as precursors. The as-prepared CDs were comprehensively characterized, revealing abundant surface functional groups, uniform particle size, a negatively charged surface, and excitation-independent emission behavior. By varying the ratio of the precursors, the isoelectric point of the CDs could be adjusted, thereby regulating their binding capacity with other substances. A positively charged fluorescent probe was constructed by electrostatically assembling the CDs with the cationic polymer polyethylenimine (PEI). This post‑synthetic surface modification strategy offered better controllability and tunability compared to directly using PEI as a precursor during CD synthesis. Furthermore, this probe was applied to RNA detection, demonstrating a good linear response with a detection limit of 65 µg/mL. Notably, it exhibited negligible response to structurally similar DNA, confirming its high selectivity for RNA. This study provides ideas for exploring and improving surface modification methods for CDs, and has broad prospects.